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DHCP FAQ

Saturday, 24 September 2005 13:34

Don't know what DHCP is?  Chances are your computer is using it right now.

Please see below for answers to many DHCP questions.

Answers

  1. General
    1. What is DHCP?

      DHCP stands for "Dynamic Host Configuration Protocol".

    2. What is DHCP's purpose?

      DHCP's purpose is to enable individual computers on an IP network to extract their configurations from a server (the 'DHCP server') or servers, in particular, servers that have no exact information about the individual computers until they request the information. The overall purpose of this is to reduce the work necessary to administer a large IP network. The most significant piece of information distributed in this manner is the IP address.

    3. Can DHCP work with Appletalk or IPX?

      No, it is too tied to IP. Furthermore, they don't need it since they have always had automated mechanisms for assigning their own network addresses.

    4. Who Created It? How Was It Created?

      DHCP was created by the Dynamic Host Configuration Working Group of the Internet Engineering Task Force (IETF; a volunteer organization which defines protocols for use on the Internet). As such, it's definition is recorded in an Internet RFC and the Internet Activities Board (IAB) is asserting its status as to Internet Standardization. As of this writing (June 1998), DHCP is an Internet Draft Standard Protocol and is Elective. BOOTP is an Internet Draft Standard Protocol and is Recommended. For more information on Internet standardization, see RFC2300 (May 1998)

    5. How is it different than BOOTP or RARP?

      DHCP is based on BOOTP and maintains some backward compatibility. The main difference is that BOOTP was designed for manual pre-configuration of the host information in a server database, while DHCP allows for dynamic allocation of network addresses and configurations to newly attached hosts. Additionally, DHCP allows for recovery and reallocation of network addresses through a leasing mechanism.

      RARP is a protocol used by Sun and other vendors that allows a computer to find out its own IP number, which is one of the protocol parameters typically passed to the client system by DHCP or BOOTP. RARP doesn't support other parameters and using it, a server can only serve a single LAN. DHCP and BOOTP are designed so they can be routed.

    6. How is it different than VLANs?

      DHCP and VLANs, which are very different in concept, are sometimes cited as different solutions to the same problem. While they have a goal in common (easing moves of networked computers), VLANs represent a more revolutionary change to a LAN than DHCP. A DHCP server and forwarding agents can allow you to set things up so that you can unplug a client computer from one network or subnet and plug it into another and have it come alive immediately, it having been reconfigured automatically. In conjunction to Dynamic DNS, it could automatically be given its same name in its new place. VLAN-capable LAN equipment with dynamic VLAN assignment allows you to configure things so a client computer can be plugged into any port and have the same IP number (as well as name) and be on the same subnet. The VLAN-capable network either has its own configuration that lists which MAC addresses are to belong to each VLAN, or it makes the determination from the source IP address of the IP packets that the client computer sends. Some differences in the two approaches:

      • DHCP handles changes by reconfiguring the client while a VLAN-capable network handles it by reconfiguring the network port the client is moved to.
      • DHCP dynamic reconfiguration requires a DHCP server, forwarding agent in each router, and DHCP capability in each client's TCP/IP support. The analogous capability in VLANs requires that all hubs throughout the network be VLAN-capable, supporting the same VLAN scheme. To this point VLAN support is proprietary with no vendor interoperability, but standards are being developed.
      • DHCP can configure a new client computer for you while a VLAN-capable network can't.
      • DHCP is generally aimed at giving "easy moves" capability to networks that are divided into subnets on a geographical basis, or on separate networks. VLANs are generally aimed at allowing you to set up subnets on some basis other than geographical, e.g. instead of putting everyone in one office on the same subnet, putting each person on a subnet that has access to the servers that that person requires.
      There is an issue with trying to use DHCP (or BOOTP) and VLANs at the same time, in particular, with the scheme by which the VLAN-capable network determines the client's VLAN based upon the client computer's source IP address. Doing so assumes the client computer is already configured, which precludes the use of network to get the configuration information from a DHCP or BOOTP server.
    7. What protocol and port does DHCP use?

      DHCP, like BOOTP runs over UDP, utilizing ports 67 and 68.

    8. What is an IP address?

      An IP address (also called an IP number) is a number (typically written as four numbers separated by periods, i.e. 107.4.1.3 or 84.2.1.111) which uniquely identifies a computer that is making use of the Internet. It is analogous to your telephone number in that the telephone number is used by the telephone network to direct calls to you. The IP address is used by the Internet to direct data to your computer, e.g. the data your web browser retrieves and displays when you surf the net. One task of DHCP is to assist in the problem of getting a functional and unique IP number into the hands of the computers that make use of the Internet.

    9. What is a MAC address?

      A MAC address (also called an Ethernet address or an IEEE MAC address) is a number (typically written as twelve hexadecimal digits, 0 through 9 and A through F, or as six hexadecimal numbers separated by periods or colons, i.e. 0080002012ef, 0:80:0:2:20:ef) which uniquely identifes a computer that has an Ethernet interface. Unlike the IP number, it includes no indication of where your computer is located. In DHCP's typical use, the server uses a requesting computer's MAC address to uniquely identify it.

    10. What is a DHCP lease?

      A DHCP lease is the amount of time that the DHCP server grants to the DHCP client permission to use a particular IP address. A typical server allows its administrator to set the lease time.

    11. What is a Client ID?

      What is termed the Client ID for the purposes of the DHCP protocol is whatever is used by the protocol to identify the client computer. By default, DHCP implementations typically employ the client's MAC address for this purpose, but the DHCP protocol allows other options. Some DHCP implementations have a setup option to specify the client ID you want. One alternative to the MAC address is simply a character string of your choice. In any case, in order for DHCP to function, you must be certain that no other client is using the client ID you choose, and you must be sure the DHCP server will accept it.

    12. Why shouldn't clients assign IP numbers without the use of a server?

      It is theoretically possible to develop software for client-machines that finds an unused address by picking them out of the blue and broadcasting a request of all the other client machines to see if they are using them. Appletalk is designed around this idea, and Apple's MacTCP can be configured to do this for IP. However, this method of IP address assignment has disadvantages.

      1. A computer that needs a permanently-assigned IP number might be turned off and lose its number to a machine coming up. This has problems both for finding services and for security.
      2. A network might be temporarily divided into two non-communicating networks while a network component is not functioning. During this time, two different client-machines might end up claiming the same IP number. When the network comes back, they start malfunctioning.
      3. If such dynamic assignment is to be confined to ranges of IP addresses, then the ranges are configured in each desktop machine rather than being centrally administered. This can lead both to hidden configuration errors and to difficulty in changing the range. Another problem with the use of such ranges is keeping it easy to move a computer from one subnet to another.
    13. Can DHCP support statically defined addresses?

      Yes. At least there is nothing in the protocol to preclude this and one expects it to be a feature of any DHCP server. This is really a server matter and the client should work either way. The RFC refers to this as manual allocation.

    14. How does DHCP and BOOTP handle multiple subnets?

      For the situations where there is more than one LAN, each with its own subnet number, there are two ways. First of all, you can set up a seperate server on each subnet. Secondly, a feature of some routers known as "BOOTP forwarding" to forward DHCP or BOOTP requests to a server on another subnet and to forward the replies back to the client. The part of such a router (or server acting as a router) that does this is called a "BOOTP forwarding agent". Typically you have to enable it on the interface to the subnet to be served and have to configure it with the IP address of the DHCP or BOOTP server. On a Cisco router, the address is known as the "UDP Helper Address".

    15. Can a BOOTP client boot from a DHCP server?

      Only if the DHCP server is specifically written to also handle BOOTP queries.

    16. Can a DHCP client boot from a BOOTP server?

      Only if the DHCP client were specifically written to make use of the answer from a BOOTP server. It would presumably treat a BOOTP reply as an unending lease on the IP address.

      In particular, the TCP/IP stack included with Windows 95 does not have this capability.

    17. Is a DHCP server "supposed to" be able to support a BOOTP client?

      The RFC on such interoperability (1534) is clear: "In summary, a DHCP server: ... MAY support BOOTP clients," (section 2). The word "MAY" indicates such support, however useful, is left as an option.

      A source of confusion on this point is the following statement in section 1.5 of RFC 1541: "DHCP must provide service to existing BOOTP clients." However, this statement is one in a list of "general design goals for DHCP", i.e. what the designers of the DHCP protocol set as their own goals. It is not in a list of requirements for DHCP servers.

    18. Is a DHCP client "supposed to" be able to use a BOOTP server?

      The RFC on such interoperability (1534) is clear: "A DHCP client MAY use a reply from a BOOTP server if the configuration returned from the BOOTP server is acceptable to the DHCP client." (section 3). The word "MAY" indicates such support, however useful, is left as an option.

    19. Can a DHCP client or server make a DNS server update the client's DNS entry to match the client's dynamically assigned address?

      RFCs 2136 and 2137 indicate a way in which DNS entries can be updated dynamically. Using this requires a DNS server that supports this feature and a DHCP server that makes use of it. The RFCs are very recent (as of 5/97) and implementations are few. In the mean time, there are DNS and DHCP servers that accomplish this through proprietary means.

    20. Can a DHCP server back up another DHCP server?

      You can have two or more servers handing out leases for different addresses. If each has a dynamic pool accessible to the same clients, then even if one server is down, one of those clients can lease an address from the other server.

      However, without communication between the two servers to share their information on current leases, when one server is down, any client with a lease from it will not be able to renew their lease with the other server. Such communication is the purpose of the "server to server protocol" (see next question). It is possible that some server vendors have addressed this issue with their own proprietary server-to-server communication.

    21. When will the server to server protocol be defined?

      The DHC WG of the IETF is actively investigating the issues in inter-server communication. The protocol should be defined "soon".

    22. Is there a DHCP mailing list?

      There are several:

      List Purpose
      ---- -------
      [email protected] General discussion: a good list for
      server administrators.
      [email protected] DHCP bakeoffs
      [email protected] Implementations
      [email protected] Server to server protocol
      [email protected] DNS-DHCP issues
      [email protected] DHCP for IPv6

      The lists are run by [email protected] which can be used to subscribe and sign off. Archives for the dhcp-v4 list (which used to be called the host-conf list) are stored at ftp://ftp.bucknell.edu/pub/dhcp/.
    23. In a subnetted environment, how does the DHCP server discover what subnet a request has come from?

      DHCP client messages are sent to off-net servers by DHCP relay agents, which are often a part of an IP router. The DHCP relay agent records the subnet from which the message was received in the DHCP message header for use by the DHCP server.

      Note: a DHCP relay agent is the same thing as a BOOTP relay agent, and technically speaking, the latter phrase is correct.

    24. If a single LAN has more than one subnet number, how can addresses be served on subnets other than the primary one?

      A single LAN might have more than one subnet number applicable to the same set of ports (broadcast domain). Typically, one subnet is designated as primary, the others as secondary. A site may find it necessary to support addresses on more than one subnet number associated with a single interface. DHCP's scheme for handling this is that the server has to be configured with the necessary information and has to support such configuration & allocation. Here are four cases a server might have to handle:

      1. Dynamic allocation supported on secondary subnet numbers on the LAN to which the server is attached.
      2. Dynamic allocation supported on secondary subnet numbers on a LAN which is handled through a DHCP/BOOTP Relay. In this case, the DHCP/BOOTP Relay sends the server a gateway address associated with the primary subnet and the server must know what to do with it.
      The other two cases are the same capabilities during manual allocation. It is possible that a particular server-implementation can handle some of these cases, but not all of them. See section below listing the capabilities of some servers.
    25. If a physical LAN has more than one logical subnet, how can different groups of clients be allocated addresses on different subnets?

      One way to do this is to preconfigure each client with information about what group it belongs to. A DHCP feature designed for this is the user class option. To do this, the client software must allow the user class option to be preconfigured and the server software must support its use to control which pool a client's address is allocated from.

    26. Where is DHCP defined?

      In Internet RFCs.

      RFC 2131 R. Droms, "Dynamic Host Configuration Protocol", 3/97. Supersedes RFC 1541 and RFC 1531. [Note that some of the references in this FAQ are to RFC 1541: I'll update them when I get a chance. -- Author] RFC 1534 R. Droms, "Interoperation Between DHCP and BOOTP", 10/08/1993. RFC 2132 S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor Extensions", 3/97. Supersedes RFC 1533.

      Some websites with copies of RFCs:
      http://info.internet.isi.edu/1s/in-notes/rfc/
      http://www.cis.ohio-state.edu/hypertext/information/rfc.html
      http://www.pmg.lcs.mit.edu/rfc.html

    27. What other sources of information are available?

      See the dhcp-v4 mailing list mentioned above as well as its archives.

      DHCP - Dynamic Host Configuration Protocol http://www.eg.bucknell.edu/~droms/dhcp/ Problems and Solutions of DHCP: Experiences with DHCP implementation and Operation A. Tominaga, O. Nakamura, F. Teraoka, J. Murai. http://info.isoc.org/HMP/PAPER/127/html/paper.html DHCP Resources Alan Dobkin. http://NWS.CC.Emory.Edu/WebStaff/Alan/Net-Man/Computing/DHCP/ DHCP Reading Room Eric Hall. http://www.ehsco.com/reading/dhcp.html Internet Drafts Internet drafts are works in progress intended to update the current RFCs or specify additional functionality, and sometimes there is one or more draft related to DHCP. All Internet Drafts are available from various sites: the US East Cost site is ftp://ds.internic.net/internet-drafts/; a web site is http://ds.internic.net/ds/dsintdrafts.html. The DHCP-related drafts currently have filenames of the form "draft-ietf-dhc-SOMETHING". These DHCP-related drafts are also stored at ftp://ftp.bucknell.edu/pub/dhcp/, and are available through http://www.eg.bucknell.edu/~droms/dhcp/. I cannot be more specific about the documents because they are by their nature temporary. "DHCP Clients: Do They Really Work?" Eric Hall. Network Computing, Vol. 7, No. 7, May 1, 1996, pp. 114-120. Reviews DHCP-client-function of some popular Windows IP stacks. http://www.ehsco.com/reading/19960515ncw2.html "The Heaven And Hell Of DHCP Servers" Eric Hall. Network Computing, Vol. 7, No. 8, May 15, 1996, pp. 118-121. Reviews DHCP servers. http://www.ehsco.com/reading/19960515ncw1.html
    28. Can DHCP support remote access?

      PPP has its own non-DHCP way in which communications servers can hand clients an IP address called IPCP (IP Control Protocol) but doesn't have the same flexibility as DHCP or BOOTP in handing out other parameters. Such a communications server may support the use of DHCP to acquire the IP addresses it gives out. This is sometimes called doing DHCP by proxy for the client. I know that Windows NT's remote access support does this.

      A feature of DHCP under development (DHCPinform) is a method by which a DHCP server can supply parameters to a client that already has an IP number. With this, a PPP client could get its IP number using IPCP, then get the rest of its parameters using this feature of DHCP.

      SLIP has no standard way in which a server can hand a client an IP address, but many communications servers support non-standard ways of doing this that can be utilized by scripts, etc. Thus, like communications servers supporting PPP, such communications servers could also support the use of DHCP to acquire the IP addressees to give out.

      The DHCP protocol is capable of allocating an IP address to a device without an IEEE-style MAC address, such as a computer attached through SLIP or PPP, but to do so, it makes use of a feature which may or may not be supported by the DHCP server: the ability of the server to use something other than the MAC address to identify the client. Communications servers that acquire IP numbers for their clients via DHCP run into the same roadblock in that they have just one MAC address, but need to acquire more than one IP address. One way such a communications server can get around this problem is through the use of a set of unique pseudo-MAC addresses for the purposes of its communications with the DHCP server. Another way (used by Shiva) is to use a different "client ID type" for your hardware address. Client ID type 1 means you're using MAC addresses. However, client ID type 0 means an ASCII string.

    29. Can a client have a home address and still float?

      There is nothing in the protocol to keep a client that already has a leased or permanent IP number from getting a(nother) lease on a temporary basis on another subnet (i.e., for that laptop which is almost always in one office, but occasionally is plugged in in a conference room or class room). Thus it is left to the server implementation to support such a feature. I've heard that Microsoft's NT-based server can do it.

    30. How can I relay DHCP if my router does not support it?

      A server on a net(subnet) can relay DHCP or BOOTP for that net. Microsoft has software to make Windows NT do this.

    31. How do I migrate my site from BOOTP to DHCP?

      I don't have an answer for this, but will offer a little discussion. The answer depends a lot on what BOOTP server you are using and how you are maintaining it. If you depend heavily on BOOTP server software to support your existing clients, then the demand to support clients that support DHCP but not BOOTP presents you with problems. In general, you are faced with the choice:

      1. Find a server that is administered like your BOOTP server only that also serves DHCP. For example, one popular BOOTP server, the CMU server, has been patched so that it will answer DHCP queries.
      2. Run both a DHCP and a BOOTP server. It would be good if I could find out the gotcha's of such a setup.
      3. Adapt your site's administration to one of the available DHCP/BOOTP servers.
      4. Handle the non-BOOTP clients specially, e.g. turn off DHCP and configure them statically: not a good solution, but certainly one that can be done to handle the first few non-BOOTP clients at your site.
    32. Can you limit which MAC addresses are allowed to roam?

      Sites may choose to require central pre-configuration for all computers that will be able to acquire a dynamic address. A DHCP server could be designed to implement such a requirement, presumably as an option to the server administrator. See section below on servers that implement this.

    33. Is there an SNMP MIB for DHCP?

      There is no standard MIB; creating one is on the list of possible activities of the DHCP working group. It is possible that some servers implement private MIBs.

    34. What is DHCP Spoofing?

      Ascend Pipeline ISDN routers (which attach Ethernets to ISDN lines) incorporate a feature that Ascend calls "DHCP spoofing" which is essentially a tiny server implementation that hands an IP address to a connecting Windows 95 computer, with the intention of giving it an IP number during its connection process.

    35. How long should a lease be?

      I've asked sites about this and have heard answers ranging from 15 minutes to a year. Most administrators will say it depends upon your goals, your site's usage patterns, and service arrangements for your DHCP server.

      A very relevant factor is that the client starts trying to renew the lease when it is halfway through: thus, for example, with a 4 day lease, the client which has lost access to its DHCP server has 2 days from when it first tries to renew the lease until the lease expires and the client must stop using the network. During a 2-day outage, new users cannot get new leases, but no lease will expire for any computer turned on at the time that the outage commences.

      Another factor is that the longer the lease the longer time it takes for client configuration changes controlled by DHCP to propogate.

      Some relevant questions in deciding on a lease time:

      Do you have more users than addresses? If so, you want to keep the lease time short so people don't end up sitting on leases. Naturally, there are degrees. In this situation, I've heard examples cited of 15 minutes, 2 hours, and 2 days. Naturally, if you know you will have 20 users using 10 addresses in within a day, a 2 day lease is not practical. Are you supporting mobile users? If so, you may be in the situation of having more users than addresses on some particular IP number range. See above. Do you have a typical or minimum amount of time that you are trying to support? If your typical user is on for an hour at minimum, that suggest a hour lease at minimum. How many clients do you have and how fast are the communications lines over which the DHCP packets will be run? The shorter the lease, the higher the server and network load. In general, a lease of at least 2 hours is long enough that the load of even thousands of clients is negligible. For shorter leases, there may be a point beyond which you will want to watch the load. Note that if you have a communication line down for a long enough time for the leases to expire, you might see an unusually high load it returns. If the lease-time is at least double the communication line outage, this is avoided. How long would it take to bring back up the DHCP server, and to what extent can your users live without it? If the lease time is at least double the server outage, then running clients who already have leases will not lose them. If you have a good idea of your longest likely server outage, you can avoid such problems. For example, if your server-coverage is likely to recover the server within three hours at any time that clients are using their addresses, then a six hour lease will handle such an outage. If you might have a server go down on Friday right after work and may need all Monday's work-day to fix it, then your maximum outage time is 3 days and a 6-day lease will handle it. Do you have users who want to tell other users about their IP number? If your users are setting up their own web servers and telling people how to get to them either by telling people the IP number or through a permanent DNS entry, then they are looking for an IP number that won't be changing. While some sites would manually allocate any address that people expected to remain stable, other sites want to use DHCP's ability to automate distribution of relatively permanent addresses. The relevant time is the maximum amount of time that you wish to allow the user to keep their machine turned off yet keep their address. For example, in a university, if students might have their computers turned off for as long as three weeks between semesters, and you wish them to keep their IP address, then a lease of six weeks or longer would suffice.

      Some examples of lease-times that sites have used & their rationals:

      15 minutes To keep the maximum number of addresses free for distribution in cases where there will be more users than addresses. 6 hours Long enough to allow the DHCP server to be fixed, e.g. 3 hours. 12 hours If you need to take back an address, then you know that it will only take one night for the users' lease to expire. 3 days This is apparently Microsoft's default, thus many sites use it. 6 days Long enough that a weekend server outage that gets fixed on Monday will not result in leases terminating. 4 months Long enough that students can keep their IP address over the summer hiatus. I believe this rational is workable if the summer hiatus is no more than 2 months. One year If a user has not used their address in six months, then they are likely to be gone. Allows administrator to recover those addresses after someone has moved on.
    36. How can I control which clients get leases from my server?

      There is no ideal answer: you have to give something up or do some extra work.

      • You can put all your clients on a subnet of your own along with your own DHCP server.
      • You can use manual allocation.
      • Perhaps you can find DHCP server software that allows you to list which MAC addresses the server will accept. DHCP servers that support roaming machines may be adapted to such use.
      • You can use the user class option assuming your clients and server support it: it will require you to configure each of your clients with a user class name. You still depend upon the other clients to respect your wishes.
    37. How can I prevent unauthorized laptops from using a network that uses DHCP for dynamic addressing?

      This would have to be done using a mechanism other than DHCP. DHCP does not prevent other clients from using the addresses it is set to hand out nor can it distinguish between a computer's permanent MAC address and one set by the computer's user. DHCP can impose no restrictions on what IP address can use a particular port nor control the IP address used by any client.

    38. What are the Gotcha's?
      • A malicious user could make trouble by putting up an unofficial DHCP server.
        • The immediate problem would be a server passing out numbers already belonging to some computer yielding the potential for two or more "innocent bystander" nodes ending up with the same IP number. Net result is problems using the nodes, possibly intermittent of one or the other is sometimes turned off.
        • A lot of problems are possible if a renegade server manages to get a client to accept its lease offering, and feeds the client its own version of other booting parameters. One scenario is a client that loads its OS over the network via tftp being directed to a different file (possibly on a different server), thus allowing the perpetrator to take over the client. Given that boot parameters are often made to control many different things about the computers' operation and communication, many other scenarios are just as serious.
        Note that BOOTP has the same vulnerabilities.
      • The "broadcast flag": DHCP includes a way in which client implementations unable to receive a packet with a specific IP address can ask the server or relay agent to use the broadcast IP address in the replies (a "flag" set by the client in the requests). The definition of DHCP states that implementations "should" honor this flag, but it doesn't say they "must". Some Microsoft TCP/IP implementations used this flag, which meant in practical terms, relay agents and servers had to implement it. A number of BOOTP-relay-agent implementations (e.g. in routers) handled DHCP just fine except for the need for this feature, thus they announced new versions stated to handle DHCP.
      • Some of the virtual LAN schemes, i.e., those that use the packet's IP number to decide which "virtual LAN" a client-computer is on for the purposes of TCP/IP, don't work when using DHCP to dynamically assign addresses. DHCP servers and relay agents use their knowledge of what LAN the client-station is on to select the subnet number for the client-station's new IP address whereas such switches use the subnet number sent by the client-station to decide which (virtual) LAN to put the station on.
      • Routers are sometimes configured so that one LAN on one port has multiple network (or subnet) numbers. When the router is relaying requests from such a LAN to the DHCP server, it must pass along as IP number that is associated with one of the network (or subnet) numbers. The only way the DHCP server can allocate addresses on one of the LAN's other network (or subnet) numbers is if the DHCP server is specifically written to have a feature to handle such cases, and it has a configuration describing the situation.
      • The knowledge that a particular IP number is associated with a particular node is often used for various functions. Examples are: for security purposes, for network management, and even for identifying resources. Furthermore, if the DNS's names are going to identify IP numbers, the numbers, the IP numbers have to be stable. Dynamic configuration of the IP numbers undercuts such methods. For this reason, some sites try to keep the continued use of dynamically allocatable IP numbers to a minimum.
      • With two or more servers serving a LAN, clients that are moved around (e.g. mobile clients) can end up with redundant leases. Consider a home site with two DHCP servers, a remote site with DHCP services, and a mobile client. The client first connects to the home site and receives an address from one of the two serves. He/she then travels to the remote site (without releasing the lease at the home site) and attempts to use the acquired address. It is of course NAK'ed and the client receives an address appropriate for the remote site. The client then returns home and tries to use the address from the remote site. It is NAK'ed but now the client broadcasts a DHCPDISCOVER to get a address. The server that holds the previous lease will offer the address back to the client but there is no guarantee that the client will accept that address; consequently, it is possible for the client to acquire an address on the other server and therefore have two leases within the site. The problem can be solved by using only one server per subnet/site and can be mitigated by short lease lengths. But in a very mobile environment, it is possible for these transient clients to consume more than their fair share of addresses.
      • If departments, offices, or individuals run DHCP servers with their own small address pools on LANs shared by other departments, offices, or individuals, they can find that their addresses are being used by anyone on the LAN that happens to set their IP configuration to use DHCP.
      • An easy mistake to make in setting up a DHCP server is to fail to set all the necessary global parameters. This can result in some functions working while others are not, or functions working when the client is set up manually, but failing to work when set to use DHCP.
      • Long leases can be disadvantageous in cases where you need to change a configuration parameter or withdraw an address from use. The length of the lease can mean the difference between having to go to every affected client and rebooting it, or merely waiting a certain amount of time for the leases to be renewed. (Note: one workaround is to fool with the client computer's clock).

  2. Info on Implementations
    1. What features or restrictions can a DHCP server have?

      While the DHCP server protocol is designed to support dynamic management of IP addresses, there is nothing to stop someone from implementing a server that uses the DHCP protocol, but does not provide that kind of support. In particular, the maintainer of a BOOTP server-implementation might find it helpful to enhance their BOOTP server to allow DHCP clients that cannot speak "BOOTP" to retrieve statically defined addresses via DHCP. The following terminology has become common to describe three kinds of IP address allocation/management. These are independent "features": a particular server can offer or not offer any of them:

      • Manual allocation: the server's administrator creates a configuration for the server that includes the MAC address and IP address of each DHCP client that will be able to get an address: functionally equivalent to BOOTP though the protocol is incompatible.
      • Automatic allocation: the server's administrator creates a configuration for the server that includes only IP addresses, which it gives out to clients. An IP address, once associated with a MAC address, is permanently associated with it until the server's administrator intervenes.
      • Dynamic allocation: like automatic allocation except that the server will track leases and give IP addresses whose lease has expired to other DHCP clients.

      Other features which a DHCP server may or may not have:

      • Support for BOOTP clients.
      • Support for the broadcast bit.
      • Administrator-settable lease times.
      • Administrator-settable lease times on manually allocated addresses.
      • Ability to limit what MAC addresses will be served with dynamic addresses.
      • Allows administrator to configure additional DHCP option-types.
      • Interaction with a DNS server. Note that there are a number of interactions that one might support and that a standard set & method is in the works.
      • Interaction with some other type of name server, e.g. NIS.
      • Allows manual allocation of two or more alternative IP numbers to a single MAC address, whose use depends upon the gateway address through which the request is relayed.
      • Ability to define the pool/pools of addresses that can be allocated dynamically. This is pretty obvious, though someone might have a server that forces the pool to be a whole subnet or network. Ideally, the server does not force such a pool to consist of contiguous IP addresses.
      • Ability to associate two or more dynamic address pools on separate IP networks (or subnets) with a single gateway address. This is the basic support for "secondary nets", e.g. a router that is acting as a BOOTP relay for an interface which has addresses for more than one IP network or subnet.
      • Ability to configure groups of clients based upon client-supplied user and/or vendor class. Note: this is a feature that might be used to assign different client-groups on the same physical LAN to different logical subnets.
      • Administrator-settable T1/T2 lengths.
      • Interaction with another DHCP server. Note that there are a number of interactions that one might support and that a standard set & method is in the works.
      • Use of PING (ICMP Echo Request) to check an address prior to dynamically allocating it.
      • Server grace period on lease times.
      • Ability to force client(s) to get a new address rather than renew.

      Following are some features related not to the functions that the server is capable of carrying out, but to the way that it is administered.

      • Ability to import files listing manually allocated addresses (as opposed to a system which requires you to type the entire configuration into its own input utility). Even better is the ability to make the server do this via a command that can be used in a script, rdist, rsh, etc.
      • Graphical administration.
      • Central administration of multiple servers.
      • Ability to import data in the format of legacy configurations, e.g. /etc/bootptab as used by the CMU BOOTP daemon.
      • Ability to make changes while the server is running and leases are being tracked, i.e. add or take away addressees from a pool, modify parameters.
      • Ability to make global modifications to parameters, i.e., that apply to all entries; or ability to make modifications to groups of ports or pools.
      • Maintenance of a lease audit trail, i.e. a log of the leases granted.
    2. What freeware DHCP servers are available?

      (This is not necessarily a complete list)

      950415 Bootp server:
      Bootp 2.4.3 (not DHCP, but with the "DHCP patches" mentioned
      below, can handle DHCP requests)
      ftp://ftp.mc.com/pub/bootp-2.4.3.tar.Z
      950425 Bootp server version 2.4.3 with "samba" DHCP patches
      (does manual allocation of IP addresses)
      http://www.sghms.ac.uk/~mpreston/bootp_dhcp.tar.Z
      (within http://www.sghms.ac.uk/~mpreston/tools.htm)
      950706 "samba" DHCP patches for bootp server:
      (does manual allocation of IP addresses)
      ftp://nimbus.anu.edu.au:/pub/tridge/samba/contributed/DHCP.patch
      (note: I've heard that the patched server will crash if it receives
      one particular optional packet, the DHCP Release packet)
      950711 Patched bootp server supporting DHCP-based "automatic" allocation:
      (gives addresses dynamically, but never takes them away)
      ftp://ftp.ntplx.net/pub/networking/bootp/bootp-DD2.4.3.tar.gz
      951219 BOOTP server and patches for DHCP
      ftp://africa.geomic.uni-oldenburg.de/pub/people/joey/dhcp/bootpd/
      960112 OS/2 port of BOOTP server with patches for manual DHCP support
      ftp://ftp.leo.org/pub/comp/os/os2/tcpip/systools/bootpd-243-dhcp.zip
      960130 Rose-Hulman Institute of Technology "Mondo-DB" LAN administration
      project: modified DHCP server planned
      http://www.rose-hulman.edu/~allard/Mondo-DB/index.html
      950630 WIDE Project:
      mailto:[email protected]
      WIDE Project
      Keio Univ.
      Japan
      ftp://sh.wide.ad.jp/WIDE/free-ware/dhcp/dhcp-1.2.1.tar.gz
      Check Archie for dhcp-1.2.1 because lots of sites distribute it.
      Beta version:
      ftp://sh.wide.ad.jp/WIDE/free-ware/dhcp/dhcp-1.3beta.tar.gz
      960312 Carnegie Mellon University DHCP/BOOTP server (SunOS, dhcp-3.3.7)
      ftp://ftp.net.cmu.edu/pub/dhcp/dhcp-3.3.7.tar.gz
      961104 Princeton patches to CMU dhcpd 3.7.7.
      http://www.princeton.edu/~irwin/dhcpd.html
      971204 Internet Software Consortium (ISC) DHCP/BOOTP Server
      http://www.isc.org/dhcp.html

    3. What commercial DHCP servers are available?

      (This is not necessarily a complete list)

      951010 Wollongong: included in next release of PathWay for OpenVMS which is in
      beta
      951219 Puzzle Systems: WEBserv (NLM(s) that do DHCP, BOOTP, HTTP, and FTP)
      mailto:[email protected]
      http://www.puzzle.com/
      951220 Process Software: server for OpenVMS included in TCPware for OpenVMS
      http://www.process.com/
      960130 Digital: RoamAbout Mobile IP Client/Server Network Software V2.0
      http://www.digital.com/info/Customer-Update/940620001.txt.html
      960312 Nevod Inc. Proxy IP/DHCP Server (PIP) Beta-1.0
      http://www.nevod.com/pip/index.html
      960327 Xedia: IP/Assist 1.0 feature for their switches includes DHCP service.
      http://www.xedia.com/
      960420 Competitive Automation's JOIN (415-321-4006): SunOS4.x, Solaris2.x,
      Digital Unix 3.2, 4.x, HP-UX 9 & 10 DHCP/BOOTP servers.
      http://www.join.com/
      960514 SunSoft: Solstice SolarNet PC-Admin 1.5 includes a DHCP/BOOTP server.
      http://www.sun.com/solstice/Networking-products/PC-Admin.html
      960514 Microsoft: DHCP server included in Windows NT Server 3.51
      http://www.microsoft.com/NTServer/
      http://www.microsoft.com/BackOffice/techbriefs/tech1000.htm
      ftp://ftp.microsoft.com/bussys/winnt/winnt-docs/papers/tcpipimp.doc
      960514 ON Technology: IPTrack 1.0 is a Novell Server-based DHCP/BOOTP server (NLM)
      http://www.on.com/on/onprods/iptrack.html/
      960514 FTP Software: OnNet Server 2.0 (Services OnNet Product)
      http://www.ftp.com/mkt_info/services.html
      960531 Cisco: server in development.
      http://www.cisco.com/
      960620 Farallon: a DHCP server is built into its Netopia Internet Router
      http://www.farallon.com/
      960716 Weird Solutions: BOOTP Server NT supports both BOOTP and statically
      allocated DHCP.
      http://www.mhi.se/
      960808 Novell: NetWare/IP 2.2 (free upgrade to NetWare servers)
      includes a DHCP/BOOTP server; unlike NetWare/IP 2.2 itself, this
      server will run on NetWare 3.12.
      ftp://ftp.novell.com/updates/unixconn/nwip22/nip22b.exe
      http://netware.novell.com/discover/nwip/index.htm
      960809 Silicon Graphics: 'proclaim' software for SGI workstations; part
      of IRIXpro.
      http://www.sgi.com/Products/hardware/challenge/IRIXpro/IRIXpro.html
      http://www.sgi.com/Products/hardware/challenge/IRIXpro/IRIXprospecs.html
      960829 Isotro: NetID DHCP Server (BOOTP/DHCP server)
      (No longer available from Isotro)
      960912 Cisco: (announced) DHCP/BOOTP server for Solaris, HP-UX, and
      AIX, Windows NT (Alpha & Intel) included in Cisco's DNS/DHCP
      Manager V1.0 and Cisco's Server Suite 1000 V1.0
      http://www.cisco.com/
      960917 SunSoft: (future) DHCP/BOOTP server to be bundled with Solaris 2.6
      or as hte "Internet Server Supplement" to Solaris 2.5.1.
      http://www.sun.com/
      961118 Network TeleSystems: Shadow (PC-based) also does BOOTP
      http://www.nts.com/NTS/shadow.html
      961217 Hewlett-Packard: HP-UX 10.10 and subsequent versions include a bootp
      server with DHCP extensions.
      970325 American Internet Corp: Net Registrar (for Windows NT and Solaris)
      http://www.american.com/
      970403 Microsoft: BOOTP/DHCP server in NT 4.0 SP2.
      http://www.microsoft.com/kb/articles/q161/5/71.htm
      970415 VICOM: VICOM DHCP Server (runs on Macintosh/MacOS)
      http://www.vicomtech.com/dhcp.main.html
      970415 Sonic Systems: Sonic DHCP Internet Server runs on Macintosh/MacOS,
      also does BOOTP
      http://www.sonicsys.com/dhcp.html
      970805 Process Software: MultiNet 3.5 for OpenVMS includes DHCP/BOOTP server.
      http://www.process.com/multinet/
      971217 Quadritek Systems, Inc.: QDHCP (NT or UNIX), also does BOOTP
      http://www.quadritek.com/products/qipdhcpserv.html
      980518 Billiter Consultants: ipLease DHCP server (32bit Windows)
      http://www.billiter.com/
      980331 Deerfield Communications: DHCP server included in Wingate
      Pro (2.1b) "proxy server"
      http://www.wingate.net/
      980603 IBM OS/400 Version 3 Release 7 and subsequent versions includes
      a DHCP/BOOTP server.
      http://www.as400.ibm.com/
      980611 Bay (Xylogics) Remote Annex (RA) and Remote Access Concentrator
      (RAC) communication servers have proxy DHCP client since release
      13.2, December 1995.
      http://www.baynetworks.com/
      980612 IBM: DHCP server included in AIX 4.1.4 and beyond. Includes
      BOOTP service.
      http://www.rs6000.ibm.com/
      980612 IBM: TCP/IP Version 4.1 for OS/2 Warp includes DHCP, BOOTP and
      DDNS and java-based administration.
      http://www.software.ibm.com/os/warp-server

    4. What freeware DHCP clients are available?

      (This is not necessarily a complete list)

      960809 WIDE Project includes a client for BSD and SunOS systems:
      mailto:[email protected]
      WIDE Project
      Keio Univ.
      Japan
      ftp://sh.wide.ad.jp/WIDE/free-ware/dhcp/dhcp-1.2.1.tar.gz
      Check Archie for dhcp-1.2.1 because lots of sites distribute it.
      Beta version:
      ftp://sh.wide.ad.jp/WIDE/free-ware/dhcp/dhcp-1.3beta.tar.gz
      960904 Linux bootp client: bootpc; DHCP being added over time.
      ftp://ftp.damtp.cam.ac.uk/pub/linux/bootpc/
      970415 dhcpcd (for Linux 1.2.xx, 1.3.xx, 2.0.x)
      ftp://ftp.kobe-u.ac.jp/pub/PC-UNIX/Linux/network/dhcp/
      version 0.4a:
      ftp://ftp.kobe-u.ac.jp/pub/PC-UNIX/Linux/network/dhcp/dhcpcd-0.4a.tar.gz
      version 0.5:
      ftp://ftp.kobe-u.ac.jp/pub/PC-UNIX/Linux/network/dhcp/dhcpcd-0.5.tar.gz
      version 0.5-p1:
      ftp://ftp.kobe-u.ac.jp/pub/PC-UNIX/Linux/network/dhcp/dhcpcd-0.5-p1.tar.gz
      version 0.6:
      ftp://ftp.kobe-u.ac.jp/pub/PC-UNIX/Linux/network/dhcp/dhcpcd-0.6.tar.gz
      971204 Internet Software Consortium (ISC) DHCP/BOOTP Server
      Distribution includes a client. See ISC server in section above
      on "Freeware Servers".


    5. Which vendors of client software currently support DHCP?

      (This is not necessarily a complete list)

      950417 Shiva: proxy client for remote users (in Lanrovers and Netmodems)
      950425 Hewlett-Packard
      950502 NetManage: Chameleon 4.5
      950630 Beame & Whiteside Software: resells Dirk Koeppen EDV-Beratungs-GmbH's
      TCP/IP BOOT-PROM
      950705 Microsoft: MS-TCP/IP 3.11a & MS-TCP/IP 3.11b
      950711 Microsoft: Windows NT 3.5
      950711 Microsoft: Windows for Workgroups 3.11a
      950711 Frontier Technologies(800-929-3054): in SuperTCP for Windows
      http:www.frontiertech.com
      [email protected]
      950712 Beame & Whiteside(800-720-7151): BW-Connect NFS for DOS & Windows
      950802 Wollongong: PathWay Access ver 3.2 (Windows)
      http://www.twg.com/
      950802 WRQ: Reflection Network Series products (version 5) for Windows
      http://www.wrq.com/
      950814 Competitive Automation(415-321-4006): SunOS4.x, Solaris2.x and
      DECOSF3.x,4.x clients
      950915 Stampede: included in Remote Office Gold
      951113 Persoft(800-368-5283): TCP Addition and Portable TCP
      http://www.persoft.com/
      951207 Dirk Koeppen EDV-Beratungs-GmbH: TCP/IP DHCP Boot ROMs (TCP/IP
      BOOT-PROM) www.dunkel.de/dksoft
      951220 Attachmate: IRMA TCP Suite Version 3.1
      960130 Digital: RoamAbout Mobile IP Client/Server Network Software V2.0
      http://www.digital.com/info/Customer-Update/940620001.txt.html
      960209 FTP Software: included in OnNet 2.0 (Windows)
      http://www.ftp.com/
      960209 FTP Software: PC/TCP 4.0 (DOS)
      http://www.ftp.com/
      960312 Core Systems: Internet-Connect for Windows 95 Version 2.1 has DHCP
      proxy client.
      http://ns1.win.net/~core/Coresys/homepage.html
      960313 Apple: Open Transport 1.1 included with System 7.5.3 & runs on
      68030, 68040, and PowerPC Macintoshes.
      960314 Apple: Open Transport 1.1 shrink wrap version will be offered.
      960408 IBM: Client DHCP software for Windows 3.x.
      960408 IBM: Client DHCP software for MS/PC-DOS.
      960501 SunSoft: included in PC-NFS Pro 2.0 for Windows
      960501 NetManage: included in ChameleonNFS 4.6
      960503 FTP Software: included in OnNet32, Version 1.0 (Windows 95 and NT)
      http://www.ftp.com/
      960514 Novell: Client32 for DOS/Windows 3.1 (beta) will use either DHCP
      or BOOTP to get IP parameters.
      960514 Novell: NetWare/IP for DOS, Windows 3.1, Windows 95, and
      Windows NT uses DHCP to obtain IP parameters.
      960514 Novell: NetWare/IP servers can use DHCP to auto-configure their
      IP parameters.
      960809 Silicon Graphics: included in IRIX since version 5.3.
      http://www.sgi.com/Products/software/IRIX6.2/IRIX62DS.html
      http://www.sgi.com/Products/hardware/challenge/IRIXpro/IRIXpro.html
      960917 Sun: Solaris 2.6.
      http://www.sun.com/
      961118 Network TeleSystems TCP Pro 3.0 for Windows
      http://www.nts.com/NTS/tcp_pro.html
      970805 Cisco: DHCP & BOOTP for Windows 3.1 included in Cisco TCP/IP
      Suite 100 for Windows (formerly MultiNet for Windows) V2.0
      For Windows 95, uses the native support.
      http://www.cisco.com/
      980331 Deerfield Communications: DHCP server included in Wingate
      Pro (2.1b) "proxy server"
      http://www.wingate.net/
      980611 IBM: OS/2 WARP Version 4 (Merlin) has DHCP client capability in the
      basic package.
      http://www.software.ibm.com/os/warp-client
      980612 IBM's DOS/Windows LAN Services (for IBM PC-DOS, Microsoft
      MS-DOS, and/or Microsoft Windows 3.x)
      980612 IBM's line of NetworkStations are all DHCP clients (or BOOTP)
      http://www.as400.ibm.com/networkstation/
      980612 IBM: AIX 4.1.4 client and server packages include a DHCP client.
      http://www.rs6000.ibm.com/

    6. What are the DHCP plans of major client-software vendors? Apple MacOS MacTCP's successor, Open Transport, supports DHCP. Open Transport 1.1 ships with System 7.5 Update 2.0 (which updates MacOS to version 7.5.3, released March 11, 1996) and supports any 68030, 68040, or PowerPC Macintosh. A shrink wrap version of Open Transport is planned. Microsoft Windows95 supports it and does not support BOOTP. I heard a rumor that BOOTP support will be added. Novell LAN Workplace for DOS For supporting DOS/Windows 3.1, Client32 for DOS/Windows, due in June 1996, will provide the TCP/IP stack functions and will support DHCP and BOOTP. For Windows 95 and Windows NT, the native stack will be used so that DHCP is supported. IBM OS/2 Warp supports it.
    7. What Routers forward DHCP requests?

      (This is not necessarily a complete list).

      Note that in general, these routers probably already had BOOTP forwarding, but lacked the support for the BOOTP broadcast flag (see "broadcast flag" under What are the Gotcha's? above). It is likely that many other routers also support BOOTP forwarding.

      Cisco (from Cisco FAQ) Routers running GSYS version 9.21(4) and 10.0(3) as well as later releases. Wellfleet/Bay (from Wellfleet FAQ) DHCP is supported by enabling BOOTP support (with transmission and/or reception as needed). Starting with version 9.00 of their routing software BayRs. 3Com Netbuilder Version 7.2 software can support DHCP relaying through the use of its generic UDP Helper service. Version 8.0 and later officially supports DHCP. Xyplex Version 5.5 of their routing software supports DHCP. ALANTEC The switches' "router" function has have been handling BOOTP forwarding since around 1993. Support for the broadcast flag introduced in a maintenance release of 2.5 of their software and is in version 2.6 and later. IBM 2210 I've confirmed that Version 1 Release 2 has a BOOTP relay agent. I haven't found out anything about support for the broadcast flag. ACC Version 7.2 (about 1994) and later support DHCP relaying. Proteon/Digital I'm not sure what is the first version that has this support. Novell MPR The same as for their server. IBM 6611 Supports BOOTP forwarding.
    8. What Routers include DHCP servers?

      DHCP requires disk storage (or some other form of reliable non-volatile storage), making the task of DHCP service more compatible with servers than with dedicated routers. The large-scale routers (i.e., those of Cisco, Bay, Fore) don't an will probably never will have a DHCP server function.

      But there are a number of types of servers that can be configured to route and serve DHCP. This includes Novell servers and computers running Unix. There are also units designed to handle two or more aspects of your Internet connection, e.g. routing between a LAN and a leased line as well as doing other functions to allow computers on the LAN to reach the Internet (or corporate intranet as the case may be). One example is Farallon's Netopia Internet Router mentioned above under commercial servers.

    9. What Routers use DHCP to configure their IP addresses?

      The DHCP RFC specifically says that DHCP is not intended for use in configuring routers. The reason is that in maintaining and troubleshooting routers, it is important to know its exact configuration rather than leaving that to be automatically done, and also that you do not want your router's operation to depend upon the working of yet another server.

      It may be possible to configure some types of more general-purpose computers or servers to get their addresses from DHCP and to act as routers. Also, there are remote access servers, often which are usually not true routers, which use DHCP to acquire addresses to hand out to their clients.

    10. What Servers forward DHCP requests?
      • Windows NT's 3.51 Service Pack 3 (and 4) includes a BOOTP (& DHCP) relay agent as part of "Multi Protocol Router". 3.51).
      • For Novell servers, there are NLMs that forward BOOTP requests, thus DHCP requests. The "BOOTPFWD NLM" is included in NetWare 4.1. You can get this support in NetWare 3.11 and 3.12 also by applying the TCPN01.EXE patch which is located at ftp://ftp.novell.com/updates/inet/mpr211/tcpn01.exe and on Netwire. Two other such NLMs (possibly old versions of the same) that are available online:
      • Also for Novell servers, the DHCP server that comes with NetWare/IP 2.2 can be configured to be just a BOOTP/DHCP forwarding agent.
      • AIX, through its dhcprd daemon.
      • Warp Server Version 4.
    11. Which implementations support or require the broadcast flag?

      The broadcast flag is an optional element of DHCP, but a client which sets it works only with a server or relay that supports it.

      • Clients Microsoft Windows NT DHCP client support added with version 3.5 sets the broadcast flag. Version 3.51 and later no longer set it. The exception is in the remote access support: it sets the flag when it uses DHCP to acquire addresses to hand out to its PPP clients. tcp/ip-32 for Microsoft Windows for Workgroups (WFW) Version 3.11a sets it, but version 3.11B doesn't. Microsoft Windows 95 Does not set the broadcast flag.
    12. What servers support secondary subnet numbers?

      (These are not complete lists) The following servers can handle dynamic allocation on secondary subnet numbers:

      • IPTrack version 2.0
      • ISC
      • JOIN
      • SGI's DHCP Server under IRIX 6.2
      • Cisco (previously TGV)
      • NetID
      • Microsoft Windows NT 4.0 (since service pack 2)
      • Sonic
      • QDHCP
      • ipLease
      • IBM Warp Server Version 4
      • IBM AIX

      The following can serve manually allocated addresses on secondary subnet numbers:

      • IPTrack version 2.0
      • ISC
      • JOIN
      • QDHCP

      The following cannot support secondary subnet numbers:

      • Microsoft Windows NT 3.51 and 4.0 (through RC1)
      • WIDE
      • Sonic DHCP Server
    13. What servers support RFC-based dynamic DNS update?

      The following DHCP servers include the ability to make use of the RFC 2136/2137 DNS feature to make dynamic updates to the DNS. To make use of this ability, you need a DNS server that supports this feature. A likely use is to create temporary DNS records that associate a fully qualified DNS name derived from the client's netbios name with the client's leased IP number. Another use might be to associate DNS names with MAC addresses. These products might support one or both of these uses.

      • American Internet Corp Net Registrar
      • QDHCP
      • IBM's Warp Server (version 4 and after)
      • IBM's AIX server (version 4.1 and after)
    14. How can I run Windows 95 without a DHCP server?

      Not really a DHCP question, but it has been asked a lot, particularly by sites for which changing from BOOTP represents a lot of work. Some choices:

      • Use no server at all for the Windows 95 clients: set the addresses in each client's setup.
      • Install a non-Microsoft TCP/IP stack for Windows 95 that supports BOOTP.
      • Switch from your current BOOTP server to one that supports both BOOTP and DHCP.
      • The 'billgPC' program uses BOOTP (instead of DHCP) to configure Windows 95's native IP stack: http://www.panix.com/~perin/ (note: it also works with Windows NT).

      A Document that addresses this question is the Windows 95tm Networking FAQ, http://www-leland.stanford.edu/~llurch/win95netbugs/faq.html

    15. Do any servers limit the MAC addresses that may roam?
      • IBM's AIX and OS/2 WARP DHCP servers.
      • ISC.
    16. What analyzers decode DHCP?
      • Release 5.0 of Network General Corporation's Sniffer software.
      • I believe one of the free Unix implementations has included in its distribution a program that captures and decodes BOOTP and DHCP negotiations.
      • Microsoft's SMS includes a protocol analyzer called "Network Monitor" that decodes DHCP. All NT software includes a remote agent for it.
      • NetXRay, software that runs under Windows NT adn 95. http://ngcwebgate.ngc.com/product_info/netxray/netxray.html
      • PacketView (LAN), SerialView (PPP and SLIP), and ISDNView (PPP over ISDN) all are DOS programs that fully decode DHCP packets. href="http://www.klos.com/
    17. What administration tools administer DHCP configurations?
      • Quadritek's QIP network administration product includes an interface to Competitive Automation's JOIN DHCP server and IBM's DHCP server and their own server.
    18. How do I make a client give up its lease?

      This is a general question, but the answer is of necessity specific to the client-implementation. Naturally, one way to avoid the problem is to keep leases short enough that you are not obliged to do this.

      • One method mentioned is to temporarily change the clock on the client.
      • For a Win95 client, the winipcfg.exe program can do it.
    19. What are the Gotcha's specific to various implementations?

      In many cases, new releases have solved the problems that have been identified with various DHCP implementations.

      • An extra server feature is required to handle the allocation of addresses on the secondary IP addresses associated with a router port. You may find out after the fact that you have such secondary addresses
      • There have been servers that are inflexible as to the list of configuration parameters they were able to serve. If your client requires certain parameters, you could find such a server unusable.
      • I hate to cast wide suspicions, but I've heard occasional word on client DHCP implementations that do not implement the entire protocol. Doing so requires that the software module be able to wake up again after a specified period of time and "renew the lease", i.e., ask to continue using the IP number. This is at least one feature of DHCP that is very hard to implement in some simpler systems.
      • A specific complaint about Microsoft's Windows 95 dhcp client: it times out its requests much more quickly than the times specified by RFC1541 section 4.1. Among the circumstances that can turn this into a practical problem are the latencies due to relay agents and a server's use of ICMP echo to doublecheck the address. While it works with Microsoft's own NT-based server, the problem prevents interoperation with some other DHCP servers under some conditions. Microsoft is rumored to have developed an updater named VDHCPUPD.EXE to patch this problem, once available through the following patch:
        File: Vdhcp.386
        File Last Modified Date: 02/12/96
        File Size: 27,985 bytes
        File Version Information: 4.00.951
        It consists of 2 files, vdhcpupd.exe and vdhcpupd.txt. I've since been told that a newer version is 4.00.954. I've also been told that the exe file is on the net at http://www.halcyon.com/cerelli/software/vdhcpupd.exe
      • There are a number of issues regarding the patched bootp servers. These have been reported to re DD2.4.3:
        • 'When run from inetd, I had problems with "Could not bind port" and DHCP request failure. I don't know why, and the problem went away when bootpd is run as a daemon.'
        • 'Unless you set "dl" to some value in the bootptab file, the DHCP lease time, renewal time and prebinding time will be rubbish, which will cause occasional renewal problems.' One symptom you might see is Microsoft DHCP implementations using 5-minute leases, which is their default. Other implementations may not run at all.
      • Early Microsoft DHCP client implementations required the broadcast bit. Current ones do not.
      • I have heard a vague complaints about the Microsoft implementations of DHCP: that it does not follow the standards. I could use details.
      • Early Apple Open Transport implementations did not always fill out packets to BOOTP's 300-byte minimum, thus BOOTP forwarding agents that follow the BOOTP RFC and discard such packets end up discarding such DHCP packets, causing some of the functions to fail. Open Transport 1.1 fixes this.
      • Pre 1.1 versions of Open Transport experienced interoperability problems with the Microsoft NT DHCP server.
      • The very first announced release of Carnegie Mellon's server, dhcp-3.3.6, circa March 1996 has shown signs of needing to be shaken out to be more easily compiled outside of its development environment.
      • Windows NT server v3.51 allows the administrator to specify addresses within its assignment range to be excluded, but does not always exclude them.
      • Report: Novell's NetwareIP 2.2 server refuses to hand out dynamic bootp assignments to hosts mentioned in the local /etc/hosts file, even if configured to do so.
      • I've heard a report that some combinations of versions of Unix & the ISC server will transmit packets to the subnet broadcast address rather than the default broadcast address (255.255.255.255), which impedes interoperability with some clients.
      • Windows 95 DHCP client answers pings from an IP address even after the the client's lease has expired. Thus a server that uses ping to check to see that an IP number is unused before reassigning it may find that it is still in use.
      • Windows 95 DHCP client cannot handle a lease renewal offered by a different server.
      • Some clients have no way to configure a class option, which can be a showstopper if you need to use the class option to help decide what pool of addresses the client uses.
      • I've heard reports that Windows 95, or at least some versions will use an address after the lease has expired under some circumstances, even when renewal requests have been turned down. With properly behaving clients, an IP administrator can safely make the following statement: "As long as all the clients are set to get their addresses through DHCP, I can tell which addresses are not being used by the clients simply by checking the server to see which IP addresses have no outstanding leases." The reports suggest that Windows 95 implementations won't allow this statement to be assumed.
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