Sunday, May 26, 2024

My ssh/sshd Configurations

Let’s look at my SSH configurations!

File Layout

Starting with Ubuntu 22.04 LTS and Debian 12, the OpenSSH version in the distribution is new enough that the Include directive is supported, and works properly with Match blocks in included files.  Therefore, most of the global stuff ends up in /etc/ssh/sshd_config.d/01-security.conf and further modifications are made at higher numbers.

Core Security

To minimize surface area, I turn off features I don’t use, if possible:

GSSAPIAuthentication no
HostbasedAuthentication no
PasswordAuthentication no
PermitEmptyPasswords no

AllowTcpForwarding no
X11Forwarding no
Compression no

PermitUserRC no
# Debian and derivatives
DebianBanner no

Some of these are defaults, unless the distribution changes them, which means “explicit is better than implicit” is strongly advised.

Next, I use a group to permit access, allowing me to explicitly add the members to the group without needing to edit the ssh config when things change.  Don’t forget to groupadd ssh-users (once) and gpasswd -a USER ssh-users (for each user.) Then, permit only that group:

AllowGroups ssh-users
# extra paranoia
PermitRootLogin no

Note that all of the above may be overridden in Match blocks, where required. TCP forwarding may also be more finely controlled through PermitListen and PermitOpen directives.

Note also that my systems are essentially single-user.  The group doesn't permit any sharing (and doesn't participate in quotas or anything) that would otherwise be forbidden.


Machines I use for ssh and sshd are all amd64, so for personal usage, I bump the AES algorithms to the front of the list:

Ciphers ^,aes256-ctr


The biggest trouble is the SFTP subsystem.  I comment that out in the main config, then set it in my own:

# /etc/ssh/sshd_config:
#Subsystem sftp ...

# /etc/ssh/sshd_config/02-sftp.conf:
Subsystem sftp internal-sftp
Match group sftp-only
    # ForceCommand, ChrootDirectory, etc.

I forget the details of what goes in that Match block.  It’s work stuff, set up a while ago now.

Ongoing Hardening

I occasionally run ssh-audit and check out what else shows up.  Note that you may need to run it with the --skip-rate-test option these days, particularly if you have set up fail2ban (guess how I know.)

There are also other hardening guides on the internet; I have definitely updated my moduli to only include 3072-bit and up options.  Incidentally, if you wonder how that works:

awk '$5 >= 3071' ...

The default action for awk is print, so that command prints lines that fulfill the condition.  The fifth field is the length of the modulus, so that’s what we compare to.  The actual bit count is 3071 instead of 3072, because the first digit must be 1 to make a 3072-bit number, so there are only 3071 bits that aren’t predetermined.

Client Config Sample

Host site-admin
  # [HostName, Port, User undisclosed]
  IdentityFile ~/.ssh/id_admin
  IdentitiesOnly yes

Host 192.168.*
  # Allow talking to Dropbear 2022.83+ on this subnet
  KexAlgorithms +curve25519-sha256,
  MACs +hmac-sha2-256

Host *
  GSSAPIAuthentication no

It’s mostly post-quantum, or assigning a very specific private key to the administrative user on my Web server.

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