Cache DNS Server Selection on the Dual-stack Home Network

When clients or CEs know several IP addresses of IPv4 DNS servers and IPv6 DNS servers in the home network environment, there is no rule in a cache DNS server selection method that clients or CEs refer to. The selection method depends on implementations of clients or CEs. This situation causes some problems as noted at . This document describes five methods for solving these problems by modifying only clients and/or CEs. By adopting these methods to clients or CEs, it is possible to optimize the procedure to resolve names in this situation. Finally, two methods are recommended.

Equipment which is installed in a user's home network such as Broadband Router or Home Gateway. The basic features are required by , and the design considerations are described in . Cache DNS server which has A RR and AAAA RR, and accept IPv4 transport only. Cache DNS server which has A RR and AAAA RR, and accept IPv6 transport only.

This document picks up two typical topologies as dual-stack home network.

A client PC connects directly to the both IPv4 and IPv6 Internet. This topology is composed without CE.

Home network topology without CE

A client PC connects to both IPv4 and IPv6 Internet through a CE.

Home network topology with CE In this network, CE works as a proxy DNS server for clients. Other configurations are same as .

Clients or CEs always refer to a cache DNS server. Usually the address of the cache DNS server is set by DHCPv4 and DHCPv6, RA or manually. At that time, clients or CEs know two or more different DNS servers' addresses of IPv4 and IPv6. There are no standards or guidelines that indicates which DNS server is referred to and how it is selected on the clients or CEs. Thus there are some implementations of DNS selection method on the clients or CEs. The viewpoints of the problems are described as follows.

It is not efficient for vendors to implement all the options. Users need to select one option if vendor implements many options, and it's not realistic to require users to select one option.

When there are several options to select a cache DNS server on clients and CEs, ISPs or network operators have to adjust each option to operate comfortably. If the queries go to one cache DNS server, the load of the DNS servers is on one side. If CEs send queries to both IPv4 DNS and IPv6 DNS, ISP will receive twice as many queries as current.

When a user accesses to the same target by some clients, the behaviors are different according to the implementations of clients or CEs. It is a critical issue for application developers.

CDN providers control their own traffic by returning different address of contents servers for every cache DNS server. When the queries from CEs are on one side, CDN providers cannot control the contents delivery traffic well.

|cache DNS| -> | | www.example.com +------+ <= +---------+ <= | | S1 S1 | | +----+ +---------+ | | | S1 | |cache DNS| -> | | +----+ +---------+ <= | contents | S2 | DNS | +----+ +------+ +---------+ | | | S2 | |client| -> |cache DNS| -> | | +----+ +------+ <= +---------+ <= | | S1 S1 | | +----+ +---------+ | | | S3 | |cache DNS| -> | | +----+ +---------+ <= +-----------+ S3 -> : query <= : answer Sx : contents server ]]> Traffic control of using DNS on CDN

When a client's network is adapted for IPv6 and an ISP doesn't have global reachability to the IPv6 Internet because of some reasons, IPv6/IPv4 TCP fallback problem which is stated at and occurs. For example, assuming a scenario, one client doesn't have a connectivity to the IPv6 Internet, and a harmful node advertises rogue RA to the client. At that time, the client gets IPv6 global prefix and tries to connect to the IPv6 Internet by using IPv6. But it cannot access to the IPv6 Internet, then the client falls back to IPv4. If taking a method which doesn't answer AAAA record when a client has no connectivity to the IPv6 Internet, the IPv6/IPv4 TCP fallback problem doesn't occur in any environment. To solve this problem, CEs will return an answer only available transport records to users. By returning an answer only available transport, clients decline to access to the destination on the transport which doesn't have a connectivity to the Internet.

In this section, five different options are described as a method of referring to a cache DNS server to solve the problems.

The referred cache DNS server is fixed to IPv4 DNS regardless of the query type which is A or AAAA. When clients refer to an IPv4 cache DNS server, also fix the using transport protocol as IPv4. This function will be implemented on clients or CEs. It is not a problem to send an AAAA query to a cache DNS server on the IPv4 transport. Pros: The behavior is simple, troubleshooting is easier than other options generally. Cons: The mismatch between RR type and transport arises. When a client refers IPv6 query to an IPv4 DNS, some troubles on the IPv4 network influence the IPv6 network. This method is against deployment of IPv6 in the world.

The referred DNS server is fixed to IPv6 DNS regardless of the query type which is A or AAAA. When clients refer to an IPv6 cache DNS server, also fix the using transport protocol as IPv6. This function will be implemented on clients or CEs. It is not a problem that a resolver send A query to a cache DNS server over the IPv6 transport. Pros: The behavior is simple, troubleshooting is easier than other options generally. Cons: The mismatch between RR type and transport arises. When a client refers IPv4 query to an IPv6 DNS, some troubles on the IPv6 network influence the IPv4 network. There are bad influences to users who don't have IPv6 connectivity to the IPv6 Internet.

Clients and CEs send query to both IPv4 and IPv6 cache DNS servers at same time. When clients or CEs send to IPv4 DNS, use IPv4 transport. When clients or CEs send to IPv6 DNS, use IPv6 transport. The answer which returns faster is used for a client. This function will be implemented on clients or CEs. Pros: The response time is to be faster. IPv6/IPv4 TCP fallback problem will be solved. Cons: The number of queries increases twice. Referred DNS server is not constant.

The transport protocol from a CE to a cache DNS server depends on the query type from clients to a CE. The CE maps query type onto a transport protocol toward a cache DNS server. When the query type from a client is A RR, the CE uses IPv4 transport to the IPv4 cache DNS server. When the query type from a client is AAAA RR, the CE uses IPv6 transport to the IPv6 cache DNS server. This function will be implemented on CEs. Pros: IPv6/IPv4 TCP fallback problem will be solved. Cons: Other query types need to be considered about to be able to handle them. IPv6 network topology is not same as IPv4 network, so it is not appropriate for depending on the query type. The relationship between a query and a transport protocol is stated in Section 1: The IP protocol version used for querying resource records is independent of the protocol version of the resource records; e.g., IPv4 transport can be used to query IPv6 records and vice versa. A query type and a transport protocol are independent, and it is not a problem that a transport protocol depends on a query type.

DNS server which is selected depends on the transport protocol from clients to CEs. When the transport is IPv4, CE selects IPv4 DNS server. When the transport is IPv6, CE selects IPv6 DNS server. Pros: A client and a CE select same DNS server. Cons: The mismatch between RR type and transport arises. When ISP networks become native IPv6, but home network is still IPv4, user cannot access to the destination. All queries from clients which doesn't have IPv6 transport for DNS, like Windows XP, go to IPv4 DNS.

According to above discussion, this document recommends two selection methods of cache DNS servers. First method, which is noted at , is that a CE sends queries to both IPv4 and IPv6 cache DNS servers at same time. Second recommended method is noted at , which indicates that a CE selects a cache DNS server depends on query type from a client.

There are no new security considerations.

There are no IANA considerations.