Last updated Jul 21, 2021

Published under MIT licence

How to self-host hardened Borg server

Requirements

Caveats

  • When copy/pasting commands that start with $, strip out $ as this character is not part of the command
  • When copy/pasting commands that start with cat << "EOF", select all lines at once (from cat << "EOF" to EOF inclusively) as they are part of the same (single) command

Setup guide

Step 1: create borg SSH key pair (on computer)

When asked for file in which to save key, enter borg.

When asked for passphrase, use output from openssl rand -base64 24 (and store passphrase in password manager).

$ mkdir ~/.ssh

$ cd ~/.ssh

$ ssh-keygen -t rsa -C "borg"
Generating public/private rsa key pair.
Enter file in which to save the key (/Users/sunknudsen/.ssh/id_rsa): borg
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in borg.
Your public key has been saved in borg.pub.
The key fingerprint is:
SHA256:9DzU/jDPyR/vGe8k2Yn1p31wF8UxLzCmYEj//D6+oYk borg
The key's randomart image is:
+---[RSA 3072]----+
|     ...o   +  +.|
|      .o . + o  =|
|        o o . . o|
|       . * .   o |
|        S * +  ..|
|           o B++=|
|            o.O**|
|         . +.. *O|
|        E o.+o.+O|
+----[SHA256]-----+

$ cat borg.pub
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABgQCwCdu7RCOmISZQ5cr43lDRPrFoxCXcVfCREYsdTIBEoQrIwyg1jZyzQMf9kORIGcNe5+olIj1aK9qVg0hCEeDSJosSsMP5o8tQJzNu5aYCtnADlZ+AuCgp5CpL1vECMaQsfQV9nju3ScE/+0C/MSYVvDx5sbRvi1XuutBbCAZtlUa7Rn7S8/X08XLFasM7KhFz7AH2Hvvi1i3Cg1WqkRKzpXE/uxntZ/qZxBdpa2WEN/phD4LgmmCbzKJYflhJNKJnYQZxGveGsdexdrDpEbajVECBw/0ntS5/YYaLxzqCrNGyCRdAajIccuOLQjRGzr9U5mdzVpHhkCLjbIDQ1JHxtb9nHxNgvGep7z0UCqawdcJN2nEr1D7Khu7Mh8mryR7iBxqEdPfdARuQn3kMFH+YA5NASTus9p/MR1cavJmBq3u88oNje8q+szkBsQDb1h0n0eAzjjDXRSxgm8bdtpi07TjTNCc+AmhYiym+MYXmbxqMO6pnjjE1I+ht3a8zUU0= borg

Step 2: create borg-append-only SSH key pair (on computer)

When asked for file in which to save key, enter borg-append-only.

When asked for passphrase, leave field empty for no passphrase.

$ ssh-keygen -t rsa -C "borg-append-only"
Generating public/private rsa key pair.
Enter file in which to save the key (/Users/sunknudsen/.ssh/id_rsa): borg-append-only
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in borg-append-only.
Your public key has been saved in borg-append-only.pub.
The key fingerprint is:
SHA256:Se6MQbWpFg0lWI2+fJ1IVPtUCs/ZRYrgtpz4F3hi2ow borg-append-only
The key's randomart image is:
+---[RSA 3072]----+
|         +....   |
|         .B o..  |
|         ooB.o ..|
|         .E.....+|
|        S. o. oo+|
|        . o o.o+=|
|       . o = +**+|
|        o o o.*=B|
|         . . o o=|
+----[SHA256]-----+

$ cat borg-append-only.pub
ssh-rsa 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 borg-append-only

Step 3: generate SSH authorized keys heredoc (on computer)

Set Borg storage quota environment variable

BORG_STORAGE_QUOTA="10G"

Generate heredoc (the output of following command will be used at step 8)

cat << EOF
cat << "_EOF" > /home/borg/.ssh/authorized_keys
command="borg serve --restrict-to-repository /home/borg/backup --storage-quota $BORG_STORAGE_QUOTA",restrict $(cat ~/.ssh/borg.pub)
command="borg serve --append-only --restrict-to-repository /home/borg/backup --storage-quota $BORG_STORAGE_QUOTA",restrict $(cat ~/.ssh/borg-append-only.pub)
_EOF
EOF

Step 4: log in to server or Raspberry Pi

Replace server-admin@185.112.147.115 with SSH destination of server and ~/.ssh/server with path to associated private key.

ssh server-admin@185.112.147.115 -i ~/.ssh/server

Step 5: switch to root

When asked, enter root password.

su -

Step 6: create borg user

When asked for password, use output from openssl rand -base64 24 (and store password in password manager).

All other fields are optional, press enter to skip them and then press Y.

$ adduser borg
Adding user `borg' ...
Adding new group `borg' (1000) ...
Adding new user `borg' (1000) with group `borg' ...
Creating home directory `/home/borg' ...
Copying files from `/etc/skel' ...
New password:
Retype new password:
passwd: password updated successfully
Changing the user information for borg
Enter the new value, or press ENTER for the default
	Full Name []:
	Room Number []:
	Work Phone []:
	Home Phone []:
	Other []:
Is the information correct? [Y/n] Y

Step 7: update APT index

apt update

Step 8: install Borg

apt install -y borgbackup

Step 9: configure borg SSH authorized keys

Create .ssh directory

mkdir -p /home/borg/.ssh

Create /home/borg/.ssh/authorized_keys using heredoc generated at step 2

cat << "_EOF" > /home/borg/.ssh/authorized_keys
command="borg serve --restrict-to-repository /home/borg/backup --storage-quota 10G",restrict ssh-rsa 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 borg
command="borg serve --append-only --restrict-to-repository /home/borg/backup --storage-quota 10G",restrict ssh-rsa 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 borg-append-only
_EOF

Change ownership of /home/borg/.ssh

chown -R borg:borg /home/borg/.ssh

👍

Contributors:Sun KnudsenSun Knudsen
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