How to verify host fingerprint in Openssh

Your ssh server is providing sha256 public key hashes, which is far more secure than md5 hashes.

You then have to specify to ssh-keygen that you want sha256 instead of md5 hashes. Try executing the command on your home-machine:

for pubkey_file in /etc/ssh/*.pub; do ssh-keygen -lf ${pubkey_file} -E sha256; done


Commands used

  • Display ascii-art of the public host key stored on the server (to be done on server side, the one you connect TO via ssh):

    ssh-keygen -l -v -E md5 -f /etc/ssh/ssh_host_ecdsa_key.pub

    -l: Show fingerprint of specified public key file.

    -v: visual (ascii-art)

    -E md5: the hash algorithme used to calculate the fingerprint ("md5" or "sha256"). ("sha256" is prefered if available). (may not be available in old versions of ssh-keygen)

    -f: file

  • Display ascii-art of remote server public host key (to be done on client side, the one you connect FROM via ssh):

    ssh -o visualhostkey=yes -o FingerprintHash=md5 <host_server_to_connect>

    -o: option

    visualhostkey: visual (ascii-art)

    FingerprintHash: hash algo to use (use the same as the one you obtain from the server: md5 or sha256)

What to do to check the authenticity of a host/server

First, 1. is to be done locally on the server (the one you want to connect TO via ssh ): it will give you a first ascii-art. Print it or take a picture.

Second, 2. is to be done at the first SSH connexion; it will display a second ascii-art. If the ascii-art is the same, then you can answer yes to the "do I trust?" question (i.e. Are you sure you want to continue connecting (yes/no)).

Example

  • Server side
$ ssh-keygen -l -v -E md5 -f /etc/ssh/ssh_host_ecdsa_key.pub
256 2e:a6:b3:27:14:12:0b:79:df:9a:7f:bd:4d:b1:e0:b6   (ECDSA)
+--[ECDSA  256]---+
| .               |
|o o              |
| o + .           |
|  o o .          |
|   . +  S . .    |
|    +  . . . o   |
|   . .o ..o o    |
|    ooo....+     |
|    o= .  E..    |
+-----------------+
  • Client side
$ ssh -o visualhostkey=yes -o FingerprintHash=md5 192.168.12.211
The authenticity of host '192.168.12.211 (192.168.12.211)' can't be established.
ECDSA key fingerprint is MD5:2e:a6:b3:27:14:12:0b:79:df:9a:7f:bd:4d:b1:e0:b6.
+---[ECDSA 256]---+
| .               |
|o o              |
| o + .           |
|  o o .          |
|   . +  S . .    |
|    +  . . . o   |
|   . .o ..o o    |
|    ooo....+     |
|    o= .  E..    |
+------[MD5]------+
Are you sure you want to continue connecting (yes/no)?

Some more explanation

The first command will display the ascii-art corresponding to the fingerprint of the file you give as input (and the fingerprint itself). The file you give as input is the public host key of the server. When a client connect (not only for the first time), the server will sent its public host key. This public host key will be searched in ~/.ssh/known_hosts. If the public key is in the file, then it's ok: the host (server) is known, so we move on to the next step to authentificate the user (user auth is not described in this post). If the public key is not in the file, then the client will compute the fingerprint of this public host key with a hash algorithm (a different hash algo will give a different fingerprint). This fingerprint previously calculated is displayed (along with the ascii-art if corresponding option provided) and you will have to answer yes or no depending on you recognising this fingerprint or no (this fingerprint is the image/hash of the public host key of the server). If you say yes, then the bublic key of the server (not its fingerprint) will be added to the file ~/.ssh/known_hosts.

We can notice that ~/.ssh/known_hosts is under you home (~) directory, because you trust this host (server), but a different user may not trust the same as you. Also, the host public key of the server is not user-dependent, so it is stored in /etc/ssh/.

The second command will display the fingerprint and the ascii-art of the public key received from the host_server_to_connect (according to the hash algo given in options). It is the same as doing only ssh, but with more visual options, so the connection will continue the same way as a normal ssh connexion.

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