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Re: PHP socket and NAT [message #177122 is a reply to message #177120]

Thu, 23 February 2012 10:52

Erwin Moller
Messages: 228
Registered: September 2010

Karma:

Senior Member

On 2/23/2012 11:32 AM, Erwin Moller wrote:
> On 2/23/2012 9:58 AM, SL wrote:
>> This is not strictly a PHP question, it just springs into my mind while
>> coding sockets in PHP.
>>
>> Assuming that I am communicating with 2 persons who are in the same
>> LAN, say
>> A& B.
>>
>> When I send some data to A, do I have to take care of the NAT (network
>> address translation) ?
>>
>> Will the sockets take care of it automatically ?
>>
>> Thanks.
>>
>>
>
> Hi,
>
>
> NAT typically is used to change IP-addresses (and maybe port).
> If A and B are in the same LAN I expect they can address each other
> directly.
> For example:
> A IP: 192.168.178.22
> B IP: 192.168.178.25
>
> Once you go outside your LAN, you (might) need NAT.
>
> And sockets have little to do with NAT.
> A IP package arrives at a certain IP on a certain port: If some socket
> is listening on that port on that machine, the package will be handled
> by the listening process.
>
> NAT itself is performed by routers and such, not by some service
> listening to a port.
>
> SO in short: No you have to care about it.
>
> Disclaimer: If you are talking IP6, things are different.
>
> Regards,
> Erwin Moller
>

An additional example of NAT that makes things clear.
This is how it was explained to me when I wanted to know about NAT.

Suppose you have 10 computers on your LAN, and 1 internet gateway (some
ADSL modem for example).
All you 10 computers can connect to the internet because they use the
routers IP address as the gateway.

The router has an INTERNAL (LAN-side) IP address.
Let's say this LAN-IP is 192.168.178.1

The router also has an EXTERNAL (Internet-side) IP address.
Let's say internet-side IP is 69.147.94.94. (I made the numbers up)

Now, suppose a few people on the LAN are visiting some websites.
Suppose compA has IP address 192.168.178.22. (On the LAN)
Suppose compA has IP address 192.168.178.50. (On the LAN)

CompA (192.168.178.22): requests www.google.com
CompB: (192.168.178.50)requests www.php.net

The router receives these requests from the LAN side, and sends the
requests to www.google.com and www.php.net.
The servers from Google and PHP answer the requests, and send them back
to the router's IP address (being 69.147.94.94).

Now the problem: How should the router handle these answers? Should it
send them to 192.168.178.22 or 192.168.178.50, or even another IP on the
LAN?

This is where NAT comes into the picture.
It works like this:
1) Router receives request from 192.168.178.22 (=LAN)for www.google.com.
2) Router thinks up some random port that is not is use, eg 33000.
3) Router stores this portnumber (33000) in a table together with the
requesting LAN IP-address (192.168.178.22).
4) Router sends the request to www.google.com TOGETHER with the
instruction to answer on port 33000.
5) When Google answers, the router looks up the portnumber, ans sees
what (internal) IP-address made this request. This way the router can
see which internal IP-adress made what request because they all get
random assigned portnumbers.

That is NAT in short.

So I don't expect you have to worry about that in your case.

Regards,
Erwin Moller

--
"That which can be asserted without evidence, can be dismissed without
evidence."
-- Christopher Hitchens

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[Message index]

		PHP socket and NAT By: SL on Thu, 23 February 2012 08:58
		Re: PHP socket and NAT By: Erwin Moller on Thu, 23 February 2012 10:32
		Re: PHP socket and NAT By: Captain Paralytic on Thu, 23 February 2012 10:41
		Re: PHP socket and NAT By: Erwin Moller on Thu, 23 February 2012 10:53
		Re: PHP socket and NAT By: Erwin Moller on Thu, 23 February 2012 10:52
		Re: PHP socket and NAT By: SL on Thu, 23 February 2012 13:04
		Re: PHP socket and NAT By: The Natural Philosoph on Thu, 23 February 2012 13:37
		Re: PHP socket and NAT By: The Natural Philosoph on Thu, 23 February 2012 11:40
		Re: PHP socket and NAT By: Jerry Stuckle on Thu, 23 February 2012 13:00
		Re: PHP socket and NAT By: SL on Thu, 23 February 2012 13:09
		Re: PHP socket and NAT By: Captain Paralytic on Thu, 23 February 2012 13:19
		Re: PHP socket and NAT By: The Natural Philosoph on Thu, 23 February 2012 13:53
		Re: PHP socket and NAT By: Captain Paralytic on Thu, 23 February 2012 14:25
		Re: PHP socket and NAT By: Michael Fesser on Thu, 23 February 2012 17:28
		Re: PHP socket and NAT By: The Natural Philosoph on Thu, 23 February 2012 20:42
		Re: PHP socket and NAT By: M. Strobel on Fri, 24 February 2012 10:58
		Re: PHP socket and NAT By: SL on Thu, 23 February 2012 16:03
		Re: PHP socket and NAT By: The Natural Philosoph on Thu, 23 February 2012 20:44
		Re: PHP socket and NAT By: M. Strobel on Fri, 24 February 2012 11:07
		Re: PHP socket and NAT By: The Natural Philosoph on Fri, 24 February 2012 13:22
		Re: PHP socket and NAT By: Tim Streater on Fri, 24 February 2012 14:20
		Re: PHP socket and NAT By: The Natural Philosoph on Fri, 24 February 2012 15:39
		Re: PHP socket and NAT By: Tim Streater on Fri, 24 February 2012 18:53
		Re: PHP socket and NAT By: The Natural Philosoph on Fri, 24 February 2012 21:04
		Re: PHP socket and NAT By: Tim Streater on Fri, 24 February 2012 21:41
		Re: PHP socket and NAT By: Peter H. Coffin on Sun, 26 February 2012 18:50
		Re: PHP socket and NAT By: Erwin Moller on Thu, 23 February 2012 13:37
		Re: PHP socket and NAT By: The Natural Philosoph on Thu, 23 February 2012 13:50

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