IP Forwarding and Network Routing

If you plan to use PPP to create a wide area network connection over telephone lines, it may be necessary to disable IP forwarding. IP forwarding is enabled at boot time when the system detects more than one interface (i.e. Ethernet and PPP). In this case, the system will also be configured as a router. This is undesirable because other routers on the network will attempt to route network traffic through your system. In addition to the performance impact on your system and the network, it is possible to cause network routing failures that are difficult to detect and correct.

To avoid this, establish an /etc/defaultrouter file that contains the name of one of the routers serving your network or subnetwork. You can determine a candidate router by inspecting the route table with the netstat command while your system is connected to the network.

# netstat -r
Routing Table:
 Destination          Gateway           Flags  Ref   Use   Interface
----------------- ------------------ ----- ---- ------ ---------
localhost	localhost	UH	0	20	lo0
slivan	ethr17a-11	UGH	0	0
single	ethr17a-11	UGH	0	0
mtnview	ethr17a-11	UG	0	9
net-eng-bb	fddi17b-11	UG	0	3961
net15-067-n	ethr17a-11	UG	0	0
net17-007-n	goldigger-eng	UG	0	1291
net19-174-n	fddi17b-11	UG	0	0
net17-083-n	shunt-116	UG	0	0
net21-244-n	ethr17a-11	UG	0	0
net18-148-n	ethr17a-11	UG	0	0
milpitas	ethr17a-11	UG	0	42
net11-144-n	voyager	U	2	3788	le0
224.0.0.0	voyager	U	4	0	le0

Since most routes in the above example are directed through gateway ethr17a-11, choose ethr17a-11 as your default router by creating an /etc/defaultrouter file with that name in it. The file should contain only the name of the selected default router, as in:

ethr17a-11

Note that when you configure your system to use a default router, you sacrifice the dynamic routing capability that Solaris provides. If you don't want to do this, edit the /etc/init.d/inetinit file. Change the test for "routerhood" to always fail: in /etc/init.d/inetinit, change

if [ $numifs -gt 2 -o $numptptifs -gt 0 -o -f /etc/gateways ]; then

to:

if [ `/bin/false` ]; then	# Never be a router

The disadvantage of this method is that your system can receive Routing Information Packets (RIP) across a PPP connection, which limits the available bandwidth.