Internet DRAFT - draft-bill-dnsop-source-address-selection
draft-bill-dnsop-source-address-selection
INTERNET-DRAFT Declan Ma, Ed.
Intended Status: Proposed Standard zDNS Ltd.
Expires: 2015-10-15 2015-05-22
Multi-homed DNS Server Reply Source Address Selection
draft-bill-dnsop-source-address-selection-00
Abstract
RFC 2181 collected eight independent considerations and created a single
docuement to address each of them in turn. Over the following two decades
it has become clear that each of these items should be considered and evovolve
in its own right, as suggested in RFC 2181. This document extracts the exact
text from RFC 2181 and places it into its own track.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at
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Copyright and License Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 UDP Source Address Selection . . . . . . . . . . . . . . . . . 3
4 Port Number Selection . . . . . . . . . . . . . . . . . . . . . 3
5 Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6 References . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 4
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1 Introduction
Most, if not all, DNS clients, expect the address from which a reply
is received to be the same address as that to which the query
eliciting the reply was sent. This is true for servers acting as
clients for the purposes of recursive query resolution, as well as
simple resolver clients. The address, along with the identifier (ID)
in the reply is used for disambiguating replies, and filtering
spurious responses. This may, or may not, have been intended when
the DNS was designed, but is now a fact of life.
Some multi-homed hosts running DNS servers generate a reply using a
source address that is not the same as the destination address from
the client's request packet. Such replies will be discarded by the
client because the source address of the reply does not match that of
a host to which the client sent the original request. That is, it
appears to be an unsolicited response.
This document is intended to describe IP packet header address usage
from multi-homed DNS servers.
2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3 UDP Source Address Selection
To avoid these problems, servers when responding to queries using UDP
must cause the reply to be sent with the source address field in the
IP header set to the address that was in the destination address
field of the IP header of the packet containing the query causing the
response. If this would cause the response to be sent from an IP
address that is not permitted for this purpose, then the response may
be sent from any legal IP address allocated to the server. That
address should be chosen to maximise the possibility that the client
will be able to use it for further queries. Servers configured in
such a way that not all their addresses are equally reachable from
all potential clients need take particular care when responding to
queries sent to anycast, multicast, or similar, addresses.
4 Port Number Selection
Replies to all queries must be directed to the port from which they
were sent. When queries are received via TCP this is an inherent
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part of the transport protocol. For queries received by UDP the
server must take note of the source port and use that as the
destination port in the response. Replies should always be sent from
the port to which they were directed. Except in extraordinary
circumstances, this will be the well known port assigned for DNS
queries [RFC1700].
5 Security Considerations
It is not believed that anything in this document adds to any
security issues that may exist with the DNS, nor does it do anything
to that will necessarily lessen them. Correct implementation of the
clarifications in this document might play some small part in
limiting the spread of non-malicious bad data in the DNS, but only
DNSSEC can help with deliberate attempts to subvert DNS data.
6 References
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
October 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7 Authors' Addresses
Declan Ma, Ed.
ZDNS Ltd.
4, South 4th Street, Zhongguancun,
Haidian, Beijing 100190,
China
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