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How to verify host keys using OpenSSH and DNS
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OpenSSH contains support for verifying host keys using DNS as described
in https://tools.ietf.org/html/rfc4255. The document contains very brief
instructions on how to use this feature. Configuring DNS is out of the
scope of this document.
(1) Server: Generate and publish the DNS RR
To create a DNS resource record (RR) containing a fingerprint of the
public host key, use the following command:
ssh-keygen -r hostname -f keyfile -g
where "hostname" is your fully qualified hostname and "keyfile" is the
file containing the public host key file. If you have multiple keys,
you should generate one RR for each key.
In the example above, ssh-keygen will print the fingerprint in a
generic DNS RR format parsable by most modern name server
implementations. If your nameserver has support for the SSHFP RR
you can omit the -g flag and ssh-keygen will print a standard SSHFP RR.
To publish the fingerprint using the DNS you must add the generated RR
to your DNS zone file and sign your zone.
(2) Client: Enable ssh to verify host keys using DNS
To enable the ssh client to verify host keys using DNS, you have to
add the following option to the ssh configuration file
($HOME/.ssh/config or /etc/ssh/ssh_config):
VerifyHostKeyDNS yes
Upon connection the client will try to look up the fingerprint RR
using DNS. If the fingerprint received from the DNS server matches
the remote host key, the user will be notified.
Jakob Schlyter
Wesley Griffin
$OpenBSD: README.dns,v 1.2 2003/10/14 19:43:23 jakob Exp $
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