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Description:
As of firmware version LCOS 7.80, DHCP options make it possible to send not only an IP address to a DHCP client but additional information as well
This article describes how network devices, which obtain their IP address via DHCP can be assigned additional information besides the usual IP parameters via a DHCP option.
Info

For a list of the most common DHCP options see RFC 2123.





Requirements:
LCANtools
  • LCOS as of version
7
  • 9.
80
LCOS
  • LANtools as of version
7
  • 9.
80
For a full list of DHCP options see RFC 2123 (http://www.ietf.org/rfc/rfc2132.txt).
  • Configured and functional DHCP server for the network where the DHCP option should be assigned


Procedure
Sample configuration
:
1)
In LANconfig, open
Open the
configuration dialog for the LANCOM router and navigate to the menu item Configuration -> TCP/IP -> DHCP -> DHCP options.
2) Click on the Add... button to create a new entry.
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3) As specified in RFC 2132 section 3.8 (see below), a DHCP client can be notified of one or more DNS servers.
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The information is structured as follows:
Option number = 6
Network name (as required)
Option type = IP address
Option value = e.g. 192.168.10.200 -> if just one DNS server is to be specified
Option value =
router configuration in LANconfig and go to the menu IPv4 → DHCPv4 → DHCP options.
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2) Create a new entry and modify the following parameters as needed:
  • Option number: Enter the code number of the DHCP option.
  • Sub-Options-Number: Optionally you can enter a Sub-Option-Number. For instance this is used in the DHCP option 43 to inform the connected SIP telephones where they can get the firmware and the configuration.
  • Vendor-Class-Mask: Optionally you can enter a Vendor-Class-Mask. The DHCP option is only sent when the network device sends a DHCP request with the string contained in the mask. It is possible to use the wildcards * (any number of characters) and ? (exactly one character). For LANCOM devices the string *LANCOM* would be used.
  • User-Class-Mask: Optionally you can enter a User-Class-Mask ein. The DHCP option is only sent when the network device sends a DHCP request with the string contained in the mask. It is possible to use the wildcards * (any number of characters) and ? (exactly one character). For LANCOM devices the string *LANCOM* would be used.
  • Network name: In the dropdown menu select the DHCP network where the DHCP option should be assigned.
  • Type: In the dropdown menu select the Type which fits the selected DHCP option.
    • String (default setting)
    • IP address
    • 32 bit integer
    • 16 bit integer
    • 8 bit integer
  • Value: Enter the Value which fits the used DHCP option and the selected Type. For example, this could be an IP address or a URL.
  • Append Sub-Option: If the DHCP option 43 is used and a Sub-Options-Number should be appended, the option Yes has to be selected. The Sub-Options-Number with the corresponding information then has to be entered in a separate entry. Thereby the entries are bundled.

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Examples for DHCP options:
DHCP option 6 - Domain Name Server Option:
  • Option number: Enter the number 6.
  • Network name: In the dropdown menu select the DHCP network, where the DHCP option should be assigned (in this example the network INTRANET).
  • Type: In the dropdown menu select the type IP address.
  • Value: Enter the IP address of a DNS server (in this example 192.168.10.200). If two DNS servers should be entered, these can be separated via a comma (e.g. 192.168.10.200, 217.137.150.33
-> if multiple DNS servers are to be specified
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Multiple values are separated by commas.
Information:
This enhanced system is unable to reproduce all of the possible options as some consist in part of an integer number and partly of character strings. This possibility is only available to those who can assemble the value byte-by-byte with the type "integer-8".
Other examples of options:
Time server IP address (NTP):
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NetBIOS server address (NBNS):
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Rootpath:
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There are also custom options that are manufacturer-dependent, as with the DHCP option 252 shown below.
Proxy server address:
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TFTP server:
  • The option type for the 66 option must be string and the value must be the ip address of the tftp server.
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Image-File for a PXE-Boot-Server:
Go to the menu IPv4 -> BootP -> stations and create a new entry which contains the MAC address of the PXE client (see figure below).
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  • ).
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DHCP option 17 - Rootpath:
  • Option number: Enter the number 17.
  • Network name: In the dropdown menu select the DHCP network, where the DHCP option should be assigned (in this example the network INTRANET).
  • Type: In the dropdown menu select the type String.
  • Value: Enter the URL for the path to the root disk.
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DHC option 42 - Network Time Protocol Servers Option:
  • Option number: Enter the number 42.
  • Network name: In the dropdown menu select the DHCP network, where the DHCP option should be assigned (in this example the network INTRANET).
  • Typd: In the dropdown menu select the type IP address or String.
  • Value: Enter the IP address of a time server or the DNS name of a time server.
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DHCP option 44 - NetBIOS over TCP/IP Name Server Option:
  • Option number: Enter the number 44.
  • Network name: In the dropdown menu select the DHCP network, where the DHCP option should be assigned (in this example the network INTRANET).
  • Type: In the dropdown menu select the type IP address.
  • Value: Enter the IP address of a NetBIOS server.
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DHCP option 66 - TFTP server name:
  • Option number: Enter the number 66.
  • Network name: In the dropdown menu select the DHCP network, where the DHCP option should be assigned (in this example the network INTRANET).
  • Type: In the dropdown menu select the type IP address or String.
  • Value: Enter the IP address of a TFTP server or the DNS name of a TFTP server.
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DHCP option 121 - Classless Static Route:
The procedure is described in the following Knowledge Base article:
Configuring DHCP option 121 (classless static route) in the LANCOM router

DHCP option 252 - Web Proxy Auto-Discovery Protocol:
  • Option number: Enter the number 252.
  • Network name: In the dropdown menu select the DHCP network, where the DHCP option should be assigned (in this example the network INTRANET).
  • Type: In the dropdown menu select the type String.
  • Value: Enter the URL of the web server with the proxy configuration (in this example http://server.intern/proxyconfig.pac).
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Specify an image file for a PXE boot server:
1) Go to the menu IPv4 → BootP → Boot images.
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2) Create a new entry and modify the following parameters:
  • Designation: Enter a descriptive name for the boot image.
  • Server address: Enter the IP address of the server, which provides the boot image.
  • File name: Enter the name of the file on the server, which contains the boot image.

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3) Go to the menu IPv4 → BootP → Stations.
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4) Create a new entry and modify the following parameters:
  • Station MAC address: Enter the MAC address of the PXE client in the format 00:a0:57:12:34:56.
  • Network name: In the dropdown menu select the Network, from which an IP address should be assigned to the network device.
  • IP address: Enter the IP address, which should be assigned to the network device.
  • Station name: Optionally you can enter a descriptive name for the station.
  • Boot image: In the dropdown menu select the boot image created in step 2.

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