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Re: Delegating /24's from a /19

  • From: Edward Lewis
  • Date: Wed Mar 16 16:29:09 2005

At 20:22 -0800 3/15/05, Owen DeLong wrote:

.... I'm not sure what you mean by "sideways delegations". It is
perfectly acceptable, for example, for:

a.root-servers.net returns 16.172.in-addr.arpa. IN NS ns1.arin.net.
ns1.arin.net returns 124.16.172.in-addr.apra. IN NS ns1.foobar.com.
ns1.foobar.com. returns 124.16.172.in-addr.arpa. IN NS ns1.subsidiary.com.
ns1.subsidiary.com. returns 5.124.16.172.in-addr.arpa. IN PTR foo.subsidiary.com.

This does work. This is what is being proposed.
I'm afraid that above is not an accurate or workable sequence of events.

Here is what happens *today* when a "fresh" copy of named seeks a "reverse map." First the disclaimer - I'm using named 9.3.1, not all iterating resolvers have the same strategy. I'm omitting repeated queries, as well as queries for AAAA records and retranmissions because of FORMERR. Also, if you start the server, run the query, stop the server and repeat (with an empty cache), the next result may vary as zones held vary be server.

Note too that this is from a fresh (empty) cache. Some queries are not needed when the cache is populated. It is not always as bad as I am illustrating - but it's easiest to visualize from the "0" state.

1. I ask a root-server for "162.57.173.209.in-addr.arpa./PTR". The response is a referral to the servers for 209.in-addr.arpa and I am told there are 7 of them.

2. I ask another root-server for the address of each of the 7 name servers. This means 7 new queries (14 w/AAAA's) directed to this second root server. Note that in this example, all 7 name servers are in the .net domain.

3. I ask the .net name servers the same questions as in #2. From this I get some "hybrid" answers - referrals enriched with answers - for most of the 7 name servers. I call these hybrids because, if you adhere strictly to the DNS protocol specifications, referrals are what you should get. The addition of the answer records to the referrals is a crutch to help the Internet run more smoothly. (For this we should thank Verisign engineers.)

4. The query in step 1 is issued to one of the name servers with a hybrid answer at this point. The reply received in this "step" is a referral to the servers for 57.173.209.in-addr.arpa, served up by four machines in neustar.com.

5. BIND continues seeking the glue for the name servers w/o hybrid answers in step 3. BIND does this to have these name servers available for further querying, but is not necessary for the immediate question. (This is done in parallel too - to avoid unnecessary latency.)

6. Armed with new name servers in step 4, a query for each's address is sent to another root server, which results in referrals to the servers for .com.

7. The .com servers also give the same hybrid answers (.com and .net are on the same machines) for the 4 name servers.

8. The original query is then issued to one of the servers whose address was obtained in step 7. The result of this is what we wanted all along.

9. BIND may continue seeking addresses for other servers after returning the answer, filling out its cache.

Why bother to detail all this? It's important to realize that the real iteration is done only in steps 1, 4, and 8. In step 1 I am being told who to ask. In step 4 I am also being told who to ask. The rest of the time I am trying to find out "where to ask". Steps 2,3,5 get me the addresses for the question in step 4. Steps 6,7,9 get the addresses for the question in step 8.

So - going back to the comment above:

a.root-servers.net returns 16.172.in-addr.arpa. IN NS ns1.arin.net.
The root-servers do not return NS records, the refer the querier to one of the /8 zones. (Note that not all root servers have the same zones, some might refer the querier to the in-addr.arpa. zone.)

ns1.arin.net returns 124.16.172.in-addr.apra. IN NS ns1.foobar.com.
ns1.foobar.com. returns 124.16.172.in-addr.arpa. IN NS ns1.subsidiary.com.
If the above two "situations" happened, then there's a violation of the database coherency principle that DNS tries hard (split-brain aside) to uphold. In the global DNS, no matter where you ask question, you should get the same answer.

I.e.,

dig @ns1.arin.net 124.16.172.in-addr.arpa. IN NS

and

dig @ns1.foobar.com 124.16.172.in-addr.apra. IN NS

had better return the same NS RRSet.

So, I don't think the above is workable or even realistic.

 	Thirdly I'm sick and tired of having to debug stupid
 	schemes ISP's come up with to try to avoid SWIPing the
 	nameservers in situations like this.  They don't work
 	or they don't meet the customers expectations (i.e.
 	they have a /24 and should just be able to use x.y.z.in-addr.arpa
 	and have it work reliably).

So don't debug them.  As long as ARIN has all of the /24s within the /19
pointing as NS records to the nameserver which contains the partially
populated /16 zone file (or which secondaries each of the relevant /24 zones
from their true owners), things work just fine.  Nothing really to debug.
I think the above two paragraphs are on the same side of the page.

 	Delegation is the DNS is strictly hierachical.

I don't see where the above breaks this.
It's the incoherency in your example that is the breakage.

 	You either SWIP the new servers or you slave the zones
 	from the customer.  In both cases you are following the
 	delegation heirarchy.  Note even if you slave the zones
 	you still have to ensure the delegation is correct.

I guess we will have to agree to disagree on this.  I will point out that
the above solution is working in a number of networks without problem.
Sure, if you screw it up, it doesn't work.  That's true of DNS generally.
If the delegation from the /8 zone is to the /24 level (as opposed to the /16 level) there are three options for an ISP to "transfer" the delegation to the customer.

1) Send a reassign-detailed or reallocate template (in ARIN lingo) for the space to the RIR. Then the next set of DNS zone files generated will delegate to the customer's name servers.

2) Use DNAME, RFC 2672. Good luck.

(http://www.isc.org/index.pl?/pubs/tn/index.pl?tn=isc-tn-2002-1.html)

3) Use RFC 2317. "I encourage my competitors to operate this way."

'Course, the ISP is free to have the customer just update the ISP's name servers, whether by dynamic update or be zone transfers from hidden masters.

--
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Edward Lewis +1-571-434-5468
NeuStar

Achieving total enlightenment has taught me that ignorance is bliss.




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