Online Tutoriale How to set up a home DNS server, part II
Posted by ascultradio on August 14, 2009
Welcome back
In the first part of this series on the Domain Name System (DNS), we set up a caching nameserver that allowed our clients to take advantage of faster network operations by caching frequently requested DNS queries. In this article, we will extend our caching nameserver to a master nameserver that is responsible for managing the authoritative information for our internal client hostnames.
Overview
As with our caching-only nameserver, we will see that BIND RPMS packaged by Red Hat® Enterprise Linux® and Fedora ease the process of configuring our master nameserver. Adding authoritative responsibility to the caching-only nameserver only requires us to add two more files and modify the existing named.conf
file. For the purpose of this article we will assume the following:
- The BIND 9.x RPMS discussed in Part 1 are installed on the machine that will serve as a nameserver.
- Our internal network is in the 192.168.15.0/24 subnet. You will need to substitute your subnet if different.
- The master nameserver will only allow DNS queries from internal clients in the 192.168.15.0/24 subnet.
- The master nameserver will continue to forward any DNS requests it can’t answer to your upstream ISP nameserver(s).
- We will use the domain
hughes.lan
as our internal domain name.
You might notice that we selected a mock top-level domain (sometimes referred as a TLD) named lan
. Our internal domain name can be as creative as we wish since the domain is only known inside our home network. The naming convention for a public nameserver is not as relaxed, since we would need to follow certain rules that would allow our nameserver to respond to other nameservers requesting host information from around the world.
Zones
Nameservers store information about a domain namespace in files called zone data files. A zone data file contains all the resource records that describe the domain represented in the zone. The resource records further describe all the hosts in the zone. We will need to modify our existing named.conf to reference two zone files for our domain name:
- Forward zone definitions that map hostnames to IP addresses.
- Reverse zone definitions that map IP addresses to hostnames.
Open /var/named/chroot/etc/named.conf
and add the following forward and reverse zone file directives:
# Forward Zone for hughes.lan domain zone "hughes.lan" IN { type master; file "hughes.lan.zone"; }; # Reverse Zone for hughes.lan domain zone "15.168.192.in-addr.arpa" IN { type master; file "192.168.15.zone"; };
Both the forward and reverse zones contain the type master
indicating that our nameserver is the master or primary nameserver for the hughes.lan
domain. The file
keyword indicates which zone file contains the resource records for the corresponding zone. Note that the reverse zone contains a special domain named in-addr-arpa
. DNS uses this special domain to support IP to hostname mappings. Reverse lookups are backwards since the name is read from the leaf to the root (imagine a domain name as a tree structure) so the resultant domain name has the topmost element at the right-hand side. For a home network the reverse lookup zones can be considered optional but we will include them for completeness.
Included with the BIND RPMs is a root zone nameservers use when a query is unresolvable by any other configured zones. The root zone directive is named “.”, has a type of hint
and references a file named named.ca
that contains a list of 13 root name servers located around the world. We will not directly use the root servers since we are forwarding any unresolvable queries to our ISP nameservers.
We need to modify the named.conf
global options to allow our internal clients to query the nameserver. Modify the existing global options block to the following:
acl hughes-lan { 192.168.15.0/24; 127.0/8; }; options { directory "/var/named"; allow-query { hughes-lan; }; forwarders { xxx.xxx.xxx.xxx; xxx.xxx.xxx.xxx; }; # ISP primary/secondary forward-only; # Rely completely on ISP for cache misses };
The acl
statement above sets up a range of IP addresses we can reference throughout the named.conf
file. The allow-query
specifies IP addresses of hosts that can query our nameserver. The forwarders
statement tells our nameserver to forward any unresolvable queries to our upstream nameservers. The forward-only
statement restricts our nameserver to only rely on our ISP nameservers and not contact other nameservers to find information that our ISP can not provide. It’s very rare for a primary and secondary ISP nameserver to be down at the same time but you can comment forward-only
if you want your nameserver to try the root nameservers when the upstream ISP nameservers cannot resolve a hostname.
Zone files
We are now ready to start defining our hostname mappings in the zone files we referenced in the named.conf
configuration. Zone files need to be placed in the /var/named/chroot/var/named
directory, have 644 permissions with an owner and group of named
:
cd /var/named/chroot/var/named touch hughes.lan.zone chown named:named hughes.lan.zone chmod 644 hughes.lan.zone
Let’s take a look at an example zone file for the hughes.lan
forward zone and then dive into the various parts:
$TTL 1D hughes.lan. IN SOA velma.hughes.lan. foo.bar.tld. ( 200612060 ; serial 2H ; refresh slaves 5M ; retry 1W ; expire 1M ; Negative TTL ) @ IN NS velma.hughes.lan. velma.hughes.lan. IN A 192.168.15.10 ; RHEL server fred.hughes.lan. IN A 192.168.15.1 ; router scooby.hughes.lan. IN A 192.168.15.2 ; upstairs WAP shaggy.hughes.lan. IN A 192.168.15.3 ; downstairs WAP scooby-dum.hughes.lan. IN A 192.168.15.4 ; Fedora desktop daphne.hughes.lan. IN A 192.168.15.5 ; network printer mysterymachine IN A 192.168.15.6 ; mail server scrappy IN A 192.168.15.7 ; Windows box ; aliases www IN CNAME velma.hughes.lan. ; WWW server virtual IN CNAME velma ; virtual WWW tests mail IN CNAME mysterymachine ; sendmail host ; DHCP Clients dhcp01.hughes.lan. IN A 192.168.15.100 dhcp02.hughes.lan. IN A 192.168.15.101 dhcp03.hughes.lan. IN A 192.168.15.102 dhcp04.hughes.lan. IN A 192.168.15.103 dhcp05.hughes.lan. IN A 192.168.15.104 @ IN MX 10 mail.hughes.lan.
The very first line in the hughes.lan.zone
contains the TTL (Time To Live) value and is set to one day. TTL is used by nameservers to know how long to cache nameserver responses. This value would have more meaning if our nameserver was public and had other external nameservers depending on our domain information. Notice the ‘D’ in the TTL value stands for Day. Bind also uses ‘W’ for weeks, ‘H’ for hours, and ‘M’ for minutes.
The first resource record is the SOA
(Start Of Authority) Record which indicates that this nameserver is the best authoritative resource for the hughes.lan
domain. The IN
stands for Internet Class and is optional. The first hostname after the SOA is the name of our master or primary nameserver. The second name, “foo.bar.tld.”, is the email address for the person in charge of this zone. Notice the ‘@’ is replaced with a ‘.’ and also ends with a ‘.’. The third value is the serial number that indicates the current revision, typically in the YYYYMMDD format with a single digit at the end indicating the revision number for that day. The fourth, fifth, sixth, and seventh values can be ignored for the purposes of this article.
The NS
record lists each authoritative nameserver for the current zone. Notice the first ‘@’ character in this line. The ‘@’ character is a short-hand way to reference the domain, hughes.lan
, that was referenced in the named.conf
configuration file for this zone.
The next block of A
records contains our hostname to address mappings. The CNAME
records act as aliases to previously defined A
records. Notice how fully qualified domains end with a ‘.’. If the ‘.’ is omitted then the domain, hughes.lan
, is appended to the hostname. In our example the hostname, scrappy
, will become scrappy.hughes.lan
If you want to reference an internal mail server, then add a MX
record that specifies a mail exchanger. The MX value “10” in our example indicates the preference value (number between 0 and 65535) for this mail exchanger’s priority. Clients try the highest priorty exchanger first.
The reverse zone file, 192.168.15.zone
, is similar to our forward zone but contains PTR
records instead of A
records:
$TTL 1D @ IN SOA velma.hughes.lan. foo.bar.tld. ( 200612060 ; serial 2H ; refresh slaves 5M ; retry 1W ; expire 1M ; Negative TTL ) IN NS velma.hughes.lan. 10 IN PTR velma.hughes.lan. 1 IN PTR fred.hughes.lan. 2 IN PTR scooby.hughes.lan. 3 IN PTR shaggy.hughes.lan. 4 IN PTR scooby-dum.hughes.lan. 5 IN PTR daphne.hughes.lan. 6 IN PTR mysterymachine.hughes.lan. 7 IN PTR scrappy.hughes.lan. 100 IN PTR dhcp01.hughes.lan. 101 IN PTR dhcp02.hughes.lan. 102 IN PTR dhcp03.hughes.lan. 103 IN PTR dhcp04.hughes.lan. 104 IN PTR dhcp05.hughes.lan.
Testing
Save your zone files, make sure you have the correct permissions and check the syntax using named-checkzone
:
named-checkzone hughes.lan hughes.lan.zone named-checkzone 15.168.192.in-addr.arpa 192.168.15.zone
Correct any syntax errors reported by named-checkzone
.
Restart the nameserver:
service named restart
Browse through the tail of the /var/log/messages
file and confirm the domain loaded successfully.
Make the following DNS queries (substituting your domain):
dig scooby.hughes.lan
dig -x 192.168.15.2
Your output should be similar to the following:
. . . ;; QUESTION SECTION: ;scooby.hughes.lan. IN A ;; ANSWER SECTION: scooby.hughes.lan. 86400 IN A 192.168.15.2 ;; AUTHORITY SECTION: hughes.lan. 86400 IN NS velma.hughes.lan. ;; ADDITIONAL SECTION: velma.hughes.lan. 86400 IN A 192.168.15.10 . . .
Continue to test each host you added to the zone file and then enjoy your new master nameserver.
Conclusion
The goal of this series of DNS articles was to pick the high-level features DNS can offer to improve the efficiency and management of the home network. In addition to the performance improvement we saw with the caching nameserver, the master nameserver helps manage both static and dynamic clients using human-friendly hostnames instead of IP addresses. For readers interested in learning more about DNS or expanding the nameservers discussed in this series, checkout the following resources:
- BIND user documenation located in
/usr/share/doc/bind-9.x.x
- DNS and BIND (5th Edition)
About the author
Shannon Hughes is a Red Hat Network (RHN) engineer who enjoys using open source software to solve the most demanding software projects. When he is not cranking out code, tweaking servers, or coming up with new RHN projects, you can find him trying to squeeze in yet another plant in the yard/garden with his wife, watching Scooby Doo reruns with his two kids and dog, or incorporating the latest open source projects for his church.
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