[Bug 278458] ng_bridge(4) states that uplinkX is valid, but fails if uplink0 is given

From: <bugzilla-noreply_at_freebsd.org>
Date: Fri, 19 Apr 2024 11:26:38 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278458

            Bug ID: 278458
           Summary: ng_bridge(4) states that uplinkX is valid, but fails
                    if uplink0 is given
           Product: Documentation
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Manual Pages
          Assignee: bugs@FreeBSD.org
          Reporter: parakleta@darkreality.org
                CC: doc@FreeBSD.org

Most examples show ng_bridge being connected to ng_ether with :lower being
connected to :link0 (see /usr/share/examples/netgraph/virtual.lan among
others).

The ng_bridge man page suggests that it is desirable to connect the :lower hook
of ng_ether to an uplink hook of the form :uplinkX.

Unfortunately, when combining these two instructions we get an illegal
arrangement which responds with `ngctl: send msg: Invalid argument` and no
further explanation.

I had to read through the code changes in https://reviews.freebsd.org/D23963 to
see that the uplink form was disallowed when the link number is 0 (see
`sys/netgraph/ng_bridge.c:360` in that commit).

It is not clear to me from that change request why 0 was disallowed, and this
seems contrary to the common guidance on how to connect ng_bridge to ng_ether,
so either this restriction should be removed or the man page should be updated
to be explicit that this is disallowed (and perhaps provide an example scenario
which could be copied).

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