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Re: pebkac routing problem

From: "Martin P. Hellwig" <mhellwig@xxxxxxxxx>
Date: Fri, 07 Oct 2005 22:19:53 +0200

Martin P. Hellwig wrote:
<cut problem>

Thanks to hints from Matt I solved my routing problem concerning multiple gateways on non routing uplinks using IPFW, I scribbled my progress down and like to share that with you guys.

Here you go:

The limitation of a single default gateway in combination with a multi homed computer on non routing networks will prevent the other connection from working correctly.

The problem is that when a IP traffic is initiated from an outside source via the other route, the reply is still done over the default gateway.
All correctly configured gateways on a non routing network will filter these packages out because the gateway has no knowledge that these originating IP addresses are not spoofed.

The solution is to use the gateway related to the IP address/network where your packages is coming from.
There are different ways to achieve this most of them rely on a package filter/manager, others on multiple routing tables.

On my DragonFlyBSD system IPFW is suitable to do this task, for FreeBSD it should be the same.

First I need to configure my system to use IPFW:

% echo 'firewall_enable="YES"'>> /etc/rc.conf
% echo 'firewall_script="/etc/rc.firewall"' >> /etc/rc.conf
% echo 'firewall_type="OPEN"' >> /etc/rc.conf

The last line prevents me from logging myself out permantly and lets the machine behave as if there is no firewall. In /etc/rc.firewall "Open" is define with a couple of (indirect) rule sets.
Although the machine is within immidialty physical reach (as it should if you're doing network adaption) I administrate it over a secure remote shell.

If you have a machine doing NAT you probably have a recompiled kernel with built in IPFW and "options IPDIVERT", thus IPFW is already present on your system and you don't need to configure the above part. If you have other firewall/nat configuration, this explanation might do you more harm then good.

starting IPFW if it's not already started:
% /etc/rc.d/ipfw start

At this point all network connections are dropped and I have to relogon.
Let's see what the current "Open" IPFW rules are:
% ipfw list
00100 allow ip from any to any via lo0
00200 deny ip from any to
00300 deny ip from to any
65000 allow ip from any to any
65535 deny ip from any to any

Now I have to add a rule that when my 'other' IP address sends a package back that it does that over the 'other' gateway except when it's a address on the local bus. But first I need to configure my second IP address and get some info from my existing configuration.

My configuration for my default (and first) network:
IP and netmask with gateway

The other network I like to add: with gateway

Both networks are directly reachable from one network card (fxp0) i.e. the same physical network, so I added a alias:
% ifconfig fxp0 alias
At this moment all machines can reach my machine over the added IP address and only machines on receive my reply originating from the added alias.
If you have your second network on a separate physical network, configure your NIC as usual.

With this information the rules I created and executed are:
% ipfw add 110 fwd ip from to not
% ipfw add 120 fwd ip from to not

Strictly speaking the last rule is not (yet) necessary because thats the behavior of the default gateway however in my case, my default gateway will change so I included the rule already now.
Do notice that this is an unnecessary system burden although probably not too much!

At this moment my machine does exactly what I want, however if I reboot it, all configurations are lost, to make it permantly:
echo 'ifconfig_fxp0_alias0="inet"' >> /etc/rc.conf

Then open /etc/rc.conf in a editor and searched for the definition of "Open".
I found it at:
case ${firewall_type} in

I added the below rules under "allow_rest":
${fwcmd} add 110 fwd ip from to not
${fwcmd} add 120 fwd ip from to not

So that the configuration is now:
case ${firewall_type} in
${fwcmd} add 110 fwd ip from to not
${fwcmd} add 120 fwd ip from to not

-- mph

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