LWP Use Multiple IP Addresses On A Multihomed Machine

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Let LWP Use Multiple IP Addresses On A Machine

Subjects > Computers > Internet

Stepping through finding a solution

By Garnet

Remember how I was wondering how to send pings, and web requests, and other net requests, from something other than the first IP address bound to a multihomed machine?

After years of wondering how to do this, I decided to spend a few hours single stepping through the perl libraries, since the main place I want to do this from is from the [spiders] that I write. I got rid of some of my internet servers recently, so I now have less places to run spiders from. Sometimes I need to run a lot of copies of a given spider so I can get thousands of requests finished faster.

After a couple of hours, I finally discovered a place in the perl runtime libraries where I could specify a localaddress which could be one of the addresses of the machine. So my click11 server, that has a dozen IP addresses bound to it, I can send a web get request from a perl program from any of the IP addresses on the machine.

Then I was trying to figure out how to hack this into the libraries, since at first I just hard coded an alternate address into the perl library. I finally figured out what a somewhat odd thing was that I’d been stepping over. Then finally how to set the value it was returning. Now that I know what to do, I did a search for that variable, EXTRA_SOCK_OPTS, and found what I had been looking for.

 binding LWP to a specific interface 
 After search and googleing for an answer a nice fellow in #perl (undernet)
 came up with the following:
 @LWP::Protocol::http::EXTRA_SOCK_OPTS = ( LocalAddr  => q(ip.addr) ); 
 Posted by sps at 10:54 AM

Too bad I never figured out that way to ask the question on google, or maybe I could have found the solution a lot sooner…..

So this doesn’t help me for stuff run from the command line like ping, or for the .NET framework spiders I write in C#, but maybe I can find something for them too. Turns out it looks like at the socket level, the magic thing is to bind to a specific address before the connect statement actually does the web request. It appears the default is to specify ANY_ADDR and let the winsock layer choose something.

Benefits of Perl and Open Source To Finding Solutions

Going through and finding this kind of thing in Perl was a lot easier than with any other language I use. Once I had a suspicion of where I needed to specify a localaddress, it was very easy to hack the statement into the perl run time libraries. I didn’t need to worry about recompiling or anything like that. I don’t know how I’ll be able to replicate this process with .NET, since it doesn’t like to expose it’s assemblies to such easy hacking, since everything there is compiled.

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I think I just heard (August 7 2005) something about Mountain View NASA opening their doors to people who want to watch the shuttle landing tonight….

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