Configuring a Linux Wireless Access Point

4.3 Setting Up Name Services

Problem

Your LAN is going to have a combination of hosts with static IP addresses and DHCP clients that come and go, especially wireless clients. And, you want DHCP clients to automatically be entered into DNS so they can be accessed by hostname just like the hosts with static IP addresses.

Solution

You don’t want much. Fortunately, you can have it all. Pyramid comes with dnsmasq, which handles DHCP and DNS, and automatically enters DHCP clients into DNS. This requires the clients to send their hostnames when they are requesting a DHCP lease. Windows clients do this by default. Most Linux clients do not, so go to Recipe 4.5 to learn about client configuration.

Now, we’ll edit /etc/dnsmasq.conf on your Pyramid box. First make the filesystem writeable by running/sbin/rw. Copy this example, using your own network name instead of alrac.net, whatever DHCP range you prefer, and your own upstream nameservers:

  pyramid:~# /sbin/rw
  pyramid:~# nano /etc/dnsmasq.conf

  domain-needed
  bogus-priv
  local=/alrac.net/
  expand-hosts
  domain=alrac.net
  interface=br0
  listen-address=127.0.0.1

  #upstream nameservers
  server=22.33.44.2
  server=22.33.44.3

  dhcp-range=lan,192.168.1.100,192.168.1.200,12h
  dhcp-lease-max=100

Next, add all of your hosts that already have static IP addresses to /etc/hosts on your Pyramid box, using only their hostnames and IP addresses. At a minimum, you must have an entry for localhost and your Pyramid router:

  ## /etc/hosts
  127.0.0.1      localhost
  192.168.1.50   pyramid
  192.168.1.10   xena
  192.168.1.74   uberpc

Restart dnsmasq:

  pyramidwrap:~# killall dnsmasq

To test your new nameserver, ping your LAN hosts from each other:

  $ ping pyramid
  $ ping xena
  $ ping uberpc

You should see responses like this:

  PING pyramid.alrac.net (192.168.1.50) 56(84) bytes of data.
  64 bytes from pyramid.alrac.net (192.168.1.50): icmp_seq=1 ttl=64 time=0.483 ms
  64 bytes from pyramid.alrac.net (192.168.1.50): icmp_seq=2 ttl=64 time=0.846 ms

You should be able to ping both wired and wireless clients, and DHCP clients should be entered automatically into the DNS table as well.

Finally, verify that their domain names are correctly assigned by DNS:

  $ hostname
  xena
  $ hostname -f
  xena.alrac.net
  $ dnsdomainname
  alrac.net

Discussion

Pyramid Linux mounts a number of files into a temporary, writeable filesystem, like /etc/resolv.conf. You can see which ones they are by looking in /rw, or running ls -l /etc to see which ones are symlinked to /rw. These are copied over from /ro on boot. It’s designed to keep flash writes down. So, you can either edit /ro, or make the files in /etc immutable.

dnsmasq.conf crams a lot of functionality into a few lines, so let’s take a closer look:

domain-needed

Do not forward requests for plain hostnames that do not have dots or domain parts to upstream DNS servers. If the name is not in /etc/hosts or DHCP, it returns a “not found” answer. This means that incomplete requests (for example, “google” or “oreilly” instead of google.com or oreilly.com) will be cut off before they leave your network.

bogus-priv

Short for “bogus private lookups.” Any reverse lookups for private IP ranges (such as 192.168.x.x) are not forwarded upstream. If they aren’t found in /etc/hosts, or the DHCP leases file, “no such domain” is the answer. Usingdomain-neededandbogus-privare simple options for practicing good Netizenship.

local=/alrac.net/

Put your local domain name here so queries for your local domain will only be answered from /etc/hosts and DHCP, and not forwarded upstream. This is a nice bit of magic that lets you choose any domain name for your private network and not have to register it. To make this work right, you also need theexpand-hostsanddomainoptions.

expand-hosts

This automatically adds the domain name to the hostnames.

domain=alrac.net

expand-hostslooks here for the domain name.

interface

Define which interface dnsmasq should listen to. Use one line per interface, if you have more than one.

listen-address=127.0.0.1

This tells dnsmasq to also use its own local cache instead of querying the upstream nameservers for every request. This speeds up lookups made from the router, and it also allows the router to use your local DNS. You can verify this by pinging your LAN hosts from the router by their hostnames or FQDNs.

server

The server option is used for several different purposes; here, it defines your upstream DNS servers.

dhcp-range=lan,192.168.1.100,192.168.1.200,12h

Define your pool of DHCP leases and lease time, and define a network zone called “lan.” Using named zones lets you assign servers and routes to groups of clients and different subnets; see Recipe 3.13 to see this in action.

dhcp-max-lease

Maximum limit of total DHCP leases. The default is 150. You may have as many as your address range supports.

See Also

1. Recipe 4.12 for an example of using named zones
   
2. man 8 dnsmasq contains a wealth of helpful information about all the available command-line options, many of which are also dnsmasq.conf options
   
3. dnsmasq.conf is also a great help resource
   
4. dnsmasq home page is where you’ll find mailing list archives and excellent help documents: http://www.thekelleys.org.uk/dnsmasq/doc.html
   
5. Chapter 24, “Managing Name Resolution,” in Linux Cookbook, by Carla Schroder (O’Reilly)