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Backing up in pull mode

Typically the borg client connects to a backup server using SSH as a transport when initiating a backup. This is referred to as push mode.

If you however require the backup server to initiate the connection or prefer it to initiate the backup run, one of the following workarounds is required to allow such a pull mode setup.

A common use case for pull mode is to backup a remote server to a local personal computer.

SSHFS

Assuming you have a pull backup system set up with borg, where a backup server pulls the data from the target via SSHFS. In this mode, the backup client’s file system is mounted remotely on the backup server. Pull mode is even possible if the SSH connection must be established by the client via a remote tunnel. Other network file systems like NFS or SMB could be used as well, but SSHFS is very simple to set up and probably the most secure one.

There are some restrictions caused by SSHFS. For example, unless you define UID and GID mappings when mounting via sshfs, owners and groups of the mounted file system will probably change, and you may not have access to those files if BorgBackup is not run with root privileges.

SSHFS is a FUSE file system and uses the SFTP protocol, so there may be also other unsupported features that the actual implementations of sshfs, libfuse and sftp on the backup server do not support, like file name encodings, ACLs, xattrs or flags. So there is no guarantee that you are able to restore a system completely in every aspect from such a backup.

Warning

To mount the client’s root file system you will need root access to the client. This contradicts to the usual threat model of BorgBackup, where clients don’t need to trust the backup server (data is encrypted). In pull mode the server (when logged in as root) could cause unlimited damage to the client. Therefore, pull mode should be used only from servers you do fully trust!

Warning

Additionally, while being chrooted into the client’s root file system, code from the client will be executed. Thus, you should only do that when fully trusting the client.

Warning

The chroot method was chosen to get the right user and group name-id mappings, assuming they only come from files (/etc/passwd and group). This assumption might be wrong, e.g. if users/groups also come from ldap or other providers. Thus, it might be better to use --numeric-ids and not archive any user or group names (but just the numeric IDs) and not use chroot.

Creating a backup

Generally, in a pull backup situation there is no direct way for borg to know the client’s original UID:GID name mapping of files, because Borg would use /etc/passwd and /etc/group of the backup server to map the names. To derive the right names, Borg needs to have access to the client’s passwd and group files and use them in the backup process.

The solution to this problem is chrooting into an sshfs mounted directory. In this example the whole client root file system is mounted. We use the stand-alone BorgBackup executable and copy it into the mounted file system to make Borg available after entering chroot; this can be skipped if Borg is already installed on the client.

# Mount client root file system.
mkdir /tmp/sshfs
sshfs root@host:/ /tmp/sshfs
# Mount BorgBackup repository inside it.
mkdir /tmp/sshfs/borgrepo
mount --bind /path/to/repo /tmp/sshfs/borgrepo
# Make borg executable available.
cp /usr/local/bin/borg /tmp/sshfs/usr/local/bin/borg
# Mount important system directories and enter chroot.
cd /tmp/sshfs
for i in dev proc sys; do mount --bind /$i $i; done
chroot /tmp/sshfs

Now we are on the backup system but inside a chroot with the client’s root file system. We have a copy of Borg binary in /usr/local/bin and the repository in /borgrepo. Borg will back up the client’s user/group names, and we can create the backup, retaining the original paths, excluding the repository:

borg create --exclude borgrepo --files-cache ctime,size /borgrepo::archive /

For the sake of simplicity only borgrepo is excluded here. You may want to set up an exclude file with additional files and folders to be excluded. Also note that we have to modify Borg’s file change detection behaviour – SSHFS cannot guarantee stable inode numbers, so we have to supply the --files-cache option.

Finally, we need to exit chroot, unmount all the stuff and clean up:

exit # exit chroot
rm /tmp/sshfs/usr/local/bin/borg
cd /tmp/sshfs
for i in dev proc sys borgrepo; do umount ./$i; done
rmdir borgrepo
cd ~
umount /tmp/sshfs
rmdir /tmp/sshfs

Thanks to secuser on IRC for this how-to!

Restore methods

The counterpart of a pull backup is a push restore. Depending on the type of restore – full restore or partial restore – there are different methods to make sure the correct IDs are restored.

Partial restore

In case of a partial restore, using the archived UIDs/GIDs might lead to wrong results if the name-to-ID mapping on the target system has changed compared to backup time (might be the case e.g. for a fresh OS install).

The workaround again is chrooting into an sshfs mounted directory, so Borg is able to map the user/group names of the backup files to the actual IDs on the client. This example is similar to the backup above – only the Borg command is different:

# Mount client root file system.
mkdir /tmp/sshfs
sshfs root@host:/ /tmp/sshfs
# Mount BorgBackup repository inside it.
mkdir /tmp/sshfs/borgrepo
mount --bind /path/to/repo /tmp/sshfs/borgrepo
# Make borg executable available.
cp /usr/local/bin/borg /tmp/sshfs/usr/local/bin/borg
# Mount important system directories and enter chroot.
cd /tmp/sshfs
for i in dev proc sys; do mount --bind /$i $i; done
chroot /tmp/sshfs

Now we can run

borg extract /borgrepo::archive PATH

to partially restore whatever we like. Finally, do the clean-up:

exit # exit chroot
rm /tmp/sshfs/usr/local/bin/borg
cd /tmp/sshfs
for i in dev proc sys borgrepo; do umount ./$i; done
rmdir borgrepo
cd ~
umount /tmp/sshfs
rmdir /tmp/sshfs

Full restore

When doing a full restore, we restore all files (including the ones containing the ID-to-name mapping, /etc/passwd and /etc/group). Everything will be consistent automatically if we restore the numeric IDs stored in the archive. So there is no need for a chroot environment; we just mount the client file system and extract a backup, utilizing the --numeric-ids option:

sshfs root@host:/ /mnt/sshfs
cd /mnt/sshfs
borg extract --numeric-ids /path/to/repo::archive
cd ~
umount /mnt/sshfs

Simple (lossy) full restore

Using borg export-tar it is possible to stream a backup to the client and directly extract it without the need of mounting with SSHFS:

borg export-tar /path/to/repo::archive - | ssh root@host 'tar -C / -x'

Note that in this scenario the tar format is the limiting factor – it cannot restore all the advanced features that BorgBackup supports. See borg export-tar for limitations.

socat

In this setup a SSH connection from the backup server to the client is established that uses SSH reverse port forwarding to transparently tunnel data between UNIX domain sockets on the client and server and the socat tool to connect these with the borg client and server processes, respectively.

The program socat has to be available on the backup server and on the client to be backed up.

When pushing a backup the borg client (holding the data to be backed up) connects to the backup server via ssh, starts borg serve on the backup server and communicates via standard input and output (transported via SSH) with the process on the backup server.

With the help of socat this process can be reversed. The backup server will create a connection to the client (holding the data to be backed up) and will pull the data.

In the following example borg-server connects to borg-client to pull a backup.

To provide a secure setup sockets should be stored in /run/borg, only accessible to the users that run the backup process. So on both systems, borg-server and borg-client the folder /run/borg has to be created:

sudo mkdir -m 0700 /run/borg

On borg-server the socket file is opened by the user running the borg serve process writing to the repository so the user has to have read and write permissions on /run/borg:

borg-server:~$ sudo chown borgs /run/borg

On borg-client the socket file is created by ssh, so the user used to connect to borg-client has to have read and write permissions on /run/borg:

borg-client:~$ sudo chown borgc /run/borg

On borg-server, we have to start the command borg serve and make its standard input and output available to a unix socket:

borg-server:~$ socat UNIX-LISTEN:/run/borg/reponame.sock,fork EXEC:"borg serve --append-only --restrict-to-path /path/to/repo"

Socat will wait until a connection is opened. Then socat will execute the command given, redirecting Standard Input and Output to the unix socket. The optional arguments for borg serve are not necessary but a sane default.

Note

When used in production you may also use systemd socket-based activation instead of socat on the server side. You would wrap the borg serve command in a service unit and configure a matching socket unit to start the service whenever a client connects to the socket.

Now we need a way to access the unix socket on borg-client (holding the data to be backed up), as we created the unix socket on borg-server Opening a SSH connection from the borg-server to the borg-client with reverse port forwarding can do this for us:

borg-server:~$ ssh -R /run/borg/reponame.sock:/run/borg/reponame.sock borgc@borg-client

Note

As the default value of OpenSSH for StreamLocalBindUnlink is no, the socket file created by sshd is not removed. Trying to connect a second time, will print a short warning, and the forwarding does not take place:

Warning: remote port forwarding failed for listen path /run/borg/reponame.sock

When you are done, you have to manually remove the socket file, otherwise you may see an error like this when trying to execute borg commands:

Remote: YYYY/MM/DD HH:MM:SS socat[XXX] E connect(5, AF=1 "/run/borg/reponame.sock", 13): Connection refused
Connection closed by remote host. Is borg working on the server?

When a process opens the socket on borg-client, SSH will forward all data to the socket on borg-server.

The next step is to tell borg on borg-client to use the unix socket to communicate with the borg serve command on borg-server via the socat socket instead of SSH:

borg-client:~$ export BORG_RSH="sh -c 'exec socat STDIO UNIX-CONNECT:/run/borg/reponame.sock'"

The default value for BORG_RSH is ssh. By default Borg uses SSH to create the connection to the backup server. Therefore Borg parses the repo URL and adds the server name (and other arguments) to the SSH command. Those arguments can not be handled by socat. We wrap the command with sh to ignore all arguments intended for the SSH command.

All Borg commands can now be executed on borg-client. For example to create a backup execute the borg create command:

borg-client:~$ borg create ssh://borg-server/path/to/repo::archive /path_to_backup

When automating backup creation, the interactive ssh session may seem inappropriate. An alternative way of creating a backup may be the following command:

borg-server:~$ ssh \
   -R /run/borg/reponame.sock:/run/borg/reponame.sock \
   borgc@borg-client \
   borg create \
   --rsh "sh -c 'exec socat STDIO UNIX-CONNECT:/run/borg/reponame.sock'" \
   ssh://borg-server/path/to/repo::archive /path_to_backup \
   ';' rm /run/borg/reponame.sock

This command also automatically removes the socket file after the borg create command is done.

ssh-agent

In this scenario borg-server initiates an SSH connection to borg-client and forwards the authentication agent connection.

After that, it works similar to the push mode: borg-client initiates another SSH connection back to borg-server using the forwarded authentication agent connection to authenticate itself, starts borg serve and communicates with it.

Using this method requires ssh access of user borgs to borgc@borg-client, where:

  • borgs is the user on the server side with read/write access to local borg repository.

  • borgc is the user on the client side with read access to files meant to be backed up.

Applying this method for automated backup operations

Assume that the borg-client host is untrusted. Therefore we do some effort to prevent a hostile user on the borg-client side to do something harmful. In case of a fully trusted borg-client the method could be simplified.

Preparing the server side

Do this once for each client on borg-server to allow borgs to connect itself on borg-server using a dedicated ssh key:

borgs@borg-server$ install -m 700 -d ~/.ssh/
borgs@borg-server$ ssh-keygen -N '' -t rsa  -f ~/.ssh/borg-client_key
borgs@borg-server$ { echo -n 'command="borg serve --append-only --restrict-to-repo ~/repo",restrict '; cat ~/.ssh/borg-client_key.pub; } >> ~/.ssh/authorized_keys
borgs@borg-server$ chmod 600 ~/.ssh/authorized_keys

install -m 700 -d ~/.ssh/

Create directory ~/.ssh with correct permissions if it does not exist yet.

ssh-keygen -N '' -t rsa  -f ~/.ssh/borg-client_key

Create an ssh key dedicated to communication with borg-client.

Note

Another more complex approach is using a unique ssh key for each pull operation. This is more secure as it guarantees that the key will not be used for other purposes.

{ echo -n 'command="borg serve --append-only --restrict-to-repo ~/repo",restrict '; cat ~/.ssh/borg-client_key.pub; } >> ~/.ssh/authorized_keys

Add borg-client’s ssh public key to ~/.ssh/authorized_keys with forced command and restricted mode. The borg client is restricted to use one repo at the specified path and to append-only operation. Commands like delete, prune and compact have to be executed another way, for example directly on borg-server side or from a privileged, less restricted client (using another authorized_keys entry).

chmod 600 ~/.ssh/authorized_keys

Fix permissions of ~/.ssh/authorized_keys.

Pull operation

Initiating borg command execution from borg-server (e.g. init):

borgs@borg-server$ (
  eval $(ssh-agent) > /dev/null
  ssh-add -q ~/.ssh/borg-client_key
  echo 'your secure borg key passphrase' | \
    ssh -A -o StrictHostKeyChecking=no borgc@borg-client "BORG_PASSPHRASE=\$(cat) borg --rsh 'ssh -o StrictHostKeyChecking=no' init --encryption repokey ssh://borgs@borg-server/~/repo"
  kill "${SSH_AGENT_PID}"
)

Parentheses around commands are needed to avoid interference with a possibly already running ssh-agent. Parentheses are not needed when using a dedicated bash process.

eval $(ssh-agent) > /dev/null

Run the SSH agent in the background and export related environment variables to the current bash session.

ssh-add -q ~/.ssh/borg-client_key

Load the SSH private key dedicated to communication with the borg-client into the SSH agent. Look at man 1 ssh-add for a more detailed explanation.

Note

Care needs to be taken when loading keys into the SSH agent. Users on the borg-client having read/write permissions to the agent’s UNIX-domain socket (at least borgc and root in our case) can access the agent on borg-server through the forwarded connection and can authenticate using any of the identities loaded into the agent (look at man 1 ssh for more detailed explanation). Therefore there are some security considerations:

  • Private keys loaded into the agent must not be used to enable access anywhere else.

  • The keys meant to be loaded into the agent must be specified explicitly, not from default locations.

  • The borg-client’s entry in borgs@borg-server:~/.ssh/authorized_keys must be as restrictive as possible.

echo 'your secure borg key passphrase' | ssh -A -o StrictHostKeyChecking=no borgc@borg-client "BORG_PASSPHRASE=\$(cat) borg --rsh 'ssh -o StrictHostKeyChecking=no' init --encryption repokey ssh://borgs@borg-server/~/repo"

Run the borg init command on borg-client.

ssh://borgs@borg-server/~/repo refers to the repository repo within borgs’s home directory on borg-server.

StrictHostKeyChecking=no is used to automatically add host keys to ~/.ssh/known_hosts without user intervention.

kill "${SSH_AGENT_PID}"

Kill ssh-agent with loaded keys when it is not needed anymore.

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