ChorusOS man pages section 4CC: Files

dhclient.cf(4CC)

NAME

dhclient.cf- DHCP client configuration file

DESCRIPTION

The dhclient.cf file contains configuration information for dhclient, the Internet Software Consortium DHCP Client.

The dhclient.cf file is a free-form ASCII text file. It is parsed by the recursive-descent parser built into dhclient(1M). The file may contain extra tabs and newlines for formatting purposes. Keywords in the file are case-insensitive. Comments may be placed anywhere within the file (except within quotes). Comments begin with the # character and end at the end of the line.

The dhclient.cf file can be used to configure the behavior of the client in a wide variety of ways: protocol timing, information requested from the server, information required of the server, defaults to use if the server does not provide certain information, values with which to override information provided by the server, or values to prepend or append to information provided by the server. The configuration file can also be preinitialized with addresses to use on networks that do not have DHCP servers.

WARNINGS

Many options are defined for DHCP. The implementation of dhclient in this release takes only the subnet-mask option into account. Other options are silently ignored.

PROTOCOL TIMING

You do not need to configure the timing behavior of the client. If no timing configuration is provided by the user, a reasonable timing behavior is be used by default, allowing timely updates without placing an inordinate load on the server.

The following statements can be used to adjust the timing behavior of the DHCP client if required, however:

The `timeout` statement

timeout time ;

The timeout statement determines the amount of time that must pass between the time that the client begins to determine its address and the time that it shall be unable to contact a server. By default, this timeout is sixty seconds. After the timeout has passed, if there are any static leases defined in the configuration, the client loops through these, attempting to validate them. If it finds a valid lease, it uses that lease's address. If there are no valid static leases, the client restarts the protocol after the defined retry interval.

The `retry` statement

retry time ;

The retry statement determines the time that must pass after the client has determined that there is no DHCP server present before it tries again to contact a DHCP server. By default, this is five minutes.

The `select-timeout` statement

select-timeout time ;

It is desirable for there to be more than one DHCP server serving any given network. In this case, it is possible that a client may be sent more than one offer in response to its initial lease discovery message. One of these offers can be preferable to the other For example, one offer may have the address the client previously used, and the other may not.

The select-timeout is the time after the client sends its first lease discovery request at which it stops waiting for offers from servers, assuming that it has received at least one such offer. If no offers have been received by the time the select-timeout has expired, the client accepts the first offer that arrives.

By default, the select-timeout is zero seconds -- that is, the client takes the first offer it sees.

The `reboot` statement

reboot time ;

When the client is restarted, it first tries to reacquire the last address it had. This is called the INIT-REBOOT state. If it is still attached to the same network it was attached to when it last ran, this is the quickest way to get started. The reboot statement sets the time that must elapse after the client first tries to reacquire its old address before it gives up and tries to discover a new address. By default, the reboot timeout is ten seconds.

The `backoff-cutoff` statement

backoff-cutoff time ;

The client uses an exponential backoff algorithm with some randomness, so that if many clients try to configure themselves at the same time, they do not make their requests in lockstep. The backoff-cutoff statement determines the maximum amount of time that the client is allowed to back off. It defaults to two minutes.

The `initial-interval` statement

initial-interval time ;

The initial-interval statement sets the amount of time between the first attempt to reach a server and the second attempt to reach a server. Each time a message is sent, the interval between messages is incremented by twice the current interval multiplied by a random number between zero and one. If it is greater than the backoff-cutoff amount, it is set to that amount. It defaults to ten seconds.

LEASE REQUIREMENTS AND REQUESTS

The DHCP protocol allows the client to request that the server send it specific information, and not send it other information that it is not prepared to accept. The protocol also allows the client to reject offers from servers if they do not contain information the client needs, or if the information provided is not satisfactory.

There is a variety of data contained in offers that DHCP servers send to DHCP clients. The data that can be specifically requested is what are called DHCP Options. DHCP Options are defined in dhcp-options.

The `request` statement

request

-option

The request statement causes the client to request that any server responding to the client send the client its values for the specified options. Only the option names, and not the options parameters, should be specified in the request statement.

The `require` statement

require

-option

The require statement lists options that must be sent for an offer to be accepted. Offers that do not contain all the listed options are ignored.

The `send` statement

send

-option

The `lease` statement

lease

-lease-declaration

The DHCP client may decide after some period of time (see PROTOCOL TIMING) that it is not going to succeed in contacting a server. It is possible to define one or more fixed leases in the client configuration file for networks where there is no DHCP or BOOTP service, so that the client can still automatically configure its address. This is done with the lease statement.

A lease statement consists of the lease keyword, followed by a left curly brace, {, followed by one or more lease declaration statements, followed by a right curly brace, }. The following lease declarations are possible:

bootp;

The bootp statement is used to indicate that the lease was acquired using the BOOTP protocol rather than the DHCP protocol. It is never necessary to specify this in the client configuration file. The client uses this syntax in its lease database file.

interface "string";

The interface lease statement is used to indicate the interface on which the lease is valid. If set, this lease is only be tried on a particular interface. When the client receives a lease from a server, it always records the interface number on which it received that lease. If predefined leases are specified in the dhclient.cf file, the interface should also be specified, although this is not required.

fixed-address IP_address;

The fixed-address statement is used to set the ip address of a particular lease. This is required for all lease statements. The IP address must be specified as a dotted quad (for example, 12.34.56.78).

filename "string";

The filename statement specifies the name of the boot filename to use. This is not used by the standard client configuration script, but is included for completeness.

server-name "string";

The server-name statement specifies the name of the boot server name to use. This is also not used by the standard client configuration script.

option option-declaration;

The option statement is used to specify the value of an option supplied by the server, or, in the case of predefined leases declared in dhclient.cf, the value that the user wishes the client configuration script to use if the predefined lease is used.

script script-name;

The script statement is used to specify the pathname of the dhcp client configuration script. This script is used by the dhcp client to set each interface's initial configuration prior to requesting an address, to test the address once it has been offered, and to set the interface's final configuration once a lease has been acquired. If no lease is acquired, the script is used to test predefined leases, if any, and also called once if no valid lease can be identified.

medium "media setup";

The medium statement can be used on systems where network interfaces cannot automatically determine the type of network to which they are connected. The media setup string is a system-dependent parameter that is passed to the DHCP client configuration script when initializing the interface. On Unix and Unix-like systems, the argument is passed on the ifconfig command line when configuring the interface.

The DHCP client automatically declares this parameter if it used a media type (see the media statement) when configuring the interface to obtain a lease. This statement should be used in predefined leases only if the network interface requires media type configuration.

renew date; rebind date; expire date;

The renew statement defines the time at which the DHCP client should begin trying to contact its server to renew a lease that it is using. The rebind statement defines the time at which the DHCP client should begin to try to contact any DHCP server to renew its lease. The expire statement defines the time at which the DHCP client must stop using a lease if it has been unable to contact a server to renew it.

These declarations are automatically set in leases acquired by the DHCP client, but must also be configured in predefined leases; a predefined lease whose expiry time has passed is not used by the DHCP client.

Dates are specified as follows:

weekday
year/month/day
hour:minute:second:

The weekday is present to make it easy for the user to tell when a lease expires. It is specified as a number from zero to six, with zero being Sunday. When declaring a predefined lease, it can always be specified as zero. The year is specified with the century, so it should generally be four digits except for very long leases. The month is specified as a number starting with 1 for January. The day of the month is likewise specified starting with 1. The hour is a number between 0 and 23; the minute a number between 0 and 59, and the second also a number between 0 and 59.

ALIAS DECLARATIONS

alias

-declaration

Some DHCP clients running TCP/IP roaming protocols may require that in addition to the lease they may acquire via DHCP, their interface also be configured with a predefined IP alias. This is to ensure that they have a permanent IP address even while roaming. The Internet Software Consortium DHCP client does not support roaming with fixed addresses directly, but to facilitate this, the DHCP client can be set up to configure an IP alias using the alias declaration.

The alias declaration resembles a lease declaration, except that options other than the subnet-mask option are ignored by the standard client configuration script. Expiry times are also ignored. A typical alias declaration includes an interface declaration, a fixed-address declaration for the IP alias address, and a subnet-mask option declaration. A medium statement should never be included in an alias declaration.

OTHER DECLARATIONS

reject IP-address;

The reject statement causes the DHCP client to reject offers from servers who use the specified address as a server identifier. This can be used to avoid being configured by rogue or misconfigured DHCP servers. Note that it is preferable to track down the bad DHCP server and fix it.

interface "name" { declarations ... }

A client with more than one network interface may require different behavior depending on which interface is being configured. All timing parameters and declarations other than lease and alias declarations can be enclosed in an interface declaration, and those parameters are then used only for the interface that matches the specified name. Interfaces for which there is no interface declaration use the parameters declared outside of any interface declaration, or the default settings.

media "media setup" [ , "media setup", ... ];

The media statement defines one or more media configuration parameters that can be tried while attempting to acquire an IP address. The DHCP client cycles through each media setup string on the list, configuring the interface using that setup and attempting to boot, and then trying the next one. This can be used for network interfaces that are not capable of sensing the media type unaided. Whichever media type succeeds in getting a request to the server and hearing the reply is likely to be the correct one.)

The media setup is only used for the initial phase of address acquisition (the DHCPDISCOVER and DHCPOFFER packets). Once an address has been acquired, the DHCP client records it in its lease database and records the media type used to acquire the address. Whenever the client tries to renew the lease, it uses that same media type. The lease must expire before the client goes back to cycling through media types.

EXAMPLES

The following configuration file is used on a laptop running NetBSD 1.3. The laptop has an IP alias of 192.5.5.213, and has one interface, ep0 (a 3com 3C589C). Booting intervals have been shortened from the default, because the client is known to spend most of its time on networks with little DHCP activity. The laptop roams to multiple networks.

timeout 60; retry 60; reboot 10; select-timeout 5;
initial-interval 2; reject 192.33.137.209;

interface "ep0" {
    send host-name "andare.fugue.com"; send
    dhcp-client-identifier 1:0:a0:24:ab:fb:9c; send
    dhcp-lease-time 3600; supersede domain-name "fugue.com
    rc.vix.com home.vix.com"; prepend domain-name-servers
    127.0.0.1; request subnet-mask, broadcast-address,
    time-offset, routers,
     domain-name, domain-name-servers, host-name;
    require subnet-mask, domain-name-servers;
    media "media 10baseT/UTP", "media 10base2/BNC";
}

alias {
  interface "ep0"; fixed-address 192.5.5.213; option subnet-mask
  255.255.255.255;
}

This is a complicated dhclient.cf file. In many cases, you need no dhclient.cf file; the defaults are usually sufficient.

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving

AUTHOR

dhclient was written by Ted Lemon <mellon@vix.com> under a contract with Vixie Labs. Funding for this project was provided by the Internet Software Consortium. Information about the Internet Software Consortium can be found at http://www.isc.org/isc.

ChorusOS 5.0 Last Revised December 2001

dhclient.cf(4CC)

NAME

DESCRIPTION

WARNINGS

PROTOCOL TIMING

The timeout statement

The retry statement

The select-timeout statement

The reboot statement

The backoff-cutoff statement

The initial-interval statement

LEASE REQUIREMENTS AND REQUESTS

The request statement

The require statement

The send statement

OPTION MODIFIERS

The default statement

The supercede statement

The prepend statement

The append statement

LEASE DECLARATIONS

The lease statement

ALIAS DECLARATIONS

OTHER DECLARATIONS

EXAMPLES

ATTRIBUTES

SEE ALSO

AUTHOR

The `timeout` statement

The `retry` statement

The `select-timeout` statement

The `reboot` statement

The `backoff-cutoff` statement

The `initial-interval` statement

The `request` statement

The `require` statement

The `send` statement

The `default` statement

The `supercede` statement

The `prepend` statement

The `append` statement

The `lease` statement