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I want to change the my datapackage from port 25 to another internal address(192.168.2.253).

iptables -A OUTPUT -o em1 -p tcp --dport 25 -j SNAT --to 192.168.2.253

But I get the output is:

root@gateway:~# iptables -A OUTPUT -o em1 -p tcp --dport 25 -j SNAT --to 192.168.2.253
iptables: Invalid argument. Run `dmesg' for more information.

how to do it correctly?

The dmesg output is:

https://gist.githubusercontent.com/thinksource/af08acd451380823257b/raw/5d720ce48bcb5ea27699bfcde63e4ccd92e2118c/dmesg%20output

user504909
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  • Well, what did dmesg have to say about it? – EEAA Mar 25 '15 at 04:05
  • @EEAA please click the link – user504909 Mar 25 '15 at 04:20
  • What are you exactly wanting to do ? Why rewrite the source address when the destination port is SMTP ? – Xavier Lucas Mar 25 '15 at 10:18
  • @Xavier Lucas there are two ip addresses on the same network adapter one for internal network, one for external network. Actually, I want use the external ip address send emails, while everytime the computer always use the internal ip address. – user504909 Mar 26 '15 at 17:13

2 Answers2

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To add a NAT rule you need to do so in the NAT table; add "-t nat" to the iptables command line. Further, you can only do SNAT on the INPUT NAT chain (not OUTPUT), and on an inbound interface (-i, not -o). You will obviously have to alter which interface that is, to the one where the packet is arriving, not the one where it is leaving

Craig Miskell
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The SNAT target is only meaningful in the POSTROUTING chain of the nat table.

A valid version of your rule would be :

iptables -t nat -A POSTROUTING -o em1 -p tcp --dport 25 -j SNAT --to-source 192.168.2.253

By the way if your (output) interface holds the IP 192.168.2.253, then rather use : 

iptables -t nat -A POSTROUTING -o em1 -p tcp --dport 25 -j MASQUERADE
Xavier Lucas
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  • I do not want holds ip address, the simple reason is I have two ip address on one network adapter. I need to use one of them to send email, while every-time the machine always use another one. – user504909 Mar 26 '15 at 17:20
  • That's exactly what my answer's first rule is doing... I'm giving up on your case, you seem to make no effort at all. – Xavier Lucas Mar 26 '15 at 17:48
  • I can not use nat, since the net address is not assemble by the server. – user504909 Mar 28 '15 at 03:01