Ubuntu Manpage: ssh-keyscan — gather SSH public keys from servers

Provided by: openssh-client_9.3p1-1ubuntu3.6_amd64

NAME

     ssh-keyscan — gather SSH public keys from servers

SYNOPSIS

     ssh-keyscan [-46cDHv] [-f file] [-O option] [-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.

Hosts to be scanned may be specified by hostname, address or by CIDR network range (e.g.
192.168.16/28). If a network range is specified, then all addresses in that range will be
scanned.

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. Names read
from a file must start with an address, hostname or CIDR network range to be
scanned. Addresses and hostnames may optionally be followed by comma-separated name
or address aliases that will be copied to the output. For example:

192.168.11.0/24
10.20.1.1
happy.example.org
10.0.0.1,sad.example.org

-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.

-O option
Specify a key/value option. At present, only a single option is supported:

hashalg=algorithm
Selects a hash algorithm to use when printing SSHFP records using the -D
flag. Valid algorithms are “sha1” and “sha256”. The default is to print
both.

-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”, “ecdsa-sk”, “ed25519-sk”, or “rsa”. Multiple values
may be specified by separating them with commas. The default is to fetch “rsa”,
“ecdsa”, “ed25519”, “ecdsa-sk”, and “ed25519-sk” 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

     Search a network range, printing all supported key types:

           $ ssh-keyscan 192.168.0.64/25

     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 -

AUTHORS