Domain Name System (DNS)

Contents

Client
- Client settings
- Accelerating
Reference

Client

For Linux, domain name lookup is handled by glibc. NSCD (Name Server Cache Daemon) is part of glibc to add caching service.

Client settings

Domain name servers are set in /etc/resolve.conf:
```
search example.com
nameserver 1.2.3.4
```
The order of looking up a domain name is defined in /etc/nsswitch.conf:
```
hosts:      files dns
```

Accelerating

To accelerate DNS look up speed under linux, you can use NSCD or setup a local cache only DNS server (like dnsmasq).

Windows has builtin DNS cache service called "DNS Client".

Windows DNS Client service

To display the DNS resolver cache:
```
C:\>ipconfig /displaydns
```
To flush the cache:
```
C:\>ipconfig /flushdns
```
To turn off the DNS cache service, you can use either of these two commands:
```
C:\>net stop dnscache
C:\>sc <servername> stop dnscache
```
This will disable DNS caching until the next reboot. To make the change permanent, use the Service Controller tool or the Services tool to set the DNS Client service startup type to Disabled.
You can tune the DNS cache service parameters by setting two registry entries under the HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesDnscacheParameters registry key.
- MaxCacheTtl: represents the maximum time that the results of a DNS lookup will be cached. The default value is 86,400 seconds. If you set this value to 1, DNS entries will only be cashed for a single second.
- MaxNegativeCacheTtl: represents the maximim time that the results of a failed DNS lookup will be cached. The default value is 900 seconds. If you set this value to 0, failed DNS lookups will not be cached.

NSCD (Name Server Cache Daemon)

/etc/nscd.conf:

#       logfile                 /var/log/nscd.log
#       threads                 6
#       max-threads             128
        server-user             nscd
#       stat-user               nocpulse
        debug-level             0
#       reload-count            5
        paranoia                no
#       restart-interval        3600

        enable-cache            passwd          yes
        positive-time-to-live   passwd          600
        negative-time-to-live   passwd          20
        suggested-size          passwd          211
        check-files             passwd          yes
        persistent              passwd          yes
        shared                  passwd          yes
        max-db-size             passwd          33554432
        auto-propagate          passwd          yes

        enable-cache            group           yes
        positive-time-to-live   group           3600
        negative-time-to-live   group           60
        suggested-size          group           211
        check-files             group           yes
        persistent              group           yes
        shared                  group           yes
        max-db-size             group           33554432
        auto-propagate          group           yes

        enable-cache            hosts           yes
        positive-time-to-live   hosts           3600
        negative-time-to-live   hosts           20
        suggested-size          hosts           211
        check-files             hosts           yes
        persistent              hosts           yes
        shared                  hosts           yes
        max-db-size             hosts           33554432

Start:
```
# /etc/init.d/nscd start
```
Stop:
```
# /etc/init.d/nscd stop
```
Flush the cache:
```
# /etc/init.d/nscd restart
```
Restart without flush the cache:
```
# /etc/init.d/nscd reload
```

Dnsmasq

Install dnsmasq if your distribution doesn't have it.
Change /etc/dnsmasq.conf:
```
listen-address=127.0.0.1
```
Start:
```
# /etc/init.d/dnsmasq start
```
Stop:
```
# /etc/init.d/dnsmasq stop
```
Flush the cache:
```
# /etc/init.d/dnsmasq restart
```
Restart without flush the cache:
```
# /etc/init.d/dnsmasq reload
```

Change /etc/resolv.conf:

search example.com
nameserver 127.0.0.1
nameserver 1.2.3.4

Reference

Domain Name System (DNS): http://en.wikipedia.org/wiki/Domain_Name_System
Dynamic DNS: http://en.wikipedia.org/wiki/Dynamic_DNS
dnsmasq: http://www.thekelleys.org.uk/dnsmasq/
Bind: http://www.isc.org/software/bind