changing MTU size for Netgraph pseudo devices

Denis Guerrero dguerrero at
Wed Oct 13 16:41:42 PDT 2004

I recently came across the documentation provided at
for implementing the Netgraph Fast-Channel kernel module on FreeBSD 
systems it works great. One thing that I have not been able to figure 
out is how to change the mtu on the pseudo device to anything greater 
than 1500(I want mtu 9000). If I do

#ifconfig fec0 mtu 1000 (this works if its any value less than 1500)

but, if I do

#ifconfig fec0 mtu 9000 (I get the following: " ifconfig: ioctl (set 
mtu): Invalid argument ")

I also tried changing the "NG_FEC_MTU_Default 1500" line in the 
ng_fec.h file to

NG_FEC_MTU_Default 9000

and then do a build/install new kernel and build/install world. Still 
have no success. Any suggestions would be greatly appreciated.

#! /bin/sh

  echo "load module";
  cd /usr/src/sys/modules/netgraph/fec/

  kldload /usr/src/sys/modules/netgraph/fec/ng_fec.ko

  echo "kldstat results";


  echo "create psuedo fec interfaces";

  ngctl mkpeer fec dummy fec

  echo "bind physical interfaces to pseudo devices";

  ngctl msg fec0: add_iface '"em0"'
  ngctl msg fec0: add_iface '"em1"'

  echo "set capture mode for each pseudo interface";
  ngctl msg fec0: set_mode_inet

  echo "set all physical and pseudo interfaces to promiscuous mode";
  ifconfig fec0 promisc
  ifconfig em0 promisc
  ifconfig em1 promisc

  echo "bring up pseudos";
  ifconfig fec0 up



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