DHCP Option

[Karsten Kosmala]

Hi,

I'm trying to craft a DHCP packet to test my DHCP server.

I recorded some dhcp-requests with a mac and my linux pc with wireshark and now wanted to replay these and want to know why the server sometimes isn't responding.

This is my code:

  PacketSender sender("enp0s25");

  NetworkInterface iface = NetworkInterface::default_interface();

  NetworkInterface::Info info = iface.addresses();

  IP pkt = IP("192.168.0.75") / UDP(67, 68) / DHCP();

  pkt.rfind_pdu<DHCP>().chaddr(info.hw_addr);

  pkt.rfind_pdu<DHCP>().type(DHCP::REQUEST);

  pkt.rfind_pdu<DHCP>().add_option(DHCP::DHCP_CLIENT_IDENTIFIER);

  pkt.rfind_pdu<DHCP>().end();

  sender.send(pkt);

Everything is fine and the server get the message, but now I want to add some data to the option DHCP_CLIENT_IDENTIFIER.

How can I do that?

Karsten

[Matias Fontanini]

DHCP::option is an instantiation of the PDUOption class template. So any of the constructors you see on its documentation can be used. In your code, you're implicitly calling the constructor that takes an identifier and optionally a data and data size parameters.

For example, you could do the following:

// Fill a vector with the data you want.

std::vector<uint8_t> data = ...; 

// Create a DHCP::option 

DHCP::option opt(DHCP::DHCP_CLIENT_IDENTIFIER, data.begin, data.end());

// Add the option to the DHCP PDU 

dhcp.add_option(opt);

That should work. Let me know if you have any troubles.

[Karsten Kosmala]

Hi Matias,

thanks for your answer.

What do I have to put here: std::vector<uint8_t> data = ???;

I want to transmit the hardware type (1 for ethernet) and my clients mac address.

Karsten

[Matias Fontanini]

I thought there was a wrapper implemented for this option, but apparently not. It's should be easy to generate the option's payload anyway (I haven't tested this, but it should work unless my understanding of the option's format is not correct):

std::vector<uint8_t> data(7);

data[0] = 1; // ethernet

Tins::HWAddress<6> mac_address = "00:01:02:03:04:05"; // your client's mac address

mac_address.copy(&data[1]); // copy this address into data

// now add the option like in the previous post