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

NAME
     ssh-keyscan -- gather ssh public keys

SYNOPSIS
     ssh-keyscan [-46Hv] [-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 suit-able suitable
     able 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.

     -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 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 con-nection connection
             nection 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.  Mul-tiple Multiple
             tiple values may be specified by separating them with commas.  The default is to fetch ``rsa''
             and ``ecdsa'' 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 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 ``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 -f ssh_hosts | \
             sort -u - ssh_known_hosts | diff ssh_known_hosts -SEE ssh_known_hostsSEE

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.

BSD                            October 11, 2013                            BSD

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