Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!rutgers!topaz!hedrick
From: hedrick@topaz.RUTGERS.EDU (Charles Hedrick)
Newsgroups: comp.unix.wizards,comp.protocols.tcp-ip
Subject: Re: IP fragmentation, and how to avoid it
Message-ID: <11947@topaz.RUTGERS.EDU>
Date: Wed, 13-May-87 04:14:40 EDT
Article-I.D.: topaz.11947
Posted: Wed May 13 04:14:40 1987
Date-Received: Sat, 16-May-87 01:11:53 EDT
References: <12640@teknowledge-vaxc.ARPA>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 73
Xref: mnetor comp.unix.wizards:2288 comp.protocols.tcp-ip:173

Now and then we run into machines that can't reassemble.  Note that
the 1006 limit on imp0 isn't a problem with the VAX.  It is the
limit allowed by the Arpanet.  There are more elegant solutions,
but if you don't have source, here is a program that will let you
change the MTU on the fly.  We have used it on both Pyramid and
Sun, changing only the name of the kernel variable.  I.e.
the string "_il_softc", which is the name appropriate for il0 on
the Pyramid.  I just checked and it looks like _le_softc will work
for a Sun 3/50.  At least this will let you see whether your problem
is really a reassembly problem.  You should try "mtu 1006" or
maybe some slightly smaller number.  (We typically use 900 for
testing.)

#include <sys/types.h>
#include <sys/stat.h>
#include <a.out.h>
#include <stdio.h>

struct nlist nl[2];

short mtu;
int kmem;
struct stat statblock;
char *kernelfile;

main(argc,argv)
char *argv[];
{
	if (argc < 2) {
		fprintf(stderr,"usage: mtu <n> {<kernelfile>}\n");
		exit(2);
	}

	if ((kmem = open("/dev/kmem",2))<0) {
		perror("open /dev/kmem");
		exit(1);
	}
	if (argc > 2) {
		kernelfile = argv[2];
	} else {
		kernelfile = "/vmunix";
	}
	if (stat(kernelfile,&statblock)) {
		fprintf(stderr,"%s not found.\n",kernelfile);
		exit(1);
	}
	initnlistvars(atoi(argv[1]));
	exit(0);
}

initnlistvars(on)
register int on;
{
	nl[0].n_un.n_name = "_il_softc";
	nl[1].n_un.n_name = "";
	nlist(kernelfile,nl);
	if (nl[0].n_type == 0) {
		fprintf(stderr, "%s: No namelist\n", kernelfile);
		exit(4);
	}
	(void) lseek(kmem,(nl[0].n_value)+6,0);
	if (read(kmem,&mtu,2) != 2) {
		perror("read kmem");
		exit(5);
	}
	fprintf(stderr,"mtu was: %d is now: %d\n",mtu,on);
	(void) lseek(kmem,(nl[0].n_value)+6,0);
	mtu = on;
	if (write(kmem,&mtu,2) != 2) {
		perror("write kmem");
		exit(6);
	}
}