1.\" $OpenBSD: ssh-keyscan.1,v 1.45 2019/11/30 07:07:59 jmc Exp $ 2.\" 3.\" Copyright 1995, 1996 by David Mazieres <dm@lcs.mit.edu>. 4.\" 5.\" Modification and redistribution in source and binary forms is 6.\" permitted provided that due credit is given to the author and the 7.\" OpenBSD project by leaving this copyright notice intact. 8.\" 9.Dd $Mdocdate: November 30 2019 $ 10.Dt SSH-KEYSCAN 1 11.Os 12.Sh NAME 13.Nm ssh-keyscan 14.Nd gather SSH public keys from servers 15.Sh SYNOPSIS 16.Nm ssh-keyscan 17.Op Fl 46cDHv 18.Op Fl f Ar file 19.Op Fl p Ar port 20.Op Fl T Ar timeout 21.Op Fl t Ar type 22.Op Ar host | addrlist namelist 23.Sh DESCRIPTION 24.Nm 25is a utility for gathering the public SSH host keys of a number of 26hosts. 27It was designed to aid in building and verifying 28.Pa ssh_known_hosts 29files, 30the format of which is documented in 31.Xr sshd 8 . 32.Nm 33provides a minimal interface suitable for use by shell and perl 34scripts. 35.Pp 36.Nm 37uses non-blocking socket I/O to contact as many hosts as possible in 38parallel, so it is very efficient. 39The keys from a domain of 1,000 40hosts can be collected in tens of seconds, even when some of those 41hosts are down or do not run 42.Xr sshd 8 . 43For scanning, one does not need 44login access to the machines that are being scanned, nor does the 45scanning process involve any encryption. 46.Pp 47The options are as follows: 48.Bl -tag -width Ds 49.It Fl 4 50Force 51.Nm 52to use IPv4 addresses only. 53.It Fl 6 54Force 55.Nm 56to use IPv6 addresses only. 57.It Fl c 58Request certificates from target hosts instead of plain keys. 59.It Fl D 60Print keys found as SSHFP DNS records. 61The default is to print keys in a format usable as a 62.Xr ssh 1 63.Pa known_hosts 64file. 65.It Fl f Ar file 66Read hosts or 67.Dq addrlist namelist 68pairs from 69.Ar file , 70one per line. 71If 72.Sq - 73is supplied instead of a filename, 74.Nm 75will read from the standard input. 76Input is expected in the format: 77.Bd -literal 781.2.3.4,1.2.4.4 name.my.domain,name,n.my.domain,n,1.2.3.4,1.2.4.4 79.Ed 80.It Fl H 81Hash all hostnames and addresses in the output. 82Hashed names may be used normally by 83.Xr ssh 1 84and 85.Xr sshd 8 , 86but they do not reveal identifying information should the file's contents 87be disclosed. 88.It Fl p Ar port 89Connect to 90.Ar port 91on the remote host. 92.It Fl T Ar timeout 93Set the timeout for connection attempts. 94If 95.Ar timeout 96seconds have elapsed since a connection was initiated to a host or since the 97last time anything was read from that host, the connection is 98closed and the host in question considered unavailable. 99The default is 5 seconds. 100.It Fl t Ar type 101Specify the type of the key to fetch from the scanned hosts. 102The possible values are 103.Dq dsa , 104.Dq ecdsa , 105.Dq ed25519 , 106or 107.Dq rsa . 108Multiple values may be specified by separating them with commas. 109The default is to fetch 110.Dq rsa , 111.Dq ecdsa , 112and 113.Dq ed25519 114keys. 115.It Fl v 116Verbose mode: 117print debugging messages about progress. 118.El 119.Pp 120If an ssh_known_hosts file is constructed using 121.Nm 122without verifying the keys, users will be vulnerable to 123.Em man in the middle 124attacks. 125On the other hand, if the security model allows such a risk, 126.Nm 127can help in the detection of tampered keyfiles or man in the middle 128attacks which have begun after the ssh_known_hosts file was created. 129.Sh FILES 130.Pa /etc/ssh/ssh_known_hosts 131.Sh EXAMPLES 132Print the RSA host key for machine 133.Ar hostname : 134.Pp 135.Dl $ ssh-keyscan -t rsa hostname 136.Pp 137Find all hosts from the file 138.Pa ssh_hosts 139which have new or different keys from those in the sorted file 140.Pa ssh_known_hosts : 141.Bd -literal -offset indent 142$ ssh-keyscan -t rsa,dsa,ecdsa,ed25519 -f ssh_hosts | \e 143 sort -u - ssh_known_hosts | diff ssh_known_hosts - 144.Ed 145.Sh SEE ALSO 146.Xr ssh 1 , 147.Xr sshd 8 148.Rs 149.%D 2006 150.%R RFC 4255 151.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints 152.Re 153.Sh AUTHORS 154.An -nosplit 155.An David Mazieres Aq Mt dm@lcs.mit.edu 156wrote the initial version, and 157.An Wayne Davison Aq Mt wayned@users.sourceforge.net 158added support for protocol version 2. 159