Linux DHCP Server Configuration

Dynamic Host Configuration Protocol (DHCP) automatically assigns IP addresses and other network configuration information (subnetmask, broadcast address, etc) to computers on a network. A client configured for DHCP will send out a broadcast request to the DHCP server requesting an address. The DHCP server will then issue a "lease" and assign it to that client. The time period of a valid lease can be specified on the server. DHCP reduces the amount of time required to configure clients and allows one to move a computer to various networks and be configured with the appropriate IP address, gateway and subnet mask. For ISP's it conserves the limited number of IP addresses it may use. DHCP servers may assign a "static" IP address to specified hardware. Microsoft NetBios information is often included in the network information sent by the DHCP server.

 

DHCP assignment:

1. Lease Request: Client broadcasts request to DHCP server with a source address of 0.0.0.0 and a destination address of 255.255.255.255. The request includes the MAC address which is used to direct the reply.
2. IP lease offer: DHCP server replies with an IP address, subnet mask, network gateway, name of the domain, name servers, duration of the lease and the IP address of the DHCP server.
3. Lease Selection: Client recieves offer and broadcasts to al DHCP servers that will accept given offer so that other DHCP server need not make an offer.
4. The DHCP server then sends an ack to the client. The client is configured to use TCP/IP.
5. Lease Renewal: When half of the lease time has expired, the client will issue a new request to the DHCP server.

DHCP server installation:

• Red Hat/CentOS/Fedora: rpm -ivh dhcp-x.xxx.elx.i386.rpm
• Ubuntu/Debian 8: apt-get install dhcp3-server
  ( Later releases of Ubuntu (11.04) used the busybox release known as udhcpd and the configuration is NOT shown here)

Starting DHCP server:

• Red Hat/CentOS/Fedora: service dhcpd start
  (or /etc/rc.d/init.d/dhcpd start for Red Hat, Fedora and CentOS Linux distributions)
• Ubuntu/Debian: /etc/init.d/networking restart

Sample DHCP server config file: (DHCP v3.0.1)

• Red Hat/CentOS/Fedora: /etc/dhcpd.conf
  (See /usr/share/doc/dhcp-3.X/dhcp.conf.sample)
  [Potential Pitfall]: Its /etc/dhcpd.conf NOT /etc/dhcp.conf !!
• Ubuntu/Debian: /etc/default/dhcp3-server

Test configuration file for errors with the following command: /etc/rc.d/init.d/dhcpd configtest
(Other distributions may use: /usr/sbin/dhcpd -f)

Note: The MAC addresses for the static address name server (ns2.your-domain.com), can be obtained with either of the two commands:

• /sbin/ip addr show:

  1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
      link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
      inet 127.0.0.1/8 scope host lo
         valid_lft forever preferred_lft forever
      inet6 ::1/128 scope host 
         valid_lft forever preferred_lft forever
  2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
      link/ether 00:02:c3:d0:e5:83 brd ff:ff:ff:ff:ff:ff
      inet 192.168.42.214/24 brd 192.168.42.255 scope global dynamic eth0
         valid_lft 82646sec preferred_lft 82646sec
      inet6 f080::477:3e0e:d5fd:803a/64 scope link 
         valid_lft forever preferred_lft forever
      

  OR
• /sbin/ifconfig:

  eth0      Link encap:Ethernet  HWaddr 00:02:C3:D0:E5:83
            inet addr:192.168.42.214  Bcast:192.168.42.255  Mask:255.255.255.0
            inet6 addr: fe80::202:b3ff:fef0:e484/64 Scope:Link
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:4070 errors:0 dropped:0 overruns:0 frame:0
            TX packets:3878 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:1000
            RX bytes:3406445 (3.2 MiB)  TX bytes:439612 (429.3 KiB)
      

When dhcpd is running it will generate entries in the file: /var/lib/dhcp/dhcpd.leases

lease 192.168.1.128 {
 starts 2 2004/12/01 20:07:05;
 ends 3 2004/12/02 08:07:05;
 hardware ethernet 00:00:e8:4a:2c:5c;
 uid 01:00:00:e8:4c:5d:31;
 client-hostname "Node1";
}

Options:

• ddns-update-style:
  • interim: allows your DHCP server to update a DNS server whenever it hands out a lease. Allows your DNS server to know which IP addresses are associated with which computers in your network. Requires that your DNS server support DDNS (Dynamic DNS).
  • none: to disable dynamic DNS updates or DNS is not supporting DDNS.
  • ad-hoc: been deprecated and shouldn't be used
• Default options (Red Hat/CentOS/Fedora) are set in /etc/sysconfig/dhcpd

LANs separated by routers: In order to have your DHCP broadcast pass through a router on the the next network, one must configure the router to allow DHCP relay. (Cisco: ip-helper address, Juniper: dhcp-relay) The local lan subnet{ } configuration must come before the configuration directives of the remote LANs.

Look for errors in /var/log/messages

See dhcp-options man page below.

• Client: dhclient: DHCP client daemon (replaces dhcpcd)
  • dhclient.conf: DHCP client configuration file (/etc/dhcp/dhcpd.conf)
  • dhclient.leases: DHCP client lease database (/var/lib/dhcpd/dhcpd.leases)
  • dhclient-script: DHCP client network configuration script
• Server: dhcpd: Dynamic Host Configuration Protocol Server daemon
  • dhcpd.conf: dhcpd configuration file
  • dhcpd.leases: dhcpd DHCP client lease database
  • dhcp-options: dhcpd Dynamic Host Configuration Protocol options
• dhcrelay: bootp relay agent. One DHCP server to service multiple network segments.
• omshell: OMAPI Command Shell - interactive way to connect to, query, and possibly change, the ISC DHCP Server's state via OMAPI One can make the changes while the server is running.

DHCP RFC's: RFC2131, RFC1541 (obsolete), RFC2132

Note: DHCP client will overwrite your /etc/resolv.conf file with new information recieved from the DHCP server.

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