Service (SRV) Records
Service (SRV) records are RRs that indicate
which resources perform a particular service. Domain controllers in AD
DS are referenced by SRV records that define specific services, such as
the Global Catalog (GC), Lightweight Directory Access Protocol (LDAP),
and Kerberos. SRV records are a relatively new addition to DNS, and did
not exist in the original implementation of the standard. Each SRV
record contains information about a particular functionality that a
resource provides. For example, an LDAP server can add an SRV record,
indicating that it can handle LDAP requests for a particular zone. SRV
records can be very useful for AD DS because domain controllers can
advertise that they handle Global Catalog requests, as illustrated in Figure 3.
Figure 3. Sample SRV record for an AD GC entry.
Note
Because SRV records are a relatively new
addition to DNS, they are not supported by several down-level DNS
implementations, such as UNIX BIND 4.1.x and NT 4.0 DNS. It is,
therefore, critical that the DNS environment that is used for Windows
Server 2012’s AD DS has the capability to create SRV records. For UNIX
BIND servers, version 8.1.2 or later is recommended.
Mail Exchanger (MX) Records
A Mail Exchanger (MX) record
indicates which resources are available for Simple Mail Transfer
Protocol (SMTP) mail reception. MX records can be set on a domain basis
so that mail sent to a particular domain will be forwarded to the
server or servers indicated by the MX record. For example, if an MX
record is set for the domain companyabc.com, all mail sent to [email protected] will be automatically directed to the server indicated by the MX record.
Pointer (PTR) Records
Reverse queries to DNS are
accomplished through the use of Pointer (PTR) records. In other words,
if a user wants to look up the name of a resource that is associated
with a specific IP address, he would do a reverse lookup using that IP
address. A DNS server would reply using a PTR record that would
indicate the name associated with that IP address. PTR records are most
commonly found in reverse lookup zones.
Canonical Name (CNAME) Records
A Canonical Name (CNAME) record
represents a server alias, and allows any one of a number of servers to
be referred to by multiple names in DNS. The record essentially
redirects queries to the A record for that particular host. CNAME
records are useful when migrating servers and for situations in which
friendly names, such as mail.companyabc.com, are required to point to
more complex server-naming conventions, such as
sfoexch01.companyabc.com.
Other DNS Record Types
Other, less-common forms of records that
might exist in DNS have specific purposes, and there might be cause to
create them. The following is a sample list, but is by no means
exhaustive:
• AAAA—Maps a standard IP address into a 128-bit IPv6 address. This type of record will become more prevalent as IPv6 is adopted.
• ISDN—Maps a specific DNS name to an ISDN telephone number.
• KEY—Stores a public key used for encryption for a particular domain.
• RP—Specifies the person responsible for a domain.
• WKS—Designates a particular well-known service.
• MB—Indicates which host contains a specific mailbox.
