Try to be a bit more specific about what happens when link0 is used.

link0 will reset the interface and all configuration is lost (e.g. -inet6 or
rdomain settings) so use link0 as first command without any other arguments
to get a deterministic behaviour.
With a lot of help by jmc@ (actually commiting his version because it is much
nicer, as ususal).

1 files changed, 15 insertions, 7 deletions

diff --git a/share/man/man4/tun.4 b/share/man/man4/tun.4
index 7f534cce6e9..ebbd1a90713 100644
--- a/share/man/man4/tun.4
+++ b/share/man/man4/tun.4
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: tun.4,v 1.36 2007/05/31 19:19:52 jmc Exp $
+.\"	$OpenBSD: tun.4,v 1.37 2010/08/26 14:04:46 claudio Exp $
 .\"
 .\" Copyright (c) 2003 Marcus D. Watts  All rights reserved.
 .\"
@@ -26,7 +26,7 @@
 .\" TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 .\" USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: May 31 2007 $
+.Dd $Mdocdate: August 26 2010 $
 .Dt TUN 4
 .Os
 .Sh NAME
@@ -53,18 +53,26 @@ interface can be created at runtime using the
 command or by opening the character special device
 .Pa /dev/tunN .
 .Pp
-Both layer 3 and layer 2 tunneling is supported.
-Layer 3 tunneling is the default mode; to enable layer 2 tunneling mode the
+Both layer 3 and layer 2 tunneling is supported;
+layer 3 tunneling is the default mode.
+To enable layer 2 tunneling mode,
+where the
+.Nm
+interface simulates an Ethernet network interface,
+the
 .Ar link0
 flag needs to be set with
-.Xr ifconfig 8 ,
+.Xr ifconfig 8
 or by setting up a
 .Xr hostname.if 5
 configuration file for
 .Xr netstart 8 .
-In layer 2 mode the
+Note that setting or unsetting the
+.Ar link0
+flag causes
 .Nm
-interface is simulating an Ethernet network interface.
+to lose any configuration settings,
+and that it is not advisable to use the flag with any other parameters.
 .Pp
 Each device has the exclusive open property; it cannot be opened
 if it is already open and in use by another process.