Changing ethernet and IP parameters

[Rice Don D]

In trying to work around a router problem, I need to change the default
networking parameters on this end (Ultrix 4.0, DS5000).  Unfortunately, it
looks like the only way to change any of these is by fooling with the
kernel configuration and building a new kernel.  Is there a better way to
modify packet lengths, window sizes, etc?

What I'm trying to do right now is reduce the MTU from 1500 to 576.  I've
located ETHERMTU in /sys/net/inet/if_ether.h and in /sys/io/netif/if_ether.h.
I changed its value from 1500 to 576 in both files, did "make clean", "make
depend", and "make" as per TFM, and booted from the new kernel, but netstat
and pfstat say that MTU is still 1500.  What else needs to be changed?

Thanks, 
Don Rice                                  Internet: ddr at flux.gi.alaska.edu
Geophysical Institute                     E-mail:   fnddr at alaska.bitnet
University of Alaska                      Phone:    (907) 474-7569
Fairbanks, AK 99775                       Loran:    64.86N 212.16E