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I have an Debian Jessie Server and I would like to verify the SSH HostKey about the DNS SSHFP record. I only provide the ed25519 and rsa algorithm to connect to the server. I have masquerade the server FQDN to kronos.example.local.

I used this commands to generate the fingerprint for the ed25519 algorithm

root@kronos:~# ssh-keygen -r kronos.example.local -f /etc/ssh/ssh_host_ed25519_key
kronos.example.local IN SSHFP 4 1 f6291b4afb2b18cb3042e9dc01bd15efca7f5d5e
kronos.example.local IN SSHFP 4 2 7b2e9286d0969618ccb88d954587ac82f2ad471c34d07d25d91f9905b86f152d

and this one for rsa

root@kronos:~# ssh-keygen -r kronos.example.local -f /etc/ssh/ssh_host_rsa_key
kronos.example.local IN SSHFP 1 1 365f966468f59ed889fb9d94c25f8f6cb858aa7a
kronos.example.local IN SSHFP 1 2 162f0117baae8380427698ba8800bdef36e150e6014f220b568d85445832d050

I used dig to check the records on my client machine.

volker@vm23 ~ $ dig SSHFP kronos.example.local
;; ANSWER SECTION:
kronos.example.local.   600 IN  SSHFP   1 2 162F0117BAAE8380427698BA8800BDEF36E150E6014F220B568D8544 5832D050
kronos.example.local.   600 IN  SSHFP   4 2 7B2E9286D0969618CCB88D954587AC82F2AD471C34D07D25D91F9905 B86F152D
kronos.example.local.   600 IN  SSHFP   4 1 F6291B4AFB2B18CB3042E9DC01BD15EFCA7F5D5E
kronos.example.local.   600 IN  SSHFP   1 1 365F966468F59ED889FB9D94C25F8F6CB858AA7A

The SSHFP Record is correct and I add the settings StrictHostKeyChecking and VerifyHostKeyDNS in my ssh config to verify the ssh fingerprint about the SSHFP record. 

Host                    kronos
Hostname                kronos.example.local
Port                    30
User                    root
Compression             yes        
IdentityFile            ~/.ssh/ssh.key      
PasswordAuthentication  no
PubkeyAuthentication    yes
RSAAuthentication       no
StrictHostKeyChecking   yes
VerifyHostKeyDNS        yes

Now, when I want to connect to the Server I get an error, that my ssh-client can't establish an connection because the fingerprint is wrong?

volker@vm23 ~ $ ssh kronos
No ED25519 host key is known for [kronos.example.local]:30 and you have requested strict checking.
Host key verification failed.

Why I get this error, I don't know what I make wrong.

EDIT:

When I flush the known_hosts file and connect again, I get this message with the provided fingerprint.

The authenticity of host '[kronos.example.local]:30 ([192.168.2.100]:30)' can't be established.
ED25519 key fingerprint is SHA256:ey6ShtCWlhjMuI2VRYesgvKtRxw00H0l2R+ZBbhvFS0.
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  • Are the records already in the DNS? Can you verify that using dig?
    – Jakuje
    Commented May 5, 2017 at 16:07
  • Yes, the records are in the DNS. I used first my local DNS Server and second explicit the DNS from the example.local zone. (dig kronos.example.local SSHFP @ns1.example.local)
    – Volker Raschek
    Commented May 5, 2017 at 16:22
  • 3
    StrictHostKeyChecking yes prevents openssh from automatically adding new HostKeys to the known_hosts file. You will need to add it to known_hosts manually or set StrictHostKeyChecking no for your first connection. You will then be asked if you want to add it. Afterwards you can set it back to StrictHostKeyChecking yes.
    – lsmooth
    Commented May 5, 2017 at 16:30
  • Thanks Ismooth, it's works fine. At the first connection I get now this message Matching host key fingerprint found in DNS. when I have StrictHostKeyChecking no. When I change it to yes I can connect with verifying the fingerprint. Thanks!
    – Volker Raschek
    Commented May 5, 2017 at 19:22
  • @lsmooth Please provide that in answer format so that the Q&A can be closed and credit properly attributed. Thanks!
    – Andrew B
    Commented Sep 19, 2017 at 21:19

1 Answer 1

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StrictHostKeyChecking yes prevents openssh from automatically adding new HostKeys to the known_hosts file. You will need to add it to known_hosts manually or set StrictHostKeyChecking no for your first connection. You will then be asked if you want to add it. Afterwards you can set it back to StrictHostKeyChecking yes.

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