MTU

[Zdenek Prchal]

>
> : pak muze zmenseni PMTU dosahnout libovolny uzel po ceste,
> dokonce i ten
> : posledni -- kdyz misto prijeti paketu posle zpatky
> patricnou ICMP zpravu.
> : Ale mirne pochybuji o tom, ze to nekdo, specialne Linux, dela, je-li
> : konecnym cilem paketu.
>
> Tusi niekto, ci sa k tomu da donutit? Rychlym grepom cez zdrojaky
> sa zda, ze asi nie, ale mozno existuje nejaky trik.
>
Hmm, mozna by to castecne slo pres iptables, viz man iptables:

   TCPMSS
       This  target  allows to alter the MSS value of TCP SYN packets, to
con-
       trol the maximum size for that connection (usually limiting it to
your
       outgoing  interface's MTU minus 40).  Of course, it can only be used
in
       conjunction with -p tcp.
       This target is used to overcome criminally braindead  ISPs  or
servers
       which  block  ICMP  Fragmentation Needed packets.  The symptoms of
this
       problem are that everything works fine from your Linux
firewall/router,
       but machines behind it can never exchange large packets:
        1) Web browsers connect, then hang with no data received.
        2) Small mail works fine, but large emails hang.
        3) ssh works fine, but scp hangs after initial handshaking.
       Workaround:  activate  this option and add a rule to your firewall
con-
       figuration like:
        iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
                    -j TCPMSS --clamp-mss-to-pmtu

       --set-mss value
              Explicitly set MSS option to specified value.

       --clamp-mss-to-pmtu
              Automatically clamp MSS value to (path_MTU - 40).

       These options are mutually exclusive.

Zdenek Prchal