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Dynamic Mangle rule for reducing MSS value

Posted: Sat Dec 28, 2013 9:18 pm
by sjoram
Hi all,

Previously used my RB750 on a MPoA connection but have recently moved to PPPoA.
Have a Draytek Vigor 120 acting as PPPoA to PPPoE bridge.
Have a PPPoE client configured on my RB750 to login to my ISP and this acts as my dialer interface.
Had some problems which with the help of http://forum.mikrotik.com/viewtopic.php?f=2&t=53952 I identified to be down to the dynamic mangle rule added by the PPPoE client to change the MSS value of any TCP SYN packet where it was above a set value.
It appeared to be using MTU minus 40 bytes, however I saw on the above thread a suggestion that the subtraction should be larger (44 bytes not 40). I turned off the dynamic rules and added 2 static rules to ensure that MSS was never larger than MTU minus 44 bytes.
Since doing so, the issues I was having seem to have disappeared.
Using ROS v 5.4 - is this a bug in the dynamic rule having the wrong value?