Preventing Vexation and Woe: DNS Fundamentals, Part 2 - Page 3

PTR

Pointers are reverse lookups, resolving IPs back to hostnames. They are not required but are useful when certain applications use reverse queries for authentication. In tinydns, the equals sign creates an A record and PTR:

=www.foosite.com:1.2.3.4

The plus sign creates a forward-lookup only.

NS

The authoritative name server for the domain. Every domain is required to have at least two authoritative name servers. You may have as many as you like.

SOA

Start of authority defines the primary name server and global variables. Every zone file must contain one SOA. A zone file is simply the DNS settings for a particular domain.

TTL

Time to live tells a querying name server how long to keep a record in its cache before coming back and pestering your name server again.

MX

Mail exchange provides the hostname for your mail server. MX records, like CNAMEs, must point to an A record -- never ever an IP address (which is a common mistake).

Zone Transfers

This means keeping a particular DNS database, or zone file, synchronized between two nameservers. This provides redundancy in case one should fail. To quote Ed Sawicki again: "Zone files are plain text files, so it doesn't take much sophistication to move them between computers." There's no shortage of file transfer protocols, yet BIND uses a file transfer protocol that is unique to it. With djbdns, administrators are told to simply push the updated zone file to the secondary using whatever combination of protocols/software does the job:

• rsync
• rsync over SSL
• rsync over SSH
• scp
• cp over a VPN
• etc... "

djbdns provides axfrdns for zone transfers between tinydns and BIND. Zone transfers are more of an issue if you run a public nameserver; redundancy and transfers are not required for private name servers.

Page 4: Separating DNS Caches From DNS Servers