How DNS works

named queries use UDP protocol and port 53. Responses are returned via UDP unless they are greater than 512K, which uses TCP. Both the server's "zone transfer" use TCP.

Delegation: All name servers know the root server. The root server knows the gTLD, and goes down this chain. Edu knows fudan.edu, com knows china.com... and so on. Example: Suppose I want to query the machine cs.military.china.com from the machine marco.comp.fudan.edu, which assumes that there is no cache of any information needed here other than the name and IP address of the root server before the query. The order is this (host marco asks its local name server ns.comp.fudan.edu, and ns.comp.fudan.edu is a recursive name server, so it will be queried instead of host marco. Since it doesn't know This address, and I don't know anything about military.china.com or china.com or even .com. But it knows some servers in the root domain, the root domain knows about .com data, so send a reference to ns.comp .fudan.edu, the local name server will then send the query to .com, return a recommendation of china.com, and repeat the query to china.com, return to the recommendation of the military.china.com server, so military.china.com For this query information is authoritative, and returns the address of cs. This completes the whole process, oh, no, there is a point, ns.comp.fudan.edu will tell the cache host cs address in the process, while Cache a list of servers on .com, china.com, military.china.com.

Cache and efficiency: The cache mechanism was only used for affirmative answers in the past, if the hostname could not be found This is the case, and RFC 1034 and RFC 2308 have defined and upgraded the scheme for negating the cache. It is an optional feature in BIND 8.2 and has become a mandatory feature in 9. It will save the negative type of response (the first two types of negative data are cached for 1-3 hours, other types are cached for 5 minutes):

There is no host or domain that matches the name to be queried < Br>

The host does not have the data type to be queried

The server that is queried is not responding

The server is unreachable due to network problems

Extended DNS protocol: 90 At the end of the decade, EDNS0 (Extended DNS, Version 0) solved some of the shortcomings of the DNS protocol in the Internet today. It allows speakers to announce that their assembly buffers are laughing, and support options and communication protocol versions. If the recipient's name server is In response to an error message, the sender then returns to use the original DNS protocol. BIND9 implements EDNS0 in both the server and the parser