Network Working Group E. Warnicke
Internet-Draft Cisco Systems
Expires: August 2, 2003 February 2003
DNS Resolution of Networks and Gateways
draft-warnicke-network-dns-resolution-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies a method given an IP address to perform a
series of DNS lookups to determine the network that contains that IP
address, the netmask of that network, and the gateway(s) on that
network. This method allows for variable length subnet masks,
delegation of subnets on non-octet boundaries, and multiple gateways
per subnet.
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1. Introduction
As a variety of new devices are introduced to the network, many of
them not traditional workstations or routers, there are some
applications arising which require the participation of the first hop
router on the edge in the provision of some network service for a
host.
There are some mechanisms provided for the resolution of gateways for
classful networks in RFC 1035 [1], and subnets in RFC 1101 [2]. RFC
1101 suffers from a number of defects, chief among which are that it
does not support variable length subnet masks, which are in commonly
deployed in the internet.
Since the writing of RFC 1101 mechanisms for dealing with classless
networks in DNS have arisen, for example RFC 2317 [3]. This document
describes a mechanism of using the notation similar to RFC 2317 to
specify a series of PTR records enumerating the subnets of a given
network in the RFC 2317 notation. This lookup process continues
until the contents of the PTR records are not an in-addr.arpa.
derived domain name. These terminal PTR record values are treated as
the hostname(s) of the router(s) on that network. This RFC also
specifies an extension to the method of RFC 2317 to allow for
delegation at non-octet boundaries.
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2. Generic format of a network domain name
Using the Augmented BNF of RFC 2234 [4] we can describe a generic
domain name for a network as follows:
networkdomainname = maskedoctet "." *( decimaloctet / maskedoctet ".")
".in-addr.arpa."
maskedoctet = decimaloctet "-" mask
mask = (*1( "1" / "2" ) DIGIT ) / "3" ("1" / "2" )
decimaloctet = ( *1("1") DIGIT DIGIT ) /
( "2" ( "1" / "2" / "3" / "4" ) DIGIT ) /
( "2" "5" ( "1" / "2" /"3" / "4" / "5" ) )
The by way of reference an IPv4 CIDR notation network address would
be written
IPv4CIDR = decimaloctet "." decimaloctet "." decimaloctet "."
decimaloctet "/" mask
A "-" is used as a delimiter in a maskedoctet instead of a "/" as in
RFC 2317 out of concern about compatibility with existing DNS
servers, many of which do not consider "/" to be a valid character in
a hostname.
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3. Non-octet boundary delegation
In RFC 2317 there is no mechanism for non-octet boundary delegation.
Networks would be represented as being part of the domain of the next
octet.
Examples:
10.100.2.0/26 -> 0-26.2.100.10.in-addr.arpa.
10.20.128.0/23 -> 128-23.20.10.in-addr.arpa.
10.192.0.0/13 -> 192-13.10.in-addr.arpa.
In the event that the entity subnetting does not actually own the
network being subnetted on an octet break, a mechanism needs to be
available to allow for the specification of those subnets. The
mechanism is to allow the use of maskedoctet labels as delegation
shims.
For example, consider an entity A which controls a network
10.1.0.0/16. Entity A delegates to entity B the network 10.1.0.0/18.
In order to avoid having to update entries for entity B whenever
entity B updates subnetting, entity A delegates the
0-18.1.10.in-addr.arpa domain ( with an NS record in A's DNS tables
as usual ) to entity B. Entity B then subnets off 10.1.0.0/25. It
would provide a domain name for this network of
0-25.0.0-18.1.10.in-addr.arpa ( in B's DNS tables).
In order to speak about the non-octet boundary case more easily it is
useful to define a few terms.
Network domain names which do not contain any maskedoctets after the
first ( leftmost ) label are hereafter referred to as canonical
domain names for that network. 0-25.0.1.10.in-addr.arpa. is the
canonical domain name for the network 10.1.0.0/25.
Network domain names which do contain maskedoctet labels after the
first ( leftmost ) label can be reduced to a canonical domain name by
dropping all maskedoctet labels after the first ( leftmost ) label.
They are said to be reducible to the canonical network domain name.
So for example 0-25.0.0-18.1.10.in-addr.arpa. is reducible to
0-25.0.1.10.in-addr.arpa. Note that a network domain name
represents the same network as the canonical domain name to which it
can be reduced.
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4. Lookup procedure for a network given an IP address
4.1 Procedure
1. Take the initial IP address x.y.z.w and create a candidate
network by assuming a 24 bit subnet mask. Thus the initial
candidate network is x.y.z.0/24.
2. Given a candidate network of the form x.y.z.n/m create an
in-addr.arpa candidate domain name:
1. If the number of mask bits m is greater than or equal to 24
but less than or equal to 32 then the candidate domain name
is n-m.z.y.x.in-addr.arpa.
2. If the number of mask bits m is greater than or equal to 16
but less than 24 then the candidate domain name is
z-m.y.x.in-addr.arpa.
3. If the number of mask bits m is greater than or equal to 8
but less than 16 then the candidate domain name is
y-m.x.in-addr.arpa.
4. The notion of fewer than 8 mask bits is not reasonable.
3. Perform a DNS lookup for a PTR record for the candidate domain
name.
4. If the PTR records returned from looking up the candidate domain
name are of the form of a domain name for a network as defined
previously (Section 2), then for each PTR record reduce that
returned domain name to the canonical form
p1-q1.z1.y1.x1.in-addr.arpa. This represents a network
x1.y1.z.1.p1/q1.
1. If one of the x1.y1.z1.p1/q1 subnets contains the original IP
address x.y.z.w then the PTR record return becomes the new
candidate domain name. Repeat steps 3-4.
2. If none of the x1.y1.z1.p1/q1 subnets contain the original IP
address x.y.z.w then this process has failed.
5. If the PTR record(s) for the candidate network is not of the form
of a network domain name then they are presumed to be the
hostname(s) of the gateway(s) for the subnet being resolved.
6. If the PTR lookup fails ( no PTR records are returned )
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1. If no candidate network PTR lookup for this IP address has
succeeded in the past and the netmask for the last candidate
network was 24 or 16 bits long then presume a netmask of 8
fewer bits for the candidate network and repeat steps 2-4.
2. If no candidate network PTR lookup for this IP address has
succeeded in the past and the netmask for the last candidate
network was not 24 of 16 bits long, then increase the netmask
by 1 bit and repeat steps 2-4.
3. If a candidate network PTR lookup for this IP address has
succeeded in the past or the netmask of the last candidate
network was 32 bits then this process has failed.
7. Perform a DNS A record lookup for the domain name of the gateway
to determine the IP number of the gateway.
4.2 Example
Imagine we begin with the IP number 10.15.162.3.
1. Form a candidate network of 10.15.162.0/24.
2. Form a domain name 0-24.162.15.10.in-addr.arpa.
3. Lookup the PTR records for 0-24.162.15.10.in-addr.arpa.
4. Suppose the lookup fails ( no PTR records returned ), then
5. Form a new candidate network 10.15.0.0/16.
6. Form a domain name 0-16.15.10.in-addr.arpa.
7. Lookup the PTR records for 0-16.15.10.in-addr.arpa.
8. Lookup returns:
1. 0-17.15.10.in-addr.arpa.
2. 128-18.15.10.in-addr.arpa.
3. 192-18.15.10.in-addr.arpa.
9. So 10.15.0.0/16 is subnetted into 10.15.0.0/17, 10.15.128.0/18,
and 10.15.192.0/18.
10. Since 10.15.162.3 is in 10.15.128.0/18, the new candidate domain
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name is 128-18.15.10.in-addr.arpa.
11. Lookup the PTR records for 128-18.15.10.in-addr.arpa.
12. Lookup returns
1. 128-19.128-18.15.10.in-addr.arpa.
2. 0-25.160.128-18.15.10.in-addr.arpa.
3. 128-25.160.128-18.15.10.in-addr.arpa.
4. 0-24.161.128-18.15.10.in-addr.arpa.
5. 162-23.128-18.15.10.in-addr.arpa.
13. The canonical network domains for these returned records are
1. 128-19.15.10.in-addr.arpa.
2. 0-25.160.15.10.in-addr.arpa.
3. 128-25.160.15.10.in-addr.arpa.
4. 0-24.161.15.10.in-addr.arpa.
5. 162-23.15.10.in-addr.arpa.
14. So the network 10.15.128.0/18 is subnetted into 10.15.128.0/19,
10.15.160.0/25, 10.15.160.128/25, 10.15.161.0/25, 10.15.162.0/
23.
15. Since 10.15.162.3 is in 10.15.162.0/23 the new candidate domain
name is 162-23.128-18.15.10.in-addr.arpa.
16. Lookup the PTR records for 162-23.128-18.15.10.in-addr.arpa.
17. Lookup returns:
1. gw1.example.com.
2. gw2.example.com.
18. Lookup the A records for gw1.example.com. and gw2.example.com.
19. Lookup returns
1. gw1.example.com: 10.15.162.1
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2. gw2.example.com: 10.15.162.2
So the 10.15.162.3 is in network 10.15.162.0/23 which has gateways
10.15.162.1 and 10.15.162.2.
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5. Needed DNS Entries
The example in the Lookup procedure (Section 4) section would require
DNS records as follows:
In entity A's DNS zone files:
;; provide entries for the subnets of 10.15.0.0/16
0-16.15.10.in-addr.arpa. IN PTR 0-17.15.10.in-addr.arpa.
0-16.15.10.in-addr.arpa. IN PTR 128-18.15.10.in-addr.arpa.
0-16.15.10.in-addr.arpa. IN PTR 192-18.15.10.in-addr.arpa.
;; delegate a shim zone for each of the subnets
0-17.15.10.in-addr.arpa. IN NS 10.15.0.50
128-18.15.10.in-addr.arpa. IN NS 10.15.128.50
192-18.15.10.in-addr.arpa. IN NS 10.15.192.50
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In entity B's DNS zone files:
;; provide entries for the subnets of 10.15.128.0/18
128-18.15.10.in-addr.arpa. IN PTR 128-19.128-18.15.10.in-addr.arpa.
128-18.15.10.in-addr.arpa. IN PTR 0-25.160.128-18.15.10.in-addr.arpa.
128-18.15.10.in-addr.arpa. IN PTR \
128-25.160.128-18.15.10.in-addr.arpa.
128-18.15.10.in-addr.arpa. IN PTR 0-24.161.128-18.15.10.in-addr.arpa.
128-18.15.10.in-addr.arpa. IN PTR 162-23.128-18.15.10.in-addr.arpa.
;; provide entries pointing to non in-addr.arpa hostnames for
;; terminal network
162-23.128-18.15.10.in-addr.arpa. IN PTR gw1.example.com.
162-23.128-18.15.10.in-addr.arpa. IN PTR gw2.example.com.
;; provide A records for the gateways.
gw1.example.com. IN A 10.15.162.1
gw2.example.com. IN A 10.15.162.2
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6. Security Considerations
Any revelation of information to the public internet about the
internal structure of your network may make it easier for nefarious
persons to mount diverse attacks upon a network. Consequently care
should be exercised in deciding which ( if any ) of the DNS resource
records described in this draft should be made visible to the public
internet.
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References
[1] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
SPECIFICATION", RFC 1035, November 1987.
[2] Mockapetris, P., "DNS Encoding of Network Names and Other
Types", RFC 1101, April 1989.
[3] Eidnes, H., de Groot, G. and P. Vixie, "Classless IN-ADDR.ARPA
delegation", RFC 2317, March 1998.
[4] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", November 1997.
Author's Address
Edward A. Warnicke
Cisco Systems Inc.
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709-4987
US
Phone: (919) 392-8489
EMail: eaw@cisco.com
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Appendix A. Acknowledgements
The author gratefully acknowledges the assistance of Murry Gavin and
Josh Littlefield in reviewing this document.
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