RE: How to override the default source ipv4 address on packets originating from a router

Linux Advanced Routing and Traffic Control

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Your problem is one I've come up against a number of times in the past. You are definitely on the right route using "policy routing" (otherwise known as utilizing IP Rules), but the snag you're hitting is that the source address of a locally generated packet isn't chosen until a destination route has already been selected. Using the from statement in an ip rules table is only useful when you're routing packets from other subnets.

Now, just so you know, the local table should normally be left alone. It's used for locally connected subnets only. It is given the highest priority on purpose... don't mess with it! :)

There are a few methods you could try here, but I'm only going to recommend one of them. Have your main routing table hold the routes that your local machine will use, and then have a separate table for all routed hosts. Then, use ip rule statements to force any traffic coming in from a routed host into that routing table.

IE: on firewall-router (assuming eth1 is facing your static subnet hosts):

# ip rule list
0:  from all lookup local
32765:  iif eth1 lookup static-hosts
32766:  from all lookup main
32767:  from all lookup default

Have table main include all of the routes your local firewall-router should use, and have static-hosts contain all of the routes that your static-subnet-hosts should use.

Joel Gerber
Network Specialist
Network Operations
Eastlink
E: Joel.Gerber@xxxxxxxxxxxxxxxx T: 519.786.1241
-----Original Message-----
From: lartc-owner@xxxxxxxxxxxxxxx [mailto:lartc-owner@xxxxxxxxxxxxxxx] On Behalf Of Brian Burch
Sent: December-20-13 7:05 AM
To: Linux Advanced Routing
Subject: How to override the default source ipv4 address on packets originating from a router

The mailing list has been dormant for 2 years, so I wonder whether anyone is still listening for new questions?

My broadband router runs PPPoA and is dynamically assigned a single ipv4 internet address by my ISP. I have a static subnet which I host on a linux router/firewall (called chenin). The linux firewall and the adsl router communicate via a non-internet-addressable private subnet. Here is the topology:

Internet -- adsl-router-ppp0-ipv4-dynamic
          -- adsl-router-eth0-172.16.101.1
          --
          -- firewall-router-eth0-172.16.101.2
          -- firewall-router-217.154.193.209
          --
          -- static-subnet-hosts-217.154.193.154.208/28

Each of the hosts on the static subnet use 217.154.193.209 as their own default route. The adsl router forwards all incoming packets to the firewall/router's eth0. The firewall/router forwards all incoming packets to the static subnet via its own eth1. The firewall/router does not need to perform NAT, but it implements a simple set of iptables rules for blacklisting, etc. /All this works perfectly./

My problem is that I need to download software updates (debian apt-get
http) for the firewall/router from a repository out on the internet.

The firewall/router can successfully ping the repo-server when I force the source address like this:

     ping -I 217.154.193.209 163.1.221.67

... but a simple "ping 163.1.221.67" (i.e. using the default source address selection algorithm) fails. Wireshark confirms these unanswered packets go out on eth0 with a source address of 172.16.101.2.

I believe I should be able to resolve this problem with iproute2 policy routing, but so far I have not been successful. I've tried several variations, but they all give me the same "wrong" source address.


Here is my simplest effort:

brian@chenin:~$ ip rule list
0:  from all lookup local
32765:  from 172.16.101.2 lookup CHENIN_ONLY
32766:  from all lookup main
32767:  from all lookup default

brian@chenin:~$ ip route list table CHENIN_ONLY
163.1.221.67 via 172.16.101.1 dev eth0  src 217.154.193.209

brian@chenin:~$ sudo ip route flush cache brian@chenin:~$ ip route get 163.1.221.67
163.1.221.67 via 172.16.101.1 dev eth0  src 172.16.101.2
     cache

This shows me the source address in my policy rule has NOT been used, or the routing table entry does not work the way I think.


The only rule table with higher priority than CHENIN_ONLY is local, 
which contains routes for addresses local to the firewall/router - 
nothing about remote addresses, i.e.

brian@chenin:~$ ip route list table local
broadcast 127.0.0.0 dev lo  proto kernel  scope link  src 127.0.0.1
local 127.0.0.0/8 dev lo  proto kernel  scope host  src 127.0.0.1
local 127.0.0.1 dev lo  proto kernel  scope host  src 127.0.0.1
broadcast 127.255.255.255 dev lo  proto kernel  scope link  src 127.0.0.1
broadcast 172.16.101.0 dev eth0  proto kernel  scope link  src 172.16.101.2
local 172.16.101.2 dev eth0  proto kernel  scope host  src 172.16.101.2
broadcast 172.16.101.255 dev eth0  proto kernel  scope link  src 
172.16.101.2
broadcast 217.154.193.208 dev eth1  proto kernel  scope link  src 
217.154.193.209
local 217.154.193.209 dev eth1  proto kernel  scope host  src 
217.154.193.209
broadcast 217.154.193.223 dev eth1  proto kernel  scope link  src 
217.154.193.209

and the main table looks like this:

brian@chenin:~$ ip route list table main
default via 172.16.101.1 dev eth0
169.254.0.0/16 dev eth0  scope link  metric 1000
172.16.101.0/24 dev eth0  proto kernel  scope link  src 172.16.101.2
217.154.193.208/28 dev eth1  proto kernel  scope link  src 217.154.193.209


I wonder whether my unsuccessful "naked" pings to 163.1.221.67 are 
passing through the tables three times:

1.  My explicit policy should be applied to select a source address
     of 217.154.193.209.
2. The main table is used to select the default route via 172.16.101.1:
3. The main table then provides the explicit route to the default
    router, which seems to rewrite the source address to 172.16.101.2.

I have read the latest lartc HOWTO sections relevant to policy routing, 
but didn't see anything similar to my situation. I also didn't see how 
to get a verbose (i.e. blow by blow) output from the "ip route get" 
command to show how it is selecting its route.

Am I trying to do the impossible here, or am I just making a mistake in 
the way I am doing it?

I hope this strikes someone as an interesting question. If I can achieve 
a solution, I would be happy to add the scenario to the HOWTO.

Brian
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