[Borealid]

SSH has *ANOTHER* built-in solution, in the form of the SSHFP DNS record.

If the DNS record for the host has an SSHFP (SSH FingerPrint) record, SSH will compare it to the retrieved public key(s) and refuse the connection if there is a mismatch. It can be configured to require DNSSec for this, or to only reject if it gets a secure rejection (to prevent DoS).

It works perfectly, has no notable down sides (just add a DNS record when you generate the host's SSH key), and has been around for many years.

[beagle3]

It is very insecure unless you use dnssec, isn’t it?

Just means an attacker also needs to mitm DNS if you MITM the host. Not trivial, but depending on setup might not be harder.

[Borealid]

I recommend reading the description of the option `VerifyHostKeyDns` in the `ssh_config` man page.

If set to `yes`, you get automatic trust-on-first-use (no user prompt) if you use DNSSec, and you get the current asking-the-user behavior if your DNSSec is broken or you are under attack.

Obviously it's more secure if you use DNSSec, because that way you can reflexively deny any request to manually verify a host key, but it provides value regardless.

[egberts1]

Correct. Very insecure unless your client app goes out of its way to perform DnSSEC.

But wait, there's more: SSH config, resolv.conf, DNS RR setup.

A lomg checklist for successful SSHFP deployment:

https://egbert.net/blog/articles/dns-rr-sshfp.html

[Borealid]

That site doesn't mention that when DNSSec is absent, the behaviour of SSH is identical to what happens if you hadn't used the SSHFP record at all, except that for unsophisticated attackers it also displays "no matching host key found in DNS".

So even without DNSSec using the SSHFP records is an improvement over not using them because some of the time it tells you for certain you're being interfered with.

There is no situation in which an insecure DNS response is auto-trusted by the SSH client.

[cromka]

Is there something like this for Wireguard?

[Borealid]

Wireguard has no key distribution mechanism.

You can use software like Headscale/Tailscale/Netbird on top.