Bind 9 Administrator Reference Manual

Chapter 1. Introduction

zone. To make the DNS tolerant of server and network failures, most zones have two or more

authoritative servers.

Responses from authoritative servers have the "authoritative answer" (AA) bit set in the response

packets. This makes them easy to identify when debugging DNS conﬁgurations using tools like dig

(Section 3.4.1.1).

1.4.4.1. The Primary Master

The authoritative server where the master copy of the zone data is maintained is called the primary

master server, or simply the primary. It loads the zone contents from some local ﬁle edited by humans or

perhaps generated mechanically from some other local ﬁle which is edited by humans. This ﬁle is called

the zone ﬁle or master ﬁle.

1.4.4.2. Slave Servers

The other authoritative servers, the slave servers (also known as secondary servers) load the zone

contents from another server using a replication process known as a zone transfer. Typically the data are

transferred directly from the primary master, but it is also possible to transfer it from another slave. In

other words, a slave server may itself act as a master to a subordinate slave server.

1.4.4.3. Stealth Servers

Usually all of the zone’s authoritative servers are listed in NS records in the parent zone. These NS

records constitute a delegation of the zone from the parent. The authoritative servers are also listed in the

zone ﬁle itself, at the top level or apex of the zone. You can list servers in the zone’s top-level NS records

that are not in the parent’s NS delegation, but you cannot list servers in the parent’s delegation that are

not present at the zone’s top level.

A stealth server is a server that is authoritative for a zone but is not listed in that zone’s NS records.

Stealth servers can be used for keeping a local copy of a zone to speed up access to the zone’s records or

to make sure that the zone is available even if all the "ofﬁcial" servers for the zone are inaccessible.

A conﬁguration where the primary master server itself is a stealth server is often referred to as a "hidden

primary" conﬁguration. One use for this conﬁguration is when the primary master is behind a ﬁrewall

and therefore unable to communicate directly with the outside world.