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2sshfp

Very basic shell script to create SSHFP DNS records.

Description

I wrote this very basic script to create SSHFP DNS records when I realized sshfp tool didn't support ECDSA (i.e. ecdsa-sha2-nistp256) and my patch would take years before getting in to production.

It creates SSHFP record out of /etc/ssh/ssh_host_*_key.pub keys.

Features

  • Supports rsa, dsa, ecdsa, ed25519 and xmss ciphers and sha1/sha256 hash algorithms.
  • Tested on OpenBSD, FreeBSD and Linux

Requirements

openssl, sed, tr and awk commands - basic stuff

SSHFP records?

SSHFP are basically host ssh key fingerprints stored in DNS records. If you can trust DNS query (i.e. DNSSEC) you can validate SSH's host fingerprint automatically.

Here's one: soundwave.mantor.org. IN SSHFP 1 1 F48459337A91E833FA259C8F95D751D22D8541C2

The first number refers to the cipher of the key (1=rsa, 2=dsa, 3=ecdsa, 4=ed25519, 5=xmss), the second number is the hash algorithm (1=sha1, 2=sha256) used and the last long string is the hash of the key itself.

What is it used for?

Merge the resulting DNS records in your zone and use them:

  • configure SSH client to verify SSHFP records via VerifyHostKeyDNS yes;
  • make sure you're using DNSSEC ;)

Usage

> ./2sshfp vortex
vortex IN SSHFP 1 1 BBE600FEB1200CB02D5A2912DB37648F65B4A2FE
vortex IN SSHFP 1 2 64C62A33E3FDD2EB94A40B376C2AD4691BB215403217C5D2A92B166581880377
vortex IN SSHFP 2 1 9DDCD0CDE23225BC7EB0051D4FB3928BB17AE4BE
vortex IN SSHFP 2 2 2A58473A4AEC6E1943F8A0E0FDA05269B7CC77347621BBBAA813E2D00D287624
vortex IN SSHFP 3 1 3635F4E90C969C01E5DEA94F26F8268DDC334E25
vortex IN SSHFP 3 2 D109F93C739BC6401D74412F0A638877F1B5B7C1B94602BA20E6EB264EEB8754
vortex IN SSHFP 4 1 C01A3E12F70139C56EACC2DE93B0E5C7CC8D6BB4
vortex IN SSHFP 4 2 197D56859D92B89003456E30782AE449EE8A136766831C482C81344ADFCD5E4E

license

Public domain - A gift to the Internet