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

     ssh-keyscan - gather ssh public keys

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

     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.  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 effi-
     cient.  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 ssh.  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      Forces ssh-keyscan to use IPv4 addresses only.

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

     -f file
             Read hosts or addrlist namelist pairs from this file, one per line.  If - is supplied instead of a file-
             name, ssh-keyscan will read hosts or addrlist namelist pairs from the standard input.

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

     -p port
             Port to connect to on the remote host.

     -T timeout
             Set the timeout for connection attempts.  If timeout seconds have elapsed since a connection was initi-
             ated to a host or since the last time anything was read from that host, then the connection is closed and
             the host in question considered unavailable.  Default is 5 seconds.

     -t type
             Specifies the type of the key to fetch from the scanned hosts.  The possible values are "rsa1" for proto-
             col version 1 and "dsa", "ecdsa" or "rsa" for protocol version 2.  Multiple values may be specified by
             separating them with commas.  The default is "rsa".

     -v      Verbose mode.  Causes ssh-keyscan to print debugging messages about its 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 created.

     Input format:,,name,,n,,

     Output format for rsa1 keys:

     host-or-namelist bits exponent modulus

     Output format for rsa, dsa and ecdsa keys:

     host-or-namelist keytype base64-encoded-key

     Where keytype is either "ecdsa-sha2-nistp256", "ecdsa-sha2-nistp384", "ecdsa-sha2-nistp521", "ssh-dss" or


     Print the rsa host key for machine hostname:

     $ ssh-keyscan hostname

     Find all hosts from the file ssh_hosts which have new or different keys from those in the sorted file

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

     ssh(1), sshd(8)

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

     It generates "Connection closed by remote host" messages on the consoles of all the machines it scans if the
     server is older than version 2.9.  This is because it opens a connection to the ssh port, reads the public key,
     and drops the connection as soon as it gets the key.

BSD                            February 19, 2018                           BSD