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Fingerprints

In order to prevent a Man-In-The-Middle attack (MITM)?, SSH requires you to check the fingerprints of the server you connect to.

Your SSH Fingerprints

When ssh server is installed, it stores its keys in /etc/ssh. You can run this script to quickly get the ssh fingerprints for all your keys:

ssh-keygen -E md5 -lf /etc/ssh/ssh_host_dsa_key.pub
ssh-keygen -E md5 -lf /etc/ssh/ssh_host_ecdsa_key.pub
ssh-keygen -E md5 -lf /etc/ssh/ssh_host_ed25519_key.pub
ssh-keygen -E md5 -lf /etc/ssh/ssh_host_rsa_key.pub
ssh-keygen -lf /etc/ssh/ssh_host_dsa_key.pub
ssh-keygen -lf /etc/ssh/ssh_host_ecdsa_key.pub
ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub
ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub

The first four fingerprints use MD5 hashing, which is used by PuTTY. The last four uses SHA256 hashing, which is used by OpenSSH ?.

Publish SSHFP

A convenient place to publish ssh fingerprints is in DNS using SSHFP records:

$ ssh-keygen -r example.com.
example.com IN SSHFP 1 1 7251d06cf5cf9312b502388edd93ff924c52a73a
example.com IN SSHFP 1 2 a0f433e68e5ba29f23825b21a23660d94a5b8a814cd71827fb75cfb4e84e4c49
example.com IN SSHFP 2 1 22ccda0cafee42f3e2cc53d5f695244677a1a88f
example.com IN SSHFP 2 2 88fbc099391d1e37330409978e68bdeebc50fe9bc41c5e2fd4a2d29ecde20409
example.com IN SSHFP 3 1 c9a19b42a7165596f0d0e5bfa947232978901dcb
example.com IN SSHFP 3 2 6a9facbb8693644063b1eee91cfce24ada5536ff52df98210fae3d350fffaf34
example.com IN SSHFP 4 1 4dc3d59ef28733c89f83e0e078b10a4a816e2a04
example.com IN SSHFP 4 2 a1f1388dff27d02f942ea5a9e2cb6008ae3e0a61622e5ff2b1ce746b32049152

Replace example.com. with your domain, making sure to include the final period for a fully qualified domain name (FQDN). ssh will generate all of your SSHFP records for you, which can then be added to your nameserver's zone files.

SSHFP records follow this format:

<Name> [<TTL>] [<Class>] SSHFP <Algorithm> <Type> <Fingerprint>

Keyword	Meaning
TTL	Time to live (seconds)
Protocol	IN for Internet
Algorithm	0: reserved; 1: RSA; 2: DSA, 3: ECDSA; 4: Ed25519
Type	Hash -- 0: reserved; 1: SHA-1; 2: SHA-256)
Fingerprint	Hexadecimal of hash