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This target allows to alter the MSS value of TCP SYN packets, to control
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the maximum size for that connection (usually limiting it to your
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outgoing interface's MTU minus 40). Of course, it can only be used
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It is only valid in the
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This target is used to overcome criminally braindead ISPs or servers
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which block ICMP Fragmentation Needed packets. The symptoms of this
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problem are that everything works fine from your Linux
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firewall/router, but machines behind it can never exchange large
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Web browsers connect, then hang with no data received.
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Small mail works fine, but large emails hang.
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ssh works fine, but scp hangs after initial handshaking.
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Workaround: activate this option and add a rule to your firewall
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iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\
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-j TCPMSS --clamp-mss-to-pmtu
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.BI "--set-mss " "value"
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Explicitly set MSS option to specified value.
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.B "--clamp-mss-to-pmtu"
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Automatically clamp MSS value to (path_MTU - 40).
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These options are mutually exclusive.