* dhcpd *
20030607

INTRO:

DHCP, or dynamic host configuration protocol, is used so that clients can
obtain IP addresses automatically by querying a server (via dhcp arp).  The
DHCP server may give arbitrary IPs to all querying hosts or it may give
specific IPs to hosts with specific hardware (MAC) addresses.  This document
assumes the latter, as that is how we have been using it.  In theory, all
DHCP clients have the same behavior; in reality, different OSes treat DHCP
slightly differently.  This document covers the ISC DHCP distribution
(dhcpd).  For more information man dhcpd and dhcpd.conf, read the docs in
/usr/share/doc/dhcp/, and see:
  http://www.isc.org/products/DHCP/ 
  http://www.tldp.org/HOWTO/mini/DHCP/.


SETUP:

The package is dhcp; install it with 'apt-get install dhcpd'.  dhcpd won't
start running until you explicitly tell it to by modifying /etc/init.d/dhcp,
which is a good thing because you never want to have two dhcp servers on the
same (physical) network.

The main configuration file is /etc/dhcpd.conf (more on this later). You
also need to tell dhcp what interface(s) to run on in /etc/default/dhcp. 
The daemon is dhcpd and can be started with '/etc/init.d/dhcp start' or
'/usr/sbin/dhcpd eth0' (assuming eth0 is the correct interface).

The configuration of /etc/dhcpd.conf is not very complicated, but needs to
be done with some amount of care, as dhcpd will not start if the syntax of
the file is not correct.  More specifically, use a semicolon (';') at the
end of each statement and group declarations with curly braces ('{' and
'}').  Lines beginning with '#' are comments. Do not omit any information.

dhcpd.conf consists of three parts: lease information, subnet declarations,
and assigned leases.  

* lease information: Lease information includes  default-lease-time,
max-lease-time, and any options which will apply to all subnets.  For
example, guardian's dhcpd.conf file begins as follows:

  # dhcpd.conf
  default-lease-time 36000;
  max-lease-time 108000;
  option domain-name "example.com"; 
  option domain-name-servers 192.168.0.1,192.168.0.2;

* subnet declarations: A declaration for the shared-network follows the
leaase information, with sub-declarations for each subnet.  Options include
(but need not be limited to): routers (typically first address in the
subnet), broadcast-address (last address in the subnet), and subnet-mask
(netmask).  An excerpt of guardian's dhcpd.conf, serving dhcp to three
different subnets, follows:

  shared-network CAMELID {

  	# IP group 1:
  	subnet 192.168.1.0 netmask 255.255.255.0 {
  		option broadcast-address 192.168.1.255;
  		option routers 192.168.1.1;
  		option netbios-name-servers 192.168.1.1;
  	}

  	# IP group 2:
  	subnet 64.209.122.0 netmask 255.255.255.0 {
  		option broadcast-address 192.168.2.255;
  		option routers 192.168.2.1;
  		option netbios-name-servers 192.168.2.1;
  	}
  }

* assigned leases: Finally, there are assigned leases.  Host can be assigned
IPs "randomly" by specifying a range of available IPs.  This is accomplished
by adding a range line to the subnet declaration.  For example, to specify
that the IPs from 192.168.2.20 to 192.168.2.250 are available for dhcp
clientsm, add the line:

  range 192.168.2.20 192.168.2.250;

Alternatively/additionally, hosts can be assigned IPs/hostnames which are
tied to their hardware address.  There needs to be a separate host entry for
each such hosts.  For example, to assign the address camel.example.com 
to mac address 00:01:02:03:04:05:

  host camel {
    hardware ethernet 00:01:02:03:04:05;
    fixed-address camel.example.com;
  }


MAINTENANCE:

* logs: dhcp logs lease activity (DHCPDISCOVER, DHCPREQUEST, DHCPACK) to
/var/log/daemon.log.  To find out when a particular machine (IP or mac
adddress) obtained a lease, 'grep DHCPACK /var/log/daemon.log | grep
searchstring'.  Dyanmic leases, if allowed, are recorded in
/var/lib/dhcp/dhcpd.leases.  If dynamic leases are not allowed, you can find
unregistered machines with 'grep -B1 free /var/log/daemon.log'.  In the
event of multiple DHCP servers on the same subnet, there should be DHCPNAK
lines in /var/log/daemon.log. 

* process management: dhcpd must be restarted after configurations are made. 
The process id of dhcpd is recorded in /var/run/dhcpd.pid (as in, 'kill `cat
/var/run/dhcpd.pid` && /usr/sbin/dhcpd eth0').  dhcpd is started at boot
time with '/etc/init.d/dhcp start' (or rather by running /etc/rc*.d/S*dhcp). 
When restarting dhcp, run 'tail -f /var/log/daemon.log' to look for errors.