Lua/C DHCP example#2

dhcp.lua

-- C definitions
local ffi = require('ffi')
local csym = ffi.C
ffi.cdef[[
/* DHCP header format */
struct __attribute__((packed)) dhcp_msg {
	/* Header */
	uint8_t op;
	uint8_t htype;
	uint8_t hlen;
	uint8_t hops;
	uint32_t xid;
	uint16_t secs;
	uint16_t flags;
	/* Address fields */
	char _ciaddr[4], _yiaddr[4], _siaddr[4], _giaddr[4];
	char _chaddr[16];
};
/* Constants */
struct dhcp_op {
	static const int DISCOVER = 0x01;
	static const int OFFER    = 0x02;
};
/* C functions */
void yiaddr_set(char *msg, const char *yiaddr);
]]

-- Message meta type
local char_ptr = ffi.typeof('char *')
local dhcp_op = ffi.new('struct dhcp_op')
local dhcp_msg_t = ffi.typeof('struct dhcp_msg')
ffi.metatype( dhcp_msg_t, {
	__index = {
		chaddr = function(msg)
			return ffi.string(msg._chaddr, 16)
		end,
		yiaddr = function(msg, addr)
			if addr then
				csym.yiaddr_set(msg, addr)
			else
				return ffi.string(msg._yiaddr, 4)
			end
		end,
	},
})
-- Export public API
local dhcp = {
	-- This is a way to export constants
	op = dhcp_op,
	-- Export types
	msg_t = function (udata) return ffi.cast('struct dhcp_msg *', udata) end,
	-- API functions
	is_request = function (msg) return msg.op == dhcp_op.DISCOVER end,
}

return dhcp

ldhcp-ex2.c

#include <stdio.h>
#include <string.h>
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <stdint.h>
#include <arpa/inet.h>

/* @note Trivial example of a C callback */
void yiaddr_set(char *msg, const char *yiaddr)
{
	inet_pton(AF_INET, yiaddr, msg + 16);
}

static int on_recv(lua_State *L, int cb_ref, char *buf, size_t len)
{
	lua_rawgeti(L, LUA_REGISTRYINDEX, cb_ref);
	lua_pushlightuserdata(L, buf);
	lua_pushinteger(L, len);
	return lua_pcall(L, 2, 1, 0);
}

/* Convenience stuff */
static void close_state(lua_State **L) { lua_close(*L); }
#define cleanup(x) __attribute__((cleanup(x)))
#define auto_lclose cleanup(close_state)

int main(int argc, char *argv[])
{
	/* Create VM state */
	auto_lclose lua_State *L = luaL_newstate();
	if (!L)
		return 1;
	luaL_openlibs(L);
	/* Load config file */
	if (argc > 1) {
		luaL_loadfile(L, argv[1]); /* (1) */
		int ret = lua_pcall(L, 0, 0, 0);
		if (ret != 0) {
			fprintf(stderr, "%s\n", lua_tostring(L, -1));
			return 1;
		}
	}
	/* Read out config */
	lua_getglobal(L, "address"); /* (2) */
	lua_getglobal(L, "port");
	printf("address: %s, port: %ld\n", /* (3) */
		lua_tostring(L, -2), lua_tointeger(L, -1));
	lua_settop(L, 0); /* (4) */
	/* Register bindings */
	lua_getglobal(L, "callback");
	int cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
	/* Prepare mock datagram. */
	char dhcp_req[512] = {
		0x01, 0x01, 0x06, 0x00
	};
	char dgram[sizeof(dhcp_req)];
	/* Call a few times, no I/O yet. */
	const size_t nr_calls = 1000000;
	for (unsigned i = 0; i < nr_calls; ++i) {
		dgram[0] = 0x01; /* reset to DHCPDISCOVER */
		int ret = on_recv(L, cb_ref, dgram, sizeof(dgram));
		if (ret != 0) {
			printf("%s\n", lua_tostring(L, -1));
			return 1;
		}

		lua_pop(L, 1);
	}
	return 0;
}

test.lua

address = '255.255.255.0'
port = 67
-- Require dhcp bindings
local dhcp = require('dhcp')
-- Only allow hwaddr with these prefixes
local allowed_prefix = { '\0\0', '\5\3' }
local function acl(hwaddr)
    for i,v in ipairs(allowed_prefix) do
        if hwaddr:sub(0, #v) == v then
            return true
        end
    end
    return false
end
-- Flip some bits in the buffer and return length
local lease = {}
function callback(buf, len)
    local msg = dhcp.msg_t(buf)
    if dhcp.is_request(msg) then -- DHCPDISCOVER
        msg.op = dhcp.op.OFFER -- DHCPOFFER
        local hwaddr = msg:chaddr()
        if not acl(hwaddr) then return 0 end
        local client_ip = lease[hwaddr]
        if client_ip then -- keep leased addr
            msg:yiaddr(client_ip)
        else -- stub address range (:
            client_ip = '192.168.1.1'
            msg:yiaddr(client_ip)
            lease[hwaddr] = client_ip
        end
    end
    return len
end