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Backup with Borg

May 27, 2021



Borg is a client server system allowing to backup remotely to repositories

Risk analysis

Scenario: Backup server compromised

  • Confidentiality of all client data in danger if no encryption is used. Client encryption is a must, could even be at repository level if the client shares different backup users with different kinds of data to backup.
  • Integrity compromised at repository level, the structure of the backup data could be partly or completely damaged (mitigate by monitoring and report suspicious changes to the system).
  • Availability (not much to do about that then), a second level backup is necessary

Scenario: Backup client compromised

  • Confidentiality is compromised for the data on the client system and accessible by the client on the remote backup server. That means clients should be segregated into separate repositories to avoid a compromised client to be able to sneak/peek in another client's data.
  • Integrity: compromised the client could add or delete data from the backup server it has access to and can be mitigated by setting an append-only mode. But BEWARE with borg a pruning takes place automatically the first time a client wihout append-only accesses the repository.
  • Availability: mitigate by preparing a redundant system

Recovery / Access

Using borg mount to remotely mount snapshots to select data to be restored



Install software

sudo apt install borgbackup

Create borg user

sudo useradd -m borg
sudo passwd borg

Make directories for each of the clients

sudo mkdir -p /srv/borg/client1
# ...
sudo mkdir -p /srv/borg/clientN

# set permissions
sudo chown -R borg:borg /srv/borg


sudo apt install borgbackup

Create a key pair (without password)

ssh-keygen -t rsa -b 4096 -f ~/.ssh/id_borg
# private key: ~/.ssh/id_borg.
# public key: ~/.ssh/id_borg.pub.

copy the SSH public key to the borg server

ssh-copy-id -i ~/.ssh/id_borg borg@<server>

Configure server SSH access restrictions

Login as user borg on the server

# edit the authorized_keys file to restrict each client
vi ~/.ssh/authorized_keys

# each of the client should follow this format:
# command="cd /srv/borg/<client>; borg serve --append-only --restrict-to-path /srv/borg/<client>"

# for example, here is the entry for client1
# command="cd /srv/borg/client1; borg serve --append-only --restrict-to-path /srv/borg/client1" ssh-ecdsa  root@<server>

Configure client for initial backup

Common variables:

export BORG_RSH="ssh -i ~/.ssh/id_borg"
export BORG_REPO=borg@<server>:<repos>

Initialize repository:

borg init --encryption=keyfile --append-only $BORG_REPOS
borg key export $BORG_REPOS ~/borg-server-backup.key # Store this and passphrase into KeePass

The key can be found here: ~/.config/borg/keys

Backup example daily:

# with heuristic compression, uses lzma if data compresses well (but is roughly 4x as slow)
borg create \
  --verbose --filter AME \
  --list --stats --show-rc \
  --compression auto,lzma --exclude-caches \
  $BORG_REPOS::'daily-{utcnow:%Y-%m-%d_%H:%M:%S}' \
  /path1 \

In order to automate the backup task it can sound good to use some environment variables to avoid the interaction. A better approach would be an ansible script run once and deleted after the backup completed or some secret management server like hashi corp Vault (in that case the use of BORG_PASSCOMMAND is indicated). That would avoid to keep the password at rest visible. But the misuse if the client is compromised can be performed on the data itself and the backup integrity is ensured through the use of append-only.

PATH_TO_BACKUP=<absolute path>

export BORG_RSH="ssh -i ~/.ssh/id_borg"
export BORG_PASSPHRASE='***'

# use lz4 compression and log into file
borg create \
  --verbose --filter AME \
  --list --stats --show-rc \
  --compression lz4 \
  $BORG_REPOS::'daily-{utcnow:%Y-%m-%d_%H:%M:%S}' \
  2>> "borg_backup_${REPOS}_$(date --iso-8601).log"


Recover data

borg mount can be used to mount the data provided you have the key and passphrase and ssh access to the server holding the encrypted data.