Copyright © 2003-2008, 2017 Thomas M. Eastep
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2023/02/18
Table of Contents
This page covers Shorewall configuration to use with Squid running as a Transparent Proxy or as a Manual Proxy.
This article applies to Shorewall 4.0 and later. If you are running a version of Shorewall earlier than Shorewall 4.0.0 then please see the documentation for that release.
If your firewall is dual-stack, there are risks to using either Transparent Proxy or TPROXY. Both break PMTU discovery for local clients and can cause slow page loading and/or inability to connect to some sites.
This section gives instructions for transparent proxying of HTTP. HTTPS (normally TCP port 443) cannot be proxied transparently (stop and think about it for a minute; if HTTPS could be transparently proxied, then how secure would it be?).
Please observe the following general requirements:
In all cases, Squid should be configured to run as a transparent proxy as described at http://wiki.squid-cache.org/SquidFaq/InterceptionProxy.
The bottom line of that article is that if you are running Squid 2.6 or later, then you simply need to add the word transparent to your http_port specification:
http_port 3128 transparent
In earlier Squid versions, you need to set several options:
http_port 3128 httpd_accel_host virtual httpd_accel_port 80 httpd_accel_with_proxy on httpd_accel_uses_host_header on
Depending on your distribution, other Squid configuration changes may be required. These changes typically consist of:
Adding an ACL that represents the clients on your local network.
Example:
ACL my_networks src 192.168.1.0/24 192.168.2.0/24
Allowing HTTP access to that ACL.
Example:
http_access allow my_networks
See your distribution's Squid documentation and http://www.squid-cache.org/ for details.
It is a good idea to get Squid working as a manual proxy first before you try transparent proxying.
The following instructions mention the file /etc/shorewall/start - if you don't have that file, simply create it.
When the Squid server is in the local zone, that zone must be defined ONLY by its interface -- no /etc/shorewall/hosts file entries. That is because the packets being routed to the Squid server still have their original destination IP addresses.
You must have iptables installed on your Squid server.
In the instructions below, only TCP Port 80 is opened from the system running Squid to the Internet. If your users require browsing sites that use a port other than 80 (e.g., http://www.domain.tld:8080) then you must open those ports as well.
Three different configurations are covered:
| Squid (transparent) Running on the Firewall |
| Squid (transparent) Running in the local Network |
| Squid (transparent) Running in a DMZ |
You want to redirect all local www connection requests EXCEPT those to your own http server (206.124.146.177) to a Squid transparent proxy running on the firewall and listening on port 3128. Squid will of course require access to remote web servers.
In /etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST ACCEPT $FW net tcp www REDIRECT loc 3128 tcp www - !206.124.146.177
There may be a requirement to exclude additional destination hosts or networks from being redirected. For example, you might also want requests destined for 130.252.100.0/24 to not be routed to Squid.
If needed, you may just add the additional hosts/networks to the ORIGDEST column in your REDIRECT rule.
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST REDIRECT loc 3128 tcp www - !206.124.146.177,130.252.100.0/24
People frequently ask How can I exclude certain internal systems from using the proxy? I want to allow those systems to go directly to the net.
Suppose that you want to exclude 192.168.1.5 and 192.168.1.33 from the proxy. Your rules would then be:
#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST
ACCEPT $FW net tcp www
REDIRECT loc:!192.168.1.5,192.168.1.33\
3128 tcp www - !206.124.146.177,130.252.100.0/24
ACCEPT loc net tcp wwwThe last rule may be omitted if your loc->net policy is ACCEPT.
In some cases (when running an LTSP server on the Shorewall system), you might want to transparently proxy web connections that originate on the firewall itself. This requires care to ensure that Squid's own web connections are not proxied.
First, determine the user id that Squid is running under:
gateway:/etc/shorewall# ps aux | fgrep -i squid | fgrep -v fgrep root 10085 0.0 0.0 23864 700 ? Ss Apr22 0:00 /usr/sbin/squid -D -YC proxy 10088 0.0 0.9 40512 19192 ? S Apr22 10:58 (squid) -D -YC gateway:/etc/shorewall#
In this case, the proxy process (squid) is running under the proxy user Id. We add these rules:
#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST RATE USER
ACCEPT $FW net tcp www
REDIRECT $FW 3128 tcp www - - - !proxyYou want to redirect all local www connection requests to a Squid transparent proxy running in your local zone at 192.168.1.3 and listening on port 3128. Your local interface is eth1. There may also be a web server running on 192.168.1.3. It is assumed that web access is already enabled from the local zone to the Internet.
Add this entry to your /etc/shorewall/providers file.
#NAME NUMBER MARK DUPLICATE INTERFACE GATEWAY OPTIONS Squid 1 202 - eth1 192.168.1.3 loose,notrack
In /etc/shorewall/mangle add:
#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST MARK(202):P eth1:!192.168.1.3 0.0.0.0/0 tcp 80
If you are still using a tcrules file, you should consider switching to using a mangle file (shorewall update -t (shorewall update on Shorewall 5.0 and later) will do that for you). Corresponding /etc/shorewall/tcrules entries are:
#MARK SOURCE DEST PROTO DPORT 202:P eth1:!192.168.1.3 0.0.0.0/0 tcp 80
In :/etc/shorewall/interfaces
#ZONE INTERFACE OPTIONS
loc eth1 routeback,routefilter=0,logmartians=0 On 192.168.1.3, arrange for the following command to be executed after networking has come up
iptables -t nat -A PREROUTING -i eth0 ! -d 192.168.1.3 -p tcp --dport 80 -j REDIRECT --to-ports 3128 If you are running RedHat on the server, you can simply execute the following commands after you have typed the iptables command above:
iptables-save > /etc/sysconfig/iptables
chkconfig --level 35 iptables on You have a single system in your DMZ with IP address 192.0.2.177. You want to run both a web server and Squid on that system.
In /etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT SPORT ORIGDEST DNAT loc dmz:192.0.2.177:3128 tcp 80 - !192.0.2.177
Assume that the dmz is connected through eth2 and that your local lan interfaces through eth1
Add this entry to your /etc/shorewall/providers file.
#NAME NUMBER MARK DUPLICATE INTERFACE GATEWAY OPTIONS Squid 1 202 - eth2 192.0.2.177 loose,notrack
In /etc/shorewall/mangle add:
#ACTION SOURCE DEST PROTO DPORT MARK(202):P eth1 0.0.0.0/0 tcp 80
Corresponding /etc/shorewall/tcrules entries are:
#MARK SOURCE DEST PROTO DPORT 202:P eth1 0.0.0.0/0 tcp 80
In :/etc/shorewall/interfaces
#ZONE INTERFACE OPTIONS
loc eth2 routefilter=0,logmartians=0 On 172.0.2.177, arrange for the following command to be executed after networking has come up
iptables -t nat -A PREROUTING -i eth0 ! -d 192.0.2.177 -p tcp --dport 80 -j REDIRECT --to-ports 3128 If you are running RedHat on the server, you can simply execute the following commands after you have typed the iptables command above:
iptables-save > /etc/sysconfig/iptables
chkconfig --level 35 iptables on Assume that Squid is running in zone SZ and listening on port SP; all web sites that are to be accessed through Squid are in the “net” zone. Then for each zone Z that needs access to the Squid server.
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT ACCEPT Z SZ tcp SP ACCEPT SZ net tcp 80,443
Example 1. Squid on the firewall listening on port 8080 with access from the “loc” zone:
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT ACCEPT loc $FW tcp 8080 ACCEPT $FW net tcp 80,443
Shorewall 4.5.4 contains support for TPROXY. TPROXY differs from REDIRECT in that it does not modify the IP header and requires Squid 3 or later. Because the IP header stays intact, TPROXY requires policy routing to direct the packets to the proxy server running on the firewall. This approach requires TPROXY support in your kernel and iptables and Squid 3. See http://wiki.squid-cache.org/Features/Tproxy4.
Support for the TPROXY action in shorewall-tcrules(5) and the
local option in shorewall-providers(5) has been
available since Shoreall 4.4.7. That support required additional rules
to be added in the 'start' extention script to make it work reliably.
Beginning with Shorewall 4.6.0, TPROXY in shorewall-tcrules(5) and
in shorewall-mangle(5) work as
described here.
The following configuration works with Squid running on the firewall itself (assume that Squid is listening on port 3129 for TPROXY connections).
/etc/shorewall/interfaces:
#ZONE INTERFACE OPTIONS - lo -
/etc/shorewall/providers:
#NAME NUMBER MARK DUPLICATE INTERFACE GATEWAY OPTIONS COPY Tproxy 1 - - lo - tproxy
Notice that the MARK, DUPLICATE and GATEWAY columns are empty and
that the only option is tproxy.
/etc/shorewall/mangle (assume loc interface is
eth1 and net interface is eth0):
#ACTION SOURCE DEST PROTO DPORT SPORT DIVERT eth0 0.0.0.0/0 tcp - 80 TPROXY(3129) eth1 0.0.0.0/0 tcp 80
Corresponding /etc/shorewall/tcrules
are:
#MARK SOURCE DEST PROTO DPORT SPORT DIVERT eth0 0.0.0.0/0 tcp - 80 TPROXY(3129) eth1 0.0.0.0/0 tcp 80
The DIVERT rules are used to avoid unnecessary invocation of TPROXY for request packets after the connection is established and to direct response packets back to Squid3.
If you run a web server on the Shorewall system that also listens on port 80, then you need to exclude it from TPROXY. Suppose that your web server listens on 192.0.2.144; then:
#MARK SOURCE DEST PROTO DPORT SPORT DIVERT eth0 0.0.0.0/0 tcp - 80 TPROXY(3129) eth1 !192.0.2.144 tcp 80 -
/etc/shorewall/rules:
#ACTION SOURCE DEST PROTO DPORT ACCEPT loc $FW tcp 80 ACCEPT $FW net tcp 80
/etc/squid3/squid.conf:
... http_port 3129 tproxy ...
If you use TPROXY with both IPv4 and IPv6, then both your local hosts and the gateway must have the same DNS view. If a client resolves a website URL to an IPv6 address and the server can only resolve to an IPv4 address, then Squid will attempt to connect to the IPv4 address using the local client's IPv6 address. That clearly doesn't work.