Linux DHCP Server Configuration

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.

• 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)

• 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

• 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

ddns-update-style interim; # Required for dhcp 3.0+ / Red Hat 8.0+ ignore client-updates; subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.128 192.168.1.254; # Range of IP addresses to be issued to DHCP clients option subnet-mask 255.255.255.0; # Default subnet mask to be used by DHCP clients option broadcast-address 192.168.1.255; # Default broadcastaddress to be used by DHCP clients option routers 192.168.1.1; # Default gateway to be used by DHCP clients option domain-name "your-domain.org"; option domain-name-servers 40.175.42.254, 40.175.42.253; # Default DNS to be used by DHCP clients option netbios-name-servers 192.168.1.100; # Specify a WINS server for MS/Windows clients. # (Optional. Specify if used on your network) # DHCP requests are not forwarded. Applies when there is more than one ethernet device and forwarding is configured. # option ipforwarding off; default-lease-time 21600; # Amount of time in seconds that a client may keep the IP address max-lease-time 43200; option time-offset -18000; # Eastern Standard Time # option ntp-servers 192.168.1.1; # Default NTP server to be used by DHCP clients # option netbios-name-servers 192.168.1.1; # --- Selects point-to-point node (default is hybrid). Don't change this unless you understand Netbios very well # option netbios-node-type 2; # We want the nameserver "ns2" to appear at a fixed address. # Name server with this specified MAC address will recieve this IP. host ns2 { next-server ns2.your-domain.com; hardware ethernet 00:02:c3:d0:e5:83; fixed-address 40.175.42.254; } # Laser printer obtains IP address via DHCP. This assures that the # printer with this MAC address will get this IP address every time. host laser-printer-lex1 { hardware ethernet 08:00:2b:4c:a3:82; fixed-address 192.168.1.120; } }

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 the command /sbin/ifconfig:

eth0 Link encap:Ethernet HWaddr 00:02:C3:D0:E5:83 inet addr:40.175.42.254 Bcast:40.175.42.255 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"; }

• 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.

• dhcpcd: DHCP client daemon
  • /etc/dhcpc:
  • /etc/dhcpc/dhcpcd-<interface>.info: dhcpcd stores host information in this file. (interface=eth0 or eth1,...)
  • /etc/dhcpc/dhcpcd-<interface>.cache: Cache file containing the previously assigned IP address.
• 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 - an interactive way to connect to, query, and change, the ISC DHCP Server’s state via OMAPI. One can make the changes while the server is running.
• pump: configure network interface via BOOTP or DHCP protocol

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.