ICMP Domain Names: ICMP Domain Name Messages
Two ICMP messages that ask an IP address directly for its own domain name.
Simpson, W., "ICMP Domain Name Messages", RFC 1788, DOI 10.17487/RFC1788, April 1995, <https://www.rfc-editor.org/info/rfc1788>.
@misc{rfc1788,
series = {Request for Comments},
number = 1788,
howpublished = {RFC 1788},
publisher = {RFC Editor},
doi = {10.17487/RFC1788},
url = {https://www.rfc-editor.org/info/rfc1788},
author = {W. Simpson},
title = {{ICMP Domain Name Messages}},
pagetotal = 7,
year = 1995,
month = apr,
} RFC text
Contents
Network Working Group W. Simpson
Request for Comments: 1788 Daydreamer
Category: Experimental April 1995
ICMP Domain Name Messages
Status of this Memo
This document defines an Experimental Protocol for the Internet
community. This does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
IESG Note:
An Internet Engineering Steering Group comment from the co-Area
Director for IPng: Please note well that this memo is an individual
product of the author. It presents one view of the IN-ADDR
mechanism, motivated by discussion in the IPNG WG of the difficulty
of secure, dynamic update of the reverse tree. Other IETF discussion
and ongoing standards work on this area will be found in the IP Next
Generation (ipngwg), DNS IXFR, Notification, and Dynamic Update
(dnsind), DNS Security (dnssec) working groups.
Abstract
This document specifies ICMP messages for learning the Fully
Qualified Domain Name associated with an IP address.
Table of Contents
1. Introduction .......................................... 2
1.1 Direct Query .................................... 3
1.2 Multicast ....................................... 3
1.3 Domain Names .................................... 3
1.4 Messages ........................................ 4
2. Domain Name Request ................................... 4
3. Domain Name Reply ..................................... 5
SECURITY CONSIDERATIONS ...................................... 6
REFERENCES ................................................... 6
ACKNOWLEDGEMENTS ............................................. 7
AUTHOR'S ADDRESS ............................................. 7
1. Introduction
The Domain Name System (DNS) is described in [RFC-1034]. The IN-ADDR
domain of the DNS is specified [RFC-1035] to perform address to
domain name resolution, and to facilitate queries to locate all
gateways (routers) on a particular network in the Internet.
Neither function has been remarkably successful. The IN-ADDR domain
is not reliably populated.
As multiple routers were used at boundaries and within networks, the
IN-ADDR mechanism was found to be inadequate. The location of
routers by hosts is now performed using "ICMP Router Discovery
Messages" [RFC-1256].
As network numbers migrated to "classless" routing and aggregation,
the IN-ADDR delegation granularity has fragmented, and requires
overlapping administration. The "reverse" IN-ADDR administration
frequently does not follow the same delegation as the "forward"
domain name tree. This structure is not amenable to cooperative
secure updating of the DNS.
As application servers have appeared which require the Domain Name
for user interaction and security logging, the IN-ADDR servers have
been inundated with queries. This produces long user visible pauses
at the initiation of sessions.
1.1. Direct Query
This document proposes that each unicast address be queried directly
for its corresponding Domain Name. This has the advantages that the
naming is under the same administration as the address assignment,
and the queries are distributed in the same fashion as IP routing.
In effect, the routing is used to index the database.
1.2. Multicast
Only a few well-known multicast addresses are populated in the IN-
ADDR domain. The ephemeral nature of most multicast addresses is not
conducive to cooperative secure updating of the DNS.
However, the technique described here is not useful for multicast
addresses. A query to a multicast address could result in a storm of
replies. Most multicast groups are not named, or the member nodes
are not configured with the name.
The IN-ADDR method SHOULD continue to be used for reverse lookup of
well-known multicast addresses in the range 224.0.0.0 to
224.0.255.255. Other multicast addresses are an issue for futher
study.
1.3. Domain Names
Each Domain Name is expressed as a sequence of labels. Each label is
represented as a one octet length field, followed by that number of
octets. Since every Domain Name ends with the null label of the
root, a Domain Name is terminated by a length byte of zero. The most
significant two bits of every length octet must be '00', and the
remaining six bits of the length field limit the label to 63 octets
or less.
When the most significant two bits of the length octet are '11', the
length is interpreted as a 2 octet sequence, indicating an offset
from the beginning of the message (Type field). Further details are
described in [RFC-1035] "Message Compression".
To simplify implementations, the total length of a Domain Name
(including label octets and label length octets) is restricted to 255
octets or less.
1.4. Messages
The datagram format and basic facilities are already defined for ICMP
[RFC-792].
Up-to-date values of the ICMP Type field are specified in the most
recent "Assigned Numbers" [RFC-1700]. This document concerns the
following values:
37 Domain Name Request
38 Domain Name Reply
2. Domain Name Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 37
Code 0
Checksum The ICMP Checksum.
Identifier If Code is zero, a value to aid in matching requests
and replies. For example, it might be used like a
port in TCP or UDP to identify a session. May be
zero.
Sequence Number If Code is zero, a value to aid in matching requests
and replies. For example, the number might be
incremented on each request sent. May be zero.
A separate Domain Name Request is used for each IP Destination
queried.
An ICMP Domain Name Request received with a broadcast or multicast
Destination MUST be silently discarded.
On receipt of an ICMP error message, the implementations MAY attempt
to resolve the Domain Name using the IN-ADDR method.
3. Domain Name Reply
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time-To-Live |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Names ...
+-+-+-+-+-+-+-+-
Type 38
Code 0
Checksum The ICMP Checksum.
Identifier Copied from the request.
Sequence Number Copied from the request.
Time-To-Live The number of seconds that the name may be cached.
For historic reasons, this value is a signed 2s-
complement number.
Names zero or more Fully Qualified Domain Names. The
length of this field is determined from the total
length of the datagram.
When no names are known, the field is eliminated
(zero length), but the Reply is sent as an
authoritative indication that no name is known.
When more than one name is known, all such names
SHOULD be listed.
Any name which cannot entirely fit within the Reply
MTU is not sent.
The IP Source in a Reply MUST be the same as the IP Destination of
the corresponding Request message.
Every host and router MUST implement an ICMP Domain Name server
function that receives Domain Name Requests and sends corresponding
Domain Name Replies.
A host SHOULD also implement an application- layer interface for
sending a Domain Name Request and receiving a Domain Name Reply, for
diagnostic purposes.
Security Considerations
A primary purpose of this specification is to provide a mechanism for
address to name resolution which is more secure than the IN-ADDR
reverse tree. This mechanism is amenable to use of the IP Security
Protocols for authentication and privacy.
Although the routing infrastructure to the Destination does not
provide security in and of itself, it is as least as reliable as
delivery of correspondence for the other sessions with the same peer.
A DNS cryptographic signature, located by using the reply in the
forward DNS direction, can be used to verify the reply itself.
References
[RFC-792]
Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, USC/Information Sciences Institute, September
1981.
[RFC-1034]
Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, USC/Information Sciences Institute,
November 1987.
[RFC-1035]
Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, USC/Information
Sciences Institute, November 1987.
[RFC-1256]
Deering, S., Editor, "ICMP Router Discovery Messages",
RFC 1256, Xerox PARC, September 1991.
[RFC-1700]
Reynolds, J., and J. Postel, "ASSIGNED NUMBERS", STD 2,
RFC 1700, USC/Information Sciences Institute, October 1994.
Acknowledgements
The DNSIND and IPng Working Groups contributed substantial amounts of
discussion.
Additional comments should be submitted to the
namedroppers@internic.net mailing list.
Author's Address
Questions about this memo can also be directed to:
William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071
Bill.Simpson@um.cc.umich.edu
bsimpson@MorningStar.com
AI ELI5
On this page
§1Why Reverse Lookup Needed a Change
The IN-ADDR domain of the DNS was specified to turn an address into a domain name, and to help find all gateways on a network. The memo says neither function has been remarkably successful, and gives several reasons.
- The IN-ADDR domain is not reliably populated.
- Routers are now found with ICMP Router Discovery Messages (RFC 1256) instead.
- With classless routing and aggregation, the IN-ADDR delegation has fragmented and needs overlapping administration. Reverse administration often does not follow the forward tree, which is not suited to cooperative secure updating.
- Application servers that want the name for user interaction and security logging have inundated IN-ADDR servers, causing long user-visible pauses when sessions start.
An IESG note says this is an individual product of the author and shows one view of the problem, not a working group result.
The big phone book that turns numbers into names was often out of date and too busy. So this idea was to ask the house itself what its name is.
§2Direct Query Instead of a Tree
The proposal is that each unicast address be queried directly for its domain name. The naming is then under the same administration as the address assignment, and the queries are spread out the same way IP routing is. In effect, routing is used to index the database.
It is not useful for multicast addresses, because a query to a multicast address could cause a storm of replies and most groups are not named. A request received with a broadcast or multicast destination MUST be silently discarded. The IN-ADDR method SHOULD continue to be used for the well-known multicast range 224.0.0.0 to 224.0.255.255. Other multicast addresses are left for further study.
Instead of looking up the number in a book, you knock on the right door. You do not knock on a whole crowd of doors at once, because everyone would answer together.
§3How Names Are Written
A domain name is a series of labels. Each label is a one-octet length followed by that many octets, and the name ends with a zero length byte for the root. The top two bits of a length octet are 00, so a label is 63 octets or less. When the top two bits are 11, the length is a 2-octet offset used for message compression as described in RFC 1035. The whole name, including length bytes, is limited to 255 octets to simplify implementations.
Each part of a name is written as a count followed by the letters, and a count of zero says the name is finished.
§4The Request and Reply Messages
The messages use the normal ICMP datagram format, with Type 37 for the Domain Name Request and Type 38 for the Domain Name Reply. A request has Type, Code (0), Checksum, Identifier and Sequence Number. The identifier and sequence number help match requests to replies, and may be zero. A separate request is used for each destination queried.
The reply has the same fields, with the identifier and sequence number copied from the request, then a Time-To-Live and zero or more fully qualified names. The TTL is the number of seconds the name may be cached, and for historic reasons it is a signed two's-complement number. If no name is known the names field is empty, but the reply is still sent as an authoritative statement that no name is known. If several names are known, all SHOULD be listed, and a name that does not fit entirely within the reply MTU is not sent.
The IP source of a reply MUST equal the IP destination of the request. Every host and router MUST implement the server function, and a host SHOULD offer an application interface for diagnostics. After an ICMP error, an implementation MAY fall back to the IN-ADDR method.
You send a small note saying what is your name, and the house sends back its name or names, and how long you may remember them. If it has no name, it says so clearly.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 37 | Always 37. |
CODE | Code | 1 byte | 0 | Zero. |
CHECKSUM | Checksum | 2 bytes | The ICMP checksum. | |
IDENTIFIER | Identifier | 2 bytes | Helps match requests and replies; may be zero. | |
SEQUENCE | Sequence Number | 2 bytes | Helps match requests and replies; may be zero. |
- Domain Name Request (37)Sent to the unicast address being asked about.
- Domain Name Reply (38)Carries a TTL and zero or more names.
§5Security
A main purpose is to give address-to-name resolution that is more secure than the IN-ADDR reverse tree, and the mechanism can use the IP Security Protocols for authentication and privacy. The routing path does not itself provide security, but it is at least as reliable as delivery of other traffic with the same peer. A DNS cryptographic signature, found by using the reply in the forward DNS direction, can be used to verify the reply.
You can check the answer by looking the name up the normal way and making sure it points back to the same house.