[ Pobierz całość w formacie PDF ]
.com.host 3600 IN A6 0 3ffe:8050:201:1860:42::14.8.2.1.A6 ChainsA6 records are designed to allow network renumbering.This works when an A6 record only specifies thepart of the address space the domain owner controls.For example, a host may be at a company named"company." It has two ISPs which provide IPv6 address space for it.These two ISPs fully specify theIPv6 prefix they supply.In the company s address space:$ORIGIN example.com.host 3600 IN A6 64 0:0:0:0:42::1 company.example1.net.host 3600 IN A6 64 0:0:0:0:42::1 company.example2.net.ISP1 will use:$ORIGIN example1.net.company 3600 IN A6 0 3ffe:8050:201:1860::ISP2 will use:$ORIGIN example2.net.company 3600 IN A6 0 1234:5678:90ab:fffa::Whenhost.example.comis looked up, the resolver (in the resolver daemon or caching name server)will find two partial A6 records, and will use the additional name to find the remainder of the data.4.8.2.2.A6 Records for DNS ServersWhen an A6 record specifies the address of a name server, it should use the full address rather thanspecifying a partial address.For example:$ORIGIN example.com.@ 14400 IN NS ns014400 IN NS ns134Chapter 4.Advanced Conceptsns0 14400 IN A6 0 3ffe:8050:201:1860:42::1ns1 14400 IN A 192.168.42.1It is recommended that IPv4-in-IPv6 mapped addresses not be used.If a host has an IPv4 address, use anA record, not an A6, with::ffff:192.168.42.1as the address.4.8.3.Address to Name Lookups Using Nibble FormatWhile the use of nibble format to look up names is deprecated, it is supported for backwardscompatiblity with existing IPv6 applications.When looking up an address in nibble format, the address components are simply reversed, just as inIPv4, andip6.int.is appended to the resulting name.For example, the following would providereverse name lookup for a host with address3ffe:8050:201:1860:42::1.$ORIGIN 0.6.8.1.1.2.5.8.e.f.f.3.ip6.int.1.2.4.0 14400 IN PTR host.example.com.4.8.4.Address to Name Lookups Using Bitstring FormatBitstring labels can start and end on any bit boundary, rather than on a multiple of 4 bits as in the nibbleformat.They also use ip6.arpa rather than ip6.int.To replicate the previous example using bitstrings:$ORIGIN \[x3ffe805002011860/64].ip6.arpa.\[x0042000000000001/64] 14400 IN PTR host.example.com.4.8.5.Using DNAME for Delegation of IPv6 Reverse AddressesIn IPV6, the same host may have many addresses from many network providers.Since the trailingportion of the address usually remains constant, DNAME can help reduce the number of zone files usedfor reverse mapping that need to be maintained.For example, consider a host which has two providers (example.netandexample2.net) andtherefore two IPv6 addresses.Since the host chooses its own 64 bit host address portion, the provideraddress is the only part that changes:$ORIGIN example.com.35Chapter 4.Advanced Conceptshost IN A6 64 ::1234:5678:1212:5675 cust1.example.net.IN A6 64 ::1234:5678:1212:5675 subnet5.example2.net.$ORIGIN example.net.cust1 IN A6 48 0:0:0:dddd:: ipv6net.example.net.ipv6net IN A6 0 aa:bb:cccc::$ORIGIN example2.net.subnet5 IN A6 48 0:0:0:1:: ipv6net2.example2.net.ipv6net2 IN A6 0 6666:5555:4::This sets up forward lookups.To handle the reverse lookups, the providerexample.netwould have:$ORIGIN \[x00aa00bbcccc/48].ip6.arpa.\[xdddd/16] IN DNAME ipv6-rev.example.com.andexample2.netwould have:$ORIGIN \[x666655550004/48].ip6.arpa.\[x0001/16] IN DNAME ipv6-rev.example.com.example.comneeds only one zone file to handle both of these reverse mappings:$ORIGIN ipv6-rev.example.com.\[x1234567812125675/64] IN PTR host.example.com.36Chapter 5.The BIND 9 Lightweight Resolver5.1.The Lightweight Resolver LibraryTraditionally applications have been linked with a stub resolver library that sends recursive DNS queriesto a local caching name server.IPv6 introduces new complexity into the resolution process, such as following A6 chains and DNAMErecords, and simultaneous lookup of IPv4 and IPv6 addresses.These are hard or impossible toimplement in a traditional stub resolver.Instead, BIND 9 provides resolution services to local clients using a combination of a lightweightresolver library and a resolver daemon process running on the local host.These communicate using asimple UDP-based protocol, the "lightweight resolver protocol" that is distinct from and simpler than thefull DNS protocol.5.2.Running a Resolver DaemonTo use the lightweight resolver interface, the system must run the resolver daemon lwresd.By default, applications using the lightweight resolver library will make UDP requests to the IPv4loopback address (127.1) on port 921.The address can be overridden by lwserver lines in/etc/resolv.conf.The daemon will try to find the answer to the questions "what are the addressesfor hostfoo.example.com?" and "what are the names for IPv4 address 10.1.2 [ Pobierz całość w formacie PDF ]