Source ip based networking in ubuntu

Question

I have 3 interfaces in ubuntu machine. eth0/eth1/eth2.

• eth0 is management interface with ip (10.102.x.x)
• eth1 is private interface with ip 192.168.1.x
• eth2 is another private interface with ip 192.168.2.x

I want to reach to 25.25.25.x (which is the box in some other network). Here is the topology:-

Ubuntu machine <----------------> Router <------------> End Machine
192.168.1.x                 192.168.1.x   25.25.25.x     25.25.25.x

I want to reach to 25.25.25.x using eth1 and eth2 interfaces. So for that i am trying to do source ip based routing tables. Here is the configuration applied:

ifconfig eth1 up
ifconfig eth1 192.168.1.x netmask 255.255.255.0
ip rule add from 192.168.1.x table 1 
ip route add 192.168.1.0/24 dev eth1 scope link table 1
ip route add default via 192.168.1.x dev eth1 table 1

However this is not being successful as ping 25.25.25.x -I eth1 doesn't work. However the direct route works:

ifconfig eth1 up
ifconfig eth1 192.168.1.x netmask 255.255.255.0 
route add -net 25.25.25.0/24 gw 192.168.1.x

In non-working case the linux client itself is doing broadcast for 25.25.25.x ip, which should't happen.

Here is the ip route and ip route show table 1 output:-

root@ubuntu:~# ip rule show
0:  from all lookup local 
32765:  from 192.168.1.x lookup 1 
32766:  from all lookup main 
32767:  from all lookup default 
root@ubuntu:~# ip route show table 1
default via 192.168.1.x dev eth1 
192.168.1.0/24 dev eth1 scope link

Can someone please answer this as to why first case isn't working?

Answer 1

Your configuration doesn't make sense.

ip rule add from 192.168.1.x table 1

This says that if we know a packet is from 192.168.1.x, we can use table 1 to route it.

ip route add default via 192.168.1.x dev eth1 table 1

This says that if we know we're using table 1 to route a packet, the default route is via 192.168.1.x.

See the problem? Only table 1 knows that your default route is reachable via the 192.168.1.x router. And only knowing that the default route is reachable via the 192.168.1.x tells you to use table 1. So this configuration shouldn't be expected to work as you seem to be expecting.

I don't understand why you're trying to use source based routing. You just want to route a particular way to a particular destination. It's hard to suggest changes without understanding why you're making things more complicated than they seem to need to be.

Answer 2

Use cases for such configuration:

• ensure that incoming traffic is replied via same interface, for example
• multi-homing protocols and applications, for example SCTP

Basically this works for me just fine. Contents of /etc/network/interfaces:

auto eth1
iface eth1 inet static
  address 192.168.1.19
  netmask 255.255.255.0
  gateway 192.168.1.254 # default gateway if no source address
  post-up ip link set eth1 promisc on
  post-up ip route add 192.168.1.0/24 dev eth1 table 122
  post-up ip route add default via 192.168.1.254 table 122
  post-up ip rule add from 192.168.1.19/32 table 122 priority 122
  post-up ip route flush cache
  pre-down ip rule del from 192.168.1.19/32 table 122 priority 122
  pre-down ip route flush table 122
  pre-down ip route flush cache

auto eth2
iface eth2 inet static
  address 192.168.2.19
  netmask 255.255.255.0
  post-up ip link set eth2 promisc on
  post-up ip route add 192.168.2.0/24 dev eth2 table 222
  post-up ip route add default via 192.168.2.254 table 222
  post-up ip rule add from 192.168.2.19/32 table 222 priority 222
  post-up ip route flush cache
  pre-down ip rule del from 192.168.2.19/32 table 222 priority 222
  pre-down ip route flush table 222
  pre-down ip route flush cache 

Then:

$ ip route get 8.8.8.8
8.8.8.8 via 192.168.1.254 dev eth1  src 192.168.1.19 
    cache 
$ ip route get 8.8.8.8 from 192.168.1.19
8.8.8.8 from 192.168.1.19 via 192.168.1.254 dev eth1 
    cache 
$ ip route get 8.8.8.8 from 192.168.2.19
8.8.8.8 from 192.168.2.19 via 192.168.2.254 dev eth2 
    cache