Understanding Network Connectivity in an Enterprise (part 2) - Understanding the DHCP Lease

3. Understanding the DHCP Lease

You'll almost always find at least one DHCP server used within an enterprise. It automates the process of assigning TCP/IP configuration, and once it's configured, it takes very little management to keep it running. A single DHCP server can handle as many 10,000 clients, though when you have that many clients, you'd usually add another DHCP server for redundancy and fault tolerance.

When a DHCP client turns on, four packets are exchanged over the network between the DHCP client and the DHCP server. This is often referred to as the DORA process because of the names of the IP packets: D for Discover, O for Offer, R for Request, and A for Acknowledge.

Consider Figure 2. In the figure, a single DHCP server is being used to serve three subnets. When the client in subnet C turns on, it will start the DHCP process by broadcasting the Discover packet, and the DHCP server in subnet A will respond.

Figure 2. Using DHCP on a network

Discover When the DHCP client turns on, it will broadcast a DHCPDiscover packet. It's asking, "Are there any DHCP servers out there?"

Offer When a DHCP server receives a DHCPDiscover packet, it will respond by broadcasting a DHCPOffer. This offer includes a DHCP lease, which includes the TCP/IP configuration for the client. If multiple DHCP servers are on the network, each one can respond with an offer.

Request The client sends a DHCPRequest in response to the first DHCPOffer it receives. In essence, it says, "Thank you; I'll take that lease." A DHCP lease is typically configured to last eight days on a network. If a second DHCP server sent an offer but did not receive a request, the offer will expire and the IP address will still be available to be issued.

Acknowledge When the DHCP server receives the request, it allocates the lease to ensure the IP address is not issued to any other clients and responds with the DHCPACK (Acknowledge) packet.

After 50 percent of the lease length has expired, the client requests a renewal of the lease from the DHCP server. For example, if the lease length is set to eight days, the client will try to renew the lease after four days. If the DHCP server doesn't respond, the client will continue to request a renewal until 87.5 percent of the lease length has expired (after seven days for an eight-day lease). After 87.5 percent, the client will repeat the DORA process every 5 minutes looking for any DHCP server. At 100 percent, the client will assign itself an IP address in the range of 169.254.y.z /16.

3.1. Understanding DHCP Scopes

DHCP servers are typically configured with multiple DHCP scopes. Each DHCP scope typically refers to a subnet, though it is possible to have more than one scope for a subnet. In addition, the DHCP server can configure different TCP/IP options for individual scopes.

As an example, consider Figure 3. This shows three subnets served by a single DHCP server. One router routes packets between the subnets and to the Internet via the firewall. This router has three network adapters, and each adapter is configured with an IP address (identified as the default gateway) for the subnet.

Figure 3. Using DHCP on a network

In addition, the network has a single DNS server located in subnet A. The options in the DHCP server can be set up to configure clients in all three subnets using three different scopes.

Figure 4 shows the DHCP console configured for the network shown in Figure 7.3. In the figure, the options for the Subnet A scope are shown. The 003 Router option is for the default gateway. The DNS server has an IP address of 192.168.1.10, and the domain name is Wiley.com.

Figure 4. The DHCP console showing three scopes and options

You may notice that the icon for the option for the 003 Router is a little different from the 006 DNS Servers and 015 DNS Domain Name options. The Router option is configured at the scope level and applies only to clients connecting from Subnet A. However, the 006 DNS Servers and 015 DNS Domain Name options are server-level options and apply to all clients served by this DHCP server. Server-level options are configured once for the server and apply to all scopes.

It's also possible to issue the addresses for multiple DNS servers to clients. The first DNS server would be used as the preferred DNS server, and others would be designated as alternate DNS servers. Alternate DNS Servers are queried only if the preferred DNS server does not respond.

3.2. DCHP and RFC 1542

If you know a little about networking, you may have been thinking, "Wait a minute! Broadcasts can't go through the router," as you read through the DHCP DORA process. You're absolutely correct. However, DHCP broadcasts are special broadcasts, and the router is configured to allow them on most internal networks. Either that or a DHCP server, or a DHCP relay agent, must be placed on every subnet. It's much more common to configure the router to pass the DHCP broadcasts.

RFC 1542 (Request For Comments 1542) is a TCP/IP standards document that specifies that DHCP broadcasts are to use UDP ports 67 and 68. Furthermore, routers that are RFC 1542 compliant can be configured to allow these DCHP broadcasts through UDP ports 67 and 68.

If you look back at Figure 7.2 and Figure7.3, the router would be configured to allow DHCP broadcasts through UDP ports 67 and 68 on the network interfaces connected to Subnet A, Subnet B, and Subnet C. It would not allow these broadcasts from the network interface connected to the Internet via the firewall.

3.3. APIPA

What if the DHCP server doesn't answer, or what if the DHCP lease expires? Well, TCP/IP has an app for that.

Automatic Private IP Addressing (APIPA) is used to configure clients with addresses in a special range when the DHCP server doesn't respond. The range of addresses is 169.254.0.1 through 169.245.255.254, with a subnet mask of 255.255.0.0.

As mentioned previously, you can use the IPConfig /all command at the command prompt to view the current TCP/IP configuration. Figure 5 shows what this looks like when the DHCP server can't be reached and an APIPA address is assigned.

Figure 5. Identifying an APIPA address

Several line items shown in the figure are worth mentioning here:

DHCP Enabled

The Yes setting indicates that it is configured as a DHCP client.

Autoconfiguration Enabled

The Yes setting indicates that APIPA is enabled. This is enabled by default but can be disabled with a Registry modification.

IPv4 Address

An address in the 169.254.0.0/24 range indicates an APIPA address. Because it is configured as a DCHP client and an address of 169.254.0.16 is assigned, it clearly indicates the DHCP server couldn't be reached or didn't respond.

DHCP Server

A DHCP address of 169.254.0.1 also indicates that this is an APIPA-assigned address. If a DHCP server was reached, this would show the IP address of the DHCP server, and a DHCP server would never be assigned an address in the APIPA range.