Manual: ssh-keyscan

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

NAME
ssh-keyscan — gather SSH public keys from servers

SYNOPSIS
ssh-keyscan [-46cDHqv] [-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 net‐
work 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 an 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.

-q Quiet mode: do not print server host name and banners in comments.

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

-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 cre‐
ated.

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

Manual: ssh-keyscan

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