208 lines
4.2 KiB
HTML
208 lines
4.2 KiB
HTML
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>15.7. Circumventing Path MTU Discovery issues with MSS Clamping
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(for ADSL, cable, PPPoE & PPtP users)</H1
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><P
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>As explained above, Path MTU Discovery doesn't work as well as it should
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anymore. If you know for a fact that a hop somewhere in your network has a
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limited (<1500) MTU, you cannot rely on PMTU Discovery finding this out.</P
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><P
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>Besides MTU, there is yet another way to set the maximum packet size, the so
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called Maximum Segment Size. This is a field in the TCP Options part of a
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SYN packet.</P
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><P
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>Recent Linux kernels, and a few PPPoE drivers (notably, the excellent
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Roaring Penguin one), feature the possibility to 'clamp the MSS'. </P
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><P
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>The good thing about this is that by setting the MSS value, you are telling
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the remote side unequivocally 'do not ever try to send me packets bigger
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than this value'. No ICMP traffic is needed to get this to work.</P
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><P
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>The bad thing is that it's an obvious hack - it breaks 'end to end' by
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modifying packets. Having said that, we use this trick in many places and it
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works like a charm.</P
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><P
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>In order for this to work you need at least iptables-1.2.1a and Linux 2.4.3
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or higher. The basic command line is:
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<TABLE
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><PRE
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CLASS="SCREEN"
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># iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu</PRE
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></FONT
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> </P
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><P
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>This calculates the proper MSS for your link. If you are feeling brave, or
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think that you know best, you can also do something like this:</P
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><P
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> <TABLE
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BORDER="1"
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WIDTH="100%"
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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># iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 128</PRE
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></FONT
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></TD
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></TR
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> </P
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><P
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>This sets the MSS of passing SYN packets to 128. Use this if you have VoIP
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with tiny packets, and huge http packets which are causing chopping in your
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voice calls.</P
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