SSH-KEYSCAN(1)           BSD General Commands Manual          SSH-KEYSCAN(1)

NAME         top

     ssh-keyscan — gather SSH public keys

SYNOPSIS         top

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

DESCRIPTION         top

     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:

     -4      Force ssh-keyscan to use IPv4 addresses only.

     -6      Force ssh-keyscan to use IPv6 addresses only.

     -c      Request certificates from target hosts instead of plain keys.

     -D      Print keys found as SSHFP DNS records.  The default is to print
             keys in a format usable as a ssh(1) known_hosts file.

     -f file
             Read hosts or “addrlist namelist” pairs from file, one per
             line.  If ‘-’ is supplied instead of a filename, ssh-keyscan
             will read from the standard input.  Input is expected in the


     -H      Hash all hostnames and addresses in the output.  Hashed names
             may be used normally by ssh(1) and sshd(8), but they do not
             reveal identifying information should the file's contents be

     -p port
             Connect to port on the remote host.

     -T timeout
             Set the timeout for connection attempts.  If timeout seconds
             have elapsed since a connection was initiated to a host or
             since the last time anything was read from that host, the con‐
             nection is closed and the host in question considered unavail‐
             able.  The default is 5 seconds.

     -t type
             Specify the type of the key to fetch from the scanned hosts.
             The possible values are “dsa”, “ecdsa”, “ed25519”, or “rsa”.
             Multiple values may be specified by separating them with com‐
             mas.  The default is to fetch “rsa”, “ecdsa”, and “ed25519”

     -v      Verbose mode: print debugging messages about progress.

     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

FILES         top


EXAMPLES         top

     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:

           $ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \
                   sort -u - ssh_known_hosts | diff ssh_known_hosts -

SEE ALSO         top

     ssh(1), sshd(8)

     Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
     4255, 2006.

AUTHORS         top

     David Mazieres <> wrote the initial version, and Wayne
     Davison <> added support for protocol ver‐
     sion 2.

COLOPHON         top

     This page is part of the openssh (Portable OpenSSH) project.  Informa‐
     tion about the project can be found at  If you have a bug report for
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     was obtained from the tarball openssh-7.9p1.tar.gz fetched from
     ⟨⟩ on
     2018-10-29.  If you discover any rendering problems in this HTML ver‐
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BSD                             March 5, 2018                            BSD