bind_intro(7) Miscellaneous Information Manual bind_intro(7)
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
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 Pro-
tocol (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 sepa-
rated 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:
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 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:
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.
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
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,
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 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
for information on contacting ARIN.
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 over-
loaded, or is down.
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.
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 mas-
ter 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 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 for-
warders 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.
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 220.127.116.11 nameserver 18.104.22.168
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 con-
tacts the forward-only server and asks for the host name and address. 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. 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. The forward-only server passes the result back to the gethostbyname library routine.
The gethostbyname library routine passes the result back to the application.
Commands: bindconfig(8), named(8), nslookup(8), svcsetup(8).
Files: named.conf(4), resolv.conf(4), svc.conf(4).
Network Administration delim off