Adding FQDN support

[Brian Click]

Out of the box, the NetBurner's DHCP functions do not accomodate/support FQDNs, and I've experienced DNS difficulties (with Active Directory and DDNS) that were suspected (by me) to be due to this omission. WireShark was indispensable in sniffing this out BTW!

i.e. a DNS name of "MyNetBurner" is not a FQDN while "MyNetBurner.mynetwork.com" is a FQDN, where the domain name is obviously, "mynetwork.com". This is Option 81 in the standard DNS packet structure (and is decoded/displayed as such in packet sniffers such as WireShark etc.

NetBurner Folks: It might be a good idea to include this as core NB functionality...

I added this functionality to my project and was apparently able to overcome those DDNS issues. At a minimum, this required the addition of a new parameter "pDomainName" and the associated minor changes to the project's dhcpc.cpp and dhcpinternals.h modules. Of course a practical implementation also requires web element handling functions for entering / displaying the parameter, as well as including this as a non-volatile parameter in the "save-to-user-flash" functions.

In dhcpinternals.h, add Option 81 as follows:

Code: Select all

//----- DHCP option codes -----
<snip>
#define DHCPOPT_FQDN    	 81    // Fully Qualified Domain Name - new parameter added - B.Click
</snip>

And here's my added junk from dhcpc.cpp, with running commentary of what I learned along the way

Code: Select all

 
const char *pDomainName; // for FQDN support
char FQDN[40]; //buffer for assembling a fully qualified domain name

void CreateDhcpRequestMsg( DHCPMessage &OfferMsg, DHCPMessage &NewMsg, int DhcpInterface )
{
<snip>
 
if ( pDHCPOfferName != NULL)
   {
      int nlen = strlen( pDHCPOfferName );
      NewMsg.SetOptions( opt++, 12 ); //this is the "host name" DHCP option (not FQDN though)
      NewMsg.SetOptions( opt++, nlen );
      for ( i = 0; i < nlen; i++ )
      {
         NewMsg.SetOptions( opt++, pDHCPOfferName[i] );
      }
// Adding Option 81 FQDN to the DHCP Request message. Needed for Dynamic DNS
// Note this is not needed in the initial DHCP Discover message
		if ( pDomainName[0] == 0) //no domain name available
		// Just repeat host name (this is what a windows box does if not on a domain)
		// yes this seems redundant with Option 12 but DDNS implementations do want it
		{
			sprintf (FQDN, "%s", pDHCPOfferName); //FQDN = just the host name 
		}
		else //we have a domain name entered, use it
				{	
   			sprintf (FQDN, "%s.%s", pDHCPOfferName, pDomainName); //smack together a real FQDN
		}
   			
   		int plen = strlen( FQDN ); //how full is the buffer??
   		NewMsg.SetOptions( opt++, DHCPOPT_FQDN ); // Option #81 - as now defined in dhcpinternals.h
   		NewMsg.SetOptions( opt++, plen + 3 ); //message length; name string plus Flag byte plus A and PTR record bytes
   		NewMsg.SetOptions( opt++, 0x00 ); // MSB is Reserved, LSB is Flags. Normally 0x00.
		// 0x08 for "no server update" 
   		// 0x02 for "server override" 
   		NewMsg.SetOptions( opt++, 0x00 ); // "A" record result (reverse lookup) byte
   		NewMsg.SetOptions( opt++, 0x00 ); // "PTR" record result (forward lookup) byte
   		// we just pad the message, DHCP ACK returns the server's opinion of DNS record status
   		for ( i = 0; i < plen; i++ ) //write it out
   		{
   			NewMsg.SetOptions( opt++, FQDN[i] );
   		}
   } 
 
   AddDhcpOptions(NewMsg, opt);

   NewMsg.SetOptions( opt++, DHCPOPT_END );
}  

Obviously, you'll need to populate pDomainName before calling this function...

DISCLAIMER: I have not extensively tested this under infinitely numerous networking environments etc. etc. so implement and test this yourself accordingly...