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List:       freebsd-hackers
Subject:    IP Routing
From:       Joao Carlos Mendes Luis <jonny () coppe ! ufrj ! br>
Date:       1998-03-30 6:10:13
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Hi,

  Suppose I have a physical ethernet network with two (or more) logical
IP networks above it.  Suppose also I have the following machines attached
to this physical network:

Machine A:

ifconfig fxp0 inet 10.0.0.1 netmask 255.255.255.0 up
route add -net 10.0.1.0 -netmask 255.255.255.0 -interface fxp0

Machine B:

ifconfig fxp0 inet 10.0.1.1 netmask 255.255.255.0 up
route add -net 10.0.0.0 -netmask 255.255.255.0 -interface fxp0

  Now, machine A can see machine B, over the same cable, with
1 hop distance; and vice-versa.  Here's a tcpdump output from
a test I've done here (real addresses changed for security reasons):

arp who-has 10.0.0.1 tell 10.0.1.1
arp reply 10.0.0.1 is-at 0:a0:c9:5a:6d:45
10.0.1.1 > 10.0.0.1: icmp: echo request
10.0.0.1 > 10.0.1.1 icmp: echo reply
10.0.1.1 > 10.0.0.1: icmp: echo request
10.0.0.1 > 10.0.1.1 icmp: echo reply
10.0.1.1 > 10.0.0.1: icmp: echo request
10.0.0.1 > 10.0.1.1 icmp: echo reply
10.0.1.1 > 10.0.0.1: icmp: echo request
10.0.0.1 > 10.0.1.1 icmp: echo reply

  Now, why can't I do this on machine A ?

route add default 10.0.1.1

  I've taken a quick look at /sys/net/route.c and it seems to me that
this case is not allowed.  The gateway addr must be the other end a
point2point interface or an address valid on a local interface
address/netmask list.

  If I add an alias to machine B, like this:

ifconfig fxp0 net 10.0.0.2 netmask 255.255.255.0 alias

  And change the route command on machine A to:

route add default 10.0.0.2

  Then it works.

  All this discussion come from a thread on the Brasilian FreeBSD
list, on which a user wanted to do something similar to the above
and said that it works on Linux, but cannot do the same on his
FreeBSD box.  I don't care if what he wants to do is the best
thing or not (personally, I would never do such a think this way),
but I can't see why it's wrong.  The gateway is known, and is local
to the interface.  It's just not on the same subnet.

					Jonny

--
Joao Carlos Mendes Luis			jonny@gta.ufrj.br
+55 21 290-4698				jonny@coppe.ufrj.br
Universidade Federal do Rio de Janeiro	UFRJ/COPPE/CISI
PGP fingerprint: 29 C0 50 B9 B6 3E 58 F2  83 5F E3 26 BF 0F EA 67

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