/* Copyright (C) 2020  C. McEnroe <june@causal.agency>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 * Additional permission under GNU GPL version 3 section 7:
 *
 * If you modify this Program, or any covered work, by linking or
 * combining it with OpenSSL (or a modified version of that library),
 * containing parts covered by the terms of the OpenSSL License and the
 * original SSLeay license, the licensors of this Program grant you
 * additional permission to convey the resulting work. Corresponding
 * Source for a non-source form of such a combination shall include the
 * source code for the parts of OpenSSL used as well as that of the
 * covered work.
 */

#include <netdb.h>
#include <netinet/in.h>
#include <resolv.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifndef EAI_BADHINTS
#define EAI_BADHINTS EAI_BADFLAGS
#endif

#ifndef EAI_PROTOCOL
#define EAI_PROTOCOL EAI_BADFLAGS
#endif

static uint16_t u16(uint8_t **ptr) {
	uint16_t x = *(*ptr)++;
	x = x << 8 | *(*ptr)++;
	return x;
}

static void nameSkip(uint8_t **ptr) {
	for (uint8_t len; (len = *(*ptr)++); *ptr += len) {
		if (len & 0xC0) {
			(*ptr)++;
			break;
		}
	}
}

static char *nameString(uint8_t **ptr) {
	char *name = (char *)(*ptr + 1);
	for (uint8_t len; (len = **ptr); *ptr += len) {
		*(*ptr)++ = '.';
	}
	return name;
}

/* A wrapper around getaddrinfo(3) which first performs SRV record (RFC 2782)
 * lookup. hints must be provided and hints->ai_protocol must be set. SRV
 * lookup is skipped if servname is numerical. If SRV lookup is successful, the
 * ai_canonname field of the first addrinfo structure returned is set to the
 * target name.
 */
int getservinfo(
	const char *hostname, const char *servname,
	const struct addrinfo *hints, struct addrinfo **res
) {
	if (!hints) return EAI_BADHINTS;
	if (!hints->ai_protocol) return EAI_PROTOCOL;
	if (hints->ai_flags & AI_NUMERICHOST) return EAI_BADFLAGS;
	if (hints->ai_flags & AI_NUMERICSERV) return EAI_BADFLAGS;

	char *rest;
	strtoul(servname, &rest, 10);
	if (!*rest) return getaddrinfo(hostname, servname, hints, res);

	struct protoent *proto = getprotobynumber(hints->ai_protocol);
	if (!proto) return EAI_PROTOCOL;

	char dname[256];
	int len = snprintf(
		dname, sizeof(dname), "_%s._%s.%s",
		servname, proto->p_name, hostname
	);
	if ((size_t)len >= sizeof(dname)) return EAI_OVERFLOW;

	uint8_t msg[512];
	len = res_query(dname, 1 /* IN */, 33 /* SRV */, msg, sizeof(msg));
	if (len < 0) return getaddrinfo(hostname, servname, hints, res);

	uint8_t *ptr = msg;
	u16(&ptr); // ID
	uint16_t rcode = u16(&ptr) & 0x000F;
	uint16_t qdcount = u16(&ptr);
	uint16_t ancount = u16(&ptr);
	u16(&ptr); // NSCOUNT
	u16(&ptr); // ARCOUNT

	if (rcode || !ancount) return getaddrinfo(hostname, servname, hints, res);

	for (uint16_t q = 0; q < qdcount; ++q) {
		nameSkip(&ptr); // QNAME
		u16(&ptr); // QTYPE
		u16(&ptr); // QCLASS
	}

	// Read only one answer, ignoring Priority and Weight. Does anyone actually
	// use them?
	nameSkip(&ptr); // NAME
	u16(&ptr); // TYPE
	u16(&ptr); // CLASS
	u16(&ptr); // TTL
	u16(&ptr); // TTL
	u16(&ptr); // RDLENGTH
	u16(&ptr); // Priority
	u16(&ptr); // Weight

	uint16_t port = u16(&ptr);
	hostname = nameString(&ptr);
	if (!hostname[0]) return EAI_NONAME;

	char myServ[sizeof("65535")];
	snprintf(myServ, sizeof(myServ), "%hu", port);

	struct addrinfo myHints = *hints;
#ifdef AI_DEFAULT
	if (!myHints.ai_flags) myHints.ai_flags = AI_DEFAULT;
#endif
	myHints.ai_flags |= AI_NUMERICSERV;
	myHints.ai_flags &= ~AI_CANONNAME;

	int error = getaddrinfo(hostname, myServ, &myHints, res);
	if (error) return error;

	(*res)->ai_canonname = strdup(hostname);
	if (!(*res)->ai_canonname) {
		freeaddrinfo(*res);
		return EAI_MEMORY;
	}
	return 0;
}