Subject: Re: [DNSOP] WGLC: "Considerations for the use of DNS Reverse Mapping"

From: Andrew Sullivan <[email-masked]>
Date: Tue, 1 Apr 2008 17:36:02 -0400

Dear colleagues,

On Tue, Apr 01, 2008 at 10:36:28AM -0400, Edward Lewis wrote:

> Multiple PTR records can be stored in a single PTR RRset.  If a 
> device at an IP address (v4 or v6) has multiple identities with 
> domain names, it would be good to have a PTR for each.  However, this 
> is not always practical.  In some operational situations, an address 
> may have thousands of domain names holding an address record (A or 
> AAAA) with the address as the value.
> 
> The number of address records in an PTR set before tripping the upper 
> limit on what can fit on even a TCP carried DNS message is 
> approximately 4000 for A RR only and about 2000 for AAAA RR only.
> 
> If an address has just a few corresponding forward map records, it is 
> worth entering them all.  If an address has many, a better strategy 
> is to enter a few as is needed, adding more only when there is an 
> operational request.

I'm inclined to add this text.  I'd like additional expressions of
support (or edits, or whatever) from the WG to confirm my inclination.

Thanks,

A

-- 
Andrew Sullivan
[email-masked]
+1 503 667 4564 x104
http://www.commandprompt.com/
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https://www.ietf.org/mailman/liFrom [email-masked]  Tue Apr  1 14:36:09 2008
Date: Tue, 1 Apr 2008 17:36:02 -0400
From: Andrew Sullivan <[email-masked]>
To: [email-masked]
Subject: Re: [DNSOP] WGLC: "Considerations for the use of DNS
	Reverse	Mapping"
Sender: [email-masked]

Dear colleagues,

On Tue, Apr 01, 2008 at 10:36:28AM -0400, Edward Lewis wrote:

> Multiple PTR records can be stored in a single PTR RRset.  If a 
> device at an IP address (v4 or v6) has multiple identities with 
> domain names, it would be good to have a PTR for each.  However, this 
> is not always practical.  In some operational situations, an address 
> may have thousands of domain names holding an address record (A or 
> AAAA) with the address as the value.
> 
> The number of address records in an PTR set before tripping the upper 
> limit on what can fit on even a TCP carried DNS message is 
> approximately 4000 for A RR only and about 2000 for AAAA RR only.
> 
> If an address has just a few corresponding forward map records, it is 
> worth entering them all.  If an address has many, a better strategy 
> is to enter a few as is needed, adding more only when there is an 
> operational request.

I'm inclined to add this text.  I'd like additional expressions of
support (or edits, or whatever) from the WG to confirm my inclination.

Thanks,

A

-- 
Andrew Sullivan
[email-masked]
+1 503 667 4564 x104
http://www.commandprompt.com/
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---

From: Ted Lemon <[email-masked]>
Date: Tue, 1 Apr 2008 18:10:14 -0700

> support (or edits, or whatever) from the WG to confirm my inclination.

I agree that it's worth mentioning.

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---

From: Ted Lemon <[email-masked]>
Date: Tue, 1 Apr 2008 18:10:14 -0700

---

From: Joe Abley <[email-masked]>
Date: Thu, 3 Apr 2008 00:08:54 -0500

>> AAAA) with the address as the value.

How about re-writing the last sentence as "In some operational  
situations, a single address might be used as RDATA for a large number  
of A or AAAA resource records."

>> approximately 4000 for A RR only and about 2000 for AAAA RR only.

"The maximum size of a DNS message is 65536 bytes [RFC1035]. The  
number of resource records in a single PTR RRset is correspondingly  
limited."

(The limit is surely a function of the average encoded size of RDATA  
for the PTR records in the RRset, and has nothing to do with whether  
the PTR record lives under in-addr.arpa or ip6.arpa?)

>> worth entering them all. If an address has many, a better strategy is
>> to enter a few as is needed, adding more only when there is an
>> operational request.

I don't see value in adding the text above.

Joe

---

From: Joe Abley <[email-masked]>
Date: Thu, 3 Apr 2008 00:08:54 -0500

Joe

---

From: Samuel Weiler <[email-masked]>
Date: Fri, 4 Apr 2008 11:27:32 -0400 (EDT)

I have read this document and have no objection to its publication.

That said, I share Jinmei's concern that the recommendation against 
depending on reverse mapping is too weak in the context of the rest of 
the document.  I'm in favor of much stronger language saying "don't 
depend on reverse mapping being available".

While I appreciate the spirit of the text proposed by Ed Lewis, the 

>> approximately 4000 for A RR only and about 2000 for AAAA RR only.

I believe that adding explicit mention of the dangers of too many PTR 
RRs at a name will help emphasize the "you really shouldn't depend on 
reverse mapping", which is a good thing.

-- Sam

---

From: Samuel Weiler <[email-masked]>
Date: Fri, 4 Apr 2008 11:27:32 -0400 (EDT)

-- Sam