173 lines
4.1 KiB
Groff
173 lines
4.1 KiB
Groff
.\" $OpenBSD: ssh-keyscan.1,v 1.40 2017/05/02 17:04:09 jmc Exp $
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.\"
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.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>.
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.\"
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.\" Modification and redistribution in source and binary forms is
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.\" permitted provided that due credit is given to the author and the
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.\" OpenBSD project by leaving this copyright notice intact.
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.\"
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.Dd $Mdocdate: May 2 2017 $
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.Dt SSH-KEYSCAN 1
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.Os
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.Sh NAME
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.Nm ssh-keyscan
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.Nd gather ssh public keys
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.Sh SYNOPSIS
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.Nm ssh-keyscan
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.Bk -words
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.Op Fl 46cHv
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.Op Fl f Ar file
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.Op Fl p Ar port
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.Op Fl T Ar timeout
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.Op Fl t Ar type
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.Op Ar host | addrlist namelist
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.Ar ...
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.Ek
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.Sh DESCRIPTION
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.Nm
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is a utility for gathering the public ssh host keys of a number of
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hosts.
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It was designed to aid in building and verifying
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.Pa ssh_known_hosts
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files.
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.Nm
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provides a minimal interface suitable for use by shell and perl
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scripts.
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.Pp
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.Nm
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uses non-blocking socket I/O to contact as many hosts as possible in
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parallel, so it is very efficient.
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The keys from a domain of 1,000
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hosts can be collected in tens of seconds, even when some of those
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hosts are down or do not run ssh.
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For scanning, one does not need
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login access to the machines that are being scanned, nor does the
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scanning process involve any encryption.
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.Pp
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The options are as follows:
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.Bl -tag -width Ds
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.It Fl 4
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Forces
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.Nm
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to use IPv4 addresses only.
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.It Fl 6
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Forces
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.Nm
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to use IPv6 addresses only.
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.It Fl c
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Request certificates from target hosts instead of plain keys.
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.It Fl f Ar file
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Read hosts or
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.Dq addrlist namelist
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pairs from
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.Ar file ,
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one per line.
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If
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.Pa -
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is supplied instead of a filename,
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.Nm
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will read hosts or
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.Dq addrlist namelist
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pairs from the standard input.
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.It Fl H
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Hash all hostnames and addresses in the output.
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Hashed names may be used normally by
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.Nm ssh
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and
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.Nm sshd ,
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but they do not reveal identifying information should the file's contents
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be disclosed.
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.It Fl p Ar port
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Port to connect to on the remote host.
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.It Fl T Ar timeout
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Set the timeout for connection attempts.
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If
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.Ar timeout
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seconds have elapsed since a connection was initiated to a host or since the
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last time anything was read from that host, then the connection is
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closed and the host in question considered unavailable.
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Default is 5 seconds.
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.It Fl t Ar type
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Specifies the type of the key to fetch from the scanned hosts.
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The possible values are
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.Dq dsa ,
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.Dq ecdsa ,
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.Dq ed25519 ,
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or
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.Dq rsa .
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Multiple values may be specified by separating them with commas.
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The default is to fetch
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.Dq rsa ,
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.Dq ecdsa ,
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and
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.Dq ed25519
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keys.
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.It Fl v
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Verbose mode.
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Causes
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.Nm
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to print debugging messages about its progress.
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.El
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.Sh SECURITY
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If an ssh_known_hosts file is constructed using
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.Nm
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without verifying the keys, users will be vulnerable to
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.Em man in the middle
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attacks.
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On the other hand, if the security model allows such a risk,
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.Nm
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can help in the detection of tampered keyfiles or man in the middle
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attacks which have begun after the ssh_known_hosts file was created.
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.Sh FILES
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Input format:
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.Bd -literal
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1.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4
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.Ed
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.Pp
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Output format for RSA, DSA, ECDSA, and Ed25519 keys:
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.Bd -literal
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host-or-namelist keytype base64-encoded-key
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.Ed
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.Pp
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Where
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.Ar keytype
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is either
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.Dq ecdsa-sha2-nistp256 ,
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.Dq ecdsa-sha2-nistp384 ,
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.Dq ecdsa-sha2-nistp521 ,
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.Dq ssh-ed25519 ,
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.Dq ssh-dss
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or
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.Dq ssh-rsa .
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.Pp
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.Pa /etc/ssh/ssh_known_hosts
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.Sh EXAMPLES
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Print the rsa host key for machine
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.Ar hostname :
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.Bd -literal
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$ ssh-keyscan hostname
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.Ed
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.Pp
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Find all hosts from the file
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.Pa ssh_hosts
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which have new or different keys from those in the sorted file
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.Pa ssh_known_hosts :
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.Bd -literal
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$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e
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sort -u - ssh_known_hosts | diff ssh_known_hosts -
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.Ed
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.Sh SEE ALSO
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.Xr ssh 1 ,
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.Xr sshd 8
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.Sh AUTHORS
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.An -nosplit
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.An David Mazieres Aq Mt dm@lcs.mit.edu
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wrote the initial version, and
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.An Wayne Davison Aq Mt wayned@users.sourceforge.net
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added support for protocol version 2.
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.Sh BUGS
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It generates "Connection closed by remote host" messages on the consoles
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of all the machines it scans if the server is older than version 2.9.
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This is because it opens a connection to the ssh port, reads the public
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key, and drops the connection as soon as it gets the key.
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