Dynamic Host Configuration Protocol (DHCP)

The Dynamic Host Configuration Protocol (DHCP) is a network service that enables host computers to be automatically assigned settings from a server as opposed to manually configuring each network host. Computers configured to be DHCP clients have no control over the settings they receive from the DHCP server, and the configuration is transparent to the computer's user.

note: this package was called dhcp3-server in versions prior to precise 12.04 LTS.

Installation

At a terminal prompt, enter the following command to install dhcpd:

sudo apt-get install isc-dhcp-server

You will probably need to change the default configuration by editing /etc/dhcp3/dhcpd.conf to suit your needs and particular configuration.

You also need to edit /etc/default/isc-dhcp-server to specify the interfaces dhcpd should listen to. By default it listens to eth0.

Also, you have to assign a static ip to the interface that you will use for dhcp. If you will use eth0 for providing addresses in the 192.168.1.x subnet then you should assign for instance ip 192.168.1.1 to the eth0 interface using NetworkManager. Without this step you will get an error from dhcpd when starting the service.

Configuration

The error message the installation ends with might be a little confusing, but the following steps will help you configure the service:

Most commonly, what you want to do is assign an IP address randomly. This can be done with settings as follows:

nano -w /etc/dhcp/dhcpd.conf

# Sample /etc/dhcpd.conf
# (add your comments here) 
default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.1.255;
option routers 192.168.1.254;
option domain-name-servers 192.168.1.1, 192.168.1.2;
option domain-name "mydomain.example";

subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.10 192.168.1.100;
range 192.168.1.150 192.168.1.200;
} 

This will result in the DHCP server giving a client an IP address from the range 192.168.1.10-192.168.1.100 or 192.168.1.150-192.168.1.200. It will lease an IP address for 600 seconds if the client doesn't ask for a specific time frame. Otherwise the maximum (allowed) lease will be 7200 seconds. The server will also "advise" the client that it should use 255.255.255.0 as its subnet mask, 192.168.1.255 as its broadcast address, 192.168.1.254 as the router/gateway and 192.168.1.1 and 192.168.1.2 as its DNS servers.

If you need to specify a WINS server for your Windows clients, you will need to include the netbios-name-servers option, e.g.

nano -w /etc/dhcp/dhcpd.conf

option netbios-name-servers 192.168.1.1; 

Start and stop service

 sudo service isc-dhcp-server restart
 sudo service isc-dhcp-server start
 sudo service isc-dhcp-server stop 

dhcp3-server and multiple interfaces

multiple interfaces example

Interface

nano -w /etc/network/interfaces

auto lo
iface lo inet loopback

mapping hotplug
        script grep
        map eth1

iface eth1 inet dhcp

auto eth0
iface eth0 inet static
    address 10.152.187.1
    netmask 255.255.255.0

auto wlan0
  iface wlan0 inet static
    address 192.168.1.1
    netmask 255.255.255.0
    up     /sbin/iwconfig wlan0 mode TTTTTT && /sbin/iwconfig wlan0 enc
restricted && /sbin/iwconfig wlan0 key [Y] XXXXXXXX && /sbin/iwconfig
wlan0 essid SSSSSSSS

auto eth1

Select Interface card

nano -w /etc/default/isc-dhcp-server

INTERFACES="wlan0 eth0"

Configure Subnet

nano -w /etc/dhcp3/dhcpd.conf

ddns-update-style none;
log-facility local7;

subnet 192.168.1.0 netmask 255.255.255.0 {

        option routers                  192.168.1.1;
        option subnet-mask              255.255.255.0;
        option broadcast-address        192.168.1.255;
        option domain-name-servers      194.168.4.100;
        option ntp-servers              192.168.1.1;
        option netbios-name-servers     192.168.1.1;
        option netbios-node-type 2;
        default-lease-time 86400;
        max-lease-time 86400;

        host bla1 {
                hardware ethernet DD:GH:DF:E5:F7:D7;
                fixed-address 192.168.1.2;
        }
        host bla2 {
                hardware ethernet 00:JJ:YU:38:AC:45;
                fixed-address 192.168.1.20;
        }
}

subnet  10.152.187.0 netmask 255.255.255.0 {

        option routers                  10.152.187.1;
        option subnet-mask              255.255.255.0;
        option broadcast-address        10.152.187.255;
        option domain-name-servers      194.168.4.100;
        option ntp-servers              10.152.187.1;
        option netbios-name-servers     10.152.187.1;
        option netbios-node-type 2;

        default-lease-time 86400;
        max-lease-time 86400;

        host bla3 {
                hardware ethernet 00:KK:HD:66:55:9B;
                fixed-address 10.152.187.2;
        }
}

Check Route

ip route

192.168.1.0/24 dev wlan0  scope link
82.16.TT.0/24 dev eth1  scope link
10.152.187.0/24 dev eth0  scope link
default via 82.16.TT.UU dev eth1

Permission issues with ISC-DHCP server

Sometimes upon rising DHCP server informs about permission errors like

Can't open /etc/dhcp/dhcp.conf: permission denied

or

Can't open /var/lib/dhcp/dhcpd.leases: permission denied.

If after checking the permissions are found to be correct, check apparmor profile for dhcpd:

shell# sudo apparmor_status
apparmor module is loaded.
15 profiles are loaded.
15 profiles are in enforce mode.
   /sbin/dhclient
   /usr/bin/evince
   /usr/bin/evince-previewer
   /usr/bin/evince-thumbnailer
   /usr/lib/NetworkManager/nm-dhcp-client.action
   /usr/lib/connman/scripts/dhclient-script
   /usr/lib/cups/backend/cups-pdf
   /usr/lib/telepathy/mission-control-5
   /usr/lib/telepathy/telepathy-*
   /usr/sbin/cupsd
   /usr/sbin/dhcpd
   /usr/sbin/mysqld-akonadi
   /usr/sbin/mysqld-akonadi///usr/sbin/mysqld
   /usr/sbin/tcpdump
   /usr/share/gdm/guest-session/Xsession
0 profiles are in complain mode.
4 processes have profiles defined.
4 processes are in enforce mode.
   /sbin/dhclient (1092) 
   /sbin/dhclient (1093) 
   /usr/sbin/cupsd (978) 
   /usr/sbin/mysqld-akonadi///usr/sbin/mysqld (2136) 
0 processes are in complain mode.
0 processes are unconfined but have a profile defined.

If /usr/sbin/dhcpd is in the list of profiles do the following:

  • 1.Stop apparmor deamon

sudo /etc/init.d/apparmor stop
  • 2.Edit /etc/apparmor.d/usr.sbin.dhcpd with root permissions and ensure that file has following lines:

/var/lib/dhcp/dhcpd.leases* rwl,
/var/lib/dhcp/dhcpd6.leases* rwl,
/etc/dhcp/dhcpd.conf r,
/etc/dhcp/dhcpd6.conf r,
  • /var/lib/dhcp/dhcpd6.leases and /etc/dhcp/dhcpd6.conf are needed to run DHCP server in IPV6 mode, for example:

 dhcpd -6 -cf /etc/dhcp/dhcpd6.conf -lf /var/lib/dhcp/dhcpd6.leases eth0
  • 3.Start apparmor deamon

sudo /etc/init.d/apparmor start

After this operation apparmor deamon will allow dhcp server to open /etc/dhcp/dhcpd.conf or /var/lib/dhcp/dhcpd.leases files. For more information see man apparmor

isc-dhcp-server (last edited 2015-04-14 09:08:35 by simonmcnair)