IP Routing Conifuration Dilemma

Posted on 1998-10-11
Last Modified: 2010-03-18
I'm trying to set up a linux box as a router.
I have set up zillions of linux boxes as firewalls using IP MASQ but am unable to seem to get plain-old routing setup
I have already tried:
-enabled IP forwarding ( echo 1 > /proc/sys/net/ipv4/ip_forward)
-started a routed (redundant, but...)
-tried adding IP forwarding rules via ipfwadm (i.e. ipfwadm -F -a accept -S LAN/LANMASK -D 0/0 -W worldinterface0)

whats going on?
I have a feeling I'm dong somthing exceedingly stupid.
what is it? any ideas?  things to check?
thanks much.
Question by:blundar
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Accepted Solution

ghjm earned 50 total points
ID: 1587133
- Forget routed unless you want dynamically changing routes.
- Don't do anything with ipfwadm except to make sure it's not rejecting anything. (I assume you don't want firewall functions here.)
- Add static routes as appropriate using the "route" command
- Make sure the *other* machines on the network are set up correctly

It sounds to me like what you're missing is the routing. There's no magic, routed won't figure it out for you.

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Expert Comment

ID: 1587134
did netstat -rn show appropriate routes?

Author Comment

ID: 1587135
all routes are ok... netmask (to the NAT cisco...) netmask (to LAN)

I can ping
I can ping 15.10.8.anything
I have 15.10.8.anything set up with (my machine, ethernet card 2) as the default gateway.
Whats goin on?
Im confused...
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LVL 51

Expert Comment

ID: 1587136
Do all the hosts in your net have the same netmask (M$ for example is very picky about it)?
Do you have a default route to the default gateway, or do you believe that som protocol stacks use the first IP address in the net as default gateway (I would not recommend this)?

Author Comment

ID: 1587137
I'm gonna post the whole routing tables ASAP.
FYI- If I add a ipfwadm -F -a m -S -D and set the default gateway to the NAT box, it works fine.  go figure...


Expert Comment

ID: 1587138

The Cisco doesn't know how to route back to your network. That's the problem.

Add a static route on the Cisco that lists 15.10.0.xx (your Linux box's address on the Cisco's network) as the gateway for

Your Cisco is probably either sending packets to its default gateway, or trying to ARP for them on its local Ethernet, neither of which will work.


Author Comment

ID: 1587139
Thanks Graham.  I did that in my test pseudo-Cisco (Linux box w/ Masq) and things worked ok.  Further problems will be posted if any.  Thanks much.

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