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Old 28th October 2008
dk_netsvil dk_netsvil is offline
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Default spoofing with iptables

I'm trying to do some tricky spoofing using iptables and have had some issues. I have a /27 subnet populated by a dozen or so servers and I want outbound mail from one machine on my subnet to appear to originate from another machine on the same subnet. I've been trying to craft a rule something like

iptables -t nat -A POSTROUTING -o eth0 -s $SRCHOST -p tcp --dport 25 -j SNAT --to xxx.xxx.xxx.220

where $SRCHOST has the IP xxx.xxx.xxx.216.

I'm trying to accomplish this because I'm working with a company that assists in email delivery and they want to associate all mail sent for a domain with a single IP address. Since my web and mail servers are separate and I don't want to add to the load by adding a relay I wanted to try and use iptables to spoof the webserver IP.

Am I barking up the wrong tree?
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Old 29th October 2008
J65nko J65nko is offline
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Cannot you use the rewriting capabilities of your mailer?

For postfix for example, this is discussed at http://www.postfix.org/ADDRESS_REWRITING_README.html
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Old 29th October 2008
dk_netsvil dk_netsvil is offline
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While I've used the postfix rewrite capability to rewrite a domain name I've never used those functions to spoof the sending mail server's IP address. I'm not sure that's even possible.

I have mail leaving a server at 10.254.0.1 and I need it to appear to come from 10.254.0.2 when I examine the headers. To the best of my knowledge this can't be done with address rewriting, but I'm open to suggestions.
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Old 29th October 2008
dk_netsvil dk_netsvil is offline
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Maybe a little more information would be useful:

I have 3 mailservers behind a firewall running iptables. Each mailserver has it's own private 10.254.0.x IP address and currently all outbound mail appears to come from the public IP of the firewall which I'll call xxx.xxx.xxx.210. This firewall also has the internal IP 10.254.0.1 which is the default gateway for each mailserver. Each mailserver also has an interface on the public network, but their default gateway is the internal address of the firewall.

What I had wanted to do was use iptables to spoof the IP of each mailserver's public IP for outgoing mail. What I am slowly coming to understand is that this shouldn't be possible. I don't think iptables will allow you to spoof IPs that are already in use and not assigned to the current firewall.

Is the solution to assign 3 new public IPs to the firewall as aliases and then use iptables to spoof outbound mail from each server statically mapped to each of those newly assigned alias IPs?
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Old 29th October 2008
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You have the servers behind the firewall, on a private network, but they also have public IPs assigned to the interfaces on the servers?

You need to remove the public IPs from the servers. They should only have private IPs assigned to their interfaces. The public IPs should be set on the firewall. And you should have firewall rules that do 1-to-1 NAT between the public IP and the private IP.
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Old 29th October 2008
dk_netsvil dk_netsvil is offline
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These particular machines actually need their public interfaces, but I agree that I'll have to assign additional public IPs as aliases on the firewall and use static NAT to associate outbound mail with those interfaces. I was hoping to avoid using additional IPs in this /27, but it's looking unavoidable.
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Old 29th October 2008
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Yeah, you can't have two systems on the same network using the same IP(s). At least not easily. Just think about how the return packets will know where to go.
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