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dhcpm

Sponsor

Thank you to Bluecat for sponsoring this work! dhcpm leverages dhcproto check that out for the DHCP protocol.

About

A cli tool (and dhcp script runner!) for constructing & sending mocked dhcp client messages. dhcpm won't actually mess with the IP assigned to your network interfaces, it is intended to mock dhcp messages for testing dhcp servers. It aims to support v4 & v6, though v6 support is unfinished. Allows sending dhcp messages to non-default ports, and can be scripted with rhai.

This software is a personal project and should be considered beta. I use the basic cli features often, but the scripting features are new.

Installation

You can install with

cargo install dhcpm

To include the rhai scripting feature, add

cargo install dhcpm --features "script"

Use

> dhcpm --help

Usage: dhcpm <target> [-b <bind>] [-i <interface>] [-p <port>] [-t <timeout>] [--output <output>] [--script <script>] [--no-retry <no-retry>] [<command>] [<args>]

dhcpm is a cli tool for sending dhcpv4/v6 messages

ex  dhcpv4:
        dhcpm 255.255.255.255 discover          (broadcast discover to default dhcp port)
        dhcpm 192.168.0.255 discover            (broadcast discover on interface bound to 192.168.0.x)
        dhcpm 0.0.0.0 -p 9901 discover          (unicast discover to 0.0.0.0:9901)
        dhcpm 192.168.0.1 dora                  (unicast DORA to 192.168.0.1)
        dhcpm 192.168.0.1 dora -o 118,C0A80001  (unicast DORA, incl opt 118:192.168.0.1)
    dhcpv6:
        dhcpm ::0 -p 9901 inforeq       (unicast inforeq to [::0]:9901)
        dhcpm ff02::1:2 inforeq         (multicast inforeq to default port)

Positional Arguments:
  target            ip address to send to

Options:
  -b, --bind        address to bind to [default: INADDR_ANY:0]
  -i, --interface   interface to use (requires root or `cap_net_raw`) [default:
                    None - selected by OS]
  -p, --port        which port use. [default: 67 (v4) or 546 (v6)]
  -t, --timeout     query timeout in seconds [default: 5]
  --output          select the log output format (json|pretty|debug) [default:
                    pretty]
  --no-retry        setting to "true" will prevent re-sending if we don't get a
                    response [default: false]
  --help            display usage information

Commands:
  discover          Send a DISCOVER msg
  request           Send a REQUEST msg
  release           Send a RELEASE msg
  inform            Send an INFORM msg
  decline           Send a DECLINE msg
  dora              Sends Discover then Request
  inforeq           Send a INFORMATION-REQUEST msg (dhcpv6)

Sending DHCP over arbitrary ports

This will construct a discover message and unicast to 192.168.0.1:9901:

dhcpm 192.168.0.1 -p 9901 discover

dhpcm will bind to 0.0.0.0:0 for replies, meaning the server must speak DHCP over arbitrary ports. To communicate over the standard ports, simply don't provide the --port option. dhcpm will then listen to the default port if you have suitable permissions.

dhcpm 192.168.0.1 discover

This will unicast to 192.168.0.1:67 and attempt to listen on 0.0.0.0:68. You can change which address:port dhcpm listens on with the --bind option.

Broadcast vs unicast

To send a broadcast message (with the broadcast flag set) use the network broadcast address 255.255.255.255.

dhcpm 255.255.255.255 discover

Using specific interface

You can pass the --interface/-i param to bind to a specific interface by name, for example --interface enp6s0. Using this, you will only receive/send responses over that device. Ex,

dhcpm 255.255.255.255 -i enp6s0 discover --chaddr random

You can also use ip addr on linux to get the broadcast address of a particular interface:

2: enp6s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.130/24 brd 192.168.0.255 scope global noprefixroute enp6s0

Note brd 192.168.0.255. You can pass this to dhcpm and the kernel should select that interface to broadcast on (enp6s0 in this example)

dhcpm 192.168.0.255 discover

Message parameters

Each sub-command (discover/request/release, etc) has sub-options. For example, by default dhcpm will use the default interfaces mac, you can override this by sending the appropriate Options

dhcpm 255.255.255.255 discover --chaddr "80:FA:5B:41:10:6B"

dhcpv6

With DHCPv6, many messages are sent on the multicast group ff02::1:2 but responses are often unicast back on link-local addresses (starting with fe80). dhcpm won't be able to receive this data if you've got another dhcpv6 client listening on [::0]:546, the dhcpv6 client port. The other process is will likely read the datagram first.

For example, my box has:

> sudo lsof -Pi UDP
...
NetworkMa     711            root   20u  IPv6 12173080      0t0  UDP leshowbox:546

Listening on this [::0]:546, so that process would need to be killed before dhcpm could print a reply. Still, I have often found it enough to use dhpcm to generate a message, then look at the response in wireshark or tcpdump to inspect its validity.

Specify an interface with v6, it is necessary to join the multicast group.

> sudo dhcpm ff02::1:2 -i enp6s0 inforeq

Scripting

Scripting support with rhai. Compile dhcpm with the script feature and give it a path with --script:

dhcpm 255.255.255.255 --script test.rhai

In the script, you can create new discover arguments with:

let args = discover::args_default();

You can send this message with args.send().

Message types supported in script are:

  • discover::args_default()
  • request::args_default()
  • release::args_default()
  • inform::args_default()

Be careful about what scripts you choose to run, especially if you use ports only accessible with sudo, as the scripts arbitrary code will be executed with whatever permissions you give it.