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July 19th, 2012, 11:35 AM
#1
DHCP Discovery Program - Need Help With Boot Options
So I wrote this program in C# using sharppcap with the purpose of sending out a custom DHCP discovery packet. The program works. What I need help with is understanding DHCP Options if anyone has experience/knowledge of this.
I understand what is supposed to go there, but I don't know if there is supposed to be an order to the byte fields or if anything can go in there (according to the DHCP options).
For instance, I want the DHCP server to return a packet with the subnet mask (option 1), router (option 3), DNS (option 6), and domain name (option 15).
So in the string I use in the code, I have this (ddcp being a structure): ddcp.DHCP_OPTIONS = "35370103060F";
35 = 53 (DHCP Message Type)
37 = 55 (Parameter Request List)
And then do I put codes after 37 to get specific data back, such as 01 for Subnet Mask, 06 for DNS, etc...?
If anyone can shed light on DHCP Options and how to get useful information back, that would be awesome.
I should note that I think something should go after the DHCP Message Type, but if I put a 01 or 03 after that, I get no response from my dhcp server unless I'm using the wrong codes for message type. But if I use the DHCP Options as-is, I get a response back.
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July 30th, 2012, 11:05 PM
#2
Re: DHCP Discovery Program - Need Help With DHCP Options
Ok, so I figured it out and updating this thread in case, if for some bizarre reason >_<, people like me are trying to build a custom DHCP discover packet.
So in DHCP Options, it goes: DHCP Code -> Length of values -> Values -> DHCP Code -> Length -> Values... etc..
These are the DHCP options from a test server I built running on Windows Server 2008:
3501020104FFFFFF003A04000546003B0400093A803304000A8C003604C0A801010F12636F72702E6E616E6F746563682E636F6D000304C0A8010106087F000001C0A80102FF
35 = DHCP Code
01 = Length of Values
02 = Value
01 = DHCP Code
04 = Length of Values
FF FF FF 00 = Values
3A = DHCP Code
04 = Length of Values
00 05 46 00 = Values
3B = DHCP Code
04 = Length of Values
00 09 3A 80 = Values
And so on. You can follow this pattern to the end at FF, which is DHCP Code for "End".
It took ages of staring at this to see the pattern >_<.
So there ya go!
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July 31st, 2012, 08:50 AM
#3
Re: DHCP Discovery Program - Need Help With Boot Options
Did you check this RFC: http://www.faqs.org/rfcs/rfc2132.html?
It seems to contain anything you need.
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