Dynamic Host Configuration Protocol
Table of Contents
Overview of DHCP
Managing hosts on a network can be a daunting and time consuming task. Every single computer on a modern TCP/IP based network needs to have at least four things specifically configured. An IP address, the subnet mask for the local network, a primary gateway and a name server. On their own, these four things don't seem like much, but when you have to configure them on hundreds of machines it becomes super tedious. Out of these four things, three are likely the same on just about every node on the network. The subnet mask, the primary gateway, and DNS server. But the last item an IP address needs to be different on every single node on the network. That could require a lot of tricky configuration work, and this is where DHCP or Dynamic Host Configuration Protocol comes into play. Listen up, because DHCP is critical to know as an IT support specialist when it comes to troubleshooting networks.
DHCP(Dynamic Host Configuration Protocol) is an application layer protocol that automates the configuration process of hosts on a network.
With DHCP, a machine can query a DHCP server when the computer connects to the network and receive all the networking configuration in one go.
Not only does DHCP reduce the administrative overhead of having to configure lots of network devices on a single network, it also helps address the problem of having to choose what IP to assign to what machine. Every computer on a network requires an IP for communications, but very few of them require an IP that would be commonly known. For servers or network equipment on your network, like your gateway router, a static and known IP address is pretty important. For example, the devices on a network need to know the IP of their gateway at all time. If the local DNS server was malfunctioning, network administrators would still need a way to connect to some of these devices through their IP. Without a static IP configured for a DNS server, it would be hard to connect to it to diagnose any problems if it was malfunctioning. But for a bunch of client devices like desktops or laptops or even mobile phones, it's really only important that they have an IP on the right network. It's much less important exactly which IP that is.
Using DHCP you can configure a range of IP addresses that's set aside for these client devices. This ensures that any of these devices can obtain an IP address when they need one. But solves the problem of having to maintain a list of every node on the network and its corresponding IP.
There are a few standard ways that DHCP can operate. DHCP dynamic allocation, is the most common, and it works how we described it just now.
A range of IP addresses is set aside for client devices and one of these IPs is issued to these devices when they request one.
Under a dynamic allocation the IP of a computer could be different almost every time it connects to the network. Automatic allocation is very similar to dynamic allocation, in that
A range of IP addresses is set aside for assignment purposes.
The main difference here is that, the DHCP server is asked to keep track of which IPs it's assigned to certain devices in the past. Using this information, the DHCP server will assign the same IP to the same machine each time if possible.
Finally, there's what's known as fixed allocation.
Fixed allocation requires a manually specified list of MAC address and their corresponding IPs.
When a computer requests an IP, the DHCP server looks for its MAC address in a table and assigns the IP that corresponds to that MAC address. If the MAC address isn't found, the DHCP server might fall back to automatic or dynamic allocation, or it might refuse to assign an IP altogether. This can be used as a security measure to ensure that only devices that have had their MAC address specifically configured at the DHCP server will ever be able to obtain an IP and communicate on the network.
It's worth calling out that DHCP discovery can be used to configure lots of things beyond what we've touched down here. Along with things like IP address, an primary gateway, you could also use DHCP to assign things like NTP servers. NTP stands for Network Time Protocol and is used to keep all computers on a network synchronized in time. We'll cover it in more detail in later courses, but for now it's just worth knowing that DHCP can be used for more than just IP, subnet mask, gateway and DNS server.
DHCP in Action
DHCP is an application layer protocol, which means it relies on the transport, network, data link and physical layers to operate. But you might have noticed that the entire point of DHCP is to help configure the network layer itself. Let's take a look at exactly how DHCP works and how it accomplishes communications without a network layer configuration in place. Warning, geeky stuff ahead.
The process by which a client configured to use DHCP attempts to get network configuration information is known as DHCP discovery.
The DHCP discovery process has four steps.
- First, we have the server discovery step.
The DHCP clients sends what's known as a DHCP discover message out onto the network. Since the machine doesn't have an IP and it doesn't know the IP of the DHCP server, a specially crafted broadcast message is formed instead. DHCP listens on UDP port 67 and DHCP discovery messages are always sent from UDP port 68. So the DHCPDISCOVER message is encapsulated in a UDP datagram with a destination port of 67 and a source port of 68. This is then encapsulated inside of an IP datagram with a destination IP of 255.255.255.255, and a source IP of 0.0.0.0. This broadcast message would get delivered to every node on the local area network. And if a DHCP server is present, it would receive this message.
- Next, the DHCP server would examine its own configuration and would make a decision on what, if any, IP address to offer to the client. This would depend on if it's configured to run with dynamic, automatic or fixed address allocation. The response would be sent as a DHCPOFFER message with a destination port of 68, a source port of 67, a destination broadcast IP of 255.255.255.255, and its actual IP as the source.
Since the DHCP offer is also a broadcast, it would reach every machine on the network. The original client would recognize that this message was intended for itself. This is because the DHCPOFFER has the field that specifies the MAC address of the client that sent the DHCPDISCOVER message. The client machine would now process this DHCPOFFER to see what IP is being offered to it. Technically, a DHCP client could reject this offer. It's totally possible for multiple DHCP servers to be running on the same network, and for a DHCP client to be configured to only respond to an offer of an IP within a certain range. But this is rare
- More often, the DHCP client would respond to the DHCPOFFER message with a DHCPREQUEST message. This message essentially says, yes, I would like to have an IP that you offer to me. Since the IP hasn't been assigned yet, this is again sent from an IP of 0.0.0.0, and to the broadcast IP of 255.255.255.255.
- Finally, the DHCP server receives the DHCPREQUEST message and responds with a DHCPACK or DHCP acknowledgement message. This message is again sent to a broadcast IP of 255.255.255.255, and with a source IP corresponding to the actual IP of the DHCP server. Again, the DHCP client would recognize that this message was intended for itself by inclusion of its MAC address in one of the message fields. The networking stack on the client computer can now use the configuration information presented to it by the DHCP server to set up its own network layer configuration.
At this stage, the computer that's acting as the DHCP client should have all the information it needs to operate in a full fledged manner on the network it's connected to.
All of this configuration is known as DHCP lease as it includes an expiration time. A DHCP lease might last for days or only for a short amount of time. Once a lease has expired, the DHCP client would need to negotiate a new lease by performing the entire DHCP discovery process all over again. A client can also release its lease to the DHCP server, which it would do when it disconnects from the network. This would allow the DHCP server to return the IP address that was assigned to its pool of available IPs.
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