IP Sysctl¶

/proc/sys/net/ipv4/* Variables¶

ip_forward - BOOLEAN

0 - disabled (default)
not 0 - enabled

Forward Packets between interfaces.

This variable is special, its change resets all configuration parameters to their default state (RFC1122 for hosts, RFC1812 for routers)

ip_default_ttl - INTEGER

Default value of TTL field (Time To Live) for outgoing (but not forwarded) IP packets. Should be between 1 and 255 inclusive. Default: 64 (as recommended by RFC1700)

ip_no_pmtu_disc - INTEGER

Disable Path MTU Discovery. If enabled in mode 1 and a fragmentation-required ICMP is received, the PMTU to this destination will be set to the smallest of the old MTU to this destination and min_pmtu (see below). You will need to raise min_pmtu to the smallest interface MTU on your system manually if you want to avoid locally generated fragments.

In mode 2 incoming Path MTU Discovery messages will be discarded. Outgoing frames are handled the same as in mode 1, implicitly setting IP_PMTUDISC_DONT on every created socket.

Mode 3 is a hardened pmtu discover mode. The kernel will only accept fragmentation-needed errors if the underlying protocol can verify them besides a plain socket lookup. Current protocols for which pmtu events will be honored are TCP, SCTP and DCCP as they verify e.g. the sequence number or the association. This mode should not be enabled globally but is only intended to secure e.g. name servers in namespaces where TCP path mtu must still work but path MTU information of other protocols should be discarded. If enabled globally this mode could break other protocols.

Possible values: 0-3

Default: FALSE

min_pmtu - INTEGER

default 552 - minimum Path MTU. Unless this is changed mannually, each cached pmtu will never be lower than this setting.

ip_forward_use_pmtu - BOOLEAN

By default we don’t trust protocol path MTUs while forwarding because they could be easily forged and can lead to unwanted fragmentation by the router. You only need to enable this if you have user-space software which tries to discover path mtus by itself and depends on the kernel honoring this information. This is normally not the case.

Default: 0 (disabled)

Possible values:

0 - disabled
1 - enabled

fwmark_reflect - BOOLEAN

Controls the fwmark of kernel-generated IPv4 reply packets that are not associated with a socket for example, TCP RSTs or ICMP echo replies). If unset, these packets have a fwmark of zero. If set, they have the fwmark of the packet they are replying to.

Default: 0

fib_multipath_use_neigh - BOOLEAN

Use status of existing neighbor entry when determining nexthop for multipath routes. If disabled, neighbor information is not used and packets could be directed to a failed nexthop. Only valid for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.

Default: 0 (disabled)

Possible values:

0 - disabled
1 - enabled

fib_multipath_hash_policy - INTEGER

Controls which hash policy to use for multipath routes. Only valid for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.

Default: 0 (Layer 3)

Possible values:

0 - Layer 3
1 - Layer 4
2 - Layer 3 or inner Layer 3 if present
3 - Custom multipath hash. Fields used for multipath hash calculation are determined by fib_multipath_hash_fields sysctl

fib_multipath_hash_fields - UNSIGNED INTEGER

When fib_multipath_hash_policy is set to 3 (custom multipath hash), the fields used for multipath hash calculation are determined by this sysctl.

This value is a bitmask which enables various fields for multipath hash calculation.

Possible fields are:

0x0001	Source IP address
0x0002	Destination IP address
0x0004	IP protocol
0x0008	Unused (Flow Label)
0x0010	Source port
0x0020	Destination port
0x0040	Inner source IP address
0x0080	Inner destination IP address
0x0100	Inner IP protocol
0x0200	Inner Flow Label
0x0400	Inner source port
0x0800	Inner destination port

Default: 0x0007 (source IP, destination IP and IP protocol)

fib_sync_mem - UNSIGNED INTEGER

Amount of dirty memory from fib entries that can be backlogged before synchronize_rcu is forced.

Default: 512kB Minimum: 64kB Maximum: 64MB

ip_forward_update_priority - INTEGER

Whether to update SKB priority from “TOS” field in IPv4 header after it is forwarded. The new SKB priority is mapped from TOS field value according to an rt_tos2priority table (see e.g. man tc-prio).

Default: 1 (Update priority.)

Possible values:

0 - Do not update priority.
1 - Update priority.

route/max_size - INTEGER

Maximum number of routes allowed in the kernel. Increase this when using large numbers of interfaces and/or routes.

From linux kernel 3.6 onwards, this is deprecated for ipv4 as route cache is no longer used.

neigh/default/gc_thresh1 - INTEGER

Minimum number of entries to keep. Garbage collector will not purge entries if there are fewer than this number.

Default: 128

neigh/default/gc_thresh2 - INTEGER

Threshold when garbage collector becomes more aggressive about purging entries. Entries older than 5 seconds will be cleared when over this number.

Default: 512

neigh/default/gc_thresh3 - INTEGER

Maximum number of non-PERMANENT neighbor entries allowed. Increase this when using large numbers of interfaces and when communicating with large numbers of directly-connected peers.

Default: 1024

neigh/default/unres_qlen_bytes - INTEGER

The maximum number of bytes which may be used by packets queued for each unresolved address by other network layers. (added in linux 3.3)

Setting negative value is meaningless and will return error.

Default: SK_WMEM_MAX, (same as net.core.wmem_default).

Exact value depends on architecture and kernel options, but should be enough to allow queuing 256 packets of medium size.

neigh/default/unres_qlen - INTEGER

The maximum number of packets which may be queued for each unresolved address by other network layers.

(deprecated in linux 3.3) : use unres_qlen_bytes instead.

Prior to linux 3.3, the default value is 3 which may cause unexpected packet loss. The current default value is calculated according to default value of unres_qlen_bytes and true size of packet.

Default: 101

neigh/default/interval_probe_time_ms - INTEGER

The probe interval for neighbor entries with NTF_MANAGED flag, the min value is 1.

Default: 5000

mtu_expires - INTEGER

Time, in seconds, that cached PMTU information is kept.

min_adv_mss - INTEGER

The advertised MSS depends on the first hop route MTU, but will never be lower than this setting.

fib_notify_on_flag_change - INTEGER

Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/ RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.

After installing a route to the kernel, user space receives an acknowledgment, which means the route was installed in the kernel, but not necessarily in hardware. It is also possible for a route already installed in hardware to change its action and therefore its flags. For example, a host route that is trapping packets can be “promoted” to perform decapsulation following the installation of an IPinIP/VXLAN tunnel. The notifications will indicate to user-space the state of the route.

Default: 0 (Do not emit notifications.)

Possible values:

0 - Do not emit notifications.
1 - Emit notifications.
2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.

IP Fragmentation:

ipfrag_high_thresh - LONG INTEGER

Maximum memory used to reassemble IP fragments.

ipfrag_low_thresh - LONG INTEGER

(Obsolete since linux-4.17) Maximum memory used to reassemble IP fragments before the kernel begins to remove incomplete fragment queues to free up resources. The kernel still accepts new fragments for defragmentation.

ipfrag_time - INTEGER

Time in seconds to keep an IP fragment in memory.

ipfrag_max_dist - INTEGER

ipfrag_max_dist is a non-negative integer value which defines the maximum “disorder” which is allowed among fragments which share a common IP source address. Note that reordering of packets is not unusual, but if a large number of fragments arrive from a source IP address while a particular fragment queue remains incomplete, it probably indicates that one or more fragments belonging to that queue have been lost. When ipfrag_max_dist is positive, an additional check is done on fragments before they are added to a reassembly queue - if ipfrag_max_dist (or more) fragments have arrived from a particular IP address between additions to any IP fragment queue using that source address, it’s presumed that one or more fragments in the queue are lost. The existing fragment queue will be dropped, and a new one started. An ipfrag_max_dist value of zero disables this check.

Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can result in unnecessarily dropping fragment queues when normal reordering of packets occurs, which could lead to poor application performance. Using a very large value, e.g. 50000, increases the likelihood of incorrectly reassembling IP fragments that originate from different IP datagrams, which could result in data corruption. Default: 64

bc_forwarding - INTEGER

bc_forwarding enables the feature described in rfc1812#section-5.3.5.2 and rfc2644. It allows the router to forward directed broadcast. To enable this feature, the ‘all’ entry and the input interface entry should be set to 1. Default: 0

INET peer storage¶

inet_peer_threshold - INTEGER: The approximate size of the storage. Starting from this threshold entries will be thrown aggressively. This threshold also determines entries’ time-to-live and time intervals between garbage collection passes. More entries, less time-to-live, less GC interval.
inet_peer_minttl - INTEGER: Minimum time-to-live of entries. Should be enough to cover fragment time-to-live on the reassembling side. This minimum time-to-live is guaranteed if the pool size is less than inet_peer_threshold. Measured in seconds.
inet_peer_maxttl - INTEGER: Maximum time-to-live of entries. Unused entries will expire after this period of time if there is no memory pressure on the pool (i.e. when the number of entries in the pool is very small). Measured in seconds.

TCP variables¶

somaxconn - INTEGER

Limit of socket listen() backlog, known in userspace as SOMAXCONN. Defaults to 4096. (Was 128 before linux-5.4) See also tcp_max_syn_backlog for additional tuning for TCP sockets.

tcp_abort_on_overflow - BOOLEAN

If listening service is too slow to accept new connections, reset them. Default state is FALSE. It means that if overflow occurred due to a burst, connection will recover. Enable this option _only_ if you are really sure that listening daemon cannot be tuned to accept connections faster. Enabling this option can harm clients of your server.

tcp_adv_win_scale - INTEGER

Count buffering overhead as bytes/2^tcp_adv_win_scale (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale), if it is <= 0.

Possible values are [-31, 31], inclusive.

Default: 1

tcp_allowed_congestion_control - STRING

Show/set the congestion control choices available to non-privileged processes. The list is a subset of those listed in tcp_available_congestion_control.

Default is “reno” and the default setting (tcp_congestion_control).

tcp_app_win - INTEGER

Reserve max(window/2^tcp_app_win, mss) of window for application buffer. Value 0 is special, it means that nothing is reserved.

Possible values are [0, 31], inclusive.

Default: 31

tcp_autocorking - BOOLEAN

Enable TCP auto corking : When applications do consecutive small write()/sendmsg() system calls, we try to coalesce these small writes as much as possible, to lower total amount of sent packets. This is done if at least one prior packet for the flow is waiting in Qdisc queues or device transmit queue. Applications can still use TCP_CORK for optimal behavior when they know how/when to uncork their sockets.

Default : 1

tcp_available_congestion_control - STRING

Shows the available congestion control choices that are registered. More congestion control algorithms may be available as modules, but not loaded.

tcp_base_mss - INTEGER

The initial value of search_low to be used by the packetization layer Path MTU discovery (MTU probing). If MTU probing is enabled, this is the initial MSS used by the connection.

tcp_mtu_probe_floor - INTEGER

If MTU probing is enabled this caps the minimum MSS used for search_low for the connection.

Default : 48

tcp_min_snd_mss - INTEGER

TCP SYN and SYNACK messages usually advertise an ADVMSS option, as described in RFC 1122 and RFC 6691.

If this ADVMSS option is smaller than tcp_min_snd_mss, it is silently capped to tcp_min_snd_mss.

Default : 48 (at least 8 bytes of payload per segment)

tcp_congestion_control - STRING

Set the congestion control algorithm to be used for new connections. The algorithm “reno” is always available, but additional choices may be available based on kernel configuration. Default is set as part of kernel configuration. For passive connections, the listener congestion control choice is inherited.

[see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, “name” …) ]

tcp_dsack - BOOLEAN

Allows TCP to send “duplicate” SACKs.

tcp_early_retrans - INTEGER

Tail loss probe (TLP) converts RTOs occurring due to tail losses into fast recovery (draft-ietf-tcpm-rack). Note that TLP requires RACK to function properly (see tcp_recovery below)

Possible values:

0 disables TLP

3 or 4 enables TLP

Default: 3

tcp_ecn - INTEGER

Control use of Explicit Congestion Notification (ECN) by TCP. ECN is used only when both ends of the TCP connection indicate support for it. This feature is useful in avoiding losses due to congestion by allowing supporting routers to signal congestion before having to drop packets.

Possible values are:

0

Disable ECN. Neither initiate nor accept ECN.

1

Enable ECN when requested by incoming connections and also request ECN on outgoing connection attempts.

2

Enable ECN when requested by incoming connections but do not request ECN on outgoing connections.

Default: 2

tcp_ecn_fallback - BOOLEAN

If the kernel detects that ECN connection misbehaves, enable fall back to non-ECN. Currently, this knob implements the fallback from RFC3168, section 6.1.1.1., but we reserve that in future, additional detection mechanisms could be implemented under this knob. The value is not used, if tcp_ecn or per route (or congestion control) ECN settings are disabled.

Default: 1 (fallback enabled)

tcp_fack - BOOLEAN

This is a legacy option, it has no effect anymore.

tcp_fin_timeout - INTEGER

The length of time an orphaned (no longer referenced by any application) connection will remain in the FIN_WAIT_2 state before it is aborted at the local end. While a perfectly valid “receive only” state for an un-orphaned connection, an orphaned connection in FIN_WAIT_2 state could otherwise wait forever for the remote to close its end of the connection.

Cf. tcp_max_orphans

Default: 60 seconds

tcp_frto - INTEGER

Enables Forward RTO-Recovery (F-RTO) defined in RFC5682. F-RTO is an enhanced recovery algorithm for TCP retransmission timeouts. It is particularly beneficial in networks where the RTT fluctuates (e.g., wireless). F-RTO is sender-side only modification. It does not require any support from the peer.

By default it’s enabled with a non-zero value. 0 disables F-RTO.

tcp_fwmark_accept - BOOLEAN

If set, incoming connections to listening sockets that do not have a socket mark will set the mark of the accepting socket to the fwmark of the incoming SYN packet. This will cause all packets on that connection (starting from the first SYNACK) to be sent with that fwmark. The listening socket’s mark is unchanged. Listening sockets that already have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, …) are unaffected.

Default: 0

tcp_invalid_ratelimit - INTEGER

Limit the maximal rate for sending duplicate acknowledgments in response to incoming TCP packets that are for an existing connection but that are invalid due to any of these reasons:

out-of-window sequence number,

out-of-window acknowledgment number, or

PAWS (Protection Against Wrapped Sequence numbers) check failure

This can help mitigate simple “ack loop” DoS attacks, wherein a buggy or malicious middlebox or man-in-the-middle can rewrite TCP header fields in manner that causes each endpoint to think that the other is sending invalid TCP segments, thus causing each side to send an unterminating stream of duplicate acknowledgments for invalid segments.

Using 0 disables rate-limiting of dupacks in response to invalid segments; otherwise this value specifies the minimal space between sending such dupacks, in milliseconds.

Default: 500 (milliseconds).

tcp_keepalive_time - INTEGER

How often TCP sends out keepalive messages when keepalive is enabled. Default: 2hours.

tcp_keepalive_probes - INTEGER

How many keepalive probes TCP sends out, until it decides that the connection is broken. Default value: 9.

tcp_keepalive_intvl - INTEGER

How frequently the probes are send out. Multiplied by tcp_keepalive_probes it is time to kill not responding connection, after probes started. Default value: 75sec i.e. connection will be aborted after ~11 minutes of retries.

tcp_l3mdev_accept - BOOLEAN

Enables child sockets to inherit the L3 master device index. Enabling this option allows a “global” listen socket to work across L3 master domains (e.g., VRFs) with connected sockets derived from the listen socket to be bound to the L3 domain in which the packets originated. Only valid when the kernel was compiled with CONFIG_NET_L3_MASTER_DEV.

Default: 0 (disabled)

tcp_low_latency - BOOLEAN

This is a legacy option, it has no effect anymore.

tcp_max_orphans - INTEGER

Maximal number of TCP sockets not attached to any user file handle, held by system. If this number is exceeded orphaned connections are reset immediately and warning is printed. This limit exists only to prevent simple DoS attacks, you _must_ not rely on this or lower the limit artificially, but rather increase it (probably, after increasing installed memory), if network conditions require more than default value, and tune network services to linger and kill such states more aggressively. Let me to remind again: each orphan eats up to ~64K of unswappable memory.

tcp_max_syn_backlog - INTEGER

Maximal number of remembered connection requests (SYN_RECV), which have not received an acknowledgment from connecting client.

This is a per-listener limit.

The minimal value is 128 for low memory machines, and it will increase in proportion to the memory of machine.

If server suffers from overload, try increasing this number.

Remember to also check /proc/sys/net/core/somaxconn A SYN_RECV request socket consumes about 304 bytes of memory.

tcp_max_tw_buckets - INTEGER

Maximal number of timewait sockets held by system simultaneously. If this number is exceeded time-wait socket is immediately destroyed and warning is printed. This limit exists only to prevent simple DoS attacks, you _must_ not lower the limit artificially, but rather increase it (probably, after increasing installed memory), if network conditions require more than default value.

tcp_mem - vector of 3 INTEGERs: min, pressure, max

min: below this number of pages TCP is not bothered about its memory appetite.

pressure: when amount of memory allocated by TCP exceeds this number of pages, TCP moderates its memory consumption and enters memory pressure mode, which is exited when memory consumption falls under “min”.

max: number of pages allowed for queueing by all TCP sockets.

Defaults are calculated at boot time from amount of available memory.

tcp_min_rtt_wlen - INTEGER

The window length of the windowed min filter to track the minimum RTT. A shorter window lets a flow more quickly pick up new (higher) minimum RTT when it is moved to a longer path (e.g., due to traffic engineering). A longer window makes the filter more resistant to RTT inflations such as transient congestion. The unit is seconds.

Possible values: 0 - 86400 (1 day)

Default: 300

tcp_moderate_rcvbuf - BOOLEAN

If set, TCP performs receive buffer auto-tuning, attempting to automatically size the buffer (no greater than tcp_rmem[2]) to match the size required by the path for full throughput. Enabled by default.

tcp_mtu_probing - INTEGER

Controls TCP Packetization-Layer Path MTU Discovery. Takes three values:

0 - Disabled
1 - Disabled by default, enabled when an ICMP black hole detected
2 - Always enabled, use initial MSS of tcp_base_mss.

tcp_probe_interval - UNSIGNED INTEGER

Controls how often to start TCP Packetization-Layer Path MTU Discovery reprobe. The default is reprobing every 10 minutes as per RFC4821.

tcp_probe_threshold - INTEGER

Controls when TCP Packetization-Layer Path MTU Discovery probing will stop in respect to the width of search range in bytes. Default is 8 bytes.

tcp_no_metrics_save - BOOLEAN

By default, TCP saves various connection metrics in the route cache when the connection closes, so that connections established in the near future can use these to set initial conditions. Usually, this increases overall performance, but may sometimes cause performance degradation. If set, TCP will not cache metrics on closing connections.

tcp_no_ssthresh_metrics_save - BOOLEAN

Controls whether TCP saves ssthresh metrics in the route cache.

Default is 1, which disables ssthresh metrics.

tcp_orphan_retries - INTEGER

This value influences the timeout of a locally closed TCP connection, when RTO retransmissions remain unacknowledged. See tcp_retries2 for more details.

The default value is 8.

If your machine is a loaded WEB server, you should think about lowering this value, such sockets may consume significant resources. Cf. tcp_max_orphans.

tcp_recovery - INTEGER

This value is a bitmap to enable various experimental loss recovery features.

RACK: 0x1	enables the RACK loss detection for fast detection of lost retransmissions and tail drops. It also subsumes and disables RFC6675 recovery for SACK connections.
RACK: 0x2	makes RACK’s reordering window static (min_rtt/4).
RACK: 0x4	disables RACK’s DUPACK threshold heuristic

Default: 0x1

tcp_reflect_tos - BOOLEAN

For listening sockets, reuse the DSCP value of the initial SYN message for outgoing packets. This allows to have both directions of a TCP stream to use the same DSCP value, assuming DSCP remains unchanged for the lifetime of the connection.

This options affects both IPv4 and IPv6.

Default: 0 (disabled)

tcp_reordering - INTEGER

Initial reordering level of packets in a TCP stream. TCP stack can then dynamically adjust flow reordering level between this initial value and tcp_max_reordering

Default: 3

tcp_max_reordering - INTEGER

Maximal reordering level of packets in a TCP stream. 300 is a fairly conservative value, but you might increase it if paths are using per packet load balancing (like bonding rr mode)

Default: 300

tcp_retrans_collapse - BOOLEAN

Bug-to-bug compatibility with some broken printers. On retransmit try to send bigger packets to work around bugs in certain TCP stacks.

tcp_retries1 - INTEGER

This value influences the time, after which TCP decides, that something is wrong due to unacknowledged RTO retransmissions, and reports this suspicion to the network layer. See tcp_retries2 for more details.

RFC 1122 recommends at least 3 retransmissions, which is the default.

tcp_retries2 - INTEGER

This value influences the timeout of an alive TCP connection, when RTO retransmissions remain unacknowledged. Given a value of N, a hypothetical TCP connection following exponential backoff with an initial RTO of TCP_RTO_MIN would retransmit N times before killing the connection at the (N+1)th RTO.

The default value of 15 yields a hypothetical timeout of 924.6 seconds and is a lower bound for the effective timeout. TCP will effectively time out at the first RTO which exceeds the hypothetical timeout.

RFC 1122 recommends at least 100 seconds for the timeout, which corresponds to a value of at least 8.

tcp_rfc1337 - BOOLEAN

If set, the TCP stack behaves conforming to RFC1337. If unset, we are not conforming to RFC, but prevent TCP TIME_WAIT assassination.

Default: 0

tcp_rmem - vector of 3 INTEGERs: min, default, max

min: Minimal size of receive buffer used by TCP sockets. It is guaranteed to each TCP socket, even under moderate memory pressure.

Default: 4K

default: initial size of receive buffer used by TCP sockets. This value overrides net.core.rmem_default used by other protocols. Default: 131072 bytes. This value results in initial window of 65535.

max: maximal size of receive buffer allowed for automatically selected receiver buffers for TCP socket. This value does not override net.core.rmem_max. Calling setsockopt() with SO_RCVBUF disables automatic tuning of that socket’s receive buffer size, in which case this value is ignored. Default: between 131072 and 6MB, depending on RAM size.

tcp_sack - BOOLEAN

Enable select acknowledgments (SACKS).

tcp_comp_sack_delay_ns - LONG INTEGER

TCP tries to reduce number of SACK sent, using a timer based on 5% of SRTT, capped by this sysctl, in nano seconds. The default is 1ms, based on TSO autosizing period.

Default : 1,000,000 ns (1 ms)

tcp_comp_sack_slack_ns - LONG INTEGER

This sysctl control the slack used when arming the timer used by SACK compression. This gives extra time for small RTT flows, and reduces system overhead by allowing opportunistic reduction of timer interrupts.

Default : 100,000 ns (100 us)

tcp_comp_sack_nr - INTEGER

Max number of SACK that can be compressed. Using 0 disables SACK compression.

Default : 44

tcp_slow_start_after_idle - BOOLEAN

If set, provide RFC2861 behavior and time out the congestion window after an idle period. An idle period is defined at the current RTO. If unset, the congestion window will not be timed out after an idle period.

Default: 1

tcp_stdurg - BOOLEAN

Use the Host requirements interpretation of the TCP urgent pointer field. Most hosts use the older BSD interpretation, so if you turn this on Linux might not communicate correctly with them.

Default: FALSE

tcp_synack_retries - INTEGER

Number of times SYNACKs for a passive TCP connection attempt will be retransmitted. Should not be higher than 255. Default value is 5, which corresponds to 31seconds till the last retransmission with the current initial RTO of 1second. With this the final timeout for a passive TCP connection will happen after 63seconds.

tcp_syncookies - INTEGER

Only valid when the kernel was compiled with CONFIG_SYN_COOKIES Send out syncookies when the syn backlog queue of a socket overflows. This is to prevent against the common ‘SYN flood attack’ Default: 1

Note, that syncookies is fallback facility. It MUST NOT be used to help highly loaded servers to stand against legal connection rate. If you see SYN flood warnings in your logs, but investigation shows that they occur because of overload with legal connections, you should tune another parameters until this warning disappear. See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

syncookies seriously violate TCP protocol, do not allow to use TCP extensions, can result in serious degradation of some services (f.e. SMTP relaying), visible not by you, but your clients and relays, contacting you. While you see SYN flood warnings in logs not being really flooded, your server is seriously misconfigured.

If you want to test which effects syncookies have to your network connections you can set this knob to 2 to enable unconditionally generation of syncookies.

tcp_migrate_req - BOOLEAN

The incoming connection is tied to a specific listening socket when the initial SYN packet is received during the three-way handshake. When a listener is closed, in-flight request sockets during the handshake and established sockets in the accept queue are aborted.

If the listener has SO_REUSEPORT enabled, other listeners on the same port should have been able to accept such connections. This option makes it possible to migrate such child sockets to another listener after close() or shutdown().

The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should usually be used to define the policy to pick an alive listener. Otherwise, the kernel will randomly pick an alive listener only if this option is enabled.

Note that migration between listeners with different settings may crash applications. Let’s say migration happens from listener A to B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from the requests migrated from A. To avoid such a situation, cancel migration by returning SK_DROP in the type of eBPF program, or disable this option.

Default: 0

tcp_fastopen - INTEGER

Enable TCP Fast Open (RFC7413) to send and accept data in the opening SYN packet.

The client support is enabled by flag 0x1 (on by default). The client then must use sendmsg() or sendto() with the MSG_FASTOPEN flag, rather than connect() to send data in SYN.

The server support is enabled by flag 0x2 (off by default). Then either enable for all listeners with another flag (0x400) or enable individual listeners via TCP_FASTOPEN socket option with the option value being the length of the syn-data backlog.

The values (bitmap) are

0x1	(client)	enables sending data in the opening SYN on the client.
0x2	(server)	enables the server support, i.e., allowing data in a SYN packet to be accepted and passed to the application before 3-way handshake finishes.
0x4	(client)	send data in the opening SYN regardless of cookie availability and without a cookie option.
0x200	(server)	accept data-in-SYN w/o any cookie option present.
0x400	(server)	enable all listeners to support Fast Open by default without explicit TCP_FASTOPEN socket option.

Default: 0x1

Note that additional client or server features are only effective if the basic support (0x1 and 0x2) are enabled respectively.

tcp_fastopen_blackhole_timeout_sec - INTEGER

Initial time period in second to disable Fastopen on active TCP sockets when a TFO firewall blackhole issue happens. This time period will grow exponentially when more blackhole issues get detected right after Fastopen is re-enabled and will reset to initial value when the blackhole issue goes away. 0 to disable the blackhole detection.

By default, it is set to 0 (feature is disabled).

tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs

The list consists of a primary key and an optional backup key. The primary key is used for both creating and validating cookies, while the optional backup key is only used for validating cookies. The purpose of the backup key is to maximize TFO validation when keys are rotated.

A randomly chosen primary key may be configured by the kernel if the tcp_fastopen sysctl is set to 0x400 (see above), or if the TCP_FASTOPEN setsockopt() optname is set and a key has not been previously configured via sysctl. If keys are configured via setsockopt() by using the TCP_FASTOPEN_KEY optname, then those per-socket keys will be used instead of any keys that are specified via sysctl.

A key is specified as 4 8-digit hexadecimal integers which are separated by a ‘-’ as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be omitted. A primary and a backup key may be specified by separating them by a comma. If only one key is specified, it becomes the primary key and any previously configured backup keys are removed.

tcp_syn_retries - INTEGER

Number of times initial SYNs for an active TCP connection attempt will be retransmitted. Should not be higher than 127. Default value is 6, which corresponds to 63seconds till the last retransmission with the current initial RTO of 1second. With this the final timeout for an active TCP connection attempt will happen after 127seconds.

tcp_timestamps - INTEGER

Enable timestamps as defined in RFC1323.

0: Disabled.
1: Enable timestamps as defined in RFC1323 and use random offset for each connection rather than only using the current time.
2: Like 1, but without random offsets.

Default: 1

tcp_min_tso_segs - INTEGER

Minimal number of segments per TSO frame.

Since linux-3.12, TCP does an automatic sizing of TSO frames, depending on flow rate, instead of filling 64Kbytes packets. For specific usages, it’s possible to force TCP to build big TSO frames. Note that TCP stack might split too big TSO packets if available window is too small.

Default: 2

tcp_tso_rtt_log - INTEGER

Adjustment of TSO packet sizes based on min_rtt

Starting from linux-5.18, TCP autosizing can be tweaked for flows having small RTT.

Old autosizing was splitting the pacing budget to send 1024 TSO per second.

tso_packet_size = sk->sk_pacing_rate / 1024;

With the new mechanism, we increase this TSO sizing using:

distance = min_rtt_usec / (2^tcp_tso_rtt_log) tso_packet_size += gso_max_size >> distance;

This means that flows between very close hosts can use bigger TSO packets, reducing their cpu costs.

If you want to use the old autosizing, set this sysctl to 0.

Default: 9 (2^9 = 512 usec)

tcp_pacing_ss_ratio - INTEGER

sk->sk_pacing_rate is set by TCP stack using a ratio applied to current rate. (current_rate = cwnd * mss / srtt) If TCP is in slow start, tcp_pacing_ss_ratio is applied to let TCP probe for bigger speeds, assuming cwnd can be doubled every other RTT.

Default: 200

tcp_pacing_ca_ratio - INTEGER

sk->sk_pacing_rate is set by TCP stack using a ratio applied to current rate. (current_rate = cwnd * mss / srtt) If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio is applied to conservatively probe for bigger throughput.

Default: 120

tcp_tso_win_divisor - INTEGER

This allows control over what percentage of the congestion window can be consumed by a single TSO frame. The setting of this parameter is a choice between burstiness and building larger TSO frames.

Default: 3

tcp_tw_reuse - INTEGER

Enable reuse of TIME-WAIT sockets for new connections when it is safe from protocol viewpoint.

0 - disable
1 - global enable
2 - enable for loopback traffic only

It should not be changed without advice/request of technical experts.

Default: 2

tcp_window_scaling - BOOLEAN

Enable window scaling as defined in RFC1323.

tcp_shrink_window - BOOLEAN

This changes how the TCP receive window is calculated.

RFC 7323, section 2.4, says there are instances when a retracted window can be offered, and that TCP implementations MUST ensure that they handle a shrinking window, as specified in RFC 1122.

0 - Disabled. The window is never shrunk.
1 - Enabled. The window is shrunk when necessary to remain within
the memory limit set by autotuning (sk_rcvbuf). This only occurs if a non-zero receive window scaling factor is also in effect.

Default: 0

tcp_wmem - vector of 3 INTEGERs: min, default, max

min: Amount of memory reserved for send buffers for TCP sockets. Each TCP socket has rights to use it due to fact of its birth.

Default: 4K

default: initial size of send buffer used by TCP sockets. This value overrides net.core.wmem_default used by other protocols.

It is usually lower than net.core.wmem_default.

Default: 16K

max: Maximal amount of memory allowed for automatically tuned send buffers for TCP sockets. This value does not override net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables automatic tuning of that socket’s send buffer size, in which case this value is ignored.

Default: between 64K and 4MB, depending on RAM size.

tcp_notsent_lowat - UNSIGNED INTEGER

A TCP socket can control the amount of unsent bytes in its write queue, thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll() reports POLLOUT events if the amount of unsent bytes is below a per socket value, and if the write queue is not full. sendmsg() will also not add new buffers if the limit is hit.

This global variable controls the amount of unsent data for sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change to the global variable has immediate effect.

Default: UINT_MAX (0xFFFFFFFF)

tcp_workaround_signed_windows - BOOLEAN

If set, assume no receipt of a window scaling option means the remote TCP is broken and treats the window as a signed quantity. If unset, assume the remote TCP is not broken even if we do not receive a window scaling option from them.

Default: 0

tcp_thin_linear_timeouts - BOOLEAN

Enable dynamic triggering of linear timeouts for thin streams. If set, a check is performed upon retransmission by timeout to determine if the stream is thin (less than 4 packets in flight). As long as the stream is found to be thin, up to 6 linear timeouts may be performed before exponential backoff mode is initiated. This improves retransmission latency for non-aggressive thin streams, often found to be time-dependent. For more information on thin streams, see Thin-streams and TCP

Default: 0

tcp_limit_output_bytes - INTEGER

Controls TCP Small Queue limit per tcp socket. TCP bulk sender tends to increase packets in flight until it gets losses notifications. With SNDBUF autotuning, this can result in a large amount of packets queued on the local machine (e.g.: qdiscs, CPU backlog, or device) hurting latency of other flows, for typical pfifo_fast qdiscs. tcp_limit_output_bytes limits the number of bytes on qdisc or device to reduce artificial RTT/cwnd and reduce bufferbloat.

Default: 1048576 (16 * 65536)

tcp_challenge_ack_limit - INTEGER

Limits number of Challenge ACK sent per second, as recommended in RFC 5961 (Improving TCP’s Robustness to Blind In-Window Attacks) Note that this per netns rate limit can allow some side channel attacks and probably should not be enabled. TCP stack implements per TCP socket limits anyway. Default: INT_MAX (unlimited)

tcp_ehash_entries - INTEGER

Show the number of hash buckets for TCP sockets in the current networking namespace.

A negative value means the networking namespace does not own its hash buckets and shares the initial networking namespace’s one.

tcp_child_ehash_entries - INTEGER

Control the number of hash buckets for TCP sockets in the child networking namespace, which must be set before clone() or unshare().

If the value is not 0, the kernel uses a value rounded up to 2^n as the actual hash bucket size. 0 is a special value, meaning the child networking namespace will share the initial networking namespace’s hash buckets.

Note that the child will use the global one in case the kernel fails to allocate enough memory. In addition, the global hash buckets are spread over available NUMA nodes, but the allocation of the child hash table depends on the current process’s NUMA policy, which could result in performance differences.

Note also that the default value of tcp_max_tw_buckets and tcp_max_syn_backlog depend on the hash bucket size.

Possible values: 0, 2^n (n: 0 - 24 (16Mi))

Default: 0

UDP variables¶

udp_l3mdev_accept - BOOLEAN

Enabling this option allows a “global” bound socket to work across L3 master domains (e.g., VRFs) with packets capable of being received regardless of the L3 domain in which they originated. Only valid when the kernel was compiled with CONFIG_NET_L3_MASTER_DEV.

Default: 0 (disabled)

udp_mem - vector of 3 INTEGERs: min, pressure, max

Number of pages allowed for queueing by all UDP sockets.

min: Number of pages allowed for queueing by all UDP sockets.

pressure: This value was introduced to follow format of tcp_mem.

max: This value was introduced to follow format of tcp_mem.

Default is calculated at boot time from amount of available memory.

udp_rmem_min - INTEGER

Minimal size of receive buffer used by UDP sockets in moderation. Each UDP socket is able to use the size for receiving data, even if total pages of UDP sockets exceed udp_mem pressure. The unit is byte.

Default: 4K

udp_wmem_min - INTEGER

UDP does not have tx memory accounting and this tunable has no effect.

RAW variables¶

raw_l3mdev_accept - BOOLEAN

Enabling this option allows a “global” bound socket to work across L3 master domains (e.g., VRFs) with packets capable of being received regardless of the L3 domain in which they originated. Only valid when the kernel was compiled with CONFIG_NET_L3_MASTER_DEV.

Default: 1 (enabled)

CIPSOv4 Variables¶

cipso_cache_enable - BOOLEAN

If set, enable additions to and lookups from the CIPSO label mapping cache. If unset, additions are ignored and lookups always result in a miss. However, regardless of the setting the cache is still invalidated when required when means you can safely toggle this on and off and the cache will always be “safe”.

Default: 1

cipso_cache_bucket_size - INTEGER

The CIPSO label cache consists of a fixed size hash table with each hash bucket containing a number of cache entries. This variable limits the number of entries in each hash bucket; the larger the value is, the more CIPSO label mappings that can be cached. When the number of entries in a given hash bucket reaches this limit adding new entries causes the oldest entry in the bucket to be removed to make room.

Default: 10

cipso_rbm_optfmt - BOOLEAN

Enable the “Optimized Tag 1 Format” as defined in section 3.4.2.6 of the CIPSO draft specification (see Documentation/netlabel for details). This means that when set the CIPSO tag will be padded with empty categories in order to make the packet data 32-bit aligned.

Default: 0

cipso_rbm_structvalid - BOOLEAN

If set, do a very strict check of the CIPSO option when ip_options_compile() is called. If unset, relax the checks done during ip_options_compile(). Either way is “safe” as errors are caught else where in the CIPSO processing code but setting this to 0 (False) should result in less work (i.e. it should be faster) but could cause problems with other implementations that require strict checking.

Default: 0

IP Variables¶

ip_local_port_range - 2 INTEGERS

Defines the local port range that is used by TCP and UDP to choose the local port. The first number is the first, the second the last local port number. If possible, it is better these numbers have different parity (one even and one odd value). Must be greater than or equal to ip_unprivileged_port_start. The default values are 32768 and 60999 respectively.

ip_local_reserved_ports - list of comma separated ranges

Specify the ports which are reserved for known third-party applications. These ports will not be used by automatic port assignments (e.g. when calling connect() or bind() with port number 0). Explicit port allocation behavior is unchanged.

The format used for both input and output is a comma separated list of ranges (e.g. “1,2-4,10-10” for ports 1, 2, 3, 4 and 10). Writing to the file will clear all previously reserved ports and update the current list with the one given in the input.

Note that ip_local_port_range and ip_local_reserved_ports settings are independent and both are considered by the kernel when determining which ports are available for automatic port assignments.

You can reserve ports which are not in the current ip_local_port_range, e.g.:

$ cat /proc/sys/net/ipv4/ip_local_port_range
32000       60999
$ cat /proc/sys/net/ipv4/ip_local_reserved_ports
8080,9148

although this is redundant. However such a setting is useful if later the port range is changed to a value that will include the reserved ports. Also keep in mind, that overlapping of these ranges may affect probability of selecting ephemeral ports which are right after block of reserved ports.

Default: Empty

ip_unprivileged_port_start - INTEGER

This is a per-namespace sysctl. It defines the first unprivileged port in the network namespace. Privileged ports require root or CAP_NET_BIND_SERVICE in order to bind to them. To disable all privileged ports, set this to 0. They must not overlap with the ip_local_port_range.

Default: 1024

ip_nonlocal_bind - BOOLEAN

If set, allows processes to bind() to non-local IP addresses, which can be quite useful - but may break some applications.

Default: 0

ip_autobind_reuse - BOOLEAN

By default, bind() does not select the ports automatically even if the new socket and all sockets bound to the port have SO_REUSEADDR. ip_autobind_reuse allows bind() to reuse the port and this is useful when you use bind()+connect(), but may break some applications. The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this option should only be set by experts. Default: 0

ip_dynaddr - INTEGER

If set non-zero, enables support for dynamic addresses. If set to a non-zero value larger than 1, a kernel log message will be printed when dynamic address rewriting occurs.

Default: 0

ip_early_demux - BOOLEAN

Optimize input packet processing down to one demux for certain kinds of local sockets. Currently we only do this for established TCP and connected UDP sockets.

It may add an additional cost for pure routing workloads that reduces overall throughput, in such case you should disable it.

Default: 1

ping_group_range - 2 INTEGERS

Restrict ICMP_PROTO datagram sockets to users in the group range. The default is “1 0”, meaning, that nobody (not even root) may create ping sockets. Setting it to “100 100” would grant permissions to the single group. “0 4294967294” would enable it for the world, “100 4294967294” would enable it for the users, but not daemons.

tcp_early_demux - BOOLEAN

Enable early demux for established TCP sockets.

Default: 1

udp_early_demux - BOOLEAN

Enable early demux for connected UDP sockets. Disable this if your system could experience more unconnected load.

Default: 1

icmp_echo_ignore_all - BOOLEAN

If set non-zero, then the kernel will ignore all ICMP ECHO requests sent to it.

Default: 0

icmp_echo_enable_probe - BOOLEAN

If set to one, then the kernel will respond to RFC 8335 PROBE requests sent to it.

Default: 0

icmp_echo_ignore_broadcasts - BOOLEAN

If set non-zero, then the kernel will ignore all ICMP ECHO and TIMESTAMP requests sent to it via broadcast/multicast.

Default: 1

icmp_ratelimit - INTEGER

Limit the maximal rates for sending ICMP packets whose type matches icmp_ratemask (see below) to specific targets. 0 to disable any limiting, otherwise the minimal space between responses in milliseconds. Note that another sysctl, icmp_msgs_per_sec limits the number of ICMP packets sent on all targets.

Default: 1000

icmp_msgs_per_sec - INTEGER

Limit maximal number of ICMP packets sent per second from this host. Only messages whose type matches icmp_ratemask (see below) are controlled by this limit. For security reasons, the precise count of messages per second is randomized.

Default: 1000

icmp_msgs_burst - INTEGER

icmp_msgs_per_sec controls number of ICMP packets sent per second, while icmp_msgs_burst controls the burst size of these packets. For security reasons, the precise burst size is randomized.

Default: 50

icmp_ratemask - INTEGER

Mask made of ICMP types for which rates are being limited.

Significant bits: IHGFEDCBA9876543210

Default mask: 0000001100000011000 (6168)

Bit definitions (see include/linux/icmp.h):

0

Echo Reply

3

Destination Unreachable [1]

4

Source Quench [1]

5

Redirect

8

Echo Request

B

Time Exceeded [1]

C

Parameter Problem [1]

D

Timestamp Request

E

Timestamp Reply

F

Info Request

G

Info Reply

H

Address Mask Request

I

Address Mask Reply

icmp_ignore_bogus_error_responses - BOOLEAN

Some routers violate RFC1122 by sending bogus responses to broadcast frames. Such violations are normally logged via a kernel warning. If this is set to TRUE, the kernel will not give such warnings, which will avoid log file clutter.

Default: 1

icmp_errors_use_inbound_ifaddr - BOOLEAN

If zero, icmp error messages are sent with the primary address of the exiting interface.

If non-zero, the message will be sent with the primary address of the interface that received the packet that caused the icmp error. This is the behaviour many network administrators will expect from a router. And it can make debugging complicated network layouts much easier.

Note that if no primary address exists for the interface selected, then the primary address of the first non-loopback interface that has one will be used regardless of this setting.

Default: 0

igmp_max_memberships - INTEGER

Change the maximum number of multicast groups we can subscribe to. Default: 20

Theoretical maximum value is bounded by having to send a membership report in a single datagram (i.e. the report can’t span multiple datagrams, or risk confusing the switch and leaving groups you don’t intend to).

The number of supported groups ‘M’ is bounded by the number of group report entries you can fit into a single datagram of 65535 bytes.

M = 65536-sizeof (ip header)/(sizeof(Group record))

Group records are variable length, with a minimum of 12 bytes. So net.ipv4.igmp_max_memberships should not be set higher than:

(65536-24) / 12 = 5459

The value 5459 assumes no IP header options, so in practice this number may be lower.

igmp_max_msf - INTEGER

Maximum number of addresses allowed in the source filter list for a multicast group.

Default: 10

igmp_qrv - INTEGER

Controls the IGMP query robustness variable (see RFC2236 8.1).

Default: 2 (as specified by RFC2236 8.1)

Minimum: 1 (as specified by RFC6636 4.5)

force_igmp_version - INTEGER

0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier Present timer expires.
1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if receive IGMPv2/v3 query.
2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive IGMPv1 query message. Will reply report if receive IGMPv3 query.
3 - Enforce to use IGMP version 3. The same react with default 0.

Note

this is not the same with force_mld_version because IGMPv3 RFC3376 Security Considerations does not have clear description that we could ignore other version messages completely as MLDv2 RFC3810. So make this value as default 0 is recommended.

conf/interface/*

changes special settings per interface (where interface” is the name of your network interface)

conf/all/*

is special, changes the settings for all interfaces

log_martians - BOOLEAN

Log packets with impossible addresses to kernel log. log_martians for the interface will be enabled if at least one of conf/{all,interface}/log_martians is set to TRUE, it will be disabled otherwise

accept_redirects - BOOLEAN

Accept ICMP redirect messages. accept_redirects for the interface will be enabled if:

both conf/{all,interface}/accept_redirects are TRUE in the case forwarding for the interface is enabled

or

at least one of conf/{all,interface}/accept_redirects is TRUE in the case forwarding for the interface is disabled

accept_redirects for the interface will be disabled otherwise

default:

TRUE (host)

FALSE (router)

forwarding - BOOLEAN

Enable IP forwarding on this interface. This controls whether packets received _on_ this interface can be forwarded.

mc_forwarding - BOOLEAN

Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE and a multicast routing daemon is required. conf/all/mc_forwarding must also be set to TRUE to enable multicast routing for the interface

medium_id - INTEGER

Integer value used to differentiate the devices by the medium they are attached to. Two devices can have different id values when the broadcast packets are received only on one of them. The default value 0 means that the device is the only interface to its medium, value of -1 means that medium is not known.

Currently, it is used to change the proxy_arp behavior: the proxy_arp feature is enabled for packets forwarded between two devices attached to different media.

proxy_arp - BOOLEAN

Do proxy arp.

proxy_arp for the interface will be enabled if at least one of conf/{all,interface}/proxy_arp is set to TRUE, it will be disabled otherwise

proxy_arp_pvlan - BOOLEAN

Private VLAN proxy arp.

Basically allow proxy arp replies back to the same interface (from which the ARP request/solicitation was received).

This is done to support (ethernet) switch features, like RFC 3069, where the individual ports are NOT allowed to communicate with each other, but they are allowed to talk to the upstream router. As described in RFC 3069, it is possible to allow these hosts to communicate through the upstream router by proxy_arp’ing. Don’t need to be used together with proxy_arp.

This technology is known by different names:

In RFC 3069 it is called VLAN Aggregation. Cisco and Allied Telesyn call it Private VLAN. Hewlett-Packard call it Source-Port filtering or port-isolation. Ericsson call it MAC-Forced Forwarding (RFC Draft).

shared_media - BOOLEAN

Send(router) or accept(host) RFC1620 shared media redirects. Overrides secure_redirects.

shared_media for the interface will be enabled if at least one of conf/{all,interface}/shared_media is set to TRUE, it will be disabled otherwise

default TRUE

secure_redirects - BOOLEAN

Accept ICMP redirect messages only to gateways listed in the interface’s current gateway list. Even if disabled, RFC1122 redirect rules still apply.

Overridden by shared_media.

secure_redirects for the interface will be enabled if at least one of conf/{all,interface}/secure_redirects is set to TRUE, it will be disabled otherwise

default TRUE

send_redirects - BOOLEAN

Send redirects, if router.

send_redirects for the interface will be enabled if at least one of conf/{all,interface}/send_redirects is set to TRUE, it will be disabled otherwise

Default: TRUE

bootp_relay - BOOLEAN

Accept packets with source address 0.b.c.d destined not to this host as local ones. It is supposed, that BOOTP relay daemon will catch and forward such packets. conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay for the interface

default FALSE

Not Implemented Yet.

accept_source_route - BOOLEAN

Accept packets with SRR option. conf/all/accept_source_route must also be set to TRUE to accept packets with SRR option on the interface

default

TRUE (router)

FALSE (host)

accept_local - BOOLEAN

Accept packets with local source addresses. In combination with suitable routing, this can be used to direct packets between two local interfaces over the wire and have them accepted properly. default FALSE

route_localnet - BOOLEAN

Do not consider loopback addresses as martian source or destination while routing. This enables the use of 127/8 for local routing purposes.

default FALSE

rp_filter - INTEGER

0 - No source validation.
1 - Strict mode as defined in RFC3704 Strict Reverse Path Each incoming packet is tested against the FIB and if the interface is not the best reverse path the packet check will fail. By default failed packets are discarded.
2 - Loose mode as defined in RFC3704 Loose Reverse Path Each incoming packet’s source address is also tested against the FIB and if the source address is not reachable via any interface the packet check will fail.

Current recommended practice in RFC3704 is to enable strict mode to prevent IP spoofing from DDos attacks. If using asymmetric routing or other complicated routing, then loose mode is recommended.

The max value from conf/{all,interface}/rp_filter is used when doing source validation on the {interface}.

Default value is 0. Note that some distributions enable it in startup scripts.

src_valid_mark - BOOLEAN

0 - The fwmark of the packet is not included in reverse path route lookup. This allows for asymmetric routing configurations utilizing the fwmark in only one direction, e.g., transparent proxying.
1 - The fwmark of the packet is included in reverse path route lookup. This permits rp_filter to function when the fwmark is used for routing traffic in both directions.

This setting also affects the utilization of fmwark when performing source address selection for ICMP replies, or determining addresses stored for the IPOPT_TS_TSANDADDR and IPOPT_RR IP options.

The max value from conf/{all,interface}/src_valid_mark is used.

Default value is 0.

arp_filter - BOOLEAN

1 - Allows you to have multiple network interfaces on the same subnet, and have the ARPs for each interface be answered based on whether or not the kernel would route a packet from the ARP’d IP out that interface (therefore you must use source based routing for this to work). In other words it allows control of which cards (usually 1) will respond to an arp request.
0 - (default) The kernel can respond to arp requests with addresses from other interfaces. This may seem wrong but it usually makes sense, because it increases the chance of successful communication. IP addresses are owned by the complete host on Linux, not by particular interfaces. Only for more complex setups like load- balancing, does this behaviour cause problems.

arp_filter for the interface will be enabled if at least one of conf/{all,interface}/arp_filter is set to TRUE, it will be disabled otherwise

arp_announce - INTEGER

Define different restriction levels for announcing the local source IP address from IP packets in ARP requests sent on interface:

0 - (default) Use any local address, configured on any interface
1 - Try to avoid local addresses that are not in the target’s subnet for this interface. This mode is useful when target hosts reachable via this interface require the source IP address in ARP requests to be part of their logical network configured on the receiving interface. When we generate the request we will check all our subnets that include the target IP and will preserve the source address if it is from such subnet. If there is no such subnet we select source address according to the rules for level 2.
2 - Always use the best local address for this target. In this mode we ignore the source address in the IP packet and try to select local address that we prefer for talks with the target host. Such local address is selected by looking for primary IP addresses on all our subnets on the outgoing interface that include the target IP address. If no suitable local address is found we select the first local address we have on the outgoing interface or on all other interfaces, with the hope we will receive reply for our request and even sometimes no matter the source IP address we announce.

The max value from conf/{all,interface}/arp_announce is used.

Increasing the restriction level gives more chance for receiving answer from the resolved target while decreasing the level announces more valid sender’s information.

arp_ignore - INTEGER

Define different modes for sending replies in response to received ARP requests that resolve local target IP addresses:

0 - (default): reply for any local target IP address, configured on any interface
1 - reply only if the target IP address is local address configured on the incoming interface
2 - reply only if the target IP address is local address configured on the incoming interface and both with the sender’s IP address are part from same subnet on this interface
3 - do not reply for local addresses configured with scope host, only resolutions for global and link addresses are replied
4-7 - reserved
8 - do not reply for all local addresses

The max value from conf/{all,interface}/arp_ignore is used when ARP request is received on the {interface}

arp_notify - BOOLEAN

Define mode for notification of address and device changes.

0

(default): do nothing

1

Generate gratuitous arp requests when device is brought up or hardware address changes.

arp_accept - INTEGER

Define behavior for accepting gratuitous ARP (garp) frames from devices that are not already present in the ARP table:

0 - don’t create new entries in the ARP table
1 - create new entries in the ARP table
2 - create new entries only if the source IP address is in the same subnet as an address configured on the interface that received the garp message.

Both replies and requests type gratuitous arp will trigger the ARP table to be updated, if this setting is on.

If the ARP table already contains the IP address of the gratuitous arp frame, the arp table will be updated regardless if this setting is on or off.

arp_evict_nocarrier - BOOLEAN

Clears the ARP cache on NOCARRIER events. This option is important for wireless devices where the ARP cache should not be cleared when roaming between access points on the same network. In most cases this should remain as the default (1).

1 - (default): Clear the ARP cache on NOCARRIER events
0 - Do not clear ARP cache on NOCARRIER events

mcast_solicit - INTEGER

The maximum number of multicast probes in INCOMPLETE state, when the associated hardware address is unknown. Defaults to 3.

ucast_solicit - INTEGER

The maximum number of unicast probes in PROBE state, when the hardware address is being reconfirmed. Defaults to 3.

app_solicit - INTEGER

The maximum number of probes to send to the user space ARP daemon via netlink before dropping back to multicast probes (see mcast_resolicit). Defaults to 0.

mcast_resolicit - INTEGER

The maximum number of multicast probes after unicast and app probes in PROBE state. Defaults to 0.

disable_policy - BOOLEAN

Disable IPSEC policy (SPD) for this interface

disable_xfrm - BOOLEAN

Disable IPSEC encryption on this interface, whatever the policy

igmpv2_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicited IGMPv1 or IGMPv2 report retransmit will take place.

Default: 10000 (10 seconds)

igmpv3_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicited IGMPv3 report retransmit will take place.

Default: 1000 (1 seconds)

ignore_routes_with_linkdown - BOOLEAN

Ignore routes whose link is down when performing a FIB lookup.

promote_secondaries - BOOLEAN

When a primary IP address is removed from this interface promote a corresponding secondary IP address instead of removing all the corresponding secondary IP addresses.

drop_unicast_in_l2_multicast - BOOLEAN

Drop any unicast IP packets that are received in link-layer multicast (or broadcast) frames.

This behavior (for multicast) is actually a SHOULD in RFC 1122, but is disabled by default for compatibility reasons.

Default: off (0)

drop_gratuitous_arp - BOOLEAN

Drop all gratuitous ARP frames, for example if there’s a known good ARP proxy on the network and such frames need not be used (or in the case of 802.11, must not be used to prevent attacks.)

Default: off (0)

tag - INTEGER

Allows you to write a number, which can be used as required.

Default value is 0.

xfrm4_gc_thresh - INTEGER

(Obsolete since linux-4.14) The threshold at which we will start garbage collecting for IPv4 destination cache entries. At twice this value the system will refuse new allocations.

igmp_link_local_mcast_reports - BOOLEAN

Enable IGMP reports for link local multicast groups in the 224.0.0.X range.

Default TRUE

Alexey Kuznetsov. kuznet@ms2.inr.ac.ru

Updated by:

Andi Kleen ak@muc.de
Nicolas Delon delon.nicolas@wanadoo.fr

/proc/sys/net/ipv6/* Variables¶

IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also apply to IPv6 [XXX?].

bindv6only - BOOLEAN

Default value for IPV6_V6ONLY socket option, which restricts use of the IPv6 socket to IPv6 communication only.

TRUE: disable IPv4-mapped address feature

FALSE: enable IPv4-mapped address feature

Default: FALSE (as specified in RFC3493)

flowlabel_consistency - BOOLEAN

Protect the consistency (and unicity) of flow label. You have to disable it to use IPV6_FL_F_REFLECT flag on the flow label manager.

TRUE: enabled
FALSE: disabled

Default: TRUE

auto_flowlabels - INTEGER

Automatically generate flow labels based on a flow hash of the packet. This allows intermediate devices, such as routers, to identify packet flows for mechanisms like Equal Cost Multipath Routing (see RFC 6438).

0	automatic flow labels are completely disabled
1	automatic flow labels are enabled by default, they can be disabled on a per socket basis using the IPV6_AUTOFLOWLABEL socket option
2	automatic flow labels are allowed, they may be enabled on a per socket basis using the IPV6_AUTOFLOWLABEL socket option
3	automatic flow labels are enabled and enforced, they cannot be disabled by the socket option

Default: 1

flowlabel_state_ranges - BOOLEAN

Split the flow label number space into two ranges. 0-0x7FFFF is reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF is reserved for stateless flow labels as described in RFC6437.

TRUE: enabled
FALSE: disabled

Default: true

flowlabel_reflect - INTEGER

Control flow label reflection. Needed for Path MTU Discovery to work with Equal Cost Multipath Routing in anycast environments. See RFC 7690 and: https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01

This is a bitmask.

1: enabled for established flows

Note that this prevents automatic flowlabel changes, as done in “tcp: change IPv6 flow-label upon receiving spurious retransmission” and “tcp: Change txhash on every SYN and RTO retransmit”
2: enabled for TCP RESET packets (no active listener) If set, a RST packet sent in response to a SYN packet on a closed port will reflect the incoming flow label.
4: enabled for ICMPv6 echo reply messages.

Default: 0

fib_multipath_hash_policy - INTEGER

Controls which hash policy to use for multipath routes.

Default: 0 (Layer 3)

Possible values:

0 - Layer 3 (source and destination addresses plus flow label)
1 - Layer 4 (standard 5-tuple)
2 - Layer 3 or inner Layer 3 if present
3 - Custom multipath hash. Fields used for multipath hash calculation are determined by fib_multipath_hash_fields sysctl

fib_multipath_hash_fields - UNSIGNED INTEGER

When fib_multipath_hash_policy is set to 3 (custom multipath hash), the fields used for multipath hash calculation are determined by this sysctl.

This value is a bitmask which enables various fields for multipath hash calculation.

Possible fields are:

0x0001	Source IP address
0x0002	Destination IP address
0x0004	IP protocol
0x0008	Flow Label
0x0010	Source port
0x0020	Destination port
0x0040	Inner source IP address
0x0080	Inner destination IP address
0x0100	Inner IP protocol
0x0200	Inner Flow Label
0x0400	Inner source port
0x0800	Inner destination port

Default: 0x0007 (source IP, destination IP and IP protocol)

anycast_src_echo_reply - BOOLEAN

Controls the use of anycast addresses as source addresses for ICMPv6 echo reply

TRUE: enabled
FALSE: disabled

Default: FALSE

idgen_delay - INTEGER

Controls the delay in seconds after which time to retry privacy stable address generation if a DAD conflict is detected.

Default: 1 (as specified in RFC7217)

idgen_retries - INTEGER

Controls the number of retries to generate a stable privacy address if a DAD conflict is detected.

Default: 3 (as specified in RFC7217)

mld_qrv - INTEGER

Controls the MLD query robustness variable (see RFC3810 9.1).

Default: 2 (as specified by RFC3810 9.1)

Minimum: 1 (as specified by RFC6636 4.5)

max_dst_opts_number - INTEGER

Maximum number of non-padding TLVs allowed in a Destination options extension header. If this value is less than zero then unknown options are disallowed and the number of known TLVs allowed is the absolute value of this number.

Default: 8

max_hbh_opts_number - INTEGER

Maximum number of non-padding TLVs allowed in a Hop-by-Hop options extension header. If this value is less than zero then unknown options are disallowed and the number of known TLVs allowed is the absolute value of this number.

Default: 8

max_dst_opts_length - INTEGER

Maximum length allowed for a Destination options extension header.

Default: INT_MAX (unlimited)

max_hbh_length - INTEGER

Maximum length allowed for a Hop-by-Hop options extension header.

Default: INT_MAX (unlimited)

skip_notify_on_dev_down - BOOLEAN

Controls whether an RTM_DELROUTE message is generated for routes removed when a device is taken down or deleted. IPv4 does not generate this message; IPv6 does by default. Setting this sysctl to true skips the message, making IPv4 and IPv6 on par in relying on userspace caches to track link events and evict routes.

Default: false (generate message)

nexthop_compat_mode - BOOLEAN

New nexthop API provides a means for managing nexthops independent of prefixes. Backwards compatibilty with old route format is enabled by default which means route dumps and notifications contain the new nexthop attribute but also the full, expanded nexthop definition. Further, updates or deletes of a nexthop configuration generate route notifications for each fib entry using the nexthop. Once a system understands the new API, this sysctl can be disabled to achieve full performance benefits of the new API by disabling the nexthop expansion and extraneous notifications. Default: true (backward compat mode)

fib_notify_on_flag_change - INTEGER

Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/ RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.

After installing a route to the kernel, user space receives an acknowledgment, which means the route was installed in the kernel, but not necessarily in hardware. It is also possible for a route already installed in hardware to change its action and therefore its flags. For example, a host route that is trapping packets can be “promoted” to perform decapsulation following the installation of an IPinIP/VXLAN tunnel. The notifications will indicate to user-space the state of the route.

Default: 0 (Do not emit notifications.)

Possible values:

0 - Do not emit notifications.
1 - Emit notifications.
2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.

ioam6_id - INTEGER

Define the IOAM id of this node. Uses only 24 bits out of 32 in total.

Min: 0 Max: 0xFFFFFF

Default: 0xFFFFFF

ioam6_id_wide - LONG INTEGER

Define the wide IOAM id of this node. Uses only 56 bits out of 64 in total. Can be different from ioam6_id.

Min: 0 Max: 0xFFFFFFFFFFFFFF

Default: 0xFFFFFFFFFFFFFF

IPv6 Fragmentation:

ip6frag_high_thresh - INTEGER

Maximum memory used to reassemble IPv6 fragments. When ip6frag_high_thresh bytes of memory is allocated for this purpose, the fragment handler will toss packets until ip6frag_low_thresh is reached.

ip6frag_low_thresh - INTEGER

See ip6frag_high_thresh

ip6frag_time - INTEGER

Time in seconds to keep an IPv6 fragment in memory.

conf/default/*:

Change the interface-specific default settings.

These settings would be used during creating new interfaces.

conf/all/*:

Change all the interface-specific settings.

[XXX: Other special features than forwarding?]

conf/all/disable_ipv6 - BOOLEAN

Changing this value is same as changing conf/default/disable_ipv6 setting and also all per-interface disable_ipv6 settings to the same value.

Reading this value does not have any particular meaning. It does not say whether IPv6 support is enabled or disabled. Returned value can be 1 also in the case when some interface has disable_ipv6 set to 0 and has configured IPv6 addresses.

conf/all/forwarding - BOOLEAN

Enable global IPv6 forwarding between all interfaces.

IPv4 and IPv6 work differently here; e.g. netfilter must be used to control which interfaces may forward packets and which not.

This also sets all interfaces’ Host/Router setting ‘forwarding’ to the specified value. See below for details.

This referred to as global forwarding.

proxy_ndp - BOOLEAN

Do proxy ndp.

fwmark_reflect - BOOLEAN

Controls the fwmark of kernel-generated IPv6 reply packets that are not associated with a socket for example, TCP RSTs or ICMPv6 echo replies). If unset, these packets have a fwmark of zero. If set, they have the fwmark of the packet they are replying to.

Default: 0

conf/interface/*:

Change special settings per interface.

The functional behaviour for certain settings is different depending on whether local forwarding is enabled or not.

accept_ra - INTEGER

Accept Router Advertisements; autoconfigure using them.

It also determines whether or not to transmit Router Solicitations. If and only if the functional setting is to accept Router Advertisements, Router Solicitations will be transmitted.

Possible values are:

0

Do not accept Router Advertisements.

1

Accept Router Advertisements if forwarding is disabled.

2

Overrule forwarding behaviour. Accept Router Advertisements even if forwarding is enabled.

Functional default:

enabled if local forwarding is disabled.

disabled if local forwarding is enabled.

accept_ra_defrtr - BOOLEAN

Learn default router in Router Advertisement.

Functional default:

enabled if accept_ra is enabled.

disabled if accept_ra is disabled.

ra_defrtr_metric - UNSIGNED INTEGER

Route metric for default route learned in Router Advertisement. This value will be assigned as metric for the default route learned via IPv6 Router Advertisement. Takes affect only if accept_ra_defrtr is enabled.

Possible values:

1 to 0xFFFFFFFF

Default: IP6_RT_PRIO_USER i.e. 1024.

accept_ra_from_local - BOOLEAN

Accept RA with source-address that is found on local machine if the RA is otherwise proper and able to be accepted.

Default is to NOT accept these as it may be an un-intended network loop.

Functional default:

enabled if accept_ra_from_local is enabled on a specific interface.

disabled if accept_ra_from_local is disabled on a specific interface.

accept_ra_min_hop_limit - INTEGER

Minimum hop limit Information in Router Advertisement.

Hop limit Information in Router Advertisement less than this variable shall be ignored.

Default: 1

accept_ra_min_lft - INTEGER

Minimum acceptable lifetime value in Router Advertisement.

RA sections with a lifetime less than this value shall be ignored. Zero lifetimes stay unaffected.

Default: 0

accept_ra_pinfo - BOOLEAN

Learn Prefix Information in Router Advertisement.

Functional default:

enabled if accept_ra is enabled.

disabled if accept_ra is disabled.

accept_ra_rt_info_min_plen - INTEGER

Minimum prefix length of Route Information in RA.

Route Information w/ prefix smaller than this variable shall be ignored.

Functional default:

0 if accept_ra_rtr_pref is enabled.

-1 if accept_ra_rtr_pref is disabled.

accept_ra_rt_info_max_plen - INTEGER

Maximum prefix length of Route Information in RA.

Route Information w/ prefix larger than this variable shall be ignored.

Functional default:

0 if accept_ra_rtr_pref is enabled.

-1 if accept_ra_rtr_pref is disabled.

accept_ra_rtr_pref - BOOLEAN

Accept Router Preference in RA.

Functional default:

enabled if accept_ra is enabled.

disabled if accept_ra is disabled.

accept_ra_mtu - BOOLEAN

Apply the MTU value specified in RA option 5 (RFC4861). If disabled, the MTU specified in the RA will be ignored.

Functional default:

enabled if accept_ra is enabled.

disabled if accept_ra is disabled.

accept_redirects - BOOLEAN

Accept Redirects.

Functional default:

enabled if local forwarding is disabled.

disabled if local forwarding is enabled.

accept_source_route - INTEGER

Accept source routing (routing extension header).

>= 0: Accept only routing header type 2.
< 0: Do not accept routing header.

Default: 0

autoconf - BOOLEAN

Autoconfigure addresses using Prefix Information in Router Advertisements.

Functional default:

enabled if accept_ra_pinfo is enabled.

disabled if accept_ra_pinfo is disabled.

dad_transmits - INTEGER

The amount of Duplicate Address Detection probes to send.

Default: 1

forwarding - INTEGER

Configure interface-specific Host/Router behaviour.

Note

It is recommended to have the same setting on all interfaces; mixed router/host scenarios are rather uncommon.

Possible values are:

0 Forwarding disabled

1 Forwarding enabled

FALSE (0):

By default, Host behaviour is assumed. This means:

IsRouter flag is not set in Neighbour Advertisements.
If accept_ra is TRUE (default), transmit Router Solicitations.
If accept_ra is TRUE (default), accept Router Advertisements (and do autoconfiguration).
If accept_redirects is TRUE (default), accept Redirects.

TRUE (1):

If local forwarding is enabled, Router behaviour is assumed. This means exactly the reverse from the above:

IsRouter flag is set in Neighbour Advertisements.
Router Solicitations are not sent unless accept_ra is 2.
Router Advertisements are ignored unless accept_ra is 2.
Redirects are ignored.

Default: 0 (disabled) if global forwarding is disabled (default), otherwise 1 (enabled).

hop_limit - INTEGER

Default Hop Limit to set.

Default: 64

mtu - INTEGER

Default Maximum Transfer Unit

Default: 1280 (IPv6 required minimum)

ip_nonlocal_bind - BOOLEAN

If set, allows processes to bind() to non-local IPv6 addresses, which can be quite useful - but may break some applications.

Default: 0

router_probe_interval - INTEGER

Minimum interval (in seconds) between Router Probing described in RFC4191.

Default: 60

router_solicitation_delay - INTEGER

Number of seconds to wait after interface is brought up before sending Router Solicitations.

Default: 1

router_solicitation_interval - INTEGER

Number of seconds to wait between Router Solicitations.

Default: 4

router_solicitations - INTEGER

Number of Router Solicitations to send until assuming no routers are present.

Default: 3

use_oif_addrs_only - BOOLEAN

When enabled, the candidate source addresses for destinations routed via this interface are restricted to the set of addresses configured on this interface (vis. RFC 6724, section 4).

Default: false

use_tempaddr - INTEGER

Preference for Privacy Extensions (RFC3041).

<= 0 : disable Privacy Extensions

== 1 : enable Privacy Extensions, but prefer public addresses over temporary addresses.

> 1 : enable Privacy Extensions and prefer temporary addresses over public addresses.

Default:

0 (for most devices)

-1 (for point-to-point devices and loopback devices)

temp_valid_lft - INTEGER

valid lifetime (in seconds) for temporary addresses.

Default: 172800 (2 days)

temp_prefered_lft - INTEGER

Preferred lifetime (in seconds) for temporary addresses.

Default: 86400 (1 day)

keep_addr_on_down - INTEGER

Keep all IPv6 addresses on an interface down event. If set static global addresses with no expiration time are not flushed.

>0 : enabled
0 : system default
<0 : disabled

Default: 0 (addresses are removed)

max_desync_factor - INTEGER

Maximum value for DESYNC_FACTOR, which is a random value that ensures that clients don’t synchronize with each other and generate new addresses at exactly the same time. value is in seconds.

Default: 600

regen_max_retry - INTEGER

Number of attempts before give up attempting to generate valid temporary addresses.

Default: 5

max_addresses - INTEGER

Maximum number of autoconfigured addresses per interface. Setting to zero disables the limitation. It is not recommended to set this value too large (or to zero) because it would be an easy way to crash the kernel by allowing too many addresses to be created.

Default: 16

disable_ipv6 - BOOLEAN

Disable IPv6 operation. If accept_dad is set to 2, this value will be dynamically set to TRUE if DAD fails for the link-local address.

Default: FALSE (enable IPv6 operation)

When this value is changed from 1 to 0 (IPv6 is being enabled), it will dynamically create a link-local address on the given interface and start Duplicate Address Detection, if necessary.

When this value is changed from 0 to 1 (IPv6 is being disabled), it will dynamically delete all addresses and routes on the given interface. From now on it will not possible to add addresses/routes to the selected interface.

accept_dad - INTEGER

Whether to accept DAD (Duplicate Address Detection).

0

Disable DAD

1

Enable DAD (default)

2

Enable DAD, and disable IPv6 operation if MAC-based duplicate link-local address has been found.

DAD operation and mode on a given interface will be selected according to the maximum value of conf/{all,interface}/accept_dad.

force_tllao - BOOLEAN

Enable sending the target link-layer address option even when responding to a unicast neighbor solicitation.

Default: FALSE

Quoting from RFC 2461, section 4.4, Target link-layer address:

“The option MUST be included for multicast solicitations in order to avoid infinite Neighbor Solicitation “recursion” when the peer node does not have a cache entry to return a Neighbor Advertisements message. When responding to unicast solicitations, the option can be omitted since the sender of the solicitation has the correct link- layer address; otherwise it would not have be able to send the unicast solicitation in the first place. However, including the link-layer address in this case adds little overhead and eliminates a potential race condition where the sender deletes the cached link-layer address prior to receiving a response to a previous solicitation.”

ndisc_notify - BOOLEAN

Define mode for notification of address and device changes.

0 - (default): do nothing
1 - Generate unsolicited neighbour advertisements when device is brought up or hardware address changes.

ndisc_tclass - INTEGER

The IPv6 Traffic Class to use by default when sending IPv6 Neighbor Discovery (Router Solicitation, Router Advertisement, Neighbor Solicitation, Neighbor Advertisement, Redirect) messages. These 8 bits can be interpreted as 6 high order bits holding the DSCP value and 2 low order bits representing ECN (which you probably want to leave cleared).

0 - (default)

ndisc_evict_nocarrier - BOOLEAN

Clears the neighbor discovery table on NOCARRIER events. This option is important for wireless devices where the neighbor discovery cache should not be cleared when roaming between access points on the same network. In most cases this should remain as the default (1).

1 - (default): Clear neighbor discover cache on NOCARRIER events.
0 - Do not clear neighbor discovery cache on NOCARRIER events.

mldv1_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicited MLDv1 report retransmit will take place.

Default: 10000 (10 seconds)

mldv2_unsolicited_report_interval - INTEGER

The interval in milliseconds in which the next unsolicited MLDv2 report retransmit will take place.

Default: 1000 (1 second)

force_mld_version - INTEGER

0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
1 - Enforce to use MLD version 1
2 - Enforce to use MLD version 2

suppress_frag_ndisc - INTEGER

Control RFC 6980 (Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery) behavior:

1 - (default) discard fragmented neighbor discovery packets
0 - allow fragmented neighbor discovery packets

optimistic_dad - BOOLEAN

Whether to perform Optimistic Duplicate Address Detection (RFC 4429).

0: disabled (default)
1: enabled

Optimistic Duplicate Address Detection for the interface will be enabled if at least one of conf/{all,interface}/optimistic_dad is set to 1, it will be disabled otherwise.

use_optimistic - BOOLEAN

If enabled, do not classify optimistic addresses as deprecated during source address selection. Preferred addresses will still be chosen before optimistic addresses, subject to other ranking in the source address selection algorithm.

0: disabled (default)
1: enabled

This will be enabled if at least one of conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.

stable_secret - IPv6 address

This IPv6 address will be used as a secret to generate IPv6 addresses for link-local addresses and autoconfigured ones. All addresses generated after setting this secret will be stable privacy ones by default. This can be changed via the addrgenmode ip-link. conf/default/stable_secret is used as the secret for the namespace, the interface specific ones can overwrite that. Writes to conf/all/stable_secret are refused.

It is recommended to generate this secret during installation of a system and keep it stable after that.

By default the stable secret is unset.

addr_gen_mode - INTEGER

Defines how link-local and autoconf addresses are generated.

0	generate address based on EUI64 (default)
1	do no generate a link-local address, use EUI64 for addresses generated from autoconf
2	generate stable privacy addresses, using the secret from stable_secret (RFC7217)
3	generate stable privacy addresses, using a random secret if unset

drop_unicast_in_l2_multicast - BOOLEAN

Drop any unicast IPv6 packets that are received in link-layer multicast (or broadcast) frames.

By default this is turned off.

drop_unsolicited_na - BOOLEAN

Drop all unsolicited neighbor advertisements, for example if there’s a known good NA proxy on the network and such frames need not be used (or in the case of 802.11, must not be used to prevent attacks.)

By default this is turned off.

accept_untracked_na - INTEGER

Define behavior for accepting neighbor advertisements from devices that are absent in the neighbor cache:

0 - (default) Do not accept unsolicited and untracked neighbor advertisements.
1 - Add a new neighbor cache entry in STALE state for routers on receiving a neighbor advertisement (either solicited or unsolicited) with target link-layer address option specified if no neighbor entry is already present for the advertised IPv6 address. Without this knob, NAs received for untracked addresses (absent in neighbor cache) are silently ignored.

This is as per router-side behavior documented in RFC9131.

This has lower precedence than drop_unsolicited_na.

This will optimize the return path for the initial off-link communication that is initiated by a directly connected host, by ensuring that the first-hop router which turns on this setting doesn’t have to buffer the initial return packets to do neighbor-solicitation. The prerequisite is that the host is configured to send unsolicited neighbor advertisements on interface bringup. This setting should be used in conjunction with the ndisc_notify setting on the host to satisfy this prerequisite.
2 - Extend option (1) to add a new neighbor cache entry only if the source IP address is in the same subnet as an address configured on the interface that received the neighbor advertisement.

enhanced_dad - BOOLEAN

Include a nonce option in the IPv6 neighbor solicitation messages used for duplicate address detection per RFC7527. A received DAD NS will only signal a duplicate address if the nonce is different. This avoids any false detection of duplicates due to loopback of the NS messages that we send. The nonce option will be sent on an interface unless both of conf/{all,interface}/enhanced_dad are set to FALSE.

Default: TRUE

`icmp/*`:¶

ratelimit - INTEGER

Limit the maximal rates for sending ICMPv6 messages.

0 to disable any limiting, otherwise the minimal space between responses in milliseconds.

Default: 1000

ratemask - list of comma separated ranges

For ICMPv6 message types matching the ranges in the ratemask, limit the sending of the message according to ratelimit parameter.

The format used for both input and output is a comma separated list of ranges (e.g. “0-127,129” for ICMPv6 message type 0 to 127 and 129). Writing to the file will clear all previous ranges of ICMPv6 message types and update the current list with the input.

Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml for numerical values of ICMPv6 message types, e.g. echo request is 128 and echo reply is 129.

Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)

echo_ignore_all - BOOLEAN

If set non-zero, then the kernel will ignore all ICMP ECHO requests sent to it over the IPv6 protocol.

Default: 0

echo_ignore_multicast - BOOLEAN

If set non-zero, then the kernel will ignore all ICMP ECHO requests sent to it over the IPv6 protocol via multicast.

Default: 0

echo_ignore_anycast - BOOLEAN

If set non-zero, then the kernel will ignore all ICMP ECHO requests sent to it over the IPv6 protocol destined to anycast address.

Default: 0

xfrm6_gc_thresh - INTEGER

(Obsolete since linux-4.14) The threshold at which we will start garbage collecting for IPv6 destination cache entries. At twice this value the system will refuse new allocations.

IPv6 Update by: Pekka Savola <pekkas@netcore.fi> YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>

/proc/sys/net/bridge/* Variables:¶

bridge-nf-call-arptables - BOOLEAN

1 : pass bridged ARP traffic to arptables’ FORWARD chain.
0 : disable this.

Default: 1

bridge-nf-call-iptables - BOOLEAN

1 : pass bridged IPv4 traffic to iptables’ chains.
0 : disable this.

Default: 1

bridge-nf-call-ip6tables - BOOLEAN

1 : pass bridged IPv6 traffic to ip6tables’ chains.
0 : disable this.

Default: 1

bridge-nf-filter-vlan-tagged - BOOLEAN

1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
0 : disable this.

Default: 0

bridge-nf-filter-pppoe-tagged - BOOLEAN

1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
0 : disable this.

Default: 0

bridge-nf-pass-vlan-input-dev - BOOLEAN

1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan interface on the bridge and set the netfilter input device to the vlan. This allows use of e.g. “iptables -i br0.1” and makes the REDIRECT target work with vlan-on-top-of-bridge interfaces. When no matching vlan interface is found, or this switch is off, the input device is set to the bridge interface.
0: disable bridge netfilter vlan interface lookup.

Default: 0

`proc/sys/net/sctp/*` Variables:¶

addip_enable - BOOLEAN

Enable or disable extension of Dynamic Address Reconfiguration (ADD-IP) functionality specified in RFC5061. This extension provides the ability to dynamically add and remove new addresses for the SCTP associations.

1: Enable extension.

0: Disable extension.

Default: 0

pf_enable - INTEGER

Enable or disable pf (pf is short for potentially failed) state. A value of pf_retrans > path_max_retrans also disables pf state. That is, one of both pf_enable and pf_retrans > path_max_retrans can disable pf state. Since pf_retrans and path_max_retrans can be changed by userspace application, sometimes user expects to disable pf state by the value of pf_retrans > path_max_retrans, but occasionally the value of pf_retrans or path_max_retrans is changed by the user application, this pf state is enabled. As such, it is necessary to add this to dynamically enable and disable pf state. See: https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for details.

1: Enable pf.

0: Disable pf.

Default: 1

pf_expose - INTEGER

Unset or enable/disable pf (pf is short for potentially failed) state exposure. Applications can control the exposure of the PF path state in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO sockopt. When it’s unset, no SCTP_PEER_ADDR_CHANGE event with SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info can be got via SCTP_GET_PEER_ADDR_INFO sockopt; When it’s enabled, a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming SCTP_PF state and a SCTP_PF-state transport info can be got via SCTP_GET_PEER_ADDR_INFO sockopt; When it’s diabled, no SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO sockopt.

0: Unset pf state exposure, Compatible with old applications.

1: Disable pf state exposure.

2: Enable pf state exposure.

Default: 0

addip_noauth_enable - BOOLEAN

Dynamic Address Reconfiguration (ADD-IP) requires the use of authentication to protect the operations of adding or removing new addresses. This requirement is mandated so that unauthorized hosts would not be able to hijack associations. However, older implementations may not have implemented this requirement while allowing the ADD-IP extension. For reasons of interoperability, we provide this variable to control the enforcement of the authentication requirement.

1	Allow ADD-IP extension to be used without authentication. This should only be set in a closed environment for interoperability with older implementations.
0	Enforce the authentication requirement

Default: 0

auth_enable - BOOLEAN

Enable or disable Authenticated Chunks extension. This extension provides the ability to send and receive authenticated chunks and is required for secure operation of Dynamic Address Reconfiguration (ADD-IP) extension.

1: Enable this extension.
0: Disable this extension.

Default: 0

prsctp_enable - BOOLEAN

Enable or disable the Partial Reliability extension (RFC3758) which is used to notify peers that a given DATA should no longer be expected.

1: Enable extension
0: Disable

Default: 1

max_burst - INTEGER

The limit of the number of new packets that can be initially sent. It controls how bursty the generated traffic can be.

Default: 4

association_max_retrans - INTEGER

Set the maximum number for retransmissions that an association can attempt deciding that the remote end is unreachable. If this value is exceeded, the association is terminated.

Default: 10

max_init_retransmits - INTEGER

The maximum number of retransmissions of INIT and COOKIE-ECHO chunks that an association will attempt before declaring the destination unreachable and terminating.

Default: 8

path_max_retrans - INTEGER

The maximum number of retransmissions that will be attempted on a given path. Once this threshold is exceeded, the path is considered unreachable, and new traffic will use a different path when the association is multihomed.

Default: 5

pf_retrans - INTEGER

The number of retransmissions that will be attempted on a given path before traffic is redirected to an alternate transport (should one exist). Note this is distinct from path_max_retrans, as a path that passes the pf_retrans threshold can still be used. Its only deprioritized when a transmission path is selected by the stack. This setting is primarily used to enable fast failover mechanisms without having to reduce path_max_retrans to a very low value. See: http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt for details. Note also that a value of pf_retrans > path_max_retrans disables this feature. Since both pf_retrans and path_max_retrans can be changed by userspace application, a variable pf_enable is used to disable pf state.

Default: 0

ps_retrans - INTEGER

Primary.Switchover.Max.Retrans (PSMR), it’s a tunable parameter coming from section-5 “Primary Path Switchover” in rfc7829. The primary path will be changed to another active path when the path error counter on the old primary path exceeds PSMR, so that “the SCTP sender is allowed to continue data transmission on a new working path even when the old primary destination address becomes active again”. Note this feature is disabled by initializing ‘ps_retrans’ per netns as 0xffff by default, and its value can’t be less than ‘pf_retrans’ when changing by sysctl.

Default: 0xffff

rto_initial - INTEGER

The initial round trip timeout value in milliseconds that will be used in calculating round trip times. This is the initial time interval for retransmissions.

Default: 3000

rto_max - INTEGER

The maximum value (in milliseconds) of the round trip timeout. This is the largest time interval that can elapse between retransmissions.

Default: 60000

rto_min - INTEGER

The minimum value (in milliseconds) of the round trip timeout. This is the smallest time interval the can elapse between retransmissions.

Default: 1000

hb_interval - INTEGER

The interval (in milliseconds) between HEARTBEAT chunks. These chunks are sent at the specified interval on idle paths to probe the state of a given path between 2 associations.

Default: 30000

sack_timeout - INTEGER

The amount of time (in milliseconds) that the implementation will wait to send a SACK.

Default: 200

valid_cookie_life - INTEGER

The default lifetime of the SCTP cookie (in milliseconds). The cookie is used during association establishment.

Default: 60000

cookie_preserve_enable - BOOLEAN

Enable or disable the ability to extend the lifetime of the SCTP cookie that is used during the establishment phase of SCTP association

1: Enable cookie lifetime extension.
0: Disable

Default: 1

cookie_hmac_alg - STRING

Select the hmac algorithm used when generating the cookie value sent by a listening sctp socket to a connecting client in the INIT-ACK chunk. Valid values are:

md5
sha1
none

Ability to assign md5 or sha1 as the selected alg is predicated on the configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and CONFIG_CRYPTO_SHA1).

Default: Dependent on configuration. MD5 if available, else SHA1 if available, else none.

rcvbuf_policy - INTEGER

Determines if the receive buffer is attributed to the socket or to association. SCTP supports the capability to create multiple associations on a single socket. When using this capability, it is possible that a single stalled association that’s buffering a lot of data may block other associations from delivering their data by consuming all of the receive buffer space. To work around this, the rcvbuf_policy could be set to attribute the receiver buffer space to each association instead of the socket. This prevents the described blocking.

1: rcvbuf space is per association
0: rcvbuf space is per socket

Default: 0

sndbuf_policy - INTEGER

Similar to rcvbuf_policy above, this applies to send buffer space.

1: Send buffer is tracked per association
0: Send buffer is tracked per socket.

Default: 0

sctp_mem - vector of 3 INTEGERs: min, pressure, max

Number of pages allowed for queueing by all SCTP sockets.

min: Below this number of pages SCTP is not bothered about its memory appetite. When amount of memory allocated by SCTP exceeds this number, SCTP starts to moderate memory usage.

pressure: This value was introduced to follow format of tcp_mem.

max: Number of pages allowed for queueing by all SCTP sockets.

Default is calculated at boot time from amount of available memory.

sctp_rmem - vector of 3 INTEGERs: min, default, max

Only the first value (“min”) is used, “default” and “max” are ignored.

min: Minimal size of receive buffer used by SCTP socket. It is guaranteed to each SCTP socket (but not association) even under moderate memory pressure.

Default: 4K

sctp_wmem - vector of 3 INTEGERs: min, default, max

Only the first value (“min”) is used, “default” and “max” are ignored.

min: Minimum size of send buffer that can be used by SCTP sockets. It is guaranteed to each SCTP socket (but not association) even under moderate memory pressure.

Default: 4K

addr_scope_policy - INTEGER

Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00

0 - Disable IPv4 address scoping
1 - Enable IPv4 address scoping
2 - Follow draft but allow IPv4 private addresses
3 - Follow draft but allow IPv4 link local addresses

Default: 1

udp_port - INTEGER

The listening port for the local UDP tunneling sock. Normally it’s using the IANA-assigned UDP port number 9899 (sctp-tunneling).

This UDP sock is used for processing the incoming UDP-encapsulated SCTP packets (from RFC6951), and shared by all applications in the same net namespace. This UDP sock will be closed when the value is set to 0.

The value will also be used to set the src port of the UDP header for the outgoing UDP-encapsulated SCTP packets. For the dest port, please refer to ‘encap_port’ below.

Default: 0

encap_port - INTEGER

The default remote UDP encapsulation port.

This value is used to set the dest port of the UDP header for the outgoing UDP-encapsulated SCTP packets by default. Users can also change the value for each sock/asoc/transport by using setsockopt. For further information, please refer to RFC6951.

Note that when connecting to a remote server, the client should set this to the port that the UDP tunneling sock on the peer server is listening to and the local UDP tunneling sock on the client also must be started. On the server, it would get the encap_port from the incoming packet’s source port.

Default: 0

plpmtud_probe_interval - INTEGER

The time interval (in milliseconds) for the PLPMTUD probe timer, which is configured to expire after this period to receive an acknowledgment to a probe packet. This is also the time interval between the probes for the current pmtu when the probe search is done.

PLPMTUD will be disabled when 0 is set, and other values for it must be >= 5000.

Default: 0

reconf_enable - BOOLEAN

Enable or disable extension of Stream Reconfiguration functionality specified in RFC6525. This extension provides the ability to “reset” a stream, and it includes the Parameters of “Outgoing/Incoming SSN Reset”, “SSN/TSN Reset” and “Add Outgoing/Incoming Streams”.

1: Enable extension.
0: Disable extension.

Default: 0

intl_enable - BOOLEAN

Enable or disable extension of User Message Interleaving functionality specified in RFC8260. This extension allows the interleaving of user messages sent on different streams. With this feature enabled, I-DATA chunk will replace DATA chunk to carry user messages if also supported by the peer. Note that to use this feature, one needs to set this option to 1 and also needs to set socket options SCTP_FRAGMENT_INTERLEAVE to 2 and SCTP_INTERLEAVING_SUPPORTED to 1.

1: Enable extension.
0: Disable extension.

Default: 0

ecn_enable - BOOLEAN

Control use of Explicit Congestion Notification (ECN) by SCTP. Like in TCP, ECN is used only when both ends of the SCTP connection indicate support for it. This feature is useful in avoiding losses due to congestion by allowing supporting routers to signal congestion before having to drop packets.

1: Enable ecn. 0: Disable ecn.

Default: 1

`/proc/sys/net/core/*`¶

Please see: Documentation for /proc/sys/net/ for descriptions of these entries.

`/proc/sys/net/unix/*`¶

max_dgram_qlen - INTEGER

The maximum length of dgram socket receive queue

Default: 10

IP Sysctl¶

/proc/sys/net/ipv4/* Variables¶

INET peer storage¶

TCP variables¶

UDP variables¶

RAW variables¶

CIPSOv4 Variables¶

IP Variables¶

/proc/sys/net/ipv6/* Variables¶

icmp/*:¶

/proc/sys/net/bridge/* Variables:¶

proc/sys/net/sctp/* Variables:¶

/proc/sys/net/core/*¶

/proc/sys/net/unix/*¶

`icmp/*`:¶

`proc/sys/net/sctp/*` Variables:¶

`/proc/sys/net/core/*`¶

`/proc/sys/net/unix/*`¶