Caching Nameserver
Often SpamAssassin users will find that their system is performing many DNS (Domain Name System) lookups. There are many NetworkTests in the suite of tests. The network tests significantly enhance the ability of SpamAssassin to correctly catagorize messages. One of the main network tests is to look up domain names in the DnsBlocklists.
The DNS lookups can create a lot of network activity. There are a few times when this can cause performance issues. This question is often raised on the SpamAssassin MailingLists.
- For users behind slow network connections the latency of the lookup can slow down the catagorization process.
- For large sites processing many mail messages the volume of lookups can be a problem.
Often the same information is being looked up again and again. Needing to do a completely lookup of the same data repeatedly is inefficient.
The standard solution is to install a local caching nameserver to cache repeated DNS requests. This will significantly reduce network traffic due to DNS lookups. This improves system efficiency.
This page describes installing [http://www.isc.org BIND] (Berkeley Internet Name Daemon) in a caching configuration on the system. BIND is the standard nameserver in use on the Internet today. More internet servers run BIND than any other nameserver daemon. There are also other alternative DNS nameservers in common use. The
[http://tinydns.org/ tinydns] and [http://cr.yp.to/djbdns.html djbdns] programs are typical alternatives.
Debian GNU/Linux
The [http://www.debian.org Debian] system uses APT (Advanced Package Tool) to manage the system. The following commands will install BIND (Berkeley Internet Name Daemon) version 9 on the system.
apt-get update apt-get install bind9
The default configuration for the Debian package is install a caching nameserver suitable for Internet use. After installation the daemon will be configured and running.
Red Hat and Fedora Core GNU/Linux
On [http://www.redhat.com Red Hat and Fedora Core] systems the BIND software is in the "bind" rpm package. The "caching-nameserver" rpm package contains a caching nameserver configuration suitable for Internet use. Locate those packages from your vendor and install them. The http://rpmfind.net
rpm search site is very useful for locating rpms for your system.
On Red Hat the following commands will install BIND and a caching nameserver configuration on the system. The version numbers used in the following example are purely for example. Use the current package version for your system release. This example shows a typical installation on RH9.
After installation the daemon will need to be configured and started. The following commands will configure the BIND name daemon to be started at system boot time and then will start the daemon.
rpm -Uvh bind-9.2.1-16.i386.rpm rpm -Uvh caching-nameserver-7.2-7.i386.rpm chkconfig bind on /etc/init.d/named start
GNU C library configuration
For the current glibc version 6 the host lookup ordering is configured in the /etc/nsswitch.conf file. The typical configuration would specify the local system file first and the network DNS database second.
hosts: files dns
The older glibc version 5 library configured host lookup ordering in the /etc/host.conf. This library is now obsolete. But your system will probably provide this file for compatibility to enable older programs linked against the older library.
order hosts,bind
/etc/hosts
The /etc/hosts file is the original location for DNS data. However it is not used generally for DNS lookups on modern systems. It is impossible to keep the entire Internet database there. But it is still used for a small amount of local system data.
Typically the localhost loopback address is stored there and nothing more. However it is acceptable to configure a small number of local network systems there and that is typical on small network sites. Here is an example /etc/hosts file.
127.0.0.1 localhost