Is there a way to programmatically obtain a SSH server key fingerprint without authenticating to it?

I'm trying ssh -v user@host false 2>&1 | grep "Server host key", but this hangs waiting for a password if key based auth is not setup.

up vote 58 down vote accepted

You could do this by combining ssh-keyscan and ssh-keygen:

$ file=$(mktemp)
$ ssh-keyscan host > $file 2> /dev/null
$ ssh-keygen -l -f $file
521 de:ad:be:ef:de:ad:be:ef:de:ad:be:ef:de:ad:be:ef host (ECDSA)
4096 8b:ad:f0:0d:8b:ad:f0:0d:8b:ad:f0:0d:8b:ad:f0:0d host (RSA)
$ rm $file

(unfortunately the much simpler ssh-keyscan host | ssh-keygen -l -f /dev/stdin does not work)

  • 1
    Maybe ssh-keygen -l -f - <(ssh-keyscan host) does, though? – Michael Kjörling Apr 28 '14 at 13:50
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    OpenSSH >= 7.2 ssh-keyscan is able to read from stdin: ssh-keyscan host | ssh-keygen -lf - – mykhal Mar 4 '16 at 19:14
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    Just do: ssh-keygen -l -f <(ssh-keyscan host) – Christopher Oct 28 '16 at 4:59
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    That's a rather bad expression for shell scripts, as it depends on a shell supporting it, which POSIX shell doesn't. – Andreas Wiese Oct 28 '16 at 9:22
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    ssh-keygen -l -f - does work much as expected in ssh-keygen 7.2 and above. It produces some comment lines to STDERR that can be filtered out, as mentioned in the answer by Anthony Geoghegan or ssh-keyscan host 2>/dev/null | ssh-keygen -l -f - – Cedric Knight Nov 16 '16 at 16:17

I recently had to do this myself so I thought I’d add an answer which shows how this can be done (with versions of OpenSSH 7.2 or newer) in one line using process substitution:

ssh-keygen -lf <(ssh-keyscan hostname 2>/dev/null)

The following text explains how these commands work and highlights some of the differences in behaviour between older and newer versions of the OpenSSH utilities.

Fetch public host keys

The ssh-keyscan command was developed so that users can obtain public host keys without needing to authenticate to the SSH server. From its man page:

ssh-keyscan is a utility for gathering the public ssh host keys of a number of hosts. It was designed to aid in building and verifying ssh_known_hosts files.

Key type

The type of key to be fetched is specified using the -t option.

  • rsa1 (obsolete SSH Protocol version 1)
  • rsa
  • dsa
  • ecdsa (recent versions of OpenSSH)
  • ed25519 (recent versions of OpenSSH)

In modern OpenSSH releases, the default key types to be fetched are rsa (since version 5.1), ecdsa (since version 6.0), and ed25519 (since version 6.7).

With older versions of ssh-keyscan (before OpenSSH version 5.1), the default key type was the out-dated rsa1 (SSH Protocol 1) so the key types would need to be explicitly specified:

ssh-keyscan -t rsa,dsa hostname

Get fingerprint hashes of Base64 keys

ssh-keyscan prints the host key of the SSH server in Base64-encoded format. To convert this to a fingerprint hash, the ssh-keygen utility can be used with its -l option to print the fingerprint of the specified public key.

If using Bash, Zsh (or the Korn shell), process substitution can be used for a handy one-liner:

ssh-keygen -lf <(ssh-keyscan hostname 2>/dev/null)

Note: With versions of OpenSSH before 7.2, the functions used by ssh-keygen to read files, did not handle named pipes (FIFOs) very well so this method wouldn’t work, thus requiring the use of temporary files.

Hashing algorithms

Recent versions of ssh-keygen print SHA256 fingerprint hashes of the keys. To get MD5 hashes of the server key fingerprints (the old behaviour), the -E option can be used to specify the hash algorithm:

ssh-keygen -E md5 -lf <(ssh-keyscan hostname 2>/dev/null)

Using a pipeline

If using a POSIX shell (such as dash) which doesn’t feature process substitution, the other solutions using temporary files will work. However, with newer versions of OpenSSH (since 7.2), a simple pipeline can be used since ssh-keygen will accept - as a filename for the standard input stream, allowing a one-line pipeline command.

ssh-keyscan hostname 2>/dev/null | ssh-keygen -E md5 -lf -
  • Nice and thorough answer, this is certainly better than having a temporary file! May I suggest you provide a TL;DR in the beginning with the process substitution version, to make impatient folks find it faster? :) – goncalopp Mar 9 '16 at 21:07
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    Does not seem to work on Ubuntu 14.04 LTS; I get an error "/dev/fd/63 is not a public key file". The subprocess does work. – Melle Oct 18 '16 at 7:26
  • @melleb I found the same thing on an 12.04 system that I have access to. I suspect that ssh-keygen from older versions of OpenSSH have a problem reading from the FIFO / named pipe. I'll look into this (and update my answer) when I get some free time. – Anthony Geoghegan Oct 18 '16 at 9:51
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    @melleb After spending my luch-time downloading various source code releases and inserting debugging printf statements in the do_fingerprint() function, I found that with versions of OpenSSH before 7.2, the functions used by ssh-keygen to read files, did not handle named pipes (FIFOs) very well so the process substitution method would not work. – Anthony Geoghegan Oct 18 '16 at 13:35
  • This works, but if using it to verify a fingerprint, users should be aware that there's a race condition: the fingerprint you are checking with this command isn't necessarily that of the key you fetch, unless you dump the key before calling ssh-keygen on it. – CodeGnome Aug 31 at 17:11

nmap provides this ability by using the ssh-hostkey script.

To return the key's hexadecimal fingerprint:

$ nmap [SERVER] --script ssh-hostkey

To return the key's content:

$ nmap [SERVER] --script ssh-hostkey --script-args ssh_hostkey=full

To return the key's visual bubble

$ nmap [SERVER] --script ssh-hostkey --script-args ssh_hostkey='visual bubble'

To return all of the above:

$ nmap [SERVER] --script ssh-hostkey --script-args ssh_hostkey=all

Source: nmap docs

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    Do these examples assume that SSH is always running on port 22 ? What if ssh listens on a non-standard port ? – Martin Vegter Dec 1 '14 at 21:33
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    @MartinVegter (paraphrasing Guarin42, who couldn't comment:) nmap has the -p option which can specify a port, e.g. -p 22000. It's also possible to use the -vv option to increase the verbosity (amount of information given) – goncalopp Jan 29 '15 at 11:52

filezilla displays keys hashed with md5 in hexadecimal format.

to find this on your ubuntu linux machine use this command:

ssh-keygen -l -E md5 -f <(ssh-keyscan localhost 2>/dev/null)

note: replace "localhost" with the ip of the machine you wish to check.

Here is a shell script (mainly Bourne shell but using local keyword, which is available in most modern /bin/sh) I've written to do this. Use it like ssh-hostkey hostname. It will show both the sha256 and md5 format fingerprints for all hostkeys for the given hostname or IP address. You can also manually specify "md5" or "sha256" as the second argument to only show that particular format.

It uses a temporary file instead of piping to make it compatible with older OpenSSH packages (as described in other answers). The temporary file uses /dev/shm (shared memory) if available.

#!/bin/sh
usage () {
  printf '%s\n' "Usage: ssh-hostkey HOSTNAME [FPRINTHASH]"
}

ssh_hostkey () {
  local host="$1"
  local fprinthash="$2"
  local tmp=

  case "$host" in
    -h|--help|'')
      usage >&2
      return 1
      ;;
  esac

  case "$fprinthash" in
    md5|sha256|'') true;;
    *)
      usage >&2
      printf '%s\n' "Fingerprint hash may be 'md5' or 'sha256'" >&2
      return 2
      ;;
  esac

  if test -d /dev/shm
  then tmp="$(mktemp -d -p /dev/shm)"
  else tmp="$(mktemp -d)"
  fi

  trap 'trap - INT TERM EXIT; rm -rf "$tmp"' INT TERM EXIT
  ssh-keyscan "$host" > "$tmp/f" 2> /dev/null
  case "$fprinthash" in
    sha256|'') ssh-keygen -l -f "$tmp/f" 2> /dev/null;;
  esac
  case "$fprinthash" in
    md5|'') ssh-keygen -l -E md5 -f "$tmp/f" 2> /dev/null;;
  esac

  trap - INT TERM EXIT
  rm -rf "$tmp" > /dev/null 2>&1
}

ssh_hostkey "$@"

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