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Specifying outgoing interface/IP address
https://forum.linode.com/viewtopic.php?f=19&t=4803		 Page 1 of 1
Author:  smiffy [ Thu Nov 05, 2009 7:06 pm ]
Post subject:  Specifying outgoing interface/IP address
If I have a Linode with multiple IP addresses, is there any way that I can route traffic through a specific interface/IP address, on a dynamic basis?

My scenario is that I need to access a service that has a per IP queries per hour limit. The legitimate way to overcome this limit, since the providers can't be bothered to do whitelisting, is to use multiple IP addresses.

I want, therefore, to create a round-robin system whereby I can say:

query 1 - use eth0
query 2 - use eth0:1
query 3 - use eth0:2
...etcetera.

The query will be coming from a Perl programme using IO::Socket.
Author:  Stever [ Thu Nov 05, 2009 11:11 pm ]
Post subject: 
Maybe use IO::Socket::INET and set LocalAddr?

I'm not a perl programmer, so just guessing.
Author:  smiffy [ Fri Nov 06, 2009 12:31 am ]
Post subject: 
Thanks - I'll see if that works.

Failing that, someone (via another source) has suggested a trick using iptables.

I'll post my results in case anyone else comes up against the same issue.
Author:  fukawi2 [ Fri Nov 06, 2009 1:56 am ]
Post subject: 
Code:
iptables -t nat -A POSTROUTING -m statistic --mode random --probability 0.5 -m state --state NEW -j SNAT --to-source X.X.X.X
iptables -t nat -A POSTROUTING -m state --state NEW -j SNAT --to-source Y.Y.Y.Y


Each new outgoing connection will "randomly" be Source NAT'ed to either X.X.X.X or Y.Y.Y.Y

This is assuming you have 2 IP addresses. If you have 3, you'll need to:
1) duplicate the 1st rule
2) adjust the source IP in the new rule
3) adjust all instances of 0.5 to be 0.333333333333 etc

You may wish to include a -d argument in there too so only outgoing connections to the host that's causing you problems is randomized ;)
Author:  smiffy [ Fri Nov 06, 2009 2:15 am ]
Post subject: 
Cool - that's one for the "useful" information file!

Cheers for that.
Author:  fukawi2 [ Fri Nov 06, 2009 2:18 am ]
Post subject: 
I love the random module, especially when my friend leaves his firewall logged in as root and unattended....

Code:
iptables -I FORWARD -i eth1 -o eth0 -m statistic --mode random --probability 0.5 -j DROP

:P
Author:  jed [ Fri Nov 06, 2009 12:06 pm ]
Post subject: 
fukawi2 wrote:
Code:
iptables -t nat -A POSTROUTING -m statistic --mode random --probability 0.5 -m state --state NEW -j SNAT --to-source X.X.X.X
iptables -t nat -A POSTROUTING -m state --state NEW -j SNAT --to-source Y.Y.Y.Y


Each new outgoing connection will "randomly" be Source NAT'ed to either X.X.X.X or Y.Y.Y.Y

That's really interesting. Didn't know about statistic.
Author:  mwalling [ Fri Nov 06, 2009 5:09 pm ]
Post subject: 
fukawi2 wrote:
code]iptables -I FORWARD -i eth1 -o eth0 -m statistic --mode random --probability 0.5 -j DROP[/code]


If this was StackExchange, I'd upvote you.
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