Domain Name Server Configuration

Mail config is present in the following files:

/etc/named.conf "Root" configuration file
/var/named/named.zone Zone description

Further documentation is available in the directory /usr/share/doc/packages/bind9/ on ltnb10.

`/etc/named.conf`

The named.conf file says which files describe which domains. The file starts with a pre-amble describing some generic characteristics of the name server (options), which normally does not need to be changed.

Then follow the zone descriptions. There are several kinds of zones:

The root ("hint") zone. This is used to resolve names outside of the LTNB:

zone "." {
        type hint;                      // used to be specified w/ "cache"
        file "cache.db";
};

A primary zone:

zone "ltnb.lu" IN {
        type master;                    // what used to be called "primary"
        file "ltnb.lu.zone";
};

The name after zone (here, ltnb.lu says which domain is described by this zone). The parameters within braces have the following meaning:

type master	Says this is a primary domain, rather than a secondary
file "ltnb.lu.zone";	Says that the zone is described in file `ltnb.lu.zone`. Zone files are stored in directory `/var/named.ltnb.lu.zone`. This zone description file contains the name to IP mappings for all entities in the domain.

If two zones are very similar, they can be described by a same zone file. In that case, care must be taken that the domain name is never mentioned in that file. This is how most of our org.lu Web domains are set up.

A secondary zone:

zone "ill.lu" IN {
        type slave;
        file "slave/ill.lu.zone";
        masters {
                212.56.224.1;
        };
};

This means that the nameserver acts as a backup nameserver for another zone. It fetches zone data from the other server, and caches it in a local file. If the primary server goes down, it can answer to request on its behalf.

type slave	Says this is a secondary domain, rather than a primary
file "slave/ill.lu.zone";	Says that the zone data is cached in file `/var/named/slave/ill.lu.zone`.
masters { 212.56.224.1; };	IP address of primary domain server for zone

A primary reverse zone:
zone "28.64.158.in-addr.arpa" { type master; file "158.64.28.zone"; };
This zone is used for reverse lookups, i.e. for mapping IP addresses back to host names. The zone name specified is the IP address prefix read backwars followed by .in-addr.arpa. In this example, the described prefix is 58.64.28. Parameters between braces are the same as for any other primary zone.

`/var/named/named.zone`

Explanations. The header is a so-called SOA ("start of authority") record.

Example:
ltnb.lu. 1D IN SOA ns.ltnb.lu. hostmaster.ltnb.lu. ( 2001022401 ; serial: date + 2 digits 8H ; refresh 2H ; retry 1W ; expire 1D ) ; minimum 1D IN NS ns.ltnb.lu. 1D IN NS ns.restena.lu. ; nameserver 1D IN A 158.64.28.254 1D IN MX 10 mailhost ; primary mail host 1D IN MX 20 mailbackup.ltnb.org.lu. ; backup mail host
It defines the following items:

ltnb.lu. the name of the zone. You may also put @ here, which means "the domain name, as specified in /etc/named.conf". Using an @ rather than the explicit domain name allows to write domain independant zone files, which can be used for several different domains.
1D Time-to-live (1 Day). That's basically the duration of validity of the SOA record: if a cached copy is older than one day, it must be refreshed from the origin server (here NS).
hostmaster.ltnb.lu. This is the e-mail of the DNS admninistrator for this zone. The @ is changed to a dot (.) to conform to DNS syntax. This also means that the e-mail address itself should not contain a dot in its username part, or else it would be ambiguous. The trailing dot means that named should not implicitly append the domain (ltnb.lu)
2001021402 Serial number, used by secondary name servers and caches to detect whether the domain has changed. Conventionnally, this is the date followed by two increasing digits. Update this number whenever you update the DNS. This example refers to the second update (02) on February 14th 2001.

2001021402 ; serial: date + 2 digits 8H ; refresh 2H ; retry 1W ; expire 1D ) ; minimum
The various expire times for this zone. Items after semicolon (;) are comments
1D IN NS ns.ltnb.lu.
1D IN NS ns.restena.lu. Nameservers responsible for this zone (should included this server)
1D IN A 158.64.28.254 IP Address for ltnb.lu
1D IN MX 10 mailhost
1D IN MX 20 backup-mailhost
Hosts which handle mail sent to this domain. 10 and 20 are the priority. Lowest priority is tried first. Which means that if mailhost is down, backup-mailhost will be tried.
After this header (describing the attributes of the domain itself) follow the descriptions of the (other) hosts in this domain: Example:
pollux 1D IN A 158.64.28.254 1D IN MX 10 mailhost ; primary mail host 1D IN MX 20 mailbackup-ltnb.org.lu. ; backup mail host
This defines an A record (IP address) for pollux (namely 158.64.28.254), and two MX records (mail distributors): mailhost and mailbackup-ltnb.org.lu

It is also possible to define a name as an alias (CNAME) for another one; in that case, it inherits all records of its synonym: Example:
ltnb10 1D IN CNAME ns
Here ltnb10 is defined to be a synonym of ns. Because a CNAME inherits all attributes of its master, it does not make sense to combine it with other attributes. For example, the following definition would be erroneous:
ltnb10 1D IN CNAME ns 1D IN MX 10 mailhost.ltnb.lu
A CNAME can also point to the domain itself, in that case, you can use @ to denote the empty string (equivalent to domain):
www 1D IN CNAME @
In a reverse zone, we find PTR records. Those are used to map back from ip addresses to hosts:
254 1D IN PTR pollux.ltnb.lu.
In a reverse zone, some IP addresses may actually be CNAME's pointing to another "fictive" IP address. This is done when a same IP address block is shared by two institutions:
0 1D IN NS ns.lll.lu 4 1D IN CNAME 4.0
This says that:

There is a subzone, named 0.28.64.158.in-addr.arpa handled by a different nameserver (here, ns.lll.lu)
The IP address 158.64.28.4 (represented by 4.28.64.158.in-addr.arpa, in short, 4) is a CNAME for 4.0 (in full: 4.0.28.64.158.in-addr.arpa), which is handled by the nameserver responsible for zone 0.28.64.158.in-addr.arpa (in this case, ns.lll.lu)

ltnb.lu.	the name of the zone. You may also put @ here, which means "the domain name, as specified in `/etc/named.conf`". Using an @ rather than the explicit domain name allows to write domain independant zone files, which can be used for several different domains.
1D	Time-to-live (1 `D`ay). That's basically the duration of validity of the SOA record: if a cached copy is older than one day, it must be refreshed from the origin server (here NS).
hostmaster.ltnb.lu.	This is the e-mail of the DNS admninistrator for this zone. The @ is changed to a dot (.) to conform to DNS syntax. This also means that the e-mail address itself should not contain a dot in its username part, or else it would be ambiguous. The trailing dot means that named should not implicitly append the domain (ltnb.lu)
2001021402	Serial number, used by secondary name servers and caches to detect whether the domain has changed. Conventionnally, this is the date followed by two increasing digits. Update this number whenever you update the DNS. This example refers to the second update (02) on February 14th 2001.
2001021402 ; serial: date + 2 digits 8H ; refresh 2H ; retry 1W ; expire 1D ) ; minimum	The various expire times for this zone. Items after semicolon (;) are comments
`1D IN NS ns.ltnb.lu.` `1D IN NS ns.restena.lu.`	Nameservers responsible for this zone (should included this server)
`1D IN A 158.64.28.254`	IP Address for `ltnb.lu`
`1D IN MX 10 mailhost` `1D IN MX 20 backup-mailhost`	Hosts which handle mail sent to this domain. 10 and 20 are the priority. Lowest priority is tried first. Which means that if mailhost is down, backup-mailhost will be tried.