Advanced Installation

This section goes further in explaining how to setup your bastion. You should have completed the basic installation first.

Encryption & signature GPG keys

There are 2 pairs of GPG keys being used by the bastion:

The bastion GPG key
- The private key is used by the bastion to sign the ttyrec files
- The public key is used by the admins to verify the signature and prove non-repudiation and non-tampering of the ttyrec files
The admins GPG key
- The public key is used by the bastion to encrypt the backups and the ttyrec files
- The private key is used by the admins to decrypt the backups when a restore operation is needed, and the ttyrec files

Generating the bastion GPG key

Generate a GPG key that will be used by the bastion to sign files, this might take a while especially if the server is idle:

 /opt/bastion/bin/admin/setup-gpg.sh --generate

 gpg: directory `/root/.gnupg' created
 gpg: Generating GPG key, it'll take some time.

 Not enough random bytes available.  Please do some other work to give
 the OS a chance to collect more entropy! (Need 39 more bytes)
 ..........+++++
 gpg: /root/.gnupg/trustdb.gpg: trustdb created
 gpg: key A4480F26 marked as ultimately trusted
 gpg: done
 gpg: checking the trustdb
 gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
 gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u

 Configuration file /etc/bastion/osh-encrypt-rsync.conf.d/50-gpg-bastion-key.conf updated:
 8<---8<---8<---8<---8<---8<--
 # autogenerated with /opt/bastion/bin/admin/setup-gpg.sh at Wed Mar 21 10:03:08 CET 2018
 {
     "signing_key_passphrase": "************",
     "signing_key": "5D3CFDFFA4480F26"
 }
 --->8--->8--->8--->8--->8--->8

 Done.

While it's working, you can proceed to the section below.

Generating and importing the admins GPG key

You should import on the bastion one or more public GPG keys that'll be used for encryption. If you don't already have a GPG key for this, you can generate one. As this is the admin GPG key, don't generate it on the bastion itself, but on the desk of the administrator (you?) instead.

If you're running a reasonably recent GnuPG version (and the bastion does, too), i.e. GnuPG >= 2.1.x, then you can generate an Ed25519 key by running:

 myname='John Doe'
 email='jd@example.org'
 bastion='mybastion4.example.org'
 pass=$(pwgen -sy 12 1)
 echo "The passphrase for the key will be: $pass"
 gpg --batch --pinentry-mode loopback --passphrase-fd 0 --quick-generate-key "$myname <$email>" ed25519 sign 0 <<< "$pass"
 fpr=$(gpg --list-keys "$myname <$email>" | grep -Eo '[A-F0-9]{40}')
 gpg --batch --pinentry-mode loopback --passphrase-fd 0 --quick-add-key "$fpr" cv25519 encr 0 <<< "$pass"

 gpg: key 3F379CA7ECDF0537 marked as ultimately trusted
 gpg: directory '/home/user/.gnupg/openpgp-revocs.d' created
 gpg: revocation certificate stored as '/home/user/.gnupg/openpgp-revocs.d/3DFB21E3857F562A603BD4F83F379CA7ECDF0537.rev'

If you or the bastion is using an older version of GnuPG, or you are unsure and/or prefer compatibility over speed or security, you can fallback to an RSA 4096 key:

 myname='John Doe'
 email='jd@example.org'
 bastion='mybastion4.example.org'
 pass=`pwgen -sy 12 1`
 echo "The passphrase for the key will be: $pass"
 printf "Key-Type: RSA\nKey-Length: 4096\nSubkey-Type: RSA\nSubkey-Length: 4096\n" \
   "Name-Real: %s\nName-Comment: %s\nName-Email: %s\nExpire-Date: 0\n" \
   "Passphrase: %s\n%%echo Generating GPG key\n%%commit\n%%echo done\n" \
   "$myname ($bastion)" $(date +%Y) "$email" "$pass" | gpg --gen-key --batch

 The passphrase for the key will be: ************
 gpg: Generating GPG key

 Not enough random bytes available.  Please do some other work to give
 the OS a chance to collect more entropy! (Need 119 more bytes)
 .....+++++

 gpg: key D2BDF9B5 marked as ultimately trusted
 gpg: done

Of course, in both snippets above, adjust the myname, email and bastion variables accordingly. Write down the passphrase in a secure vault. All bastions admins will need it if they are to decrypt ttyrec files later for inspection, and also decrypt the backup should a restore be needed. When the key is done being generated, get the public key with:

gpg -a --export "$myname <$email>"

Copy it to your clipboard, then back to the bastion, paste it at the following prompt:

 /opt/bastion/bin/admin/setup-gpg.sh --import

Also export the private admins GPG key to a secure vault (if you want the same key to be shared by the admins):

 gpg --export-secret-keys --armor "$myname <$email>"

Rotation, encryption & backup of ttyrec files

You should already have all the needed GPG keys at the proper places, by following "Setup the encryption & signature GPG keys" section above.

The configuration file is located in /etc/bastion/osh-encrypt-rsync.conf. You can ignore the signing_key, signing_key_passphrase and recipients options, as these have been auto-filled when you generated the GPG keys, by dropping configuration files in the /etc/bastion/osh-encrypt-rsync.conf.d directory. Any file there takes precedence over the global configuration file.

Once you are done with you configuration, you might want to test it by running:

/opt/bastion/bin/cron/osh-encrypt-rsync.pl --config-test

Or even go further by starting the script in dry-run mode:

/opt/bastion/bin/cron/osh-encrypt-rsync.pl --dry-run

Configuring keys, accounts & groups remote backup

Everything that is needed to restore a bastion from backup (keys, accounts, groups, etc.) is backed up daily in /root/backups by default. If you followed the "Setup the encryption & signature GPG keys" section above, these backups will be encrypted automatically.

If you want to push these backups to a remote location, which is warmly advised, you have to specify the remote location to scp the backup archives to. The configuration file is /etc/bastion/osh-backup-acl-keys.conf, and you should specify the PUSH_REMOTE and PUSH_OPTIONS.

To verify that the script is correctly able to connect remotely (and also validate the remote hostkey), start the script manually:

 /opt/bastion/bin/cron/osh-backup-acl-keys.sh

 Pushing backup file (/root/backups/backup-2020-05-25.tar.gz.gpg) remotely...
 backup-2020-05-25.tar.gz.gpg
 100%   21MB  20.8MB/s   00:00

Also verify that the extension is .gpg, as seen above, which indicates that the script successfully encrypted the backup.

Logs/Syslog

It is advised to use syslog for The Bastion application logs. This can be configured in /etc/bastion/bastion.conf with the parameter enableSyslog.

There is a default syslog-ng configuration provided, if you happen to use it. The file can be found as etc/syslog-ng/conf.d/20-bastion.conf.dist in the repository. Please read the comments in the file to know how to integrate it properly in your system.

Clustering (High Availability)

The bastions can work in a cluster, with N instances. In that case, there is one master instance, where any modification command can be used (creating accounts, deleting groups, granting accesses), and N-1 slave instances, where only readonly actions are permitted. Note that any instance can be used to connect to infrastructures, so in effect all instances can always be used at the same time. You may set up a DNS round-robin hostname, with all the instances IPs declared, so that clients automatically choose a random instance, without having to rely on another external component such as a load-balancer. Note that if you do this, you'll need all the instances to share the same SSH host keys.

Setting up a slave bastion

Before, setting up the slave bastion, you should have the two bastions up and running (follow the normal installation documentation).

On the slave

The sync of the passwd and group files can have adverse effects on a newly installed machine where the packages where not installed in the same order than on the master, hence having different UIDs for the same users. The following commands are known to fix all the problems that could arise in that case, on an classic Debian machine, that has puppet, postfix, ossec and bind installed (disregard any file or directory not found message):

chown -R puppet:puppet /var/lib/puppet /var/log/puppet /run/puppet
chgrp puppet /etc/puppet
chown -R postfix /var/spool/postfix /var/lib/postfix
chown root:root /var/spool/postfix
chown -R root:root /var/spool/postfix/{pid,etc,lib,dev,usr}
chgrp -R postdrop /var/spool/postfix/{public,maildrop}
chown root:postdrop /usr/sbin/postdrop /usr/sbin/postqueue
chmod g+s /usr/sbin/postdrop /usr/sbin/postqueue
chown -R ossec /var/ossec/logs /var/ossec/queue /var/ossec/stats /var/ossec/var
chgrp -R ossec /var/ossec
chown ossecr /var/ossec/queue/agent-info /var/ossec/queue/rids
chown root /var/ossec/queue/ /var/ossec/queue/alerts/execq /var/ossec/var /var/ossec/var/run
chgrp bind /var/cache/bind /var/lib/bind /etc/bind /etc/bind/named.conf.default-zones /run/named
chown -R bind:bind /etc/bind/rndc.key /run/named
chgrp allowkeeper /var/log/bastion

Then, on the slave, set the readOnlySlaveMode option in the /etc/bastion/bastion.conf file to 1:

vim /etc/bastion/bastion.conf

This will instruct the bastion to deny any modification plugin, so that changes can only be done through the master instance.

Then, append the master bastion synchronization public SSH keyfile, found in ~root/.ssh/id_master2slave.pub on the master instance, to ~bastionsync/.ssh/authorized_keys on the slave, with the following prefix: from="IP.OF.THE.MASTER",restrict

Hence the file should look like this:

from="198.51.100.42",restrict ssh-ed25519 AAA[...]

Note that if you're using an old OpenSSH before version 7.2, the prefix should be instead: from="IP.OF.THE.MASTER",no-port-forwarding,no-agent-forwarding,no-X11-forwarding,no-pty,no-user-rc.

On the master

Check that the key setup works correctly by launching the following command under the root account:

rsync -vaA --numeric-ids --dry-run --delete --filter "merge /etc/bastion/osh-sync-watcher.rsyncfilter"
--rsh "ssh -i /root/.ssh/id_master2slave" / bastionsync@IP.OF.THE.SLAVE:/

Check that it's not trying to rsync too much stuff (if you have weird things in your /home, you might want to edit /etc/bastion/osh-sync-watcher.rsyncfilter to exclude that stuff)
Once you're happy with the output, retry without the --dry-run
When it's done, run it immediately again to ensure it still work, because /etc/passwd and /etc/group will have been overwritten on the slave
Then, edit the configuration on the master:

vim /etc/bastion/osh-sync-watcher.sh

Then, configure the script to start on boot and start it manually:

systemctl enable osh-sync-watcher
systemctl start osh-sync-watcher

You can check the logs (if you configured syslog instead, which is encouraged, then the logfile depends on your syslog daemon configuration)

tail -F /var/log/bastion/osh-sync-watcher.log

Misc

Create SSHFP records

If you want to use SSHFP (for a bastion, you should), generate the records and publish them in the DNS:

awk 'tolower($1)~/^hostkey$/ {system("ssh-keygen -r bastion.name -f "$2)}' /etc/ssh/sshd_config

Harden the SSH configuration

Using our SSH templates is a good start in any case. If you want to go further, there are a lot of online resources to help you harden your SSH configuration, and audit a running SSHd server. As the field evolves continuously, we don't want to recommend one particularly here, as it might get out of date rapidly, but looking for ssh audit on GitHub is probably a good start. Of course, this also depends on your environment, and you might not be able to harden your SSHd configuration as much as you would like.

Note that for The Bastion, both sides can be independently hardened: the ingress part is handled in sshd_config, and the egress part is handled in ssh_config.

2FA root authentication

The bastion supports TOTP (Time-based One Time Password), to further secure high profile accesses. This section covers the configuration of 2FA root authentication on the bastion itself. TOTP can also be enabled for regular bastion users, but this is covered in another section. To enable 2FA root authentication, run on the bastion:

script -c "google-authenticator -t -Q UTF8 -r 3 -R 15 -s /var/otp/root -w 2 -e 4 -D" /root/qrcode

Of course, you can check the --help and adjust the options accordingly. The example given above has sane defaults, but you might want to adjust if needed. Now, flash this QR code with your phone, using a TOTP application. You might want to copy the QR code somewhere safe in case you need to flash it on some other phone, by exporting the base64 version of it:

gzip -c /root/qrcode | base64 -w150

Copy this in your password manager (for example). You can then delete the /root/qrcode file.

You have then two configuration adjustments to do.

First, ensure you have installed the provided /etc/pam.d/sshd file, or at least the corresponding line to enable the TOTP pam plugin in your configuration.
Second, ensure that your /etc/ssh/sshd_config file calls PAM for root authentication. In the provided templates, there is a commented snippet to do it. The uncommented snippet looks like this:

# 2FA has been configured for root, so we force pubkey+PAM for it
Match User root
    AuthenticationMethods publickey,keyboard-interactive:pam

Note that first, the usual publickey method will be used, then control will be passed to PAM. This is where the /etc/pam.d/sshd configuration will apply.

Now, you should be asked for the TOTP the next time you try to login through ssh as root. In case something goes wrong with the new configuration, be sure to keep your already opened existing connection to be able to fix the problem without falling back to console access.

Once this has been tested, you can (and probably should) also protect the direct root console access to your machine with TOTP, including a snippet similar to this one:

# TOTP config
auth    [success=1 default=ignore]  pam_google_authenticator.so secret=/var/otp/${USER}
auth    requisite                   pam_deny.so
# End of TOTP Config

inside your /etc/pam.d/login file.

Of course, when using TOTP, this is paramount to ensure your server is properly synchronized through NTP.