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From: tas@mcnc.UUCP (Tim Seaver)
Newsgroups: net.unix-wizards,net.bugs.4bsd
Subject: 4.3 BSD subnet bug
Message-ID: <1627@alvin.mcnc.UUCP>
Date: Tue, 9-Sep-86 10:53:33 EDT
Article-I.D.: alvin.1627
Posted: Tue Sep  9 10:53:33 1986
Date-Received: Wed, 10-Sep-86 17:43:21 EDT
Organization: Microelectronics Center of NC; RTP, NC
Lines: 30
Keywords: network,subnets
Xref: mnetor net.unix-wizards:7835 net.bugs.4bsd:926

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@mcnc.org