OPNsense Forum

English Forums => General Discussion => Topic started by: CodingFiend on March 01, 2018, 09:04:03 pm

Title: how to set up MTU properly; fragmented UDP packets getting truncated
Post by: CodingFiend on March 01, 2018, 09:04:03 pm

I am getting a 1507 byte UDP packet, and as i set up OPNSense as a transparent bridge, my igb0 and ibg1 ethernet ports were set at the typical 1500 byte limit. But when a 1507 UDP packet comes in as a fragmented packet, the firewall ends up truncating the packet. Do i need to set the bridge port  (i think called OPT1) to have an MTU of 1492? I can't find any examples of how to do this correctly, and clearly i am getting screwed because i don't have the parameters set just right. Or do you set the MTU to 9000 across the board? TCP doesn't have this problem because TCP has a mechanism for determining MTU on a path, but UDP is a one shot, one way ticket on the information superhighway, so you gotta be careful...  or is this just a known bug in OPNSense whereby fragmented UDP packets are not handled correctly, and are silently truncated?

surely i can't be the first person to bump against this. A prize to whomever helps me resolve this.

Title: Re: how to set up MTU properly; fragmented UDP packets getting truncated
Post by: namezero111111 on December 16, 2020, 01:51:33 pm

Digging this up.

Having run into this exact same problem (UDP SIP; options too large) passing through a transparent filtering bridge, have you ever found a solution to this issue?