Tuesday, May 21, 2013

tcp-mss adjustment Fortigate style

Since Fortigate's are my firewall of choice, I'm wrapping this up with a FGT sample.

Fortigate  like iptables/pf based firewalls, let's  you adjust  tcp-mss per fwpolicy entry.  You can adjust mss  in  either direction  ( send |  receive )

For the receive direction it would look like the following;

Be advise in all of these examples, ipv6 was not  demostrated  nor is it supported. As a matter of fact, on the JuniperSRX and Fortinet FWs,  you can't adjust mss within the gui or cmdline for ipv6 tcp traffic. Also  ipv6 uses a different approach with  mss setting and controls and prefer fragmentation at the client's ends.

And in the above  two fwpolicies,  you have to configure the  fortigate cmdline only. So get use to editing via " config firewall policy" :)

There you have it, my list of various ways to adjust TCP mss adjustment for  traffic.

Ken Felix
Freelance Network/Security Engineer
kfelix ---at---hyperfeed --dot--com

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