PMTU, MSS and "fragmentation needed" problem with linux?

[Nico Schottelius <nico-kernel@schottelius.org>]

Hello!

[I hope this is the correct list, if not, please tell me where to ask]

The following scenario:

Linux-client <-- Ethernet --> Linux-router <-- PPPoE --> Internet.
Linux-client has MTU==1500, so the MSS is 1460.
Linux-router has MTU==1500 on eth0 and MTU=1492 on ppp0.
The MSS is set to 1452 via pppoe command line.

My question: May the MSS on ppp0 be 1460? Imho not, as

RFC 879 defines the MSS as MTU-40, which would be 1452.
But tcpdump says 1460 on ppp0.
On another box the MTU is set to 1460 and I still see packets
with MSS 1460. Is this correct?
Should not the Linux-router send the client a packet ICMP type 3,
subtype 4, Frag needed, but DF-Bit set?

I ask htis, because of a problem I  have on the first box:
If the clients have a mtu of 1500, theycannot connect
to some servers and get timeouts.

The firewall was tested with iptables of (only MASQUERADE on), so that
all icmp packets are allowed.

Thank you for any hint!

Nico

-- 
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