SSHFP records are a defense against people blindly typing ‘yes’ when asked if they want to continue connecting to an SSH host who’s authenticity is unknown.

$ ssh some.host.tld
The authenticity of host 'some.host.tld (123.456.789.10)' can't be established.
ED25519 key fingerprint is 69:76:51:39:a4:c6:de:15:7c:50:4b:4a:a7:98:40:5e.
Are you sure you want to continue connecting (yes/no)?

This prompt is likely to be extremely familiar to you and, most people seem to just type ‘yes’ to move on with their lives, which defeats the whole purpose of this prompt.

If you use DNSSEC you can bypass this prompt entirely by publishing your server’s key fingerprints via DNS and having SSH authenticate them for you.

Generating your SSHFP record

You can get SSH to generate the DNS records for you, log in to the server in question and run the command below to get similar content.

$ ssh-keygen -r some.host.tld
some.host.tld IN SSHFP 1 1 c53bfb3d5d053280b17db76909f707f3ac9cbb47
some.host.tld IN SSHFP 1 2 56310ad73fae7a3861f87c246f1fb7c0884706f9a65e94d75be4fb14ca973275
some.host.tld IN SSHFP 4 1 fe3a67a65b71631c8c16c173c09ad9885b72bd4e
some.host.tld IN SSHFP 4 2 7dd9225ef20b806e78fca60935c8b051565ab6077d7735e2c8d23fdfd26289d2

Each line in response contains the following information.

Hostname IN SSHFP Algorithm Fingerprint type Hash
some.host.tld IN SSHFP 1 1 c53bfb3d5d053280b17db76909f707f3ac9cbb47

Algorithm

  1. RSA
  2. DSA
  3. ECDSA
  4. ED25519

Fingerprint type

  1. SHA-1
  2. SHA-2

I would advise you do not use DSA or ECDSA algorithms or SHA-1 fingerprints.

Add the relevant records to your DNS.

~/.ssh/config

Add the following to your ~/.ssh/config file.

Host *
    VerifyHostKeyDNS yes

This means SSH will always try to validate host keys from DNS.

Now, when you try to SSH to a host it should validate against SSHFP records automatically.

$ ssh -v some.host.tld
...
debug1: found 2 secure fingerprints in DNS
debug1: matching host key fingerprint found in DNS
...

If you’re not using DNSSEC then automatic validation will not happen, instead you will be told that records match but the fingerprints are insecure.

Kura

Anarchist. Pessimist. Bipolar. Hacker. Hyperpolyglot. Musician. Ex pro gamer. Cunt. They/Them.

Kura

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