dhcpdWord格式文档下载.docx

1、 This software is free software. At various times its development has been underwritten by various organizations, including the ISC and Vixie Enterprises. The development of 3.0 has been funded almost entirely by Nominum, Inc. At this point development is being shepherded by Ted Lemon, and hosted by

2、 the ISC, but the future of this project depends on you. If you have features you want, please consider implementing them.OPERATION The DHCP protocol allows a host which is unknown to the network admin- istrator to be automatically assigned a new IP address out of a pool of IP addresses for its netw

3、ork. In order for this to work, the network administrator allocates address pools in each subnet and enters them into the dhcpd.conf（5） file. On startup, dhcpd reads the dhcpd.conf file and stores a list of avail- able addresses on each subnet in memory. When a client requests an address using the D

4、HCP protocol, dhcpd allocates an address for it. Each client is assigned a lease, which expires after an amount of time chosen by the administrator （by default, one day）. Before leases expire, the clients to which leases are assigned are expected to renew them in order to continue to use the address

5、es. Once a lease has expired, the client to which that lease was assigned is no longer per- mitted to use the leased IP address. In order to keep track of leases across system reboots and server restarts, dhcpd keeps a list of leases it has assigned in the dhcpd.leases（5） file. Before dhcpd grants a

6、 lease to a host, it records the lease in this file and makes sure that the contents of the file are flushed to disk. This ensures that even in the event of a system crash, dhcpd will not forget about a lease that it has assigned. On startup, after reading the dhcpd.conf file, dhcpd reads the dhcpd.

7、leases file to refresh its memory about what leases have been assigned. New leases are appended to the end of the dhcpd.leases file. In order to prevent the file from becoming arbitrarily large, from time to time dhcpd creates a new dhcpd.leases file from its in-core lease database. Once this file h

8、as been written to disk, the old file is renamed dhcpd.leases, and the new file is renamed dhcpd.leases. If the sys- tem crashes in the middle of this process, whichever dhcpd.leases file remains will contain all the lease information, so there is no need for a special crash recovery process. BOOTP

9、support is also provided by this server. Unlike DHCP, the BOOTP protocol does not provide a protocol for recovering dynamically- assigned addresses once they are no longer needed. It is still possi- ble to dynamically assign addresses to BOOTP clients, but some adminis- trative process for reclaimin

10、g addresses is required. By default, leases are granted to BOOTP clients in perpetuity, although the network administrator may set an earlier cutoff date or a shorter lease length for BOOTP leases if that makes sense. BOOTP clients may also be served in the old standard way, which is to simply provi

11、de a declaration in the dhcpd.conf file for each BOOTP client, permanently assigning an address to each client. Whenever changes are made to the dhcpd.conf file, dhcpd must be restarted. To restart dhcpd, send a SIGTERM （signal 15） to the process ID contained in /var/run/dhcpd.pid, and then re-invok

12、e dhcpd. Because the DHCP server database is not as lightweight as a BOOTP data- base, dhcpd does not automatically restart itself when it sees a change to the dhcpd.conf file. Note: We get a lot of complaints about this. We realize that it would be nice if one could send a SIGHUP to the server and

13、have it reload the database. This is not technically impossible, but it would require a great deal of work, our resources are extremely limited, and they can be better spent elsewhere. So please dont complain about this on the mailing list unless youre prepared to fund a project to implement this fe

14、ature, or prepared to do it yourself.COMMAND LINE The names of the network interfaces on which dhcpd should listen for broadcasts may be specified on the command line. This should be done on systems where dhcpd is unable to identify non-broadcast interfaces, but should not be required on other syste

15、ms. If no interface names are specified on the command line dhcpd will identify all network inter- faces which are up, eliminating non-broadcast interfaces if possible, and listen for DHCP broadcasts on each interface. If dhcpd should listen on a port other than the standard （port 67）, the -p flag m

16、ay used. It should be followed by the udp port number on which dhcpd should listen. This is mostly useful for debugging pur- poses. To run dhcpd as a foreground process, rather than allowing it to run as a daemon in the background, the -f flag should be specified. This is useful when running dhcpd u

17、nder a debugger, or when running it out of inittab on System V systems. To have dhcpd log to the standard error descriptor, specify the -d flag. This can be useful for debugging, and also at sites where a com- plete log of all dhcp activity must be kept but syslogd is not reliable or otherwise canno

18、t be used. Normally, dhcpd will log all output using the syslog（3） function with the log facility set to LOG_DAEMON. Dhcpd can be made to use an alternate configuration file with the -cf flag, or an alternate lease file with the -lf flag. Because of the importance of using the same lease database at

19、 all times when running dhcpd in production, these options should be used only for testing lease files or database files in a non-production environment. When starting dhcpd up from a system startup script （e.g., /etc/rc）, it may not be desirable to print out the entire copyright message on startup.

20、 To avoid printing this message, the -q flag may be speci- fied. The DHCP server reads two files on startup: a configuration file, and a lease database. If the -t flag is specified, the server will simply test the configuration file for correct syntax, but will not attempt to perform any network ope

21、rations. This can be used to test the a new configuration file automatically before installing it. The -T flag can be used to test the lease database file in a similar way. The -tf and -play options allow you to specify a file into which the entire startup state of the server and all the transaction

22、s it pro- cesses are either logged or played back from. This can be useful in submitting bug reports - if you are getting a core dump every so often, you can start the server with the -tf option and then, when the server dumps core, the trace file will contain all the transactions that led up to it

23、dumping core, so that the problem can be easily debugged with -play. The -play option must be specified with an alternate lease file, using the -lf switch, so that the DHCP server doesnt wipe out your existing lease file with its test data. The DHCP server will refuse to operate in playback mode unl

24、ess you specify an alternate lease file.CONFIGURATION The syntax of the dhcpd.conf（5） file is discussed separately. This section should be used as an overview of the configuration process, and the dhcpd.conf（5） documentation should be consulted for detailed refer- ence information.Subnets dhcpd need

25、s to know the subnet numbers and netmasks of all subnets for which it will be providing service. In addition, in order to dynami- cally allocate addresses, it must be assigned one or more ranges of addresses on each subnet which it can in turn assign to client hosts as they boot. Thus, a very simple

26、 configuration providing DHCP support might look like this: subnet 239.252.197.0 netmask 255.255.255.0 range 239.252.197.10 239.252.197.250; Multiple address ranges may be specified like this: range 239.252.197.10 239.252.197.107; range 239.252.197.113 239.252.197.250; If a subnet will only be provi

27、ded with BOOTP service and no dynamic address assignment, the range clause can be left out entirely, but the subnet statement must appear.Lease Lengths DHCP leases can be assigned almost any length from zero seconds to infinity. What lease length makes sense for any given subnet, or for any given in

28、stallation, will vary depending on the kinds of hosts being served. For example, in an office environment where systems are added from time to time and removed from time to time, but move relatively infre- quently, it might make sense to allow lease times of a month of more. In a final test environm

29、ent on a manufacturing floor, it may make more sense to assign a maximum lease length of 30 minutes - enough time to go through a simple test procedure on a network appliance before pack- aging it up for delivery. It is possible to specify two lease lengths: the default length that will be assigned if a client doesnt ask for any particular lease length, and a maximum lease length. These are specified as clauses to the subnet command: default-lease-time 60