Ah Jumbo frames, the Power, the Speed, the pure goodness of MTU=9000 – well, not so fast if you want a stable SSH experience across the internet.

Here’s what I found; Debian + Jumbo Frames + AT&T Uverse = intermittent, hangs on terminal and SFTP sessions while remotely connected. Recently, I built a new Debian based (Actually, Linux Mint DE) system for the house that I plan to expose port 22 on for remote shell access. I know, pretty common stuff. During the setup of the OS, I started playing around with the MTU size, figuring like my FreeNAS box I should boost the value up from the base 1500 size. I made the config change, everything worked great – at least while I was on the LAN.

So the next week comes and I’m working down in Houston – I fire up an ssh session back to the Debian box, I connect without any problem, but then discover I can’t reliably issue even simple commands like “ls” without the session locking up, stalling, or just “hanging” there. But since the connection does not drop all together, I start troubleshooting other connection issues. It’s not till I SFTP to the box (also a port 22 action) that I begin to suspect something in the Debian side, rather then my client connectivity. My SFTP session, just like the terminal was able to connect and sustain a connection, but after maybe on directory refresh would “pause” or “hang” and exhibit the same type of behavior as the command line session.

Mulling all this over, I did the only logical thing an IT Pro would do…. Roll back my changes, setting the MTU back to the default 1500. And like-a-magic remote ssh traffic is back to normal.  So friends – learn from my greedy frame size experience.