Loopholes in a proxy and smtp server

["Niranjan Patil" <niranjan.patil () gmail com>]

Hi All,

I have noticed two significant (well, I think it is) flaws in the
design of one of the corporate proxy and SMTP servers I am consulting
for. I googled for it and checked some RFC's too but couldn't get
anything much helpful. Hope to get valuable info from you all.

1. The squid proxy is sending out its internal IP when forwarding the
http requests to the outside world. I mean if the proxy's internal IP
is 192.168.1.1 and its public IP is  1.1.1.1, it is sending both of
them out to the Internet. To check this, you can open a site like
www.dnsstuff.com, where it shows the public and private IP of your
proxy (you need have one). I guess they are not using any scripts to
check my IP. Even if they have used they could find my own machine's
IP and not my proxy's. I am not sure how to harden the proxy for this
particular flaw.

2. The SMTP servers listening for incoming mail on the Internet are
also giving out valuable information. When queries are made to it, it
accept connections only to a valid email id in its address book. I
mean when we respond to its RCPT command with an email id, it checks
and throws out a message as
'[250 recipient <name () company com> ok]' for a valid id and
'[Could not connect: Got an unknown RCPT TO response: 501 #5.1.1 bad
address  nonexistingname () company com]' for non existing email id and
closes the connection.
Using this anyone can get address book of all valid email ids of that
firm, he/she may use a simple script too. I don't think this is
normal, and need to address this soon. I checked out the popular free
email providers like gmail, yahoo or hotmail, they accept connections
for all email ids and then sends back a mailer daemon for invalid ids.

Apologise for the long mail but appreciate any help.

Regards,
Niranjan

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