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194 lines
4.7 KiB
Groff
194 lines
4.7 KiB
Groff
.\" $OpenBSD: ssh-keyscan.1,v 1.49 2023/02/10 06:41:53 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: February 10 2023 $
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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 from servers
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.Sh SYNOPSIS
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.Nm ssh-keyscan
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.Op Fl 46cDHv
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.Op Fl f Ar file
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.Op Fl O Ar option
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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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.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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the format of which is documented in
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.Xr sshd 8 .
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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
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.Xr sshd 8 .
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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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Hosts to be scanned may be specified by hostname, address or by CIDR
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network range (e.g. 192.168.16/28).
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If a network range is specified, then all addresses in that range will
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be scanned.
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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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Force
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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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Force
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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 D
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Print keys found as SSHFP DNS records.
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The default is to print keys in a format usable as a
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.Xr ssh 1
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.Pa known_hosts
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file.
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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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.Sq -
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is supplied instead of a filename,
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.Nm
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will read from the standard input.
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Names read from a file must start with an address, hostname or CIDR network
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range to be scanned.
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Addresses and hostnames may optionally be followed by comma-separated name
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or address aliases that will be copied to the output.
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For example:
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.Bd -literal
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192.168.11.0/24
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10.20.1.1
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happy.example.org
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10.0.0.1,sad.example.org
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.Ed
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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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.Xr ssh 1
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and
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.Xr sshd 8 ,
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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 O Ar option
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Specify a key/value option.
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At present, only a single option is supported:
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.Bl -tag -width Ds
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.It Cm hashalg Ns = Ns Ar algorithm
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Selects a hash algorithm to use when printing SSHFP records using the
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.Fl D
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flag.
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Valid algorithms are
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.Dq sha1
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and
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.Dq sha256 .
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The default is to print both.
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.El
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.It Fl p Ar port
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Connect to
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.Ar port
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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, the connection is
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closed and the host in question considered unavailable.
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The default is 5 seconds.
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.It Fl t Ar type
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Specify 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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.Dq ecdsa-sk ,
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.Dq ed25519-sk ,
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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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.Dq ed25519 ,
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.Dq ecdsa-sk ,
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and
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.Dq ed25519-sk
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keys.
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.It Fl v
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Verbose mode:
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print debugging messages about progress.
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.El
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.Pp
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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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.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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.Pp
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.Dl $ ssh-keyscan -t rsa hostname
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.Pp
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Search a network range, printing all supported key types:
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.Pp
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.Dl $ ssh-keyscan 192.168.0.64/25
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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 -offset indent
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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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.Rs
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.%D 2006
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.%R RFC 4255
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.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
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.Re
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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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