Public Ethernet Connections

[Chuck Bearden]

On Fri, 29 May 1998, Chris Lott wrote:

> > To prevent this, you should
> > block all traffic from your walk-in public network to port 25 (SMTP)
> > of remote hosts and of your own mail hosts.  You should also disable
> > relaying for that network on your own mail hosts, just to be on the
> > safe side.
> 
> 
> Doesn't this also prevent what is perhaps one of the top legitimate reasons
> people might want to jack into the net in the first place?

If you are asking whether blocking TCP connections to remote port 25
will prevent folks from using email, I think the answer is "not in 
most cases".  

In my experience, most folks check their email in one of three ways:

1. they telnet to a remote host where they have an email account and
where they run an app like pine or elm to read & send.  This method
uses port 23 on the remote host instead of port 25.

2. they use a webbed mail service like Hotmail or Rocketmail, in 
which case they probably are using port 80 (standard www server port),
or perhaps some high-numbered port.

3. they use a popmail client to retrieve mail (port 110, not blocked),
and probably port 25 of the remote mailserver as their mail gateway.
In this case, the user would have difficulties sending.  However, I
suspect that most ISPs don't let their users send mail through their
smtp servers from IPs not in the ISP's pool, in order to prevent
spamming.  In these cases, it doesn't matter whether or not the
library blocks outbound access to port 25, the ISP won't relay mail
from a host that is not inside of its own network (unless that mail is
destined for a user inside of the ISP's network, of course).  

In other words, the only ones prevented from using email who
otherwise could are those in option 3 whose ISPs would let them relay
outbound mail through their SMTP server from a remote site, and those
using an option other than these three and that requires port 25
(anyone know of any such options?).  

I don't know how IMAP handles the sending of mail, but for its basic
duty, it connects to port 143 of the remote server.

Chuck Bearden
Network Services Librarian
Houston Public Library
Houston, TX  77002
713/247-2264 (voice)
713/247-1182 (fax)
cbearden at hpl.lib.tx.us