Box With 3 Dedicated IP's All Returning The Same Single IP Address

onesikgypo · 01-20-2010, 08:02 PM

Hi,

I have a linux box, which was given to me by a company with 3 dedicated IP addresses. My goal was to setup ssh tunnels on all three dedicated IP Addresses.

Via Putty, i am successfully able to create a tunnel to the box's on IP1 IP2 and IP3 - that is i can make incoming connections on all three IP's.

However, my problem lies when i attempt to USE the tunnel. AIn all instances, regardless of the IP i actually tunneled into the box with, it returns IP1 as an outgoing connection, i.e if i setup IE to use the tunnel, it will return my IP as IP1, even though i have created the tunenl via IP2/IP3.

What id liek to accomplish is that when i create a tunnel via IP1, then outgoing connections go via IP1, and when i create a tunnel via IP2, then outgoping connectiosn go via Ip2 etc.

After discussing this with a colleague of mine, he infomed me he had the same problem at one point, and that some configuration on the box was required - however he was unable to recall.

Any assistance that could be given would be greatly appreciated.

Thankyou.

zordrak · 01-21-2010, 04:42 AM

Your problem is routing.

The machine has only got one default gateway. That means the IP address to send packets to if they are destined for the outside world and not the local network. The default gateway is also assigned to a specific ethernet device for the packets to be sent from.

Code:

$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.0.0.0        *               255.255.255.0   U     0      0        0 eth0
loopback        *               255.0.0.0       U     0      0        0 lo
default         10.0.0.1        0.0.0.0         UG    1      0        0 eth0

In the example above, you see that anything destined for an address in 10.0.0.x will be addressed and sent directly to that IP because that is the "localnet". No gateway is required. Anything addressed to a loopback IP will be send using the "lo" interface. If a packet is destined for any other IP address, then it will be sent to the default gateway (in this case 10.0.0.1) so that the gateway can forward it on out of your network and onto the Internet. This default gateway is bound to the "eth0" interface. This means that eth0 will be used to send all packets out to the Internet and therefore will have the source IP as whatever IP is assigned to eth0. Even if you add an adapter and add a new entry to the routing table accordingly, there will still only be one default gateway bound to one adapter.

This means that while you can listen on any IP you want for incoming connections, outgoing ones will always use the default gateway.

If you want to get around this, then you are going to need to use iproute2 and iptables to create custom rules for routing outbound packets. This will require a lot of background reading. A couple of basic starting points:

http://www.linuxjournal.com/article/7291
http://blog.khax.net/2009/11/28/mult...-and-iproute2/

onesikgypo · 01-21-2010, 04:57 AM

i was under the impresion from other research i did online that it could be solved through iptables alone with something like (though i have not had the time to implement it yet) since im just concerned with what IP address is displayed on the outgoing and not concerned with anything incoming as such:

Code:

iptables -t nat -A POSTROUTING -s IP2 -o eth0 -j SNAT --to IP2