Security Basics mailing list archives

Re: Can't Resolve from behind firewall

From: "Sumit Dhar" <ml_dhar () yahoo com>
Date: Tue, 22 Oct 2002 00:32:53 +0530

Only one thing: under CheckPoint FW with IIS 5.0 i need to open DNS-tcp
to permit to send email. I don't know why this, but without DNS-tcp I
can not to send email.


I am not hundred percent sure, but here is a possible explanation for the
behaviour you have talked about:

Though DNS usually uses UDP, it occasionally has to use TCP for some sites
(like say hotmail). Typically their MX record is too large and sending that
via UDP is not viable. UDP will need to fragment it and there is no
guarantee that each packet has been received.

It is in  such cases that TCP has to be used.

With Regards,
Sumit Dhar
http://www.rootshell.be/~dhar

Current thread:

Re: Can't Resolve from behind firewall khayes (Oct 17)
- Re: Can't Resolve from behind firewall Dickon Newman (Oct 18)
  - Re: Can't Resolve from behind firewall michel 'ziobudda' morelli (Oct 18)
    - Re: Can't Resolve from behind firewall Sumit Dhar (Oct 21)
  - Re: Can't Resolve from behind firewall Jason Kohles (Oct 21)
- <Possible follow-ups>
- RE: Can't Resolve from behind firewall Security Newsletters-TM (Oct 17)
- RE: Can't Resolve from behind firewall YashPal Singh (Oct 21)