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 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 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 connection 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. Multiple values may be specified by separating them
with commas. The default is “rsa”.
- -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:
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 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.