> I'm curious is you can point me to any information about
> Multiple PTR records and DNS being sent via TCP rather
> than UDP.. You say that if the query is over 20k it will
> use TCP. I'm curious about this.
Actually, the DNS response that was being returned was over 20k. Anything over 512
bytes has to be sent via TCP. UDP is normally used, because it is much more efficient
(1 packet is sent to the DNS server, and 1 packet comes back, versus TCP which takes a
total of 6 or more packets to do the job.
The DNS RFC1035 ( http://ftp.is.co.za/rfc/rfc1035.txt ) says in section 4.2.1 that UDP
packets are limited to 512 bytes.
Actually, what I believe happens is that the DNS resolver sends a request to the DNS
server via UDP, then the DNS server returns the first 512 bytes via UDP with a flag
indicating that there is more. I believe TCP is used at that point, so there's an
extra 2 packets overhead right there.
Either way, a 20K DNS record means that if all mail servers look up reverse DNS
entries, you're dealing about 25K of traffic to get a 5K E-mail sent.
> Second thing is you say you are only supposed to have one PTR
> record per IP.. So, how do companies like msn, yahoo, hotmail,
> etc.. do it? I guarantee that they have more than one PTR record
> per email address..
I was mistaken. As someone else pointed out, it technically is allowed, it is just
not recommended. Originally, people interpreting the DNS RFCs thought that you could
only have 1; but, a later RFC specified that you could.
Companies like MSN often have many DNS records (for example, 10 MX records for
msn.com, 20 A records for mail.msn.com, etc.). I just checked one of Hotmail's mail
servers, 216.33.151.136, and their reverse DNS only has 1 entry
(mc4.law5.hotmail.com). Same with Yahoo's 128.11.68.59 that points to
mta-v1.mail.yahoo.com.
The problem is that most programs that look up reverse DNS entries only expect (and
can handle) 1 entry. For example, SMTP servers that have a received line that
includes the reverse DNS only use 1 entry. They can't add 20K to the headers (many
mail servers can't handle headers greater than 8K).
>Once again thanks for all your help,
No problem; that's what we're here for. I even learned something new from this (that
multiple PTR records are legal according to the RFCs).
--
-Scott
Declude: Anti-virus and Anti-spam solutions for IMail. http://www.declude.com
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