I've a few pcaps that contain packets larger than standard ethernet MTU size of 1500B. I searched about this and found refs pointing towards Jumbo Frames. I now understand that since I'm on a GBE network that supports sending Jumbo frames (upto 9k), the TCP/IP stack on my Debian derivative will auto tweak itself and use such huge frame sizes (PMTU Discovery). This is really awesome!
However, I want to have some control over auto-enabling of this feature. Specifically, I want to make sure that when I capture a pcap, it is temporarily disabled and as such all captured packets are under 1500B. I need to ensure this because a tcpreplay based pcap replay framework I've fails when it sees Jumbo frames. Tcpreplay has an enhancement request, Support fragment IP packets > MTU to enable sending Jumbo frames. But it is a low priority ticket and could possibly take time to get resolved.
I read about ways to manually tweak the PMTU discovery behavior and found that "ip_no_pmtu_disc" sysctl variable under the /proc filesystem could be handy. I enabled it on both my client and server systems (although I assume enabling it on any one of them would have been sufficient) and captured a few pcaps. But they still have packets larger than 1500B. The default MTU on my network interfaces is already set to 1500B and since PMTU discovery is disabled, I assumed things would work fine. But, there is something I'm missing. I would appreciate any refs/pointers.
NOTE: I didn't restart systems as the /proc tweaks I made won't persist. I also am not in position to restart systems everytime I need to replay pcaps.