C++构造DHCP Discovery报文并使用socket发送

DHCP由BOOTP协议发展而来，而后者基于UDP、IP协议，这使得使用socket发送DHCP报文成为可能。本文示例构造了DHCP Discovery报文并调用socket接口发送，值得注意的是，由于DHCP要求发送者ip是0.0.0.0，所以必须使用raw socket进行发送（本文没有示例），更进一步的，用户层代码Linux、macOS系统可以使用libpcap，Windows系统可以使用Windivert、Winpcap/Npcap进行发送。

	builder<dhcp_hdr, char[64]> packet;
	auto dhcp    = packet.get<dhcp_hdr>();
	dopt opts[]  = { 
		packet.get<dhcp_opt>(3),
		packet.get<dhcp_opt>(9),
		packet.get<dhcp_opt>(6),
		packet.get<dhcp_opt>(8), // host name
		packet.get<dhcp_opt>(10),
		packet.get<dhcp_opt>(15)
	};
	auto optend  = packet.get(1);
	auto padcalc = calculator<>();
 	auto padding = packet.get( BOOTP_MIN_LEN - packet.length());

	dhcp->opcode     = DHCP_REQUEST;
	dhcp->htype      = ARPHRD_ETHER;
	dhcp->hlen       = ETHER_ADDR_LEN;
	dhcp->hops       = 0;
	dhcp->xid        = 0; // TODO: Transaction ID
	dhcp->secs       = 0;
	dhcp->flags      = 0; // UNICAST
	dhcp->cip        = IP_THIS_ADDR;
	dhcp->yip        = IP_THIS_ADDR;
	dhcp->sip        = IP_THIS_ADDR;
	dhcp->gip        = IP_THIS_ADDR;
	memcpy(dhcp->chaddr, ETHER_SELF_MAC, ETHER_ADDR_LEN);
	memset(&dhcp->chaddr[ETHER_ADDR_LEN], 0, sizeof(dhcp_hdr::chaddr) - ETHER_ADDR_LEN);
	memset(dhcp->sname, 0, sizeof(dhcp_hdr::sname));
	memset(dhcp->file, 0, sizeof(dhcp_hdr::file));
	dhcp->magic      = BOOTP_RMAGIC;
	opts[0]->kind    = DHCP_MESSAGETYPE;
	opts[0]->len     = 1;
	opts[0]->val[0]  = DHCP_MSGDISCOVER;
	opts[1]->kind    = DHCP_CLIENTID;
	opts[1]->len     = 7;
	opts[1]->val[0]  = ARPHRD_ETHER; // hardware address
	memcpy(&opts[1]->val[1], ETHER_SELF_MAC, ETHER_ADDR_LEN);
	opts[2]->kind    = DHCP_DISCOVERADDR;
	opts[2]->len     = IPV4_ADDR_LEN;
	memcpy(opts[2]->val, "\x00\x00\x00\x00", opts[2]->len);
	opts[3]->kind    = DHCP_HOSTNAME;
	opts[3]->len     = sizeof("Huawei") - 1;
	memcpy(opts[3]->val, "Huawei", opts[3]->len);
	opts[4]->kind    = DHCP_CLASSSID;
	opts[4]->len     = sizeof("MSFT 5.0") - 1;
	memcpy(opts[4]->val, "MSFT 5.0", opts[4]->len);
	opts[5]->kind    = DHCP_PARAMREQUEST;
	opts[5]->len     = 13;
	opts[5]->val[0]  = DHCP_SUBNETMASK;
	opts[5]->val[1]  = DHCP_ROUTER;
	opts[5]->val[2]  = DHCP_DNS;
	opts[5]->val[3]  = DHCP_DOMAINNAME;
	opts[5]->val[4]  = DHCP_DOROUTEDISC;
	opts[5]->val[5]  = DHCP_STATICROUTE;
	opts[5]->val[6]  = DHCP_VENDSPECIFIC;
	opts[5]->val[7]  = DHCP_NBNS;
	opts[5]->val[8]  = DHCP_NBTCPIP;
	opts[5]->val[9]  = DHCP_NBTCPSCOPE;
	opts[5]->val[10] = DHCP_CLSTATICROUTE;
	opts[5]->val[11] = DHCP_MSCLSTATICROUTE;
	opts[5]->val[12] = DHCP_PRIAUTODISCOVERY;
	optend[0]        = DHCP_END;

	ManagedObject<Sockets> sock = new Sockets(IPPROTO_UDP, SOCK_DGRAM);
	sock->SetOption(SO_BROADCAST, 1);
	sock->SendTo(packet.first<void>(), packet.length<int>(), IP_BCAST_ADDR, DHCP_BOOTPS_RPORT);