Dynamic Host Configuration Protocol

Dynamic Host Configuration Protocol
	Chapter 4. IP Address Allocation

What is it?

Dynamic addressing is different. Dynamic addressing purely comes from the DHCP server.

DHCP server stands for Dynamic Host Configuration Protocol.

It 's job is to provide DHCP clients with IP addresses and other relevant information, for example, to provide the default gateway, the IP address, the DNS resolver and a hostname etcetera.

The DHCP server is able to offer IP addresses, to re-claim IP addresses and even to expire IP addresses.

Boot Protocol

There is a protocol called BOOTP, which is the protocol used to boot X workstations and other types of devices. Its job is to offer up an image for the workstation to boot.

Now DHCP and BOOTP are very similar protocols. They both offer similar services and in fact DHCP can service BOOTP requests.

The only difference is that BOOTP servers can't re-claim or expire IP's.

This means that DHCP has become the standard in dynamics of controlling of IP addresses.

Essentially, DHCP was developed to alleviate the problems that arose when assigning static IP addresses throughout a really large network. (If you have 5 hosts, it 's not a problem but if you've got 5000 hosts, it could be a problem and you might re-assign IP addresses and duplicate IP addresses and so on.)

How does DHCP work?

A DHCP server listens for requests.
When you switch your workstation on, the workstation sends out a broadcast message requesting a DHCP server.
If you have more than one DHCP server on the network, the first one to respond is the one that services this request.
The DHCP server hears the broadcast and responds with an IP address and the other information that we can give the client.

The cycle

When the broadcast is put on the network the DHCP server updates its ARP table, responds to the client who also updates its ARP table. The client is offered an address, which it accepts. There is a lead time on that address and when the lead time has expired the server will force the client to get a new IP address and the whole broadcast process happens again.

Why DHCP is restricted to a broadcast domain

One of the problems with DHCP is that, if the DHCP server is enclosed by routers on either side then this becomes the broadcast domain and the broadcast domain restricts DHCP requests to this subnetwork. This implies that clients wishing to obtain DHCP requests on the opposite side of the routers will not be capable of receiving them.

If another server on the network were to request a DHCP address it would only progress as far as the router at which point it would stop.

Well, if this is a Linux machine, there are some parameters in "sysctl" (System control) that would allow you to forward DHCP requests.

The exact parameter that one needs to specify is in "net/ipv4/conf/eth0/boot_relay", I've only got one Internet address, and you'll see if you do a "sysctl -a | grep bootp" that this parameter is currently set at zero:

debian:~# sysctl -a | grep bootp
net/ipv4/conf/eth0/bootp_relay = 0
net/ipv4/conf/lo/bootp_relay = 0
net/ipv4/conf/default/bootp_relay = 0
net/ipv4/conf/all/bootp_relay = 0

When we run the following command:

debian:~# echo &quot;1&quot; &gt;/proc/sys/net/ipv4/conf/eth0/bootp_relay

The Linux router will now be able to forward BOOTP requests, where before it stopped them, it will now forward them to the DHCP server and the DHCP server will respond.

The following screen shot is the result of running the "sysctl -a | grep bootp" command after running the "echo "1" >/proc/sys/net/ipv4/conf/eth0/bootp_relay" command.

 
debian:~# sysctl -a | grep bootp
net/ipv4/conf/eth0/bootp_relay = 1
net/ipv4/conf/lo/bootp_relay = 0
net/ipv4/conf/default/bootp_relay = 0
net/ipv4/conf/all/bootp_relay = 0

The process that we've just undergone where we've echoed "1" into the boot_relay field is a dynamic process, it 's an on-the-fly configuration.

To set that field permanently you should edit the file called "etc/sysctrl.conf".

I encourage you to go and read the man pages on sysctrl.conf with "man sysctrl 5" and reading that section will show you how to set up BOOTP relay.

Explain "dhclient"

On the client machine we need an application to request the DHCP service from the server. There are a couple of options, "dhclient" is one and "pump" is another.

The workings of DHCP client and pump are not that critical to understand in order to use them.

What is more important is how to set up your address to become statically or dynamically applied each time you restart your network.

How to obtain the address of the DHCP server

In order to use DHCP to find an IP address every time you restart your interface you need to go back to your etc/network/interfaces file. In there you would put an "iface eth0 inet DHCP" which would cause your network to come up every time with a DHCP server provided valid IP address.

/etc/network/interfaces -- configuration file for ifup(8), ifdown(8)

# The loopback interface
auto lo eth0
iface lo inet loopback

#mapping eth0
#       map WORK work-eth0

iface eth0 inet DHCP
        address 192.168.10.144
        netmask 255.255.255.0
        gateway address 192.168.10.1

In Conclusion:

When working with client/server technology, a DHCP server is inter-operable, so even if your DHCP server happens to be a Windows NT server and your client happens to be a Linux client, you will still get an IP address.

The server and the client are operating independently. As long as they stick to the same protocol, we will be able to get an IP address.

This interoperability is shown in that Linux is inter-operable with other alternatives, the DHCP servers could be a Solaris machine or any other type of Unix.


Chapter 4. IP Address Allocation		Chapter 5. Basic Network Configuration