mkiss: ax2: truncating oversized transmit packet!

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I encountered strange behavior today. Syslog is fully spammed

Dec  2 17:05:28 gw kernel: [33263.528216] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.529380] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.530492] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.531606] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.533164] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.534454] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.535679] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.537071] mkiss: ax2: truncating oversized transmit packet!
Dec  2 17:05:28 gw kernel: [33263.538409] mkiss: ax2: truncating oversized transmit packet!

The main issue was that i set the mtu of ax2 to 128. I do not know why, but just don`t do it. 🙂

ax2       Link encap:AMPR AX.25  HWaddr PI1LAP-3
          inet addr:44.137.31.73  Bcast:44.137.31.95  Mask:255.255.255.224
          UP BROADCAST RUNNING  MTU:128  Metric:1
          RX packets:1267 errors:0 dropped:0 overruns:0 frame:0
          TX packets:198 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:10
          RX bytes:253763 (247.8 KiB)  TX bytes:4538 (4.4 KiB)

This is the code that generates this error. (mkiss.c)

 if (len > ax->mtu) { /* Sigh, shouldn't occur BUT ... */
// translation of above: if data size of frame is GREATER than the 
// allowed MTU of ax25, generate the printk (print kernel error) below.
 - len = ax->mtu;
 printk(KERN_ERR "mkiss: %s: truncating oversized transmit packet!\n",
 ax->dev->name);
 dev->stats.tx_dropped++;
 netif_start_queue(dev);

Brian n1uro had this to say about it

It looks like this isn't a bug but a config issue on your end if I'm
reading their code correctly. You're trying to squeeze 100 litres of
water/minute through a hose that can only handle 50 litres/min so it's
telling you that it's fragmenting your frames to fit the proper MTU.

So just set your mtu to 256.

root@gw:/etc/ax25# cat axports
ax2     PI1LAP-3        19200   256     4       BBS pi8lap

 

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