NAME

       ssh-keyscan — gather SSH public keys from servers

SYNOPSIS

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

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

       -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.

FILES

       /etc/ssh/ssh_known_hosts

EXAMPLES

       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 -

SEE ALSO

       ssh(1), sshd(8)

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

AUTHORS

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

Debian                                          November 30, 2019                                 SSH-KEYSCAN(1)