ssh-keyscan(1) - Man pages

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

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 re-
veal identifying information should the file's contents be dis-
closed.

-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 con-
nection is closed and the host in question considered unavail-
able. 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 at-
tacks. 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 -