How do I generate SSHFP records?

Question

I need to setup SSHFP records in the DNS for my host. I have done some searching but I haven't found any good example.

• What are SSHFP records?
• What does SSHFP records look like?
• How do I create SSHFP records?

Solution 1

What are SSHFP records?

SSHFP records are DNS records that contain fingerprints for public keys used for SSH. They're mostly used with DNSSEC enabled domains. When an SSH client connects to a server it checks the corresponding SSHFP record. If the records fingerprint matches the servers, the server is legit and it's safe to connect.

What does SSHFP records look like?

SSHFP records consist of three things:

1. Public key algorithm
2. Fingerprint type
3. Fingerprint (in hex)

Public key algorithm

There are five different algorithms defined in SSHFP as of 2021. Each algorithm is represented by an integer. The algorithms are:

• 1 - RSA
• 2 - DSA
• 3 - ECDSA
• 4 - Ed25519
• 6 - Ed448

Fingerprint type

Two fingerprint types are defined in SSHFP as of 2012. Each fingerprint type is represented by an integer. These are:

• 1 - SHA-1
• 2 - SHA-256

How do I generate SSHFP records?

You can use ssh-keygen to generate the records using the -r parameter, followed by the hostname (which does not affect the fingerprints so you can specify whatever you like instead)

Example

Using ssh-keygen and CentOS:

[root@localhost ~]# ssh-keygen -r my.domain.com
my.domain.com IN SSHFP 1 1 450c7d19d5da9a3a5b7c19992d1fbde15d8dad34
my.domain.com IN SSHFP 2 1 72d30d211ce8c464de2811e534de23b9be9b4dc4

Note

Sometimes ssh-keygen will ask for the location of the public certificate. If it asks, you will have to run ssh-keygen multiple times and every time specify a different certificate to make sure that you generate all necessary SSHFP records. Your public keys are usually located in /etc/ssh.

Solution 2

I'm not sure if ssh-keygen works with existing keys. If not you still can easily assemble them in your shell (which I prefer), and without fancy software or remote interfaces.

A records such as mentioned...

my.domain.com IN SSHFP 2 1 72d30d211ce8c464de2811e534de23b9be9b4dc4

...exist of 6 parts:

part 1: hostname
part 2: Usually "IN" for internet
part 3: "SSHFP", the RR name for type 44
part 4: RSA keys     = "1"
        DSA keys     = "2"
        ECDSA keys   = "3"
        Ed25519 keys = "4"
        Ed448 keys   = "6"
part 5: The algorithm type:
        SHA-1        = "1"
        SHA-256      = "2"
part 6: You can generate, for example:

        $ awk '{print $2}' /etc/ssh/ssh_host_dsa_key.pub | \
            openssl base64 -d -A | openssl sha1

To make use of it, put VerifyHostKeyDNS ask in your SSH client's config, usually ~/.ssh/config.

Solution 3

Older versions of ssh-keygen don't generate all the available keys (eg. no support for ecdsa and sha256). This script does creates all the records for all available keys in /etc/ssh/:

#!/bin/bash
#
# Creates SSHFP Records for all available keys
#

HOST="${1-$(hostname -f)}"

if [[ "$1" == "-h" || "$1" == "--help" ]]
then
  echo "Usage: sshfpgen <hostname>"
fi

if which openssl >/dev/null 2>&1
then
  if ! which sha1sum >/dev/null 2>&1
  then
    sha1sum() {
      openssl dgst -sha1 | grep -E -o "[0-9a-f]{40}"
    }
  fi
  if ! which sha256sum >/dev/null 2>&1
  then
    sha256sum() {
      openssl dgst -sha256 | grep -E -o "[0-9a-f]{64}"
    }
  fi
fi

for pubkey in /etc/ssh/ssh_host_*_key.pub /etc/ssh_host_*_key.pub
do
  case "$(cut -d _ -f3 <<< "$pubkey")"
  in
    rsa)
      echo "$HOST IN SSHFP 1 1 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha1sum  | cut -f 1 -d ' ')"
      echo "$HOST IN SSHFP 1 2 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha256sum  | cut -f 1 -d ' ')"
    ;;
    dsa)
      echo "$HOST IN SSHFP 2 1 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha1sum  | cut -f 1 -d ' ')"
      echo "$HOST IN SSHFP 2 2 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha256sum  | cut -f 1 -d ' ')"
    ;;
    ecdsa)
      echo "$HOST IN SSHFP 3 1 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha1sum  | cut -f 1 -d ' ')"
      echo "$HOST IN SSHFP 3 2 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha256sum  | cut -f 1 -d ' ')"
    ;;
    ed25519)
      echo "$HOST IN SSHFP 4 1 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha1sum  | cut -f 1 -d ' ')"
      echo "$HOST IN SSHFP 4 2 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha256sum  | cut -f 1 -d ' ')"
    ;;
    ed448)
      echo "$HOST IN SSHFP 6 1 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha1sum  | cut -f 1 -d ' ')"
      echo "$HOST IN SSHFP 6 2 $(cut -f2 -d ' ' "$pubkey" | base64 --decode | sha256sum  | cut -f 1 -d ' ')"
    ;;
  esac
done

Edit: New Version with PR from alex-dupuy with *BSD support.

https://github.com/mindfuckup/Scripts/blob/master/sshfpgen

Solution 4

If you use Puppet, facter has built in support for sshfp. Plus if you are using PuppetDB you can easily extract this info for all your hosts.

facter | grep -i sshfp
  sshfp_dsa => SSHFP 2 1 e1a3e639d6dbd48d3964ebfb772d2d11f1065682
  SSHFP 2 2 4f620ce2bc97d91ae5eff42fba621d65b677ab725f275f56b2abd1303c142b73
  sshfp_rsa => SSHFP 1 1 a78351af371faf3f19533c3a4a9e967543d7d2f5
  SSHFP 1 2 795943a6ee8b53c818cfef5781209e25a6eb4bc386813db60d3ff2c1569692fc

Solution 5

This is how I'm getting my SSHFP records through Ansible:

- name: Capture the SSHFP entries
  shell: "ssh-keygen -r {{ ansible_nodename }}|awk '{print $4, $5, $6}'"
  register: sshfp_entries

View more solutions

Comments

[Celada]

For the record, ssh-keygen -r does generate SSHFP records for existing keys despite the fact that the name of the command suggests it's only for GENerating.

[Kornelis]

ssh-keygen -r also handles ed25519 type records (using the experimental number 4 from iana iana.org/assignments/dns-sshfp-rr-parameters/… )

[Michael Mior]

The server is legit, or the DNS server is compromised.