configuration option data not in file

Simon Hobson dhcp at
Wed Jun 14 08:25:52 UTC 2006

<sandeep.vashisht at> wrote:

>if we dont have any data for a particular option in the DHCP SERVER
>configuration file.
>Can somebody tell me then how the server is expected to behave ?
>How the server will respond to the client?
>Will it send the garbage values or will it drop the complete request.

It depends on what is missing.

Some options, such as default, min, max lease times will have default values.

Some options could be inferred - eg ddns-domainname has an algorithm 
used to try and infer a value if not specified.

Most options are simply 'not present' if not specified. Should a 
client request an options list that includes missing options, the 
server will simply ignore the ones it hasn't got. This is actually 
the case most of the time with common clients such a Mac OS X and 
Windows - both request options that are generally not configured.

For example, renewing the lease on my Mac, I see a request with :

     Option 53: DHCP Message Type = DHCP Request
     Option 55: Parameter Request List
         1 = Subnet Mask
         3 = Router
         6 = Domain Name Server
         15 = Domain Name
         112 = NetInfo Parent Server Address
         113 = NetInfo Parent Server Tag
         78 = Directory Agent Information
         79 = Service Location Agent Scope
         95 = Lightweight Directory Access Protocol
         Unknown Option Code: 252
     Option 57: Maximum DHCP Message Size = 1500
     Option 61: Client identifier
         Hardware type: Ethernet
         Client MAC address: AppleCom_xx:xx:xx (00:0a:95:xx:xx:xx)
     Option 50: Requested IP Address =
     Option 51: IP Address Lease Time = 90 days
     Option 12: Host Name = "xxxxxxxxxxxxxxxxxxx"

And a reply (from a windows dhcp server) with :

     Option 53: DHCP Message Type = DHCP ACK
     Option 58: Renewal Time Value = 4 days
     Option 59: Rebinding Time Value = 7 days
     Option 51: IP Address Lease Time = 8 days
     Option 54: Server Identifier =
     Option 1: Subnet Mask =
     Option 3: Router =
     Option 6: Domain Name Server =
     Option 15: Domain Name = "xxxxxxxxxxxxxxx.local"

One of the tests a client should be doing when deciding which (if 
any) offer to accept is to decide if the options available in the 
offer are sufficient for it's needs. If there are multiple offers, 
then logically it should be taking the one that offers the fullest 


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