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bind_intro(7)
NAME
bind_intro - Introduction to the Berkeley Internet Name Domain (BIND)
service
DESCRIPTION
The Berkeley Internet Name Domain (BIND) service is a host name and address
lookup service for the Internet network. The BIND service is based on the
client-server model. It allows client systems to obtain host names and
addresses from BIND servers. In the Tru64 UNIX system, BIND is used to
distribute only the hosts database.
Note
Depending on which naming services your Local Area Network (LAN) is
running, the hosts file can be located in the /etc, /var/yp/src, or
/etc/namedb/src directory.
You can use the BIND service to replace or supplement the host table
mapping provided by the local /etc/hosts file or Network Information
Service (NIS). The BIND service is composed of a software interface
(called the resolver) and a server.
The software interface consists of a group of routines that reside in the
/usr/lib/libc.a C library. The resolver exchanges query packets with a
BIND server.
All BIND servers run a name server daemon, named, which services queries on
a given network port. The standard port for User Datagram Protocol (UDP)
and Transmission Control Protocol (TCP) is specified in the /etc/services
file.
To understand how the BIND service works, you must be familiar with
Internet Protocol (IP) addressing.
The BIND service divides the Internet into a hierarchy of domains, similar
to a tree structure. Each domain is given a label. The name of the domain
is the concatenation of all the labels of the domains, from the root to the
current domain, listed from right to left and separated by periods (.).
For example, xyz.abc.com. A label must be unique within its domain.
The entire BIND Internet hierarchy is partitioned into several zones, each
starting at a domain and extending down to the leaf domains (individual
host names), or to domains where other zones start. A zone is a subdivision
of a domain and is a discrete, nonoverlapping entity. Each zone is an area
of authority for which a master server is responsible. (See the section on
Master Servers for a discussion of master servers.) Zones usually
represent an administrative boundary.
The BIND hierarchy in the United States contains eight top-level domains
shown in the following table:
_________________________________________________________
Domain Description
_________________________________________________________
arpa The Arpanet (gradually being phased out)
com Commercial institutions
edu Educational institutions
gov U.S. government
int
International (Treaty) Organizations. This
domain currently holds the IPv6 subdomain
(ip6.int) for reverse AAAA IPv6 records.
mil Military organizations
net
Network-type organizations, such as network
centers centers, consortia, and information
centers
org
Miscellaneous organizations, such as
professional societies and similar nonprofit
organizations
_________________________________________________________
In addition to these, there are several top-level domains for individual
countries. Contact the American Registry for Internet Numbers (ARIN) for
more information about them. See Network Administration: Connections for
information on contacting ARIN.
Assuming a host name in the domain cities.dec.com is chicago, the following
is the fully qualified domain name for that host:
chicago.cities.dec.com.
In this example, com is the top level domain, cities.dec.com is a subdomain
of com, and chicago is a host name. The zone, dec.com, has one primary
server and consists of multiple domains.
The period (.) at the end of the domain name indicates that the domain name
is fully qualified and is absolute. No further BIND extensions are to be
appended to the name.
BIND Servers
A BIND server is a system running the named daemon. BIND servers perform
the following functions:
· Store information locally
· Process requests that cannot be satisfied locally
· Forward queries about top-level domains
Servers maintain databases containing information about host names and
addresses. When client systems need information they do not have, they ask
the servers for it.
The BIND service has the following types of servers:
· Master
· Slave
· Stub
· Caching-only
· Forward-only
Note
Documentation for BIND prior to Version 8.1.1 referred to the master
server as a primary server and the slave server as a secondary server.
Though the terminology has changed, master and slave servers are still
referred to as having primary and secondary authority, respectively,
for zones.
Master Servers
A master server is the authority for a zone, and maintains the zone's BIND
databases. A zone can include one or more domains.
A master server loads its database from a file on disk. This server can
also delegate to other servers in its zone the authority to answer queries
for its domain space.
One type of master server is a root server, which knows about all the top-
level domains on the Internet network. From these top-level domains,
information is gathered about hosts on subdomains. The root servers, for
example, do not necessarily know about the cities.dec.com subdomain. But
they do know which server to contact for the information.
If a client requests information about a host in a domain other than its
own, any server (other than a forward-only server) can pass along the
request to a root server.
The list of root servers changes periodically, and you should periodically
update the root servers from ARIN. See Network Administration: Connections
for information on contacting ARIN.
Slave Servers
A slave server receives its authority and its database from the master
server. When a slave server boots, it loads the data for the zone from a
backup file, if possible (assuming you configured your BIND service this
way). It then consults a master server to check that the database is still
up to date.
Once the slave server is running, it waits for the master server to
indicate that the database files have been updated. When a change occurs,
the slave server updates its local database files appropriately.
A server can be the master server for some domains and a slave server for
others. It is recommended that each BIND domain have at least one master
and one or more slave servers. The slave servers act as backup servers in
the event that the master server fails, is overloaded, or is down.
Stub Servers
A stub server is like a slave server, except that it does not retain any
information in its configuration files about the machines in a specified
subzone. It is implemented to allow the administrator for a given subzone
to change the configuration of the subzone without affecting the
configuration file on the master server.
The master server becomes a stub server for the subzone by delegating
authority for it to a server local to the subzone. Instead of searching
the master DNS database, the master server queries the local server for
information about machines in the subzone.
Caching-only Servers
All servers cache the information they receive for use until the data
expires. However, caching-only servers have no authority for any zone, so
they have no databases to maintain. These servers service BIND queries by
asking other servers who have authority, such as a master server, for the
information. Caching-only servers store the information until it expires.
The expiration is based on a time-to-live (ttl) field, which is attached to
the data when the caching server receives it.
Forward-only Servers
Forward-only servers do not have access to the Internet, so they cannot
interact directly with root servers to get information that is not in their
local cache. Instead, they use forwarders, which can be either master or
slave servers, to resolve their queries. These forwarders are able to
obtain information not held in their local caches from servers in other
zones, including root servers.
A forward-only server forwards queries to the list of forwarders specified
in its configuration (boot) file, until the list is exhausted or the query
is satisfied. As forward-only servers request new information from
forwarders, they accumulate it in their cache. Forward-only servers do not
receive complete zones from master servers, like slave servers do; they
accumulate data per request.
Because forwarders receive many requests from forward-only servers, they
tend to have a larger local cache than forward-only servers. All the hosts
on the domain benefit from this larger cache, which reduces the total
number of queries from that site to servers outside the domain. For this
reason, a LAN is typically set up so all systems forward their requests to
a caching server.
BIND Clients
A BIND client is any system that uses the BIND service to resolve host
names and addresses. BIND clients make queries, but they never resolve them
locally. Instead, BIND servers resolve the clients' requests.
BIND clients do not run the named daemon. Instead, BIND clients have a
resolver file, /etc/resolv.conf, which tells the resolver the IP address of
the BIND servers that can service the client's BIND requests. The
following is an example of a /etc/resolv.conf file:
domain dec.com
nameserver 128.11.22.33
nameserver 128.11.22.44
Resolving Queries
The following steps describe how a BIND query is resolved. In this case, an
application on a slave server generates a query for a host name and
address. The process is similar for other servers.
1. An application requests host name resolution and uses the
gethostbyname library routine.
2. The gethostbyname library routine looks at the /etc/svc.conf file to
determine which service to use to resolve the query. If the routine
has local BIND, it looks at the /etc/hosts file first. If the request
cannot be answered, the routine calls the BIND resolver code, which
checks the /etc/resolv.conf file for the name of a server. In this
case, it is localhost.
3. The library routine contacts the forward-only server and asks for the
host name and address.
4. The forward-only server receives a query for a host name resolution
and checks its own cache to see if it can answer the query. If it
cannot, it forwards the query to the servers listed as forwarders in
its BIND configuration file (the default is named.conf) one at a time,
until the query is resolved or the list is exhausted.
5. The server returns the result to the forward-only server, even if the
result shows the resolution was unsuccessful. If the result is
successful, the slave server adds the information to its local cache.
6. The forward-only server passes the result back to the gethostbyname
library routine.
7. The gethostbyname library routine passes the result back to the
application.
SEE ALSO
Commands: bindconfig(8), named(8), nslookup(8), svcsetup(8)
Files: named.conf(4), resolv.conf(4), svc.conf(4)
Networking: bind_manual_setup(7)
Network Administration: Connections
Network Administration: Services
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Index for
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