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SubjectRe: mss to pmtu clamping partially broken?
Patrick McHardy wrote:
> Andreas Steinmetz wrote:
>> Patrick McHardy wrote:
>>> Andreas Steinmetz wrote:
>>>> [...]
>>>> The tcpdump on the client shows that the mss of the incoming syn reply
>>>> packet is *NOT* clamped to the ppp interface mtu.
>>> You forgot to mention *how* you're clamping the MSS. Using
>>> TCPMSS? Do you have a rule for incoming packets?
>> The relevant iptables commands I do use for masquerading and clamping are:
>> iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
>> iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS \
>> --clamp-mss-to-pmtu
> Two things here:
> - tcpdumps on ppp0 will show unclamped packets since they haven't
> been forwarded yet

That is true, I know this.

> - assuming you have ethernet internally, the PMTU from your router
> to the internal hosts is 1500, so it won't do any clamping.

Yep, internal PMTU is 1500, still the incoming packets are clamped to
1452 on the one line and not clamped on the other.

> Does that explain it?
> A useful thing for TCPMSS for routers would be to clamp to the
> minimum of the PMTU of both directions. But thats not supported
> so far.

I wonder, as somteimes it gets clamped. If it would never have been
clamped I wouldn't have asked.

Andreas Steinmetz SPAMmers use
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