.\" (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 1.273 2008/02/11 07:58:28 jmc Exp $
-.Dd $Mdocdate: March 26 2008 $
+.\" $OpenBSD: ssh.1,v 1.277 2008/07/02 13:47:39 djm Exp $
+.Dd $Mdocdate: July 2 2008 $
.Dt SSH 1
.Os
.Sh NAME
The recommended way to start X11 programs at a remote site is with
something like
.Ic ssh -f host xterm .
+.Pp
+If the
+.Cm ExitOnForwardFailure
+configuration option is set to
+.Dq yes ,
+then a client started with
+.Fl f
+will wait for all remote port forwards to be successfully established
+before placing itself in the background.
.It Fl g
Allows remote hosts to connect to local forwarded ports.
.It Fl I Ar smartcard_device
.It User
.It UserKnownHostsFile
.It VerifyHostKeyDNS
+.It VisualHostKey
.It XAuthLocation
.El
.It Fl p Ar port
.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 already known, it can be matched
+and the key can be accepted or rejected.
+Because of the difficulty of comparing host keys
+just by looking at hex strings,
+there is also support to compare host keys visually,
+using
+.Em random art .
+By setting the
+.Cm VisualHostKey
+option to
+.Dq yes ,
+a small ASCII graphic gets displayed on every login to a server, no matter
+if the session itself is interactive or not.
+By learning the pattern a known server produces, a user can easily
+find out that the host key has changed when a completely different pattern
+is displayed.
+Because these patterns are not unambiguous however, a pattern that looks
+similar to the pattern remembered only gives a good probability that the
+host key is the same, not guaranteed proof.
+.Pp
+To get a listing of the fingerprints along with their random art for
+all known hosts, the following command line can be used:
+.Pp
+.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
+.Pp
If the fingerprint is unknown,
an alternative method of verification is available:
SSH fingerprints verified by DNS.
.%T "The Secure Shell (SSH) Public Key File Format"
.%D 2006
.Re
+.Rs
+.%T "Hash Visualization: a New Technique to improve Real-World Security"
+.%A A. Perrig
+.%A D. Song
+.%D 1999
+.%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
+.Re
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.