Chapter 48. DNS and DHCP Configuration Guide
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## Chapter 48. DNS and DHCP Configuration Guide

### John H. Terpstra

Samba Team

`<jht@samba.org>`

## Features and Benefits

There are few subjects in the UNIX world that might raise as much contention as Domain Name System (DNS) and Dynamic Host Configuration Protocol (DHCP). Not all opinions held for or against particular implementations of DNS and DHCP are valid.

We live in a modern age where many information technology users demand mobility and freedom. Microsoft Windows users in particular expect to be able to plug their notebook computer into a network port and have things “just work.”

UNIX administrators have a point. Many of the normative practices in the Microsoft Windows world at best border on bad practice from a security perspective. Microsoft Windows networking protocols allow workstations to arbitrarily register themselves on a network. Windows 2000 Active Directory registers entries in the DNS namespace that are equally perplexing to UNIX administrators. Welcome to the new world!

The purpose of this chapter is to demonstrate the configuration of the Internet Software Consortium (ISC) DNS and DHCP servers to provide dynamic services that are compatible with their equivalents in the Microsoft Windows 2000 Server products.

This chapter provides no more than a working example of configuration files for both DNS and DHCP servers. The examples used match configuration examples used elsewhere in this document.

This chapter explicitly does not provide a tutorial, nor does it pretend to be a reference guide on DNS and DHCP, as this is well beyond the scope and intent of this document as a whole. Anyone who wants more detailed reference materials on DNS or DHCP should visit the ISC Web site at http://www.isc.org. Those wanting a written text might also be interested in the O'Reilly publications on DNS, see the O'Reilly web site, and the BIND9.NET web site for details. The books are:

1. DNS and BIND, By Cricket Liu, Paul Albitz, ISBN: 1-56592-010-4

2. DNS & Bind Cookbook, By Cricket Liu, ISBN: 0-596-00410-9

3. The DHCP Handbook (2nd Edition), By: Ralph Droms, Ted Lemon, ISBN 0-672-32327-3

## Example Configuration

The DNS is to the Internet what water is to life. Nearly all information resources (host names) are resolved to their Internet protocol (IP) addresses through DNS. Windows networking tried hard to avoid the complexities of DNS, but alas, DNS won. The alternative to DNS, the Windows Internet Name Service (WINS) an artifact of NetBIOS networking over the TCP/IP protocols has demonstrated scalability problems as well as a flat, nonhierarchical namespace that became unmanageable as the size and complexity of information technology networks grew.

WINS is a Microsoft implementation of the RFC1001/1002 NetBIOS Name Service (NBNS). It allows NetBIOS clients (like Microsoft Windows machines) to register an arbitrary machine name that the administrator or user has chosen together with the IP address that the machine has been given. Through the use of WINS, network client machines could resolve machine names to their IP address.

The demand for an alternative to the limitations of NetBIOS networking finally drove Microsoft to use DNS and Active Directory. Microsoft's new implementation attempts to use DNS in a manner similar to the way that WINS is used for NetBIOS networking. Both WINS and Microsoft DNS rely on dynamic name registration.

Microsoft Windows clients can perform dynamic name registration to the DNS server on startup. Alternatively, where DHCP is used to assign workstation IP addresses, it is possible to register hostnames and their IP address by the DHCP server as soon as a client acknowledges an IP address lease. Finally, Microsoft DNS can resolve hostnames via Microsoft WINS.

The following configurations demonstrate a simple, insecure dynamic DNS server and a simple DHCP server that matches the DNS configuration.

### Dynamic DNS

The example DNS configuration is for a private network in the IP address space for network 192.168.1.0/24. The private class network address space is set forth in RFC1918.

It is assumed that this network will be situated behind a secure firewall. The files that follow work with ISC BIND version 9. BIND is the Berkeley Internet Name Daemon.

The master configuration file `/etc/named.conf` determines the location of all further configuration files used. The location and name of this file is specified in the startup script that is part of the operating system.

    # Quenya.Org configuration file

    acl mynet {
    	192.168.1.0/24;
    	127.0.0.1;
    };

    options {

    	directory "/var/named";
    	listen-on-v6 { any; };
    	notify no;
    	forward first;
    	forwarders {
    		192.168.1.1;
    		};
    	auth-nxdomain yes;
    	multiple-cnames yes;
    	listen-on {
    		mynet;
    		};
    };

    # The following three zone definitions do not need any modification.
    # The first one defines localhost while the second defines the
    # reverse lookup for localhost. The last zone "." is the
    # definition of the root name servers.

    zone "localhost" in {
    	type master;
    	file "localhost.zone";
    };

    zone "0.0.127.in-addr.arpa" in {
    	type master;
    	file "127.0.0.zone";
    };

    zone "." in {
    	type hint;
    	file "root.hint";
    };

    # You can insert further zone records for your own domains below.

    zone "quenya.org" {
    	type master;
    	file "/var/named/quenya.org.hosts";
    	allow-query {
    		mynet;
    		};
    	allow-transfer {
    		mynet;
    		};
    	allow-update {
    		mynet;
    		};
    	};

    zone "1.168.192.in-addr.arpa" {
    	type master;
    	file "/var/named/192.168.1.0.rev";
    	allow-query {
    		mynet;
    	};
    	allow-transfer {
    		mynet;
    	};
    	allow-update {
    		mynet;
    	};
    };

The following files are all located in the directory `/var/named`. This is the `/var/named/localhost.zone` file:

    $TTL 1W
    @               IN SOA  @   root (
    				42              ; serial (d. adams)
    				2D              ; refresh
    				4H              ; retry
    				6W              ; expiry
    				1W )            ; minimum

    		IN NS           @
    		IN A            127.0.0.1

The `/var/named/127.0.0.zone` file:

    $TTL 1W
    @               IN SOA          localhost.  root.localhost. (
    				42              ; serial (d. adams)
    				2D              ; refresh
    				4H              ; retry
    				6W              ; expiry
    				1W )            ; minimum

    				IN NS           localhost.
    1               IN PTR          localhost.

The `/var/named/quenya.org.host` file:

    $ORIGIN .
    $TTL 38400      ; 10 hours 40 minutes
    quenya.org      IN SOA  marvel.quenya.org. root.quenya.org. (
    				2003021832 ; serial
    				10800      ; refresh (3 hours)
    				3600       ; retry (1 hour)
    				604800     ; expire (1 week)
    				38400      ; minimum (10 hours 40 minutes)
    				)
    			NS      marvel.quenya.org.
    			MX      10 mail.quenya.org.
    $ORIGIN quenya.org.
    frodo                   A       192.168.1.1
    marvel                  A       192.168.1.2
    ;
    mail                    CNAME   marvel
    www                     CNAME   marvel

The `/var/named/192.168.1.0.rev` file:

    $ORIGIN .
    $TTL 38400      ; 10 hours 40 minutes
    1.168.192.in-addr.arpa  IN SOA  marvel.quenya.org. root.quenya.org. (
    				2003021824 ; serial
    				10800      ; refresh (3 hours)
    				3600       ; retry (1 hour)
    				604800     ; expire (1 week)
    				38400      ; minimum (10 hours 40 minutes)
    				)
    			NS      marvel.quenya.org.
    $ORIGIN 1.168.192.in-addr.arpa.
    1                       PTR     frodo.quenya.org.
    2                       PTR     marvel.quenya.org.

The configuration files shown here were copied from a fully working system. All dynamically registered entries have been removed. In addition to these files, BIND version 9 will create for each of the dynamic registration files a file that has a `.jnl` extension. Do not edit or tamper with the configuration files or with the `.jnl` files that are created.

### DHCP Server

The following file is used with the ISC DHCP Server version 3. The file is located in `/etc/dhcpd.conf`:

    ddns-updates on;
    ddns-domainname "quenya.org";
    option ntp-servers 192.168.1.2;
    ddns-update-style ad-hoc;
    allow unknown-clients;
    default-lease-time 86400;
    max-lease-time 172800;

    option domain-name "quenya.org";
    option domain-name-servers 192.168.1.2;
    option netbios-name-servers 192.168.1.2;
    option netbios-dd-server 192.168.1.2;
    option netbios-node-type 8;

    subnet 192.168.1.0 netmask 255.255.255.0 {
    	range dynamic-bootp 192.168.1.60 192.168.1.254;
    	option subnet-mask 255.255.255.0;
    	option routers 192.168.1.2;
    	allow unknown-clients;
    }

In this example, IP addresses between 192.168.1.1 and 192.168.1.59 are reserved for fixed-address (commonly called `hard-wired`) IP addresses. The addresses between 192.168.1.60 and 192.168.1.254 are allocated for dynamic use.

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