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Comparison of the Microsoft DHCP Server and the ISC DHCP daemon

by Herman Verkade

The Dynamic Host Configuration Protocol is an internet standard and as such not specific to Linux or Microsoft. If I recall correctly, a DHCP server was introduced in Windows NT 3.5, so I must have been running the Microsoft DHCP Server on my network for about eight of so years, until I recently replaced it with a Linux implementation. I don't recall many problems with the Microsoft DHCP Server during these years, so this piece of Microsoft software appears reasonably stable. For me, the only reason to migrate DHCP to Linux was not because of problems with the Microsoft implementation, but because I was simply trying to get rid of all Microsoft machines. As a consultant promoting all-Linux solutions for corporates I must of course practise what I preach and run an all-Linux environment.

Whilst researching how to implement the DHCP daemon on Linux, I discovered some interesting differences between the two implementations. Those who are running a heterogeneous environment with the Microsoft DHCP Server, may well want to consider migrating DHCP to Linux.

Avoiding Duplicate Addresses

Before the Linux DHCP daemon sends an address to a new client, it will ping the address to see whether there is already something out on the network with that address. In a properly configured and managed environment this should never be the case, but... it happens. Just one simple ping with a one second wait will add one second to the boot time of a new machine, but it also avoids problems. If the daemon gets a response to the ping, it will add the address to its 'abandoned addresses' list. It won't use that address until it has completely run out of addresses. When that happens the daemon will ping the abandoned addresses to see whether they have been freed up, remove the free ones from the abandoned list and use them for clients. The Microsoft DHCP Server will simply issue an address that is free according to its database, so it's very important to exclude all addresses that are configured statically on machines, and to never randomly pick a free address to (temporarily) configure on a machine for any sort of testing or network troubleshooting. With the Linux DHCP daemon this is much less of a problem.

DHCP Failover

The Microsoft DHCP Server does not support failover. Resilience can only be achieved by splitting a subnet in two equal parts and installing two Microsoft DHCP Servers, each of them using one half of addresses for the subnet, or by making heavy use of reservations and adding identical reservations to both servers. Both methods have serious drawbacks: The first method requires each subnet to be twice as big as really necessary, whilst the other creates a major administrative burden.

The ISC DHCP server as used on Linux support the draft DHCP failover protocol. As this is still a draft protocol, there are no guarantees that the ISC implementation will interoperate with other vendor's implemetations, but as long as you are using two DHCP servers with the same version of the ISC daemon, all should be well.

The failover protocol allows two DHCP servers to share a common address pool. Each server will use roughly one half of the address pool for new leases. The two servers will exchange information about issued leases, so that when one server goes down, the other will be available to renew leases that were issued by the failed server. When the failed server comes back, it will request updates from the failover server, and resume its original activity.

When configuring failover between two servers, you specific one server as a primary and the other as a secondary. However, this is primarily a way to get each peer to act in the opposite way of the other. It is NOT a master/slave relationship, and it doesn't matter much as to which is which, as long as one is the primary and the other is the secondary.

Although this protocol is not finalised, it is implemented in the ISC DHCP server, and works between two ISC DHCP servers. With the Microsoft DHCP Server there simply is no failover capability.

DNS Registration

When DHCP was introduced, one of the biggest issues was that DNS maintenance became difficult. As IP address for machines were assigned by DHCP, and could change over time, the old system of static DNS files posed a problem. Today, we have dynamic DNS, where entries to a DNS zone file can be added and removed on the fly.

In a Microsoft environment, each client can be (and needs to be) configured to register itself with DNS. For this to work, DNS needs to accept registrations from any device on the network, which is a small (but real) security risk.

In a Linux environment, the DHCP server can register and de-register the clients' IP addresses in DNS. When a lease is issued, the DHCP daemon registers the name and IP address of the client in DNS. When a lease expires, the DHCP daemon de-registers them. Client devices do not need write access to DNS, creating a more secure environment.

Using Expressions

Sometimes it is useful to be able to set the value of a DHCP Option for a client, based on a value that the client sent in its request to the server. This can be achieved with Linux's DHCP through the use of expressions. With Microsoft's DHCP Server it simply can't be done.

For example, a lot of companies generate names for their their workstations based on some arbitrary value, which must be unique for each machine, but also needs to be consistent for each machine. Often the asset tag (which is usually found on a sticker on the machine) is used for this. In many cases the hardware address of the network card in the machine would provide an equally unique and consistent value. When using a Linux DHCP Server, expressions can be used to generate a unique machine name for each client, based on the client's network card address, and have that machine name sent to the client as one of the DHCP options. Doing so means that there no longer will be the need to configure the machine name manually when installing machines, which removes another hurdle for unattended installations.

Do note that this does not work for Windows clients, as they ignore the machine name sent by DHCP. Windows machines must have their machine name configured locally.

Groups and Classes of clients

The final difference between Microsoft and Linux DHCP that I will highlight here, is the ability to create groups and classes of machines within the DHCP configuration file on Linux. Special options and parameters can be set for different classes or groups of machines. For example, the lease time for a dial-in client can be set much shorter than that for an office-based client, whilst the lease time for desktops can be set much longer than that for laptops. With Microsoft's DHCP implementation this can only be achieved by using separate subnets for the different classes of machines. With the Linux implementation many other factors can be used to differentiate between these types of clients and select option values based on these.

 

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