How do I generate SSHFP records?

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. Algorithm
  2. Fingerprint type
  3. Fingerprint (in hex)

Algorithm

There are four different algorithms defined in SSHFP as of 2015. Each algorithm is represented by an integer. The algorithms are:

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

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 effect 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.


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"
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.


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 ' ')"
    ;;
  esac
done

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

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

Tags:

Dns

Ssh

Openssl

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