[SpamCop-Geeks] Re: How do open proxies work ?

[Michael Lefevre]

Larry Kilgallen wrote:
> That is, does one just deliver email to their Port 25 and it comes
> out the other end sanitized ?

No. Generally, you connect to the proxy, and then issue a command to
connect to a remote host. After the proxy has made the connection to
whatever host you told it to connect to, everything sent to the proxy is
sent on, byte by byte, to the destination.  This means that the process is
pretty much independent of the protocol being used - you can use proxies
to do web transactions (which is their intended use), ftp, smtp, nntp, or
whatever else you like.

> Or is there some other constant port number involved ?

there are a variety of port numbers involved.

> Or is there some other protocol involved ?

yes, there are several...

the main two are HTTP (web) proxies, and SOCKS proxies.

SOCKS proxies are usually found on port 1080 (but, as with anything else,
can be run on any port if someone decides to do that)

there isn't a defined standard port for HTTP proxies though, which means
there are quite a wide variety of ports used for them. port 8080 is quite
common, as is 3128, which is the default for the popular Squid proxy.
there are about half a dozen other quite common ports used for HTTP
proxies...

a third type is a telnet proxy, which generally run on the telnet port
(23).

> I am talking (I think) only about "single stage" proxies.

I don't think one can have anything other than a single stage proxy,
because the connection through a proxy is immediate - unlike email relays,
it doesn't perform an email transaction with the sender and then initiate
a new transaction with a remote server. The connection from the source to
the destination is "live", but with the data being transmitted through the
proxy instead of direct.  Having said that, it's possible for a proxy to
have two different IPs, and for it to use a different IP for talking to
clients and to servers.

http://www.geocities.com/spamresources/proxy.htm has a list of pages with
further information about how proxies work and how they are abused.

-- 
Michael