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DHCP client uses the following files to control its environment:
DHCP client has one configuration file that controls DHCP client behavior. This optional file, named CLIENT.PCY, is an ASCII file located in the DHCP home directory, which is either SYS$SYSDEVICE:[TCPIP$DHCP] or a directory pointed to by the system logical TCPIP$DHCP_CONFIG. If CLIENT.PCY does not exist, DHCP client uses default values for each of the configurable parameters.
Example 8-1 shows the contents of a typical CLIENT.PCY file.
| Example 8-1 Client Startup File | |||||||
|---|---|---|---|---|---|---|---|
$ TYPE CLIENT.PCY class_id TCPVMS lease_desired 86400 # 24 hour lease request routers request host_name request dns_servers request dns_domain_name |
The format of the configuration file must adhere to the following rules:
Table 8-1 describes the configuration keywords.
| Keyword | Description |
|---|---|
| class_id | Specifies the client's class identification. Consult RFC 2131 for details. The only class supported by TCP/IP Services is TCPVMS. |
| lease_desired seconds | Specifies that a client may request a lease of a particular duration, although DHCP servers are not bound to honor the request. If the client does not want a lease of a particular duration, seconds should be set to 0. If an infinite lease is required, set seconds to -1. Otherwise, specify (in seconds) the lease duration required. The default value is 0 seconds. A DHCP server grants a client permission to use an IP address for a fixed period of time, which may be infinite. In the language of DHCP, the client is granted a lease on the IP address. |
| retries | Specifies the maximum number of DHCPDISCOVER, DHCPOFFER, DHCP_REQUEST, DHCPNAK sequences the client attempts. An offer received and then refused is an unusual event; if it occurs more than once, this indicates a problem with the server. If you do not want to limit the number of bad offers that a client is willing to accept, set the value of this parameter to 0 (zero) or a negative value. The default value is 2 attempts. |
| start_delay seconds | Specifies the maximum time (in seconds) the client delays before broadcasting DHCP packets. When the DHCP client is invoked to configure an interface it will delay for a short time before broadcasting the first DHCP packet. The delay time is randomized from a value of 0 up to the value specified by seconds. The TCP/IP commands SET INTERFACE/DHCP and the START COMMUNICATION/INITIALZE command (executed at product startup time) both execute dhcpconf start commands and experience the randomized delay. The default value for seconds is 10 seconds. |
| timeouts value,value,value.... |
Specifies how long the client should wait for replies before timing out
and retrying the broadcast. The DHCP protocol requires clients to
implement an exponential retransmission and backoff when broadcasting
discover or request packets.
Each time the client sends a DHCP protocol packet, it waits for a response until a timeout occurs after an interval (in seconds) given by a member of the list of values. If a timeout occurs, the packet retransmits with the same XID (see RFC 1541), and the timeout is set to the next positive value in the comma-separated list. The last element in the list is negative or 0 (zero). At this point, the next action depends on options to the DHCPCONF program. One option is to fail. Another option is to retry forever. If the last value in the list is negative, DHCP suspends configuration of the interface for an amount of time given by the negative number terminating the list of values. During this time, the interface is considered idle---the client is not expecting responses destined for the interface and will ignore any that arrive. When the idle time is over, the client begins retransmitting with a new XID, and a timeout value is given by the first element in the array of values. If the last value is 0 (zero), the client continues to use the same XID and timeout of the last positive value in the list of values. The default list of values is 4, 8, 16, 32, 0. |
| use_saved_config |
Specifies to use the configuration stored in
ifname.DHC
from a previous invocation of the protocol if the following
conditions exist:
|
| request parameter_name |
Specifies the parameter to request from the DHCP server. There may be
many instances of the request keyword, each with a different
parameter_name. Each parameter which is configurable through
DHCP is identified by a unique parameter. Limited size of DHCP packets
dictates that a client should not request data which it cannot use.
Different implementations of DHCP servers or differing DHCP server policies can dictate that a server return more configuration parameters than a client requests. On the other hand, some DHCP servers will not send a parameter to a client unless the client explicitly requests it. If your DHCP server is configured to deliver a particular parameter to your TCP/IP Services DHCP client and the client is not receiving the information, verify that the DHCP client has a request statement for the information in its CLIENT.PCY file. Table 8-2 lists the DHCP parameters that a TCP/IP Services DHCP client may request from a server. Note that vendor-specific options, like the ones marked with TCPVMS in columns 3 and 4 of the DHCPTAGS. file entries, may not appear in a request statement. |
Table 8-2 lists the Request statement parameters supported by the TCP/IP Services DHCP client implementation.
| Parameter Name | DHCP Option Code | This parameter requests... |
|---|---|---|
| Interface-specific parameters | ||
| broadcast_address | 28 | The broadcast address in use on the client's subnet. |
| interface_mtu | 26 | The MTU size to use when performing Path MTU discovery. |
| subnet_mask | 1 | The client's subnet mask. |
| Systemwide parameters | ||
| dns_domain_name | 15 | The domain name that the client should use when resolving host names using the Domain Name System (DNS). |
| dns_servers | 6 | A list of DNS name servers available to the client. |
| host_name | 12 | The host name of the client. |
| ip_time_to_live | 23 | The default time-to-live value the client should use on outgoing datagrams. |
| ip_forwarding | 19 | How the client should configure its IP layer for packet forwarding. |
| keepalive_interval | 38 | The time interval (in seconds) that the client TCP should wait before sending a keepalive message on a TCP connection. |
| routers | 3 | A list of IP addresses for routers on the client's subnet. Routers are listed in the order of preference. |
| static_routes | 33 | A list of static routes the client should install in its routing cache. If multiple routes to the same destination are specified, they are listed in descending order of priority. The routes consist of a list of IP address pairs. The first address is the destination address and the second address is the router for the destination. |
| tcp_default_time_to_live | 37 | The default time-to-live value that the client uses when sending TCP segments. |
When the DHCP client receives parameters to configure the interface on the client, it stores them in a file named ifname.DHC along with the IP address lease information. The ifname part of the file name is the name of the interface on which the parameters were received. For example, the file created for parameters received on interface SE0 is SE0.DHC. There is one file per interface, and the files are placed in the directory specified by the system logical name TCPIP$DHCP_CONFIG (if it is defined) or in the SYS$SYSDEVICE:[TCPIP$DHCP] directory.
The interface file is a binary file, and you can display its contents
by using the SHOWDHC utility. See Section 8.6 for information on how
to use the SHOWDHC utility.
8.2.2.3 The Host Name File
You can configure the DHCP client to suggest a host name of your choice to the DHCP server by entering the name into a file named HOSTNAME.ifname. This file contains one line of text that contains the unqualified host name to suggest. You store the file in directory specified by the system logical TCPIP$DHCP_CONFIG, if defined, or in the SYS$SYSDEVICE:[TCPIP$DHCP] directory.
If you have multiple interfaces and want to suggest a different host name for each one, put the desired interface host names into separate files called HOSTNAME.ifname, where ifname is the name of the interface. For example, if you have two interfaces, WF0 and WE0, and you want the WF0 interface to receive the host name myhostfiber and the WE0 interface to receive the name myhostether , enter the following commands:
$ CREATE SYS$SYSDEVICE:[TCPIP$DHCP]HOSTNAME.WF0 myhostfiber <CTRL-Z> $ CREATE SYS$SYSDEVICE:[TCPIP$DHCP]HOSTNAME.WE0 myhostether <CTRL-Z> |
When configuring an interface, the DHCP client will first check for a HOSTNAME.ifname file and then, if that is not found, for the HOSTNAME. file.
When you initially configure the DHCP client the value of your node's SCSNODE parameter is placed into a file called HOSTNAME. with no .ifname extension.
If you change the HOSTNAME.ifname file, you must delete the
interface.DHC file for the change to take effect.
8.2.2.4 The DHCPTAGS. File
The DHCPTAGS. file identifies the type of each parameter returned to the DHCP client by the DHCP server. Each supported option consists of the following:
The options are defined as follows:
In general, the DHCP server knows little about the semantics of the
first three options. Its only duty is to deliver those values to
clients that need them. The responsibility for understanding and using
the data rests with the client. Pseudotags have a meaning specific to
TCP/IP Services.
8.2.3 Command Files
Table 8-3 lists the command files that the DHCP client uses to start up and shut down the component.
| Command File Name | Description |
|---|---|
| TCPIP$DHCP_CLIENT_STARTUP.COM | Installs the DHCP client image. |
| TCPIP$DHCP_CLIENT_SHUTDOWN.COM | Stops DHCP client. |
Use the logicals listed in Table 8-4 to alter the behavior of the DHCP client.
| Logical Name | Purpose | ||||||
|---|---|---|---|---|---|---|---|
| TCPIP$DHCP_DEBUG | Turns on DHCP client diagnostics. Refer to Section 7.2.4 for a description of this logical. | ||||||
| TCPIP$DHCP_CONFIG directory | Specifies the directory from which to read input files (CLIENT.PCY, DHCPTAGS. and HOSTNAME.) and to which to write output files ( ifname .DHC). Note that DHCP client log files will still go to the default directory of the DHCP client account. | ||||||
| TCPIP$LOG_LEVEL value |
Writes the specified level of diagnostic information to the log file.
Ignored if TCPIP$DHCP_DEBUG is defined.
Valid numeric values are:
|
DHCP client creates a log file named TCPIP$DHCP_CLIENT_RUN.LOG in the
directory SYS$SYSDEVICE:[TCPIP$DHCP].
8.3 DHCP Client Startup and Shutdown
The DHCP client can be shut down and started independently of TCP/IP Services. This is useful when you change parameters or logical names that require the service to be restarted.
The following files are provided:
To preserve site-specific parameter settings and commands, create the following files. These files are not overwritten when you reinstall TCP/IP Services:
In order for the DHCP client to run, you must perform the following steps:
For the DHCP client to execute, at least one interface on your host must be designated as being under DHCP control. This means that the interface IP address, subnet mask, and broadcast address are set automatically by DHCP when the system invokes the command procedure TCPIP$STARTUP.COM.
To place interfaces under DHCP control, you have these options:
If you have never installed a previous version of UCX or TCP/IP Services, you may simply install TCP/IP Services and manually invoke the SYS$STARTUP:TCPIP$STARTUP.COM procedure. TCPIP$STARTUP.COM detects that you have never run TCPIP$CONFIG and asks whether you want DHCP client to configure your host for you. If you answer Yes, TCPIP$STARTUP.COM invokes TCPIP$CONFIG to configure a small set of services and sets any unconfigured interfaces to be under DHCP control. This process is done in silent mode and asks you no questions.
The services enabled when you autoconfigure are:
If you want more than the set of services configured by this option, you can configure your host with the subset of TCP/IP Services and at a later time run TCPIP$CONFIG to configure other services.
DHCP client autoconfigure puts each unconfigured IP interface under DHCP control. It employs the following rules to decide which, if any, interface should be marked as the primary interface. (See Section 8.1.1 for an explanation of the DHCP primary interface.)
If you have an existing TCP/IP installation, use TCPIP$CONFIG to place interfaces under DHCP control. To do this, perform the following steps:
INTERFACE Configuration
The Ethernet device(s) on your system are: ESA0:
Start of configuration questions for Internet interface SE0.
SE0 is the Ethernet device ESA0:
Interface: SE0
IP_Addr: 10.0.0.1 NETWRK: 255.0.0.0 BRDCST: 10.255.255.255
C_Addr: C_NETWRK: C_BRDCST:
Flags:
Receive buffer: 0
Compaq TCP/IP Services for OpenVMS Interface SE0 Reconfiguration Menu
Reconfiguration options:
1 - Configure interface manually (Current default)
2 - Let DHCP configure interface
[E] - Exit menu (Do not reconfigure interface SE0)
Enter configuration option: 2
End of configuration questions for Internet interface SE0
|
Primary DHCP Interface Configuration
DHCP Client configures system-wide parameters and
interface-specific parameters. Only one interface, the
DHCP "primary" interface, can receive system-wide
parameters.
Which interface? (SE0,NONE,HELP) [NONE]:SE0
|
In order for the DHCP client to function, DHCP client software must be configured. As with any TCP/IP service, configuration involves:
TCPIP$CONFIG.COM provides a menu option under the client menu called DHCP client. This option configures the DHCP client for you. You may choose this option explicitly, but if you put an interface under DHCP control from the Interfaces menu in TCPIP$CONFIG, this step is automatically done for you.
The DHCP client software configuration does the following:
Default route
Host name
DNS servers IP addresses
DNS domain name
The DHCP client may not coexist on the same system as a DHCP server. TCPIP$CONFIG will not allow you to configure the DHCP client on a system with DHCP server configured. |
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