Installing

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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.

Installing BIND as a Caching Nameserver

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.

Debian GNU/Linux

The 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.

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 external DNS queries since some DNSBlockLists have limits on free usage.

NOTE: A local DNS caching server should not forward to other DNS servers to ensure your queries are not combined with others. Forwarding to other DNS servers often results in URIBL_BLOCKED or similar rule hits meaning you have gone over their free usage limit. More info about this can be found in FAQ.

Wikipedia DNS Server feature matrix

Dnsmasq should not be used by SpamAssassin since it can only forward to other DNS servers.

An advanced setup is possible atleast with Unbound and BIND, where queries are forwarded by default to another DNS servers, but exceptions like Spamhaus can be made to go direct. Using global forwarders like Cloudflare (1.1.1.1) or Google (8.8.8.8) can actually improve performance, since their huge caches help with all the common stuff like DKIM, SPF, PTR/MX lookups etc.

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 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 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.

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 named on
service named start

If you have yum installed, you can use the following commands to install and enable the latest caching nameserver package. yum will take care of installing any dependencies (including the BIND named package) required.

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 update
apt-get install pdns-recursor

RHEL/CentOS:

# EPEL repository required
yum install epel-release
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

Debian/Ubuntu:

apt-get update
apt-get install bind9

RHEL/CentOS:

yum install bind bind-utils

yum install caching-nameserver
chkconfig named on
service named start

BIND Resources

...

Fedora:

dnf install bind bind-utils
systemctl enable named
systemctl start named

rbldnsd

Rbldnsd is not a recursive caching DNS server. It is an authoritative DNS server primarily used to host private/internal zones from feeds like Spamhaus, Invaluement, SORBS, etc. Typically rbldnsd will listen on an alternate port then the primary DNS server setup above would forward specific zones to rbldnsd.

Rbldnsd is a little tricky to get setup but once you do it is rock solid. You simply wget, curl, rsync the feed files and rbldnsd can detect changes then automatically reload them.

Search the Internet for current articles for your specific OS. Here's the basic setup on a systemctl-based OS taken from CentOS 7:

1. Rsync the feed files into /var/lib/rbldnsd

2. List the feed files in /etc/systemd/system/rbldnsd-dsbl.service

   No Format

   .include /etc/systemd/rbldnsd.conf [Unit] Description=DNSBL (rbldnsd) dsbl instance [Service] ExecStart=/sbin/rbldnsd -n -f -r /var/lib/rbldnsd -b 127.0.0.1/530 dul.dnsbl.sorbs.net:ip4set:dul.dnsbl.sorbs.net http.dnsbl.sorbs.net:dnset:http.dnsbl.sorbs.net smtp.dnsbl.sorbs.net:ip4set:smtp.dnsbl.sorbs.net new.spam.dnsbl.sorbs.net:ip4set:new.spam.dnsbl.sorbs.net dnsbl-1.uceprotect.net:ip4set:dnsbl-1.uceprotect.net

   
   

3. Enable and start the service

   No Format
   systemctl enable rbldnsd-dsbl systemctl start rbldnsd-dsbl

   
   

4. rbldnsd should now be listening on port 530

   No Format
   # netstat -tunlap | grep rbldns udp 0 0 127.0.0.1:530 0.0.0.0:*

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Gentoo Linux

On Gentoo the dnsmasq package is called "net-dns/dnsmasq".

emerge net-dns/dnsmasq
rc-update add add dnsmasq default

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

Installing djbdns as a Caching Nameserver

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

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.

Debian GNU/Linux

To install djbdns on Debian you need to fetch (with apt for example) the packages "daemontools-installer" and "djbdns-installer". What this packages will do is fetch the source code, compile it, and create Debian packages both for daemontools and djbdns. After installing those packages, you can issue the commands "build-daemontools" and "build-djbdns" which will create the final debian packages and prompt for installation. Example:

apt-get update
apt-get install djbdns-installer daemontools-installer
build-daemontools
build-djbdns

Note that you may keep and reuse (just not redistribute) the debian packages created with the installer packages.

After installing djdbs, you need to create the "dnscache" instance under /service. Suposing you want the cache to listen on the loopback device, you would do:

dnscache-conf dnscache dnslog /service/dnscache 127.0.0.1

Installing rbldnsd as a Caching Nameserver

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.

rbldnsd uses far less memory and CPU, and is much quicker in responding to queries than BIND. Those are reasons why rbldnsd is widely used for public and private mirroring of RBL zone files. A common rule of thumb is that the overhead of doing rbldnsd and rsync becomes worthwhile for mail systems that process more than 100,000 messages per day. Some RBLs impose a minimum daily message count before allowing rsync access for local mirroring of their zone files. Some RBLs charge a subscription fee for access. Others don't. Please check with the RBL operators as appropriate.

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.

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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

The /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 0.0.0.0

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.

1. 901/rbldnsd

   
   

2. Setup your main DNS caching server to forward to rbldnsd. This is an example for PowerDNS recursor:

   • /etc/pdns-recursor/recursor.conf

     No Format
     forward-zones-file=/etc/pdns-recursor/forward-zones

     
     

   • /etc/pdns-recursor/forward-zones

     No Format
     dul.dnsbl.sorbs.net=127.0.0.1:530 http.dnsbl.sorbs.net=127.0.0.1:530 smtp.dnsbl.sorbs.net=127.0.0.1:530 new.spam.dnsbl.sorbs.net=127.0.0.1:530 dnsbl-1.uceprotect.net=127.0.0.1:530

     
     

Using

SpamAssassin local.cf

dns_available yes

/etc/resolv.conf

search example.com
nameserver 

127.0.0.1	localhost

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:

options {
...
  forward only;
  forwarders {
    IP_DNS_1;
    IP_DNS_2;
  };
...
};

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:

NOTE: If something like NetworkManager is reverting your changes in /etc/resolv.conf or you don't have permission to update the /etc/resolv.conf, you may specify a DNS server in the local.cf:

dns_server 127.0.0.1

Testing

Use dig to test DNS queries. If you don't get a response then the local DNS server could:

1. not have proper connectivity outbound to the Internet - a firewall could be blocking UDP/TCP 53
2. have reached the free usage limit of the DNSBlockList - remove the "+short" to see more detail in the ANSWER section
3. may not be configured correctly - search for articles on how to setup your specific DNS caching server on your specific OS

Spamhaus Zen:

dig +short 2.0.0.127.zen.spamhaus.org
127.0.0.10
127.0.0.4
127.0.0.2

SORBS DUL:

# dig 2.0.0.127.dul.dnsbl.sorbs.net +short
127.0.0.10

URIBL:

dig test.uribl.com.multi.uribl.com txt +short
"permanent testpoint"

If you don't get the permanent testpoint response above, then you are most likely also hitting the URIBL_BLOCKED rule. Check your mail logs. If you are a major mail filtering provider with high volume, then you may have to disable the following rules in the local.cf or you might just get an email from them with pricing information:

score URIBL_BLACK 0
score URIBL_GREY 0
score URIBL_RED 0

/* Disable forwarding for DNSBL queries */
zone "multi.uribl.com" { type forward; forward first; forwarders {}; };
zone "dnsbl.sorbs.net" { type forward; forward first; forwarders {}; };
zone "combined.njabl.org" { type forward; forward first; forwarders {}; };
zone "activationcode.r.mail-abuse.com" { type forward; forward first; forwarders {}; };
zone "nonconfirm.mail-abuse.com" { type forward; forward first; forwarders {}; };
zone "iadb.isipp.com" { type forward; forward first; forwarders {}; };
zone "bl.spamcop.net" { type forward; forward first; forwarders {}; };
zone "fulldom.rfc-ignorant.org" { type forward; forward first; forwarders {}; };
zone "list.dnswl.org" { type forward; forward first; forwarders {}; };
zone "blackholes.mail-abuse.org" { type forward; forward first; forwarders {}; };
zone "bl.score.senderscore.com" { type forward; forward first; forwarders {}; };
zone "zen.spamhaus.org" { type forward; forward first; forwarders {}; };