TOPIC
This article describes several issues related to DHCP client implementation found in Open Transport 2.5.1 and 2.5.2. These issues may result in the disruption of IP services or other operational anomalies when using DHCP to acquire IP configuration information. The article also offers solutions that should allow you to continue to use IP services.
DISCUSSION
Summary The DHCP client implementation included in Open Transport (OT) 2.5.1, included with Mac OS 8.6 on selected systems, and in OT 2.5.2, included in Mac OS 9, has two known issues. These issues may result in the disruption of IP services or other operational anomalies when using DHCP to acquire IP configuration information. There are no known issues with static, or PPP based TCP/IP configuration. The known issues are: 1. Hang during startup. The addition of a specific lease time option in a DHCP DHCPREQUEST message causes some non-compliant DHCP servers to send a DHCPNAK in response to a lease request. This condition can result in an infinite loop causing the system to hang when starting up. 2. The DHCP client fails to renew the current lease if the renewal time is greater than one hour. The DHCP standard specifies that the default renewal time is half the lease time, so for most DHCP servers this condition will occur if the lease duration is set to more than two hours. This can manifest itself as IP address exhaustion, IP address conflict, IP based communications failure, or as an ARP storm. Systems Affected These issues affect any system using OT 2.5.1, or OT 2.5.2. OT 2.5.1 was pre-installed on the systems listed below. OT 2.5.2 is included with Mac OS 9. Any system using Mac OS 9 (OT 2.5.2) may be affected by these issues. The following systems were shipped with OT version 2.5.1, or OT version 2.5.2 pre-installed: Symptoms The observed symptoms for each of the issues vary depending on several environmental factors. Differences in network topology, as well as differences in DHCP client and server implementations can change the observed symptoms. Depending on your environment you may see one or more of the following symptoms. Failure of the system to start up Certain DHCP servers (notably those built into some ISDN modems) issue a DHCPNAK message in response to the client DHCP REQUEST message, causing the client to return to DHCP INIT state and try again, resulting in an endless loop. According to the vendors of these DHCP servers, this is because they are based on the preliminary "Proposed Standard" version of the DHCP specification, which was made obsolete in March 1997 by RFC 2131. Failure to renew the lease once the lease has expired The DHCP client fails to renew the current lease if the renewal time is greater than one hour. The DHCP standard specifies that the default renewal time is half the lease time, so for most DHCP servers this condition will occur if the lease duration is set to more than two hours. As a result, there is the possibility that the IP address used by the system will be an address that the DHCP server has returned to the unused pool. Depending on several environmental factors the IP address could be given to another client resulting in a variety of issues. We have observed the following symptoms as a result of this issue: IP Address Exhaustion When the DHCP server detects a conflict with an IP address it may change the state of that address to "BAD". A DHCP server will usually hold these addresses in the "BAD" state until a system administrator manually changes the state. Over time this can result in a depletion of available addresses. IP Address Conflict A variety of symptoms can be observed when multiple clients detect that the address they are using, or attempting to use, is in use by another system. The client IP implementation has several choices ranging from reporting an error message to the disabling of the IP stack. IP Communications Failure The lease renewal issue has also been reported to cause communications failure when using some broadband Internet access providers. Some providers use an out-of-band authentication method and attempt to ensure the DHCP lease is current before providing service. As a result, it is possible for the authentication mechanism to disrupt active services once the DHCP lease expires. ARP Storm Given the right conditions it is also possible for the lease renewal issue to cause an ARP storm. The DHCP specification states that the server may issue a "ping" packet to ensure that the offered, or requested address, is not already is use. Not all DHCP servers implement the option which can result in multiple clients attempting to use the same IP address. If you are using the built-in DHCP server included in NT 4.x and also have a mix of Macintosh, Windows 95/98, or NT clients on the same network you may see ARP storms as a result of using DHCP. The ARP storm can last from five hundred to hundreds of thousands of packets, causing disruption of service to the two systems directly involved in the storm, as well as the potential disruption of service to all systems connected to the affected network. Other combinations of clients and servers may also exhibit this behavior. Solution These issues are resolved in the Open Transport (OT) 2.6 release. OT 2.6 is available via two methods: |
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| Document Information | |
| Product Area: | Mac OS System Software |
| Category: | General OS |
| Sub Category: | General Topics |
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