you to their servers...

Odnośniki

Smutek to uczucie, jak gdyby się tonęło, jak gdyby grzebano cię w ziemi.

The local name server will then present its query for erdos to one
of these, which will nally recognize the name as belonging to its zone, and return the
corresponding IP address.
Now, this looks like a lot of trac being generated for looking up a measly IP address, but it's really only miniscule compared to the amount of data that would have to be transferred if we were still stuck with HOSTS.TXT. But there's still room for improvement with this
scheme.
To improve response time during future queries, the name server will store the informa-
tion obtained in its local cache. So the next time anyone on your local network wants to
look up the address of a host in the groucho.edu domain, your name server will not have
to go through the whole process again, but will rather go to the groucho.edu name server
directly.4
4
If it didn't, then DNS would be about as bad as any other method, because each query would involve the root name servers.
2.6. The Domain Name System
56
Of course, the name server will not keep this information forever, but rather discard it
after some period. This expiry interval is called the time to live, or TTL. Each datum in
the DNS database is assigned such a TTL by administrators of the responsible zone.
2.6.4 Domain Name Servers
Name servers that hold all information on hosts within a zone are called authoritative for this zone, and are sometimes referred to as master name servers. Any query for a host
within this zone will nally wind down at one of these master name servers.
To provide a coherent picture of a zone, its master servers must be fairly well synchro-
nized. This is achieved by making one of them the primary server, which loads its zone
information from data les, and making the others secondary servers who transfer the zone
data from the primary server at regular intervals.
One reason to have several name servers is to distribute work load, another is redundance.
When one name server machine fails in a benign way, like crashing or losing its network
connection, all queries will fall back to the other servers. Of course, this scheme doesn't protect you from server malfunctions that produce wrong replies to all DNS requests, e.g.
from software bugs in the server program itself.
Of course, you can also think of running a name server that is not authoritative for any
domain. This type of server is useful nevertheless, as it is still able to conduct DNS queries 5
for the applications running on the local network, and cache the information. It is therefore called a caching-only server.
2.6.5 The DNS Database
We have seen above that DNS does not only deal with IP addresses of hosts, but also
exchanges information on name servers. There are in fact a whole bunch of dierent types
of entries the DNS database may have.
A single piece of information from the DNS database is called a resource record, or RR
for short. Each record has a type associated with it, describing the sort of data it represents, and a class specifying the type of network it applies to. The latter accomodates the needs of dierent addressing schemes, like IP addresses (the IN class), or addresses of Hesiod
networks (used at MIT), and a few more. The prototypical resource record type is the A
record which associates a fully qualied domain name with an IP address.
5
Well, almost. A name server at least has to provide name service for localhost and reverse lookups of 127.0.0.1.
2.6. The Domain Name System
57
Of course, a host may have more than one name. However, one of these names must
be identied as the ocial, or canonical host name, while the others are simply aliases
referring to the former. The dierence is that the canocical host name is the one with an
A record associated, while the others only have a record of type CNAME which points to
the canonical host name.
We will not go through all record types here, but save them for a later chapter, but
rather give you a brief example here. Figure 2.4 shows a part of the domain database that
is loaded into the name servers for the physics.groucho.edu zone.
;
; Authoritative Information on physics.groucho.edu
@
IN
SOA
{
niels.physics.groucho.edu.
hostmaster.niels.physics.groucho.edu.
1034
; serial no
360000
; refresh
3600
; retry
3600000
; expire
3600
; default ttl
}
;
; Name servers
IN
NS
niels
IN
NS