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DHCP Server problems

This deals with the DHCP Server in the Internet Gate, distributing IP addresses to clients on the LAN.

IP address collisions

One of the nightmares on a local network (and indeed on any network) is when two PC:s or other devices (we call them hosts) start using the same IP address. The consequenses are characteristic but not so obvious for other than experienced network users: Suddenly an application, like “surfing”, stops working for some time for one host. Then it starts working again, after a minute or so, but now the problem has moved to another host on the LAN. After some time again the problem jumps back to the first host, and so on.

The reason for this jumping error is that if two hosts act as if they have the same IP address it will confuse the router, in this case the Internet Gate who is set to work as their default gateway. The Internet Gate, as any router and host on a network, has a memory, the “ARP cache”, that pairs an IP address with the host's MAC address. This memory is used for forwarding packets, destined to the particular IP address, to the correct unit. But for various reasons this memory has to be refreshed now and then, and this refresh is done by asking around on the network: “who has this particular IP address”? If there is more than one host out there answering this question, the Internet Gate may well get different answers from time to another, and as a result the traffic will jump around accordingly.

This IP address collision can happen if hosts on a LAN are assigned their IP address statically by a less careful network administrator. But if all hosts are getting their IP address dynamically by a DHCP server this should not happen, since the DHCP server is responsible in the first place for handing out unique IP addresses. Still, that can happen in two cases:

  1. The DHCP server is not the only one on the network.
  2. The DHCP server has forgotten all about the IP addresses that have been distributed.

These cases will be expanded below:

More than one DHCP server

The rule is simple enough: Avoid having more than one DHCP server on the LAN.

Using several DHCP servers would need a way for them to communicate with each other to synchronize their IP lease databases, that is, their opinions about which host that should get a particular IP address. Otherwise there is a considerable risk that the DHCP servers will give the same IP to different hosts, resulting in the IP collision scenario described above. Though there are standards proposed for such DHCP server synchronization, very few DHCP servers support this, nor does the Internet Gate.

Having more than one DHCP server may not be a deliberate action. If the Internet Gate is added to a local network that already has a working DHCP server (maybe in a PC), it is important to shut the existing DHCP off.
Or, if one wants to keep the existing DHCP server, shut the DHCP server in the Internet Gate off (this is done on the Network page).
:!: If so, a static IP address that doesn't risk to conflict with a dynamic address (but still lies within the correct IP subnet) must be set on the Internet Gate, its local interfaces can never act as DHCP clients.

Likewise, if more than one Internet Gate is connected to the LAN (possibly for testing or for configuration purposes) all but one of them must first have its DHCP server shut off.

troubleshooting/dhcp_server_problems.1290783838.txt.gz · Last modified: 2010/11/26 16:03 by mats
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