6. DNS Sample BIND Configurations

This chapter provides a number of BIND configuration samples.

1. 6.1 Sample Configuration Overview
   1. 6.1.1 Zone File Naming Convention
2. 6.2 Master (Primary) DNS
3. 6.3 Slave (Secondary) DNS
4. 6.4 Caching only DNS
5. 6.5 Forwarding (a.k.a. Proxy, Client, Remote) DNS
6. 6.6 Stealth (a.k.a. Split or DMZ) DNS
7. 6.7 Authoritative Only DNS
8. 6.8 Views based Authoritative Only DNS
9. 6.9 Split Horizon using views

6.1 Sample BIND Configuration Overview

This chapter provides sample configurations and descriptions for each of the DNS types previously described. A BIND systems consists of the following parts:

1. A named.conf file describing the functionality of the BIND system. The entries in this file are fully described.
2. Depending on the configuration one or more zone files describing the domains being managed. The entries in zone files are fully described. Zone files contain Resource Records which are fully described.
3. Depending on the configuration one or more required zone files describing the 'localhost' and root name servers.

Many BIND/DNS configurations are schizophrenic in nature - they may be 'masters' for some zones, 'slaves' for others, forward others and provide caching services for all comers. Where possible we cover alternate configurations or at least note the alternate configurations.

All the configuration files are deliberately kept simple - links are provided to the various sections that will describe more advanced parameters as appropriate. Comments are included in the files to describe functionality. The configuration used throughout is:

1. Two name servers are used one internal (ns1) and one external (ns2) to the domain
2. The mail service is external to the domain (provided by a third party)
3. FTP and WWW services are provided by the same host
4. There are two hosts named bill and fred
5. The host address are all in the class C private (RFC 1918) address range 192.168.0.0 (a slightly artificial case)

6.1.1 Zone File Naming Convention

Everyone has their own ideas on a good naming convention and thus something that is supposed to be useful becomes contentious.

Here is a convention that is in daily use. Its sole merits are; it is a convention; it makes sense to its authors.

1. All zone files are placed in /var/named/ (for Windows users this would be %systemroot%\system32\drivers\etc). The base directory contains all the housekeeping zone files (e.g. localhost, reverse-mapping, root.servers etc.) with a subdirectory structure used as follows:

   1. /var/named/master - master zone files
   2. /var/named/slave - slave zones files
   3. /var/named/views - where views are used

2. master files are named master.example.com (or master.example.net etc.) if its a sub-domain it will be master.sub-domain.example.com etc.

3. slave zone files are named slave.example.com (or slave.example.ca etc.) if its a sub-domain it will be slave.sub-domain.example.com etc.

4. The root server zone file is called root.servers (typically called named.ca or named.root in BIND distributions).

5. The reverse mapping file name uses the subnet number and .rev, that is, if the zone is '23.168.192.IN-ADDR.ARPA' the file is called 192.168.23.rev to save having to reverse the digits at 3AM in a blind panic.

6. The 'localhost' zone file is called master.localhost (typically called localhost.zone on BIND distributions). The reverse mapping file is called localhost.rev (typically called named.local in BIND distributions).

Note: For most Linux distributions you have a small overhead at the beginning to rename the supplied files but the author considers it worthwhile in the long run to avoid confusion.

Final point on this topic: Whatever your convention be rigorous in its application and don't get hung up on standard distribution file names, make them useful to you. The names were not handed down on tablets of stone.

6.2 Master (Primary) DNS Server

The functionality of the master name server was previously described.

Master Name Server Configuration

The BIND DNS configuration provides the following functionality:

1. 'master' DNS for example.com
2. provides 'caching' services for all other domains
3. provides recursive query services to local resolvers only (a closed DNS - note in configuration file shows how to Open the server if required though this should only be done after careful consideration of all consequences)

The BIND 'named.conf' is as follows (click to look at any file):

// MASTER & CACHING NAME SERVER for EXAMPLE, INC.
// maintained by: me myself alone
// CHANGELOG:
// 1. 9 july 2003 - did something
// 2. 16 july 2003 - did something else
// 3. 23 july 2003 - did something more
//
options {
  directory "/var/named";
  // version statement - inhibited for security
  // (avoids hacking any known weaknesses)	
  version "get lost";
  // optional - disables all transfers 
  // slaves allowed in zone clauses
  allow-transfer {"none";};
  // Closed DNS - permits only local IPs to issue recursive queries 
  // remove if an Open DNS required to support all users 
  // or add additional ranges 
  allow-recursion {192.168.3.0/24;};
};
//
// log to /var/log/named/example.log all events from 
// info UP in severity (no debug)
// defaults to use 3 files in rotation
// BIND 8.x logging MUST COME FIRST in this file
// BIND 9.x parses the whole file before using the log
// failure messages up to this point are in (syslog) 
// typically /var/log/messages
//
  logging{
  channel example_log{
   file "/var/log/named/example.log" versions 3 size 2m;
   severity info;
   print-severity yes;
   print-time yes;
   print-category yes;
 };
 category default{
  example_log;
 };
};
// required zone for recursive queries
zone "." {
  type hint;
  file "root.servers";
};
zone "example.com" in{
  type master;
  file "master/master.example.com";
  // enable slaves only
  allow-transfer {192.168.23.1;192.168.23.2;};
};
// required local host domain
zone "localhost" in{
  type master;
  file "master.localhost";
  allow-update{none;};
};
// localhost reverse map
zone "0.0.127.in-addr.arpa" in{
  type master;
  file "localhost.rev";
  allow-update{none;};
};
// reverse map for class C 192.168.0.0
zone "0.168.192.IN-ADDR.ARPA" in{
  type master;
  file "192.168.0.rev";
};

A Master server without the caching service is shown under Authoritative Only.

6.3 Slave (Secondary) DNS Server

The functionality of the slave name server was previously described.

Slave Name Server Configuration

The BIND DNS configuration provides the following functionality:

1. 'slave' DNS for example.com
2. provides 'caching' services for all other domains
3. provides recursive query services to local resolvers only (a closed DNS - note in configuration file shows how to Open the server if required)

Note: Since we are defining the slave the alternate sample file is used throughout this example configuration with all servers being internal to the domain.

The BIND 'named.conf' is as follows (click to look at any file):

// SLAVE & CACHING NAME SERVER for EXAMPLE, INC.
// maintained by: me myself alone
// CHANGELOG:
// 1. 9 july 2003 - did something
// 2. 16 july 2003 - did something else
// 3. 23 july 2003 - did something more
//
options {
  directory "/var/named";
  // version statement - inhibited for security
  // (avoids hacking any known weaknesses)	
  version "not currently available";
  // allows notifies only from master
  allow-notify {192.168.0.1};
  // disables all zone transfer requests
  allow-transfer{"none"};
  // Closed DNS - permits only local IPs to issue recursive queries 
  // remove if an Open DNS required to support all users 
  // or add additional ranges 
  allow-recursion {192.168.3.0/24;};
};
//
// log to /var/log//named/example.log all events 
// from info UP in severity (no debug)
// defaults to use 3 files in rotation
// BIND 8.x logging MUST COME FIRST in this file
// BIND 9.x parses the whole file before using the log
// failure messages up to this point are in (syslog)
// typically /var/log/messages
//
  logging{
  channel example_log{
  file "/var/log/named/example.log" versions 3 size 2m;
  severity info;
  print-severity yes;
  print-time yes;
  print-category yes;
 };
 category default{
  example_log;
 };
};
// required zone for recursive queries
zone "." {
  type hint;
  file "root.servers";
};
// see notes below
zone "example.com" in{
  type slave;
  file "slave/slave.example.com";
  masters {192.168.0.1;};
};
// required local host domain
zone "localhost" in{
  type master;
  file "pri.localhost";
  allow-update{none;};
};
// localhost reverse map
zone "0.0.127.in-addr.arpa" in{
  type master;
  file "localhost.rev";
  allow-update{none;};
};
// reverse map for class C 192.168.0.0 (see notes)
zone "0.168.192.IN-ADDR.ARPA" IN {
  type slave;
  file "sec.192.168.0.rev";
  masters {192.168.0.1;};
};

Notes:

1. The slave zone file 'slave/slave.example.com' is optional and allows storage of the current records - minimising load when named is restarted. To create this file initially just open and save an empty file. BIND will complain the first time it loads but not thereafter.
2. The reverse map for the network (zone 0.168.192.IN-ADDR.ARPA) is defined as a slave for administrative convenience - you need to maintain only one copy - but it could be defined as a 'master' with a standard reverse map format.
3. A single 'masters' IP address is used specifying ns1.example.com.

6.4 Caching Only DNS Server

The functionality of the Caching Only name server was previously described.

Caching Only Name Server Configuration

The BIND DNS configuration provides the following functionality:

1. The name server is not a 'master' or 'slave' for any domain
2. provides 'caching' services for all domains
3. provides query services to local resolvers only (a closed DNS - note in configuration file shows how to Open the server if required)

The BIND 'named.conf' is as follows (click to look at any file):

// CACHING NAME SERVER for EXAMPLE, INC.
// maintained by: me myself alone
// CHANGELOG:
// 1. 9 july 2003 - did something
// 2. 16 july 2003 - did something else
// 3. 23 july 2003 - did something more
//
options {
  directory "/var/named";
  // version statement - inhibited for security
  // (avoids hacking any known weaknesses)	
  version "not currently available";
  // disables all zone transfer requests
  allow-transfer{"none";};
  // Closed DNS - permits only local IPs to issue queries 
  // remove if an Open DNS required to support all users 
  // or add additional IP ranges 
  // in this case either allow-query or allow-recursion can be used
  allow-query {192.168.3.0/24;};
};
//
// log to /var/log/example.log all events 
// from info UP in severity (no debug)
// defaults to use 3 files in rotation
// BIND 8.x logging MUST COME FIRST in this file
// BIND 9.x parses the whole file before using the log
// failure messages up to this point are in (syslog)
// typically /var/log/messages
//
  logging{
  channel example_log{
   file "/var/log/named/example.log" versions 3 size 2m;
   severity info;
   print-severity yes;
   print-time yes;
   print-category yes;
 };
 category default{
  example_log;
 };
};
// required zone for recursive queries
zone "." {
  type hint;
  file "root.servers";
};
// required local host domain
zone "localhost" in{
  type master;
  file "master.localhost";
  allow-update{none;};
};
// localhost reverse map
zone "0.0.127.in-addr.arpa" in{
  type master;
  file "localhost.rev";
  allow-update{none;};
};

Notes:

1. The Caching only name server contains no zones (other than 'localhost') with 'master' or 'slave' types.
2. The reverse map zone has been omitted since it assumed that an external body (ISP etc) has the master domain DNS and is therefore also responsible for the reverse map. It could be added if required for local operational reasons.

6.5 Forwarding (a.k.a. Proxy, Client, Remote) DNS Server

The functionality of the Forwarding name server was previously described.

Forwarding Name Server Configuration

The BIND DNS configuration provides the following functionality:

1. The name server is not a 'master' or 'slave' for any domain
2. provides 'caching' services for all domains
3. forwards all queries to a remote DNS from all local resolvers (Global forwarding)
4. limits query services to local resolvers only - this statement is designed to limit forwarding which both negates the effect of the forwarding server by increasing traffic loads and passes the bogus requests to the remote DNS potentially causing a DoS/DDos attack.

The BIND 'named.conf' is as follows (click to look at any file):

// FORWARDING & CACHING NAME SERVER for EXAMPLE, INC.
// maintained by: me myself alone
// CHANGELOG:
// 1. 9 july 2003 - did something
// 2. 16 july 2003 - did something else
// 3. 23 july 2003 - did something more
//
options {
  directory "/var/named";
  // version statement - inhibited for security
  // (avoids hacking any known weaknesses)	
  version "not currently available";
  forwarders {10.0.0.1; 10.0.0.2;};
  forward only;
  // disables all zone transfer requests
  allow-transfer{"none";};
  // Closed DNS - permits only local IPs to issue queries 
  // remove if an Open DNS required to support all users 
  // or add additional IP ranges 
  // in this case either allow-query or allow-recursion can be used
  allow-query {192.168.3.0/24;};
};
// log to /var/log/example.log all events from
// info UP in severity (no debug)
// defaults to use 3 files in rotation
// BIND 8.x logging MUST COME FIRST in this file
// BIND 9.x parses the whole file before using the log
// failure messages up to this point are in (syslog)
// typically /var/log/messages
logging{
  channel example_log{
   file "/var/log/named/example.log" versions 3;
   severity info;
   print-severity yes;
   print-time yes;
   print-category yes;
 };
 category default{
  example_log;
 };
};
// required local host domain
zone "localhost" in{
  type master;
  file "pri.localhost";
  allow-update{none;};
};
// localhost reverse map
zone "0.0.127.in-addr.arpa" in{
  type master;
  file "localhost.rev";
  allow-update{none;};
};

Notes:

1. The Forwarding name server typically contains no zones (other than 'localhost') with 'master' or 'slave' types.
2. The reverse map zone has been omitted since it assumed that an external body (ISP etc) has the master domain DNS and is therefore also responsible for the reverse map. It could be added if required for local operational reasons.
3. The forward option must be used in conjunction with a forwarders option. The value 'only' will override 'recursive query' behaviour.
4. Since all queries are forwarded the root servers zone ('type hint') can be omitted.
5. Forwarding can be done on a zone basis in which case the values defined override the global options.

6.6 Stealth (a.k.a. Split or DMZ) DNS Server

The functionality of the Stealth name server was previously described. The following diagram illustrates the conceptual view of a Stealth (a.k.a. Split) DNS server system.

Figure 6.1 Split/Stealth Server configuration

The key issue in a 'Stealth' (a.k.a. Split) DNS system is that there is a clear line of demarcation between the 'Internal' Stealth server(s) and the 'External' or Public DNS servers(s). The primary difference in configuration is the 'Stealth' Servers will provide a comprehensive set of services to internal users to include caching and recursive queries and would be configured as a typical Master DNS, while the External server may provide limited services and would typically be configured as an Authoritative Only DNS server.

There are two critical points:

1. The zone file for the 'Stealth' server will contain both public and private hosts, whereas the 'Public' server's master zone file will contain only public hosts.

2. To preserve the 'Stealth' nature it is vital that the PUBLIC DNS configuration does not include options such as 'master', 'allow-notify','allow-transfer', etc. with references to the IP of the 'Stealth' server. If the Stealth servers IP where to appear in the Public DNS server and its file system were to be compromised the attacker could gain more knowledge about the organisation - they can penetrated the 'veil of privacy' by simply inspecting the 'named.conf file.

   There are a number of articles which suggest that the view statement may be used to provide similar functionality using a single server. This does not address the problem of the DNS host system being compromised and by simple 'named.conf' file inspection additional data about the organisation being discovered. In a secure environment 'view' does not provide a 'Stealth DNS' solution if there is any possibility that a filesystem compromise can happen.

6.7 Authoritative Only DNS Server

The functionality of the Authoritative name server was previously described. If security is not the primary requirement then the view statement may be used to provide 'Authoritative only' services to external users and more comprehensive services to internal users.

An example configuration is shown below.

Authoritative Only Name Server Configuration

The BIND DNS configuration provides the following functionality:

1. 'master' DNS for example.com
2. does NOT provide 'caching' services for any other domains
3. does NOT provide recursive query services for all resolvers (Iterative only)
4. optimised for maximum performance

The BIND 'named.conf' is as follows (click to look at any file):

// AUTHORITATIVE ONLY NAME SERVER for EXAMPLE, INC.
// maintained by: me myself alone
// CHANGELOG:
// 1. 9 july 2003 - did something
// 2. 16 july 2003 - did something else
// 3. 23 july 2003 - did something more
//
options {
  directory "/var/named";
  // version statement - inhibited for security
  // (avoids hacking any known weaknesses)	
  version "not currently available";
  // disable all recursion - authoritative only
	recursion no;
  // disables all zone transfer requests in this case 
  // for performance not security reasons
  allow-transfer{none;};
};
//
// log to /var/log/example.log all events
// from info UP in severity (no debug)
// defaults to use 3 files in rotation
// BIND 8.x logging MUST COME FIRST in this file
// BIND 9.x parses the whole file before using the log
// failure messages up to this point are in (syslog)
// typically /var/log/messages
//
  logging{
  channel example_log{
   file "/var/log/named/example.log" versions 3 size 2m;
   severity info;
   print-severity yes;
   print-time yes;
   print-category yes;
 };
 category default{
  example_log;
 };
};
zone "example.com" in{
  type master;
  file "master/master.example.com";
};
// reverse map for class C 192.168.0.0
zone "0.168.192.IN-ADDR.ARPA" in{
  type master;
  file "192.168.0.rev";
};
// required local host domain
zone "localhost" in{
  type master;
  file "master.localhost";
  allow-update{none;};
};
// localhost reverse map
zone "0.0.127.in-addr.arpa" in{
  type master;
  file "localhost.rev";
  allow-update{none;};
};

Notes:

1. The reverse mapping zone (zone "0.168.192.IN-ADDR.ARPA" ) was originally omitted from this server - given the use of reverse look-up by anti-spam systems we have restored the reverse look-up zone. It would - in a perfect world without spam - typically not be present on a performance oriented server.

BIND provides three more parameters to control caching, max-cache-size and max-cache-ttl neither of which will have much effect on performance in the above case and allow-recursion which uses a list of hosts that are permitted to use recursion (all others are not) - a kind of poor man's 'view'.

6.8 View based Authoritative Only DNS Server

The functionality of the Authoritative name server was previously described. If security is not the primary requirement then the view clause may be used to provide 'Authoritative only' services to external users and more comprehensive services to internal users.

View based Authoritative Only Name Server Configuration

The BIND DNS configuration provides the following functionality:

1. 'master' DNS for example.com
2. does NOT provide 'caching' services for any external users
3. does NOT provide recursive query services for any external resolvers (Iterative only)
4. provides 'caching' services for internal users
5. provides recursive query services for internal users

The BIND 'named.conf' is as follows (click to look at any file):

// VIEW BASED AUTHORITATIVE ONLY NAME SERVER for EXAMPLE, INC.
// maintained by: me myself alone
// CHANGELOG:
// 1. 9 july 2003 - did something
// 2. 16 july 2003 - did something else
// 3. 23 july 2003 - did something more
//
// global options
options {
  directory "/var/named";
  // version statement - inhibited for security
  // (avoids hacking any known weaknesses)	
  version "not currently available";
};
//
// log to /var/log/example.log all events
// from info UP in severity (no debug)
// defaults to use 3 files in rotation
// BIND 8.x logging MUST COME FIRST in this file
// BIND 9.x parses the whole file before using the log
// failure messages up to this point are in (syslog)
// typically /var/log/messages
//
  logging{
  channel example_log{
   file "/var/log/named/example.log" versions 3 size 2m;
   severity info;
   print-severity yes;
   print-time yes;
   print-category yes;
 };
 category default{
  example_log;
 };
};
// provide recursive queries and caching for internal users
view "goodguys" {
 match-clients { 192.168.0.0/24; }; // our network
  recursion yes;
  // required zone for recursive queries
  zone "." {
   type hint;
   file "root.servers";
  };
  zone "example.com" {
   type master;
   // private zone file including local hosts
   file "view/master.example.com.internal";
  };
  // required local host domain
  zone "localhost" in{
   type master;
   file "master.localhost";
   allow-update{none;};
  };
  // localhost reverse map
  zone "0.0.127.in-addr.arpa" in{
   type master;
   file "localhost.rev";
   allow-update{none;};
  };
}; // end view

// external hosts view
view "badguys" {
 match-clients {"any"; }; // all other hosts
  // recursion not supported
  recursion no;
 zone "example.com" {
   type master;
	 // only public hosts
   file "view/master.example.com.external";
  };
}; // end view

Notes:

1. All the required zones must be declared in each view.
2. The 'goodguys' view contains the root.servers, 'localhost' and reverse mapping file.
3. The 'badguys' view contains only the required zone files for which we will answer authoritatively.
4. The 'badguys' view may contain an edited version of the reverse map file.

Slave DNS Servers with View Clause When using Slave servers with view clauses it is important to recall that, even when NOTIFY is used, the Slave always initiates the zone tranfer operation using an INCOMING DNS operation (TCP on Port 53 normally). To ensure the correct domain is transferred the match-clients and/or match-destinations statements associated with the views must ensure that the requesting Slave server's IP is directed to the view containing the zone file that should be tranferred.

6.9 Split Horizon using view clause

The functionality of the Split Horizon DNS was previously described. The view clause may be used to return different IP addresses to provide, say, geographically dispersed users with minimum access latency or even a level of load-balancing if you understand the service source-ip access profile (in this case the source-ip ranges in the example would be based on some analysis of incoming traffic not just geographic location).

View based Split Horizon

The BIND DNS configuration provides the following functionality:

1. Assume we want geographic users to get the lowest possible latency from a web service with a name of www.example.com
2. Assume we have a single worldwide email server called mail.example.com
3. Assume addresses 172.16.x.x originate in Mordor and we want them to be serviced by a local web server (172.16.1.1) we have installed in that land.
4. Assume addresses 172.15.x.x and 172.14.x.x originate in Gondor and we want them to be serviced by a local web server (17.15.1.1) we have installed in that land.
5. All other IP originations will default to a www.example.com at 192.168.1.2 (essentially the else condition)
6. For simplicity we assume an authoritative only server is being configured.

The BIND 'named.conf' is as follows (click to look at any file):

// VIEW BASED GEOGRAPHIC DNS SERVER for EXAMPLE, INC.
// maintained by: me myself alone
// CHANGELOG:
// 1. 9 july 2009 - did something
// 2. 16 july 2009 - did something else
// 3. 23 july 2009 - did something more
//
// global options
options {
  directory "/var/named";
  // version statement - inhibited for security
  // (avoids hacking any known weaknesses)	
  version "Name is Bind, James Bind";
  // authors note: No idea who came up with the clever text but if you email 
  // we'd be more than happy to credit it you - you deserve it
  allow-update{none;};  // defaulted if not present
  recursion no;         // authoritative only
};
//
// log to /var/log/example.log all events
// from info UP in severity (no debug)
// defaults to use 3 files in rotation
// BIND 9.x parses the whole file before using the log
// failure messages up to this point are in (syslog)
// typically /var/log/messages
//
  logging{
  channel example_log{
   file "/var/log/named/example.log" versions 3 size 2m;
   severity info;
   print-severity yes;
   print-time yes;
   print-category yes;
 };
 category default{
  example_log;
 };
};
// map service to geographic origination
view "gondor" {
 match-clients { 172.15/16; 172.14/16; }; // originate in gondor
  zone "example.com" {
   type master;
   // zone file will return www.example.com = 172.15.1.1
   file "view/master.example.com.gondor";
  };
}; // end view

view "mordor" {
 match-clients { 172.16/16; }; // originate in mordor
  zone "example.com" {
   type master;
   // zone file will return www.example.com = 172.16.1.1
   file "view/master.example.com.mordor";
  };
}; // end view

// default for everything else lies in a default view
view "default"
 match-clients { "any"; }; // must be in the last block
 zone "example.com" {
   type master;
   // zone file will return www.example.com with default (worldwide) IP
   file "view/master.example.com.default";
  };
};

Notes:

1. All the required zones must be declared in each view.
2. For simplicity of configuration an authoritative only server is assumed and therefore contains no root.servers, 'localhost' or reverse mapping files.
3. The 'mordor' and 'gondor' view zone files map to a geographically local server in this case but this need not be true.
4. The 'default' view must be the last one. It is essentially an 'else condition' since it will accept any IP address not accepted by the previous views.
5. The configuration uses the only the match-clients statement. Two other statements, match-destination and match-recursive-only, neither of which is applicable in the example, may also be used to select which view is used.

<groveling apology> Previous versions of this sample showed the default zones outside the view clauses which worked successfully in the original tests. At some point BIND, in its infinite wisdom, removed this feature. The rule, since at least BIND 9.5, is - if there is any view clause then all zones must be in view clauses.</groveling apology>

Slave DNS Servers with View Clause When using a Slave server with view clauses it is important to recall that, even when NOTIFY is used, the Slave always initiates the zone tranfer operation using an INCOMING DNS operation (TCP on Port 53 normally). To ensure the correct domain is transferred the match-clients and/or match-destinations statements associated with the views must ensure that the requesting Slave server's IP is directed to the view containing the zone file that should be tranferred.

---

Problems, comments, suggestions, corrections (including broken links) or something to add? Please take the time from a busy life to 'mail us' (at top of screen), the webmaster (below) or info-support at zytrax. You will have a warm inner glow for the rest of the day.