The files that the primary nameservers load their zone data
from are called data files or zone files. They are also referred
to as db files, short for database files.
The data files contain resource records that describe the
zone. The resource records describe all the hosts in the zone.
Explaining DNS Database Files |
 |
This is a typical DNS zone.domain file for the domain maxx.net. (Its name would be zone.maxx.net. It will translate
from a host name to its IP address.)
;
; Addresses for the local domain |
maxx.net. IN SOA nova.maxx.net. tyager.nova.maxx.net. ( |
9602171 ; Serial
36000 ; Refresh every 10 hours
3600 ; Retry after 1 hour
360000 ; Expire after 100 hours
36000 ; Minimum TTL is 10 hours ) |
; Define name servers
;
maxx.net. IN NS nova.maxx.net.
maxx.net. IN A 204.251.17.241 |
; Define localhost
;
localhost IN A 127.0.0.1 |
; Set up hosts
;
maxx IN A 204.251.17.241
IN MX 5 nova.maxx.net. |
maxx.net. IN MX 5 nova.maxx.net.
;
; All mail for net delivered to nova
;
;* IN MX 10 nova.maxx.net.
www IN CNAME nova.maxx.net.
ftp IN CNAME nova.maxx.net.
news IN CNAME nova.maxx.net.
mail IN CNAME nova.maxx.net.
ns IN CNAME nova.maxx.net.
loghost IN CNAME nova.maxx.net.
lucy IN A 204.251.17.242
linux IN CNAME lucy.maxx.net.
lucy IN MX 10 lucy.maxx.net.
messdos IN A 204.251.17.243
messdos IN MX 10 messdos.maxx.net.
pentium IN CNAME messdos.maxx.net.
solaris IN A 204.251.17.244
solaris IN MX 10 solaris.maxx.net.
maxx4 IN CNAME solaris.maxx.net.
maxx5 IN A 204.251.17.245
maxx5 IN MX 10 maxx5.maxx.net.
maxx6 IN A 204.251.17.246
maxx6 IN MX 10 maxx6.maxx.net. |
|
Most database file entries are known as DNS resource records. Generally,
the resource records are shown in order: SOA, NS, followed by the
other types, but this ordering isn't required. The data
in each entry may be entered in upper, lower, or mixed case. All
entries in the database file must start at the beginning of the
line. Blank lines as well as any text following a semicolon is ignored.
SOA stands for Start of Authority. This acronym notifies named that operational parameters follow. The most important
one is the Serial field. Every time you make a change to a database
file, you must increment its serial number. Only by doing this will
secondary servers know they need to reach into your system and pull
out new name server data, a procedure known as a "zone
transfer." Many DNS administrators use a date-time stamp
for this field, like 9602171 for the first version on February 17, 1996.
First, focus on the SOA section:
maxx.net. IN SOA nova.maxx.net. tyager.maxx.maxx.net. |
The "maxx.net." field tells named the domain defined by this file. The name server
will automatically append it to any host name that appears in the
file. The trailing dot is not a type; it keeps named from trying to tack on your domain name. Without
it, the resolver would be confused by named's expansion
of my domain name to "maxx.net.maxx.net."
The IN stands for the "Internet" class
of data. Even though other classes exist, they aren't in
common usage. The "nova.maxx.net" field is the host on which these database
files reside. Finally, "tyager.nova.maxx.net" represents the e-mail address of the
DNS administrator, where the first dot (between tyager and nova) would be replaced by the @ symbol to create a
valid address. (The @ symbol can't be used here because
it has a reserved meaning in DNS database files.)The open parenthesis
at the end of the line allows you to split the SOA record across
physical lines for readability:
36000 ; Refresh every 10 hours
3600 ; Retry after 1 hour
360000 ; Expire after 100 hours
36000 ; Minimum TTL is 10 hours ) |
The "serial" field was discussed earlier.
The remaining four fields specify various time intervals (all
values in seconds) used by the secondary name server:
- Refresh
The time interval that must elapse between each
poll of the primary by the secondary name server (here 36,000 seconds
or 10 hours). If the "serial number" has been updated
on the primary, the secondary assumes its data is stale and requests
updated information as a "zone transfer."
- Retry
The time interval used between successive connection attempts
by the secondary to reach the primary name server in case the first
attempt failed (here 3,600 seconds or one hour). Generally, less
than the "refresh" time.
- Expire
The time interval after which the secondary expires
its data if it can't reach the primary name server (here 360,000
seconds or 100 hours). The secondary will refuse to service requests
after this interval.
- Minimum
The minimum time-to-live value, which specifies
how long other servers should cache data from the name server (here
36,000 seconds or 10 hours).
There are several types of resource records, identified by
the key word in field three of each record. You may present records
in any order, but try to organize them for clarity. The NS (name server) record tells the hosts that query
your server where the name servers for this domain can be found:
maxx.net. IN NS nova.maxx.net. |
You must include in this list at least one name server, that
is the name of the server specified in the SOA record. You can list
multiple name servers for your domain. In fact, your domain should
have at least two name servers. Your Internet service provider will
probably allow you to use their name server as a secondary for your
domain, but it must have the trailing dots!
maxx.net IN A 204.251.17.241 |
The first A record, which resolves a fully-qualified host
name to an IP address, is a special one. It defines an IP address
for unqualified queries, that is, queries for the host maxx.net.
Other A records like this one:
provide name-to-address mapping for a specific named host. The domain
defined in this file (maxx.net) is appended to the host name you show in the
first field.
The CNAME records create aliases for existing hosts. These
examples illustrate a few common uses:
www IN CNAME maxx.maxx.net.
ftp IN CNAME maxx.maxx.net. |
You can give a host any alias you like, and as many aliases
as you want. The host needn't answer to that name, that
is, the alias doesn't need to be the host's true
name as reported by hostname or uname.
The other vital type of record is MX. This tells SMTP e-mail software where to send
mail for each named host:
lucy IN MX 10 lucy.maxx.net. |
When a remote host's mail delivery program sees an
e-mail address in your domain, it will query your name server for
its applicable MX record or records. Every user on your LAN can
receive e-mail, even if not every host is running its own e-mail
software. The MX record for lucy, for instance, could easily redirect e-mail to
another host on the LAN.
The number (10 in this case) in the fourth field represents
a preference value. If you define multiple MX records for a host, delivery is attempted to lower-preference
value hosts first. The actual value isn't important, only
its relationship to other preference values.
On larger LANs it's a good idea to create backup
e-mail servers. Smaller LANs can simply rely on the fact that most
SMTP mailers will retry deliveries to the site for three days before
returning a message to its sender.
The line, shown commented out here, would arrange to redirect
e-mail for all hosts in this domain to a single machine:
;
; All mail for net delivered to nova
;
;* IN MX 10 nova.maxx.net. |
This is a very good idea for LANs that benefit from a central
e-mail repository.
Also called reverse mapping, the zone.ADDR db file allows resolvers to post queries armed
with only the IP address of a host. This reverse mapping is used,
for example, by Internet server software that prefers to log host
names rather than less informative IP addresses.
Address-to-name mapping data will be provided for a DNS server
by PTR entries in its zone.ADDR files, one for every network served by this DNS
server, and its zone.LOCAL file.
Each entry will indicate the IP address in reverse order,
then the host name. For example, for host littledog.maxx.net, whose
IP address is 204.251.17.249, in the zone.ADDR file it's PTR entry would look like:
249.17.251.204. IN PTR littledog.maxx.net. |
Why is it backwards? Recall that DNS does its parsing from
right to left, from most inclusive to most specific. For IP addresses,
it needs to parse in the same direction. But IP addresses, from
right to left, go from most specific to most inclusive. So the simple
answer is to reverse the IP address in the NDS PTR records. Now
DNS can parse in the same direction, and resolve in the same order — from
most inclusive to most specific.
A shortcut in PTR records is often used. It looks like this:
249 IN PTR littledog.maxx.net. |
If the dot is left off the IP address in the PTR record, DNS
will complete the IP address with the IP address of the domain,
specified in the file's SOA record. This is also true for
A records in name-to-address mapping db files. If the dot is left
off, DNS will automatically try to complete the name with the full
domain name in this zone. Paying attention to the terminating dot
is important.
For the zone.LOCAL file we describe the loopback address just as
you would expect it, now that we know we have to reverse it. The
PTR entry in the zone.LOCAL file would look like:
1.0.0.127. IN PTR localhost. |
or, using the shortcut:
Only one line from named.conf remains to be discussed, the "cache" entry.
This is a bit of a misnomer as it doesn't have anything
to do with local caching. Instead, it defines the master root domain
name servers for the Internet. You can retrieve this list from ftp://nic.ddn.mil/netinfo/root-servers.txt. You will need to check this site periodically
to ensure you have the latest list.
This file lists the root domain servers in human-readable
format. You'll need to reformat it for consumption by named. Here's what the cache file looks like:
; Servers from the root domain
; ftp://nic.ddn.mil/netinfo/root-servers.txt
;
. 99999999 IN NS A.ROOT-SERVERS.NET
. 99999999 IN NS B.ROOT-SERVERS.NET
. 99999999 IN NS C.ROOT-SERVERS.NET
. 99999999 IN NS D.ROOT-SERVERS.NET
. 99999999 IN NS E.ROOT-SERVERS.NET
. 99999999 IN NS F.ROOT-SERVERS.NET
. 99999999 IN NS G.ROOT-SERVERS.NET
. 99999999 IN NS H.ROOT-SERVERS.NET
. 99999999 IN NS I.ROOT-SERVERS.NET |
; Root servers by address |
A.ROOT-SERVERS.NET 99999999 IN A 198.41.0.4
B.ROOT-SERVERS.NET 99999999 IN A 128.9.0.107
C.ROOT-SERVERS.NET 99999999 IN A 192.33.4.12
D.ROOT-SERVERS.NET 99999999 IN A 128.8.10.90
E.ROOT-SERVERS.NET 99999999 IN A 192.203.230.10
F.ROOT-SERVERS.NET 99999999 IN A 192.5.5.241
G.ROOT-SERVERS.NET 99999999 IN A 192.112.36.4
H.ROOT-SERVERS.NET 99999999 IN A 128.63.2.53
I.ROOT-SERVERS.NET 99999999 IN A 192.36.148.17 |
Here, the dot (.) refers to the root domain and the 99999999 means a very long time-to-live value. The TTL value is no longer used for caching
because the data isn't discarded if it times out, but administrators
generally keep it around because it does no harm.
Your site may not have access to the Internet or may have
protected its connection via a firewall. Often in this type of DNS
configuration, one or more machines will be designated as a root
server. In this case, the cache file will contain a list of internal
root servers, and not the official Internet master root domain servers.
Perform simple checks on your name server's health
with nslookup. This utility is standard with every TCP/IP-network-aware
version of UNIX. There are other similar tools available — see "List
of Utilities" later in this section for details.
You can find the source code for dig at several anonymous FTP archive sites, including: ftp://ftp.wonderland.org/NetBSD/NetBSD-current/src/usr.sbin/named/dig/ for the NetBSD release. Use Archie to find other
sites.
The nslookup utility can be used interactively, much like other programs,
such as ftp. That is, if you invoke this program without command
line arguments, it displays a prompt and waits for your command:
Default Name Server: mpe3000.cup.hp.com Address: 15.13.199.80 |
By default, nslookup performs queries based on host names you submit;
just enter a host name after the prompt:
> romeo
Server: mpe3000.cup.hp.com
Address: 15.13.199.80 |
Name: romeo.cup.hp.com
Address: 15.13.194.242 |
> 15.12.194.242
Server: mpe3000.cup.hp.com
Address: 15.13.199.80 |
Name: romeo.cup.hp.com
Address: 15.12.194.242 |
You can check the resource records information about name
server:
> set type=ns
> mpeworld
Name Server: mpeworld.cup.hp.com
Address: 15.13.199.80 |
origin = dns.cup.hp.com
mail addr = dns-admin.dns.cup.hp.com
serial = 96092255
refresh = 10800 (3 hours)
retry = 3600 (1 hour) expire = 604800 (7 days)
minimum ttl = 86400 (1 day) |
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