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SSH-KEYSCAN


Section: User Commands (1)
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BSD mandoc

 

NAME

ssh-keyscan

– gather ssh public keys

 

SYNOPSIS

ssh-keyscan

-words

[-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
“rsa”

or
“dsa”

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 and dsa keys:


host-or-namelist keytype base64-encoded-key

Where
keytype

is either
“ssh-rsa”

or
“ssh-dss”

/etc/ssh/ssh_known_hosts

 

EXAMPLES

Print the
rsa

host key for machine
hostname


$ ssh-keyscan 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 -f ssh_hosts |
sort -u – ssh_known_hosts | diff ssh_known_hosts –

 

SEE ALSO

ssh(1),

sshd(8)

 

AUTHORS

An -nosplit

An David Mazieres Aq [email protected]

wrote the initial version, and
An Wayne Davison Aq [email protected]

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.



 

Index



NAME

SYNOPSIS

DESCRIPTION

SECURITY

FILES

EXAMPLES

SEE ALSO

AUTHORS

BUGS



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