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Obtaining public and private ip addresses

Posted on 2008-10-14
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Last Modified: 2013-12-25
When I go to: http://www.whatismyip.com/ I get my public ip address 69.132.x.x  (the ip address that is assigned to the public interface on my router.  When I go to: http://whatismyip.org/ I get my DHCP assigned private ip address of 192.168.1.41 (this is the address actually assigned to my computer's ethernet card).

I know how to get my public ip address using server side scripting in JSP, PHP or cgi scripts, but I don't know how to get my private ip address.  Originally I thought the only way to do this was with some type of client side scripting, but http://whatismyip.org/ appears to be retrieving the client side ip address using only server side scripting.

How is this done?  
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Question by:Troupe75
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Expert Comment

by:Adam314
ID: 22712528
When I go to either of those, it shows only my public IP address.

It may be that your web client is providing the information in a header.  What web client are you using?  Can you try using FireFox with the LiveHTTPHeaders add-on?  This will tell you if your web client is providing the information.
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by:Troupe75
ID: 22712561
I've tried it with Firefox, Safari, and curl so I don't think it is coming from the web browser.  I'm not using the LiveHTTPHeaders add-on in Firefox.
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by:Adam314
Adam314 earned 500 total points
ID: 22733842
Can you describe your setup a bit more?

What do you get when you do an nslookup on  whatismyip.org at a prompt?
eg: from a prompt: nslookup whatismyip.org
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Author Comment

by:Troupe75
ID: 22733936
Last login: Wed Oct 15 18:51:33 on console
~  $ nslookup whatismyip.org
Server:            192.168.1.6
Address:      192.168.1.6#53

Non-authoritative answer:
Name:      whatismyip.org
Address: 206.176.224.3


I have a DHCP assigned address.  The network goes through a Barracuda content filter and then Nat through a Firebox firewall to a T1.

I'm not sure how the private address is getting passed out.  When I go one of my websites and look at the logs, it show my public ip (the one on the firebox).  I'm not sure if there is a way to run Apache in a mode that will show the full request headers.
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Accepted Solution

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Troupe75 earned 0 total points
ID: 22733961
Ah ha.  I found it.  In the headers my browser is sending there is:

x-forwarded-for       192.168.1.41

They must be grabbing that.  I'm guessing is it being added by the Barracuda or Firebox.
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Expert Comment

by:Adam314
ID: 22734001
Yes, that must be it.  I don't think you could use that very reliably, as I don't think most NAT boxes will add it.
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