NAME

     ssh-keyscan — gather ssh public keys

SYNOPSIS

     ssh-keyscan [-46cHv] [-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.  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 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.

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

     -f file
             Read hosts or “addrlist namelist” pairs from 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 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 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 “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.  Causes ssh-keyscan to print debugging messages about its progress.

SECURITY

     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

     Input format:

     1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4

     Output format for RSA, DSA, ECDSA, and Ed25519 keys:

     host-or-namelist keytype base64-encoded-key

     Where keytype is either “ecdsa-sha2-nistp256”, “ecdsa-sha2-nistp384”, “ecdsa-sha2-nistp521”,
     “ssh-ed25519”, “ssh-dss” or “ssh-rsa”.

     /etc/ssh/ssh_known_hosts

EXAMPLES

     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,ed25519 -f ssh_hosts | \
             sort -u - ssh_known_hosts | diff ssh_known_hosts -

SEE ALSO

     ssh(1), sshd(8)

AUTHORS

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

BUGS

     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.