DHCP - Overview & Configuration Manual

Transcription

1 DHCP - Overview & Configuration Manual October 4, 2010 Prepared By KTH CSD Carenet-SomaliREN team

2 Table of Contents Introduction... 2 IP Addressing and Types... 2 Static IP-Address Assignment:... 2 Dynamic IP-Address Assignment:... 3 Dynamic Host Configuration Protocol (DHCP)... 4 Configuration Guide... 5 References

3 Introduction It is a universally accepted fact that the evolution of the Internet began from a small research project initiated at the Massachusetts Institute of Technology and the Defense Advanced Research Projects Agency (DARPA) by the name of Advanced Research Projects Agency Network (ARPANET) [1]. The goal was to establish an operational packet switching network, which later expanded into the global Internet. By then, the ARPANET was a small community comprising of a few hundred hosts. With the rapid escalation of this project into a global internet, with thousands of hosts (at initial stages); it was soon realized the manual handling of the attached hosts and other networking devices was a tough task and there was an immense need to device and design automated procedures to conduct these manually intensive tasks. One of these tasks is the assignment of IP addresses to the hosts and other network elements attached to the network, and the major procedures to perform this task are discussed in the following section. IP Addressing and Types Static IP-Address Assignment In the past, the network administrators used to perform static IP-addressing in the network. This meant that the network administrator had to physically configure the IP-addresses on the network elements in the complete network. As perceivable, the assignment of IP-addresses to all the hosts in the network was a physically intensive job and had other issues including: - Physically visiting the device and configuring the IP-address - Have the complete IP-addressing plan at hand, when configuring the devices; so that one has all the information required for configuration - The inflexible nature of the address assignment, which means that the network administrator could not change the addressing scheme of the gate-way or other servers operating in the network, which have some dependency on the network hosts. For 2

4 example, if the IP address of the DNS server/gateway is changed, the network administrator had to manually change the IP-address on all the hosts in the network - The hosts/network devices cannot be moved to a different subnet, as then the IPaddressing was required to change again Besides these disadvantages and the need of manually intensive tasks related to static IP addressing, one of the major problem from the network point of view is the low address efficient nature of static assignment. On an up and running network, all the hosts in the network are not in active state simultaneously; some hosts may be shut-down or in hibernation. With static IP-addressing, the addresses for these non-active hosts are also fixed and cannot be utilized, even though the hosts are not active. Dynamic IP-Address Assignment Dynamic IP addressing is an alternative to the per-host static IP-address configuration, which addresses the shortcomings of the static address assignment and provides flexibility in the network management procedures. In dynamic address assignment, the network administrator defines the range of network addresses in a pool which can be used by the active hosts in the network dynamically. This means that if a host is down (inactive), it will not bound or hold an IP-address and contribute to increase the network capacity. Other advantages of the dynamic addressing include the facility of self-configuration and address assignment of the hosts, without the need for human intervention. The dynamic addressing assignment can automatically provide the IP address, subnet mask and the default gateway for the hosts and network elements in the network. Furthermore, the addresses can be changed and updated at all the hosts automatically. 3

5 Dynamic Host Configuration Protocol (DHCP) To perform this automatic address assignment, the Dynamic Host Configuration Protocol (DHCP) was designed. With DHCP, the network administrator can (permanently or for a particular time) assign an IP-address to the hosts. The protocol can additionally inform the hosts about their subnet mask and the first hop router (i.e. the default gateway). DHCP is also referred to as a plug-and-play protocol because of the network flexibility and automation capabilities in the protocol. Similar to Domain Name System (DNS) server, the DHCP is also a client-server protocol, in which a newly arriving client on the network requests for IP-address from the server on the network and the server responds with all the information the client requires to have access to the Internet. Allocation of the IP-address to the client is a four-step process and a two-way handshake happens between the client and the server twice in the whole address allocation procedure. The messages exchanged between the client and server during the address assignment process are as follows: DHCP Discover Message As the clients or hosts become active in the network, they tend to find a DHCP server with which it has to interact for the address assignment. The client broadcasts a DHCP discover message on the network with the source address field containing ( ), since the client doesn t have any IP-address assigned yet. DHCP Offer Message The DHCP server on receiving the DHCP discover message, responds to the client by sending a DHCP offer message, which is again broadcasted on the network (since the client has no address yet). The offer contains an IP-address proposed for the client by the server, the network mask and the time until which the client can use this IP address (lease time). DHCP Request Message The client responds to the server by a request message accepting or rejecting the addresses offered by the server by echoing the information sent by the server back to it. 4

6 DHCP Acknowledge Message The server then responds with a DHCP ACK message to confirm the parameters to the client and complete the handshake procedure. The following figure presents the association procedure: DHCP Client Server Handshake [2] Configuration Guide Installing and configuring a DHCP server on the Linux Ubuntu platform is a relatively simple procedure. The installation can be done using the following command: Sudo apt-get install dhcp3-server 5

7 Figure 1: DHCP Installation The configuration files that have to be edited to set-up the DHCP server include: - The default configuration file, located in /etc/dhcp3/dhcpd.conf - /etc/default/dhcp3-server file to set the port to which the dhcpd process will listen for DHCP discover messages In the following scenario, we assume a /24 network /24 and assume that our DHCP server shall be capable of providing IP-addresses to a maximum of 100 hosts. Other parameters set are as follows: - Lease time The time for which the client will hold the IP address assigned by the DHCP server = 10 minutes, after which the client shall ask for the IP again, if the client does not ask for a maximum period of 2 hrs, the IP address shall be taken back from the client and assigned to another host - The server shall assign the subnet mask and inform the client about the broadcast address of the network - Finally, the server will inform the client about the network gateway and DNS server. 6

8 The following commands demonstrate how to configure the dhcpd.conf file, as per the defined scenario: # /etc/dhcpd.conf Default-lease-time 600; Max-lease-time 7200; #10 minutes # 2 hrs Option subnet-mask ; # /24 subnet mask Option broadcast-address ; Option routers ; # default gateway of the network Option domain-name-servers ; Option domain-name Somaliren.com ; # DNS server address # domain name of our domain subnet netmask { Range ; # starting from.3 because.1 and.2 is assigned to DNS # and Gateway already } References [1] - [2] - 7