Linux – How to set the preferred network interface in linux

linuxnetworkingrouteroutingtcpip

I have my network set up like this.
http://docs.google.com/Doc?docid=0AZ1YxuLE4djaZGhqN2s1NmRfMjhjNjc0Ym1meg&hl=en

In words: I have a machine (Calcium, running Arch Linux) that has two network interfaces. eth0 is hoooked up to a router, and is gigabit. Eth1 is hooked up directly to the university network over 10Megabit. The router's uplink is hooked up to the university network as well, and it is also 10Megabit.

Currently (I believe) all traffic on Calcium is going through eth0, through the router, regardless of whether it is internal or external. (How can I confirm this?)

Ideally, traffic that is destined for the internal network (192.168.10.0/24) would travel over eth0 to the router, and wherever it is going. ALL other traffic should go over eth1.

Best Answer

Your network routing table will determine how the packets are routed. You can add additional routes or change the default gateway to affect the routing.

If you would like additional assistance, please provide your routing table.

To display the routing table:

/sbin/route -n

To delete default gw:

/sbin/route del default gw $IP

To add default gw:

/sbin/route add default gw $IP

For additional details, check out the manpage. There are other ways to manipulate and display the routing table as well.

netstat -r to display and ip to manipulate further.

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Linux – One-to-One NAT using Linux IPTables Not Working

You need to confirm you have the following conditions met:

Enable the IP forwarding using: echo 1 > /proc/sys/net/ipv4/ip_forward or by setting the appropriate line in /etc/sysctl.conf.
Allow the traffic to pass through the box which should be OK according to your description.
Setup the needed NAT rules which should be OK according to your description.
Setup the needed routes (static or dynamic) if the routed IPs belong to subnets not directly connected to the Linux router. This should be also OK according to your description.

It seems that you are left with the first point!

Linux – forcing network traffic to route via a specific, non-default interface

Routes are processed from the most specific route to the least specific (aka default) route.

default via 10.31.96.1 dev em3  proto static 
10.0.0.0/8 dev em1  proto kernel  scope link  src 10.0.0.100 
10.31.96.0/22 dev em3  proto kernel  scope link  src 10.31.97.100 
10.31.96.0/22 dev em4  proto kernel  scope link  src 10.31.96.61

You said you want should be going out through em3 unless its destined for something else on the local 10.0.0.1/8 subnet. This is exactly what is happening. The IP address 10.31.45.1 is within 10.0.0.0/8 and so it is leaving via em1. The 10.0.0.0/8 route matches that address is more specific then the default route. The address doesn't match the the 10.31.96.0/22 route. Therefore the em1 route is selected.

Your real problem is that you have a subnetmask on that em1 interface that is far too large for what you probably need, and it conflicts with the other networks. Anything destined for a IP address in the 10.0.0.1-10.255.255.254 range will attempt to use em1 as if it was local, which the exception of addresses in the 10.31.96.0/22 which will leave via em3/em4.

Your solution is to either fix the em1 subnet/network so that it doesn't conflict with your other networks, or to add lots of routes.

Something like ip route add 10.31.45.0/24 via 10.31.96.1 might do what you want.

Best Answer

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Linux – One-to-One NAT using Linux IPTables Not Working

Linux – forcing network traffic to route via a specific, non-default interface

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