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Message-ID: <4684F4FB.9070807@trash.net> Date: Fri, 29 Jun 2007 14:03:07 +0200 From: Patrick McHardy <kaber@...sh.net> To: Andreas Steinmetz <ast@...dv.de> CC: Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>, netfilter-devel@...ts.netfilter.org Subject: Re: mss to pmtu clamping partially broken? 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 - assuming you have ethernet internally, the PMTU from your router to the internal hosts is 1500, so it won't do any clamping. 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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