Tag - Ip Addresses

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.

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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 paramvcers.
  • Ability to make global modifications to paramvcers, 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.

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Can DHCP support remote access

26. 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 paramvcers. 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 paramvcers 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 paramvcers 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.

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