On Tue, Mar 16, 2021 at 06:08:00AM -0600, Todd C. Miller wrote:
> I think it is helpful to tell the user what happens if the remote
> endpoint doesn't support RFC 4638.
The manual section basically says that already (last sentence):
MTU/MSS ISSUES
Problems can arise on machines with private IPs connecting to the
Internet via a machine running both Network Address Translation (NAT) and
pppoe. Standard Ethernet uses a maximum transmission unit (MTU) of 1500
bytes, whereas PPPoE mechanisms need a further 8 bytes of overhead. This
leaves a maximum MTU of 1492. pppoe sets the MTU on its interface to
1492 as a matter of course.
It doesn't hurt however to mention the specific message such that users
can tell *when* that happens; perhaps add it under DIAGNOSTICS as the
various wifi driver manuals, e.g. athn(4) do?
See diff at the end for that.
> Index: share/man/man4/pppoe.4
> ===================================================================
> RCS file: /cvs/src/share/man/man4/pppoe.4,v
> retrieving revision 1.34
> diff -u -p -u -r1.34 pppoe.4
> --- share/man/man4/pppoe.4 16 Jun 2017 10:58:43 -0000 1.34
> +++ share/man/man4/pppoe.4 16 Mar 2021 12:05:40 -0000
> @@ -201,6 +201,14 @@ With this, the previously mentioned MSS
> .Xr pf.conf 5
> are no longer necessary.
> .Pp
> +If you attemp to use an MTU larger than 1492 where the remote equipment does
"attempt"
CAVEATS says "endpoint", I'd sync with that instead of "remote equipment".
> +.Em not
> +support RFC 4638,
> +.Nm
> +will write
> +.Dq No valid PPP-Max-Payload tag received in PADO
> +to the kernel message buffer and the MTU will be clamped to 1492.
> +.Pp
> See
> .Xr pf.conf 5
> for more information on MTU, MSS, and NAT.
Index: pppoe.4
===================================================================
RCS file: /cvs/src/share/man/man4/pppoe.4,v
retrieving revision 1.34
diff -u -p -r1.34 pppoe.4
--- pppoe.4 16 Jun 2017 10:58:43 -0000 1.34
+++ pppoe.4 16 Mar 2021 12:59:00 -0000
@@ -204,6 +204,14 @@ are no longer necessary.
See
.Xr pf.conf 5
for more information on MTU, MSS, and NAT.
+.Sh DIAGNOSTICS
+.Bl -diag
+.It "pppoe0: No valid PPP-Max-Payload tag received in PADO"
+The endpoint does
+.Em not
+support RFC 4638.
+The driver will clamp the MTU to 1492.
+.El
.Sh SEE ALSO
.Xr sppp 4 ,
.Xr hostname.if 5 ,
