/* 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

/* 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. Only the first SRV record is used. Priority and weight are
 * ignored.
 */
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;

	char *rest;
	strtoul(servname, &rest, 10);
	if (!*rest || hints->ai_flags & (AI_NUMERICHOST | AI_NUMERICSERV)) {
		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 < 12) return getaddrinfo(hostname, servname, hints, res);
	uint8_t *ptr = &msg[12];

#define NAME_SKIP \
	for (uint8_t n; ptr < &msg[len] && (n = *ptr++); ptr += n) { \
		if (n & 0xC0) { \
			ptr++; \
			break; \
		} \
	}

	uint16_t qdcount = msg[4] << 8 | msg[5];
	for (uint16_t q = 0; ptr < &msg[len] && q < qdcount; ++q) {
		NAME_SKIP; // QNAME
		ptr += 4; // QTYPE, QCLASS
	}

	NAME_SKIP; // NAME
	ptr += 14; // TYPE, CLASS, TTL, RDLENGTH, Priority, Weight
	if (&msg[len] < ptr + 3) {
		return getaddrinfo(hostname, servname, hints, res);
	}

	char port[sizeof("65535")];
	snprintf(port, sizeof(port), "%d", ptr[0] << 8 | ptr[1]);
	ptr += 2;

	// Name compression is not used for Target.
	if (!ptr[0]) return EAI_NONAME;
	hostname = (const char *)&ptr[1];
	for (uint8_t n; ptr < &msg[len] && (n = *ptr); ptr += n) {
		*ptr++ = '.';
	}

	struct addrinfo myHints = *hints;
	myHints.ai_flags |= AI_NUMERICSERV;
	myHints.ai_flags &= ~AI_CANONNAME;
	int error = getaddrinfo(hostname, port, &myHints, res);
	if (error) return error;

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