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bind_intro - Introduction to the Berkeley Internet Name Domain (BIND) service  


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.


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, 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 seven top-level domains shown in the following table:


arpa The Arpanet (gradually being phased out)
com Commercial institutions
edu Educational institutions
gov U.S. government
mil Military organizations
netNetwork-type organizations, such as network centers
centers, consortia, and information centers
orgMiscellaneous organizations, such as professional
societies and similar nonprofit organizations

In addition to these, there are several top-level domains for individual countries. Contact InterNIC for more information about them. See Network Administration for information on contacting InterNIC.

Assuming a host name in the domain is chicago, the following is the fully qualified domain name for that host:

In this example, com is the top level domain, is a subdomain of com, and chicago is a host name. The zone,, 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 Root Primary Secondary Caching Slave  

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. BIND has the following types of master servers: A root server 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 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 slave) can pass along the request to a root server.
The following seven root servers currently exist in the continental United States:
The period (.) at the end of each root server name indicates that this is the absolute domain name and that no BIND name extensions are to be appended. Without the period, the server name is relative to the current domain.
InterNIC determines the root servers.
See Network Administration for information on contacting InterNIC. A primary 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. A secondary server receives its authority and its database from the primary server. When a secondary 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 primary server to check that the database is still up to date.
After the secondary server is running, it polls the primary server every five minutes (by default) to be sure that its database information is current. If the database files have been modified, it updates its information. Because of the polling interval, a lag could occur between the time when changes are made in the primary server's databases and when they are pulled over to the secondary servers.

A server can be a master server for multiple domains. It can be the primary server for some domains and a secondary server for others. It is recommended that each BIND domain have at least two master servers: one primary and one or more secondary servers. The secondary servers act as backup servers in the event that the primary server fails, is overloaded, or is down.  

Caching Servers

All servers cache the information they receive for use until the data expires. However, caching 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 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.  

Slave Servers

Slave 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, slave servers use forwarders, which can be either primary or secondary servers, to resolve their queries. A slave server forwards queries to the list of forwarders specified in its boot file, until the list is exhausted or the query is satisfied. (See the section on Forwarders for information about forwarders.)

As the slave servers request new information from forwarders, they accumulate it in their cache. Slave servers do not receive complete zones from primary servers, like secondary servers do, but accumulate data per request.  


A forwarder can be any BIND server (primary, secondary, or caching) that has Internet access. Forwarders are able to obtain information not held in their local caches from servers in other zones. Any server can make use of forwarders.

Slave servers do not have access to the Internet nor do they have a large local cache. Therefore, they rely on forwarders to resolve host name and address queries. The configuration files on the slave servers define which systems the slaves access as forwarders.

Because forwarders receive many requests from slave servers, they tend to have a larger local cache than slave 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 nameserver nameserver  

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. An application requests host name resolution and uses the gethostbyname library routine. 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. The library routine contacts the slave server and asks for the host name and address. The slave 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 boot file (the default is named.boot) one at a time, until the query is resolved or the list is exhausted. The server returns the result to the slave 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. The slave server passes the result back to the gethostbyname library routine. The gethostbyname library routine passes the result back to the application.  


Commands: bindconfig(8X), bindsetup(8), named(8), nslookup(8), svcsetup(8)

Files: resolv.conf(4), svc.conf(4)

Network Administration delim off



BIND Servers
Master Servers
Caching Servers
Slave Servers
BIND Clients
Resolving Queries

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Time: 02:40:18 GMT, October 02, 2010