ssh-keyscan

gather SSH public keys

TLDR

Retrieve all public ssh keys of a remote host

>_ ssh-keyscan [host]
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Retrieve all public ssh keys of a remote host listening on a specific port

>_ ssh-keyscan -p [port] [host]
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Retrieve certain types of public ssh keys of a remote host

>_ ssh-keyscan -t [rsa,dsa,ecdsa,ed25519] [host]
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Manually update the ssh known_hosts file with the fingerprint of a given host

>_ ssh-keyscan -H [host] >> ~/.ssh/known_hosts
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SYNOPSIS

ssh-keyscan [-46cDHv] [-f file] [-p port] [-T timeout] [-t type] [host | addrlist namelist]

DESCRIPTION

ssh-keyscan is a utility for gathering the public SSH host keys of a number of hosts. It was designed to aid in building and verifying ssh_known_hosts files, the format of which is documented in sshd(8). ssh-keyscan provides a minimal interface suitable for use by shell and perl scripts.

ssh-keyscan uses non-blocking socket I/O to contact as many hosts as possible in parallel, so it is very efficient. The keys from a domain of 1,000 hosts can be collected in tens of seconds, even when some of those hosts are down or do not run sshd(8). For scanning, one does not need login access to the machines that are being scanned, nor does the scanning process involve any encryption.

The options are as follows:

If an ssh_known_hosts file is constructed using ssh-keyscan without verifying the keys, users will be vulnerable to man in the middle attacks. On the other hand, if the security model allows such a risk, ssh-keyscan can help in the detection of tampered keyfiles or man in the middle attacks which have begun after the ssh_known_hosts file was created.

FILES

/etc/ssh/ssh_known_hosts

EXAMPLES

Print the RSA host key for machine hostname :

$ ssh-keyscan -t rsa hostname

Find all hosts from the file ssh_hosts which have new or different keys from those in the sorted file ssh_known_hosts : -literal -offset indent $ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \ sort -u - ssh_known_hosts | diff ssh_known_hosts -

AUTHORS

-nosplit David Mazieres <dm@lcs.mit.edu> wrote the initial version, and Wayne Davison <wayned@users.sourceforge.net> added support for protocol version 2.

SEE ALSO

ssh(1), sshd(8) 2006, RFC 4255, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints

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