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

     ssh-keyscan — gather SSH public keys from servers

     ssh-keyscan [-46cDHv] [-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, 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 format:


     -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 disclosed.

     -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 connection is
             closed and the host in question considered unavailable.  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 commas.  The
             default is to fetch “rsa”, “ecdsa”, and “ed25519” keys.

     -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 created.


     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 -

     ssh(1), sshd(8)

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

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

BSD                             November 30, 2019                            BSD