.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
-.\" $OpenBSD: ssh.1,v 1.249 2006/01/15 17:37:05 jmc Exp $
+.\" $OpenBSD: ssh.1,v 1.253 2006/01/30 13:37:49 jmc Exp $
.Dd September 25, 1999
.Dt SSH 1
.Os
.It Protocol
.It ProxyCommand
.It PubkeyAuthentication
+.It RekeyLimit
.It RemoteForward
.It RhostsRSAAuthentication
.It RSAAuthentication
option can be used to control logins to machines whose
host key is not known or has changed.
.Pp
-.Nm
-can be configured to verify host identification using fingerprint resource
-records (SSHFP) published in DNS.
-The
-.Cm VerifyHostKeyDNS
-option can be used to control how DNS lookups are performed.
-SSHFP resource records can be generated using
-.Xr ssh-keygen 1 .
-.Pp
When the user's identity has been accepted by the server, the server
either executes the given command, or logs into the machine and gives
the user a normal shell on the remote machine.
options above) and
the user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side.
+.Sh VERIFYING HOST KEYS
+When connecting to a server for the first time,
+a fingerprint of the server's public key is presented to the user
+(unless the option
+.Cm StrictHostKeyChecking
+has been disabled).
+Fingerprints can be determined using
+.Xr ssh-keygen 1 :
+.Pp
+.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
+.Pp
+If the fingerprint is already known,
+it can be matched and verified,
+and the key can be accepted.
+If the fingerprint is unknown,
+an alternative method of verification is available:
+SSH fingerprints verified by DNS.
+An additional resource record (RR),
+SSHFP,
+is added to a zonefile
+and the connecting client is able to match the fingerprint
+with that of the key presented.
+.Pp
+In this example, we are connecting a client to a server,
+.Dq host.example.com .
+The SSHFP resource records should first be added to the zonefile for
+host.example.com:
+.Bd -literal -offset indent
+$ ssh-keygen -f /etc/ssh/ssh_host_rsa_key.pub -r host.example.com.
+$ ssh-keygen -f /etc/ssh/ssh_host_dsa_key.pub -r host.example.com.
+.Ed
+.Pp
+The output lines will have to be added to the zonefile.
+To check that the zone is answering fingerprint queries:
+.Pp
+.Dl $ dig -t SSHFP host.example.com
+.Pp
+Finally the client connects:
+.Bd -literal -offset indent
+$ ssh -o "VerifyHostKeyDNS ask" host.example.com
+[...]
+Matching host key fingerprint found in DNS.
+Are you sure you want to continue connecting (yes/no)?
+.Ed
+.Pp
+See the
+.Cm VerifyHostKeyDNS
+option in
+.Xr ssh_config 5
+for more information.
+.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
+.Nm
+contains support for Virtual Private Network (VPN) tunnelling
+using the
+.Xr tun 4
+network pseudo-device,
+allowing two networks to be joined securely.
+The
+.Xr sshd_config 5
+configuration option
+.Cm PermitTunnel
+controls whether the server supports this,
+and at what level (layer 2 or 3 traffic).
+.Pp
+The following example would connect client network 10.0.50.0/24
+with remote network 10.0.99.0/24, provided that the SSH server
+running on the gateway to the remote network,
+at 192.168.1.15, allows it:
+.Bd -literal -offset indent
+# ssh -f -w 0:1 192.168.1.15 true
+# ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
+.Ed
+.Pp
+Client access may be more finely tuned via the
+.Pa /root/.ssh/authorized_keys
+file (see below) and the
+.Cm PermitRootLogin
+server option.
+The following entry would permit connections on the first
+.Xr tun 4
+device from user
+.Dq jane
+and on the second device from user
+.Dq john ,
+if
+.Cm PermitRootLogin
+is set to
+.Dq forced-commands-only :
+.Bd -literal -offset 2n
+tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
+tunnel="2",command="sh /etc/netstart tun1" ssh-rsa ... john
+.Ed
+.Pp
+Since a SSH-based setup entails a fair amount of overhead,
+it may be more suited to temporary setups,
+such as for wireless VPNs.
+More permanent VPNs are better provided by tools such as
+.Xr ipsecctl 8
+and
+.Xr isakmpd 8 .
.Sh ENVIRONMENT
.Nm
will normally set the following environment variables:
Contains the public key for authentication.
These files are not
sensitive and can (but need not) be readable by anyone.
-They are
-never used automatically and are not necessary: they are only provided for
-the convenience of the user.
.Pp
.It ~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged into
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
.Xr ssh-keyscan 1 ,
+.Xr tun 4 ,
.Xr hosts.equiv 5 ,
.Xr ssh_config 5 ,
.Xr ssh-keysign 8 ,
andersk / openssh.git / blobdiff