4.3 BSD subnet bug

[Tim Seaver]

The 4.3 BSD in_localaddr routine in sys/netinet/in.c is
completely broken in its handling of subnets. This causes
the wrong tcp maximum segment size to be chosen when making
connections to non-local subnets of the local network, leading
to extra fragmentation. I believe the following replacement
handles things correctly:

in_localaddr(in)
	struct in_addr in;
{
	register u_long net = ntohl(in.s_addr);
	register struct in_ifaddr *ia;

	if (subnetsarelocal) {
		for (ia = in_ifaddr; ia; ia = ia->ia_next)
			if ((net&ia->ia_netmask) == ia->ia_net)
				return (1);
	} else {
		for (ia = in_ifaddr; ia; ia = ia->ia_next)
			if ((net&ia->ia_subnetmask) == ia->ia_subnet)
				return (1);
	}
	return (0);
}


	Tim Seaver
	Microelectronics Center of North Carolina
	mcnc!tas
	tas at mcnc.org