DNS
Domain Name System(域名系统)
- distributed database implemented in hierarchy of many name servers
- application-layer protocol host, routers, name servers to communicate to resolve names(address/name translation)
- NOTE: core Internet function implemented as application-layer protocol
- complexity at network's "edge"
No centralized DNS
- single point of failure
- traffic volume
- distant centralized database
- maintenance
- no server has all name-to-IP address mappings
local name servers
- each ISP, company has local(default) name server
- host DNS query first goes to local name server
authoritative name server
- for a host: stores that host‟s IP address, name
- can perform name/address translation for that host‟s name
Root name server
- contacted by local name server that can not resolve name
- features
- contacts authoritative name server if name mapping not known
- gets mapping
- returns mapping to local name server
- dozen root name servers worldwide
Query
- recursive query
- puts burden of name resolution on contacted name server
- heavy load?
- iterated query
- contacted server replies with name of server to contact
- I don‟t know this name, but ask this server
caching and updating records
- once (any) name server learns mapping, it caches mapping
- cache entries timeout(disappear) after some time
- update/notify mechanisms under design by IETF
- RFC 2136
records
- distributed database storing Resource Records(RR)
- RR format: (name, value, type, ttl)
| type | name | value |
|---|---|---|
| A | hostname | IP address |
| NS | domain | IP address of authoritative name server for this domain |
| CNAME | an alias name for some "canonical"(the real) name | canonical name |
| MX | what the hostname is associated with | hostname of mailserver |
DNS protocol
query and reply messages, both with the same message format
- header(12 bytes)
- identification(2 bytes): uses the same for query, reply to query
- flags
- query or reply
- recursion desired
- recursion available
- reply is authoritative
- number of questions
- number of answer RRs
- number of authority RRs
- number of additional RRs
- body(all parts are variable)
- questions: name, type fields for a query
- answers: RRs in reponse to query
- authority: records for authoritative servers
- additional information: additional "helpful" info that may be used