Installing a Local Caching Nameserver

Often SpamAssassin users will find that their system is performing will perform 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 categorize 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.

lookups for NetworkTests to significantly improve scoring of messages primarily by DNSBlocklists like Spamhaus, SORBS, etc. This information needs to be cached locally to improve performance and limit the number of DNS queries since some DNSBlockLists have limits on free usage.

NOTE: A local DNS server should not forward to other servers to ensure your queries are not combined with others. Forwarding to other DNS servers usually results in URIBL_BLOCKED rule hit meaning you have gone over their free usage limit.

DNSmasq should not be used since it can only forward to other DNS servers.

UNBOUND

Packaging varies slightly between distributions so refer Internet articles for details and current information for your OS version. The default configuration files should give us a desired caching non-forwarding DNS server listening locally only.

Debian/Ubuntu:

apt-get install unbound
systemctl enable unbound
systemctl start unbound

RHEL/CentOS:

yum install unbound
chkconfig unbound on
service unbound start

Fedora:

dnf install unbound
systemctl enable unbound
systemctl start unbound

PowerDNS Recursor

Default PowerDNS Recursor installs should be the desired non-forwarding caching only DNS server listening only on localhost. Refer to other online articles for details about the config files and settings specific to your OS version.

Debian/Ubuntu:

apt-get install pdns-recursor
systemctl enable pdns-recursor
systemctl start pdns-recursor

RHEL/CentOS:

yum install pdns-recursor
chkconfig pdns-recursor on
service pdns-recursor start

Fedora:

dnf install pdns-recursor
systemctl enable pdns-recursor
systemctl start pdns-recursor

BIND

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This section describes installing BIND (Berkeley Internet Name Domain) 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. Several alternative DNS nameservers in common use are described in their own sections below.

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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

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GNU/Linux

On Red Hat and Fedora 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.

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The daemon can be configured with the files /etc/conf.d/dnsmasq and /etc/dnsmasq.conf.

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djbdns

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djbdns/tinydns is D. J. Bernstein's DNS daemon.

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No Format
dnscache-conf dnscache dnslog /service/dnscache 127.0.0.1

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rbldnsd

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rbldnsd is a small and fast DNS daemon written by Michael Tokarev which is especially made to serve DNSBL zones. This daemon was inspired by Dan J. Bernstein's rbldns program found in the djbdns package. The SURBL links page under "Mirroring RBL zone files locally" references several How-Tos for setting up rbldnsd and rsnyc in different environments including FreeBSD, Solaris, etc. NJABL also has a document about setting up rbldnsd and rsync for use with RBLs.

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If you have a good guide to the commands required to install this on a typical system, please edit this page and fill out this section.

Setting up the system to use the Caching Nameserver

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/resolv.conf

Using the Local Caching Nameserver

SpamAssassin local.cf

dns_available yes

/etc/resolv.confThe /etc/resolv.conf file configures the nameserver used to look up DNS data. A typical system contains a search line to specify the local domain. It also contains up to three nameserver lines to configure nameservers. Because we are setting up a local caching nameserver only one entry is needed. The 0.0.0.0 entry specifies that the nameserver on the local host will be contacted for DNS lookups.

search example.com
nameserver 0127.0.0.0

Note that if your host uses a client configuration with DHCP to configure networking that this file may be overwritten by the DHCP client on the local host every time the network is enabled. See your DHCP documentation for more information.

/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.

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NOTE: Make sure DHCP is not changing the nameserver setting in the /etc/resolv.conf away from

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127.0.0.1

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Using only your ISP DNS servers

Sometimes is better to always use your ISP DNS servers, because you may have a faster connection to your ISP's DNS servers than to the DNS Root servers and it helps create a large site-wide cache and reduces traffic to outside nameservers.

Here are the BIND named.conf options to only resolve using your ISP DNS Servers, the response is always cached in your local DNS Caching server:

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However, if you have a large ISP or are using a public DNS server having many users who are also doing DNSBL lookups, and the ISP / DNS host has not registered with the DNSBL provider as a paid client, the aggregate traffic from those nameservers may exceed the free usage limits imposed by the DNSBL provider and lookups may start returning invalid results. This could lead to large numbers of incorrectly-classified messages.

Non-forwarding

If you have a large ISP or are using large public DNS provider(s) it is recommended you not forward mail-related DNS traffic through their DNS servers (though non-mail DNS traffic from your site shouldn't have problems.) With bind, this means not having any "forwarders" listed. Or, at a minimum, you could create exemptions by defining empty forwarders for DNSBL zones, like this:

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