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                     [host | addrlist namelist] ...


         ssh-keyscan is a utility for gathering the public ssh host keys of a num-
         ber 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 pos-
         sible 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 ssh.  For scanning, one does not need login
         access to the machines that are being scanned, nor does the scanning pro-
         cess 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 filename, 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 identi-
                 fying 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 initiated 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 protocol 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 veri-
         fying the keys, users will be vulnerable to man in the middle attacks.
         host-or-namelist keytype base64-encoded-key
         Where keytype is either "ecdsa-sha2-nistp256", "ecdsa-sha2-nistp384",
         "ecdsa-sha2-nistp521", "ssh-dss" or "ssh-rsa".


         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_known_hosts:
         $ 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


         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 October 23, 2018 BSD


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