Proxy related question 1

[electron]

Hi There:

It is said that when a proxy server is used there is no way to identify your terminal from the outside. The proxy do that replacing my terminal address with the proxy address. There has to be a way the proxy keeps a reference code related to my terminal address to identify the packets comming in to my terminal from the outside. Is this code hidden in the packet, is the code changes every time there is a new connection?? Is that the case?? The proxy puts a code in the packet going out?? In which layer this code is inserted??
If not how the proxy identifies the packets comming in if this packets does not contain the address of my terminal???

Thanks in advance

Rafael

 

[richs]

Hi-

This is "ripped" from the fine link:

http://www.tldp.org/HOWTO/Firewall-HOWTO-2.html

The linux system used in the examples is a little out
of date, but the general information is still quite
useful.

"Proxies are mostly used to control, or monitor, outbound traffic. Some application proxies cache the requested data. This lowers bandwidth requirements and decreases the access the same data for the next user. It also gives unquestionable evidence of what was transferred.

There are two types of proxy servers.

1. Application Proxies - that do the work for you.
2. SOCKS Proxies - that cross wire ports. "
<end of ripping>

In most cases, the proxy servers remember "pages" and
"files" for awhile. This allows them to repost these
"pages" to save themselves some bandwidth by returning
common "pages" to multiple users without having to
re-get them from the internet. This is proxy caching.
Note that web pages can ask for "no-proxy" setting in
the header of the files which should have the proxy
servers get them every time.

As to the second part of your question, the proxy and/or
firewall uses one form or another of "Network Address
Translation" or NAT. Indeed, it keeps record of who
(on the inside of the firewall or proxy) has sent a
request (actually to the IP address of the proxy server).
This is either served by the proxy server from it's cache,
or it goes out to the big bad outside internet and gets
the stuff requested. It uses it's own IP address to
send out the level 3 (TCP/IP) packet to the IP address
of the requesting device (usually using a unique port
numbers of the TCP packet during connection establishment).

It then sends it back to the
requesting IP address the stuff that it has received,
again using the connection with the inside of the firewall
IP and port address,
unless there are proxy "rules" which might have it do
something different (say like "Shame on you for going
to something sex something dot com". Or some other
silly thing.

Squid is a very popular web proxy caching system that is
open source and free. If you want to invest a little time
and effort, you can "obtain" a rather low performance
PC (i.e. cast off by somebody) and install linux and squid
on it and play to your hearts content.

If you find that you like the features of squid, maybe
a little newer, faster, more memory and disk space PC
and you can generate a squid proxy server for your
department and amaze your friends!


I'm pretty sure that this is the straight stuff. This is
all coming from memory (except the links) late at night.

Hope this helps!

Cheers,

Rich S.