dns.c

/*
 * Copyright (C) 2014 John Crispin <blogic@openwrt.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation
 *
 * 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.
 */

#include <sys/types.h>
#include <sys/stat.h>

#include <fcntl.h>
#include <ifaddrs.h>
#include <time.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include <stdlib.h>
#include <string.h>

#include <libubox/uloop.h>
#include <libubox/usock.h>
#include <libubox/utils.h>

#include "announce.h"
#include "util.h"
#include "dns.h"
#include "cache.h"
#include "service.h"
#include "interface.h"

static char name_buffer[MAX_NAME_LEN + 1];
static char dns_buffer[MAX_NAME_LEN];
static struct blob_buf ans_buf;

const char*
dns_type_string(uint16_t type)
{
	static const struct {
		uint16_t type;
		char str[5];
	} type_str[] = {
		{ TYPE_A, "A" },
		{ TYPE_AAAA, "AAAA" },
		{ TYPE_PTR, "PTR" },
		{ TYPE_TXT, "TXT" },
		{ TYPE_SRV, "SRV" },
		{ TYPE_ANY, "ANY" },
	};
	int i;

	for (i = 0; i < ARRAY_SIZE(type_str); i++) {
		if (type == type_str[i].type)
			return type_str[i].str;
	}

	return "N/A";
}

void
dns_send_question(struct interface *iface, struct sockaddr *to,
		  const char *question, int type, int multicast)
{
	static struct dns_header h;
	static struct dns_question q;
	static struct iovec iov[] = {
		{
			.iov_base = &h,
			.iov_len = sizeof(h),
		},
		{
			.iov_base = dns_buffer,
		},
		{
			.iov_base = &q,
			.iov_len = sizeof(q),
		}
	};
	int len;

	h.questions = cpu_to_be16(1);
	q.class = cpu_to_be16((multicast ? 0 : CLASS_UNICAST) | 1);
	q.type = cpu_to_be16(type);

	len = dn_comp(question, (void *) dns_buffer, sizeof(dns_buffer), NULL, NULL);
	if (len < 1)
		return;

	iov[1].iov_len = len;

	DBG(1, "Q <- %s %s\n", dns_type_string(type), question);
	if (interface_send_packet(iface, to, iov, ARRAY_SIZE(iov)) < 0)
		perror("failed to send question");
}


struct dns_reply {
	int type;
	struct dns_answer a;
	uint16_t rdlength;
	uint8_t *rdata;
	char *buffer;
};

static int dns_answer_cnt;

void
dns_init_answer(void)
{
	dns_answer_cnt = 0;
	blob_buf_init(&ans_buf, 0);
}

void
dns_add_answer(int type, const uint8_t *rdata, uint16_t rdlength, int ttl)
{
	struct blob_attr *attr;
	struct dns_answer *a;

	attr = blob_new(&ans_buf, 0, sizeof(*a) + rdlength);
	a = blob_data(attr);
	a->type = cpu_to_be16(type);
	a->class = cpu_to_be16(1);
	a->ttl = cpu_to_be32(ttl);
	a->rdlength = cpu_to_be16(rdlength);
	memcpy(a + 1, rdata, rdlength);

	dns_answer_cnt++;
}

void
dns_send_answer(struct interface *iface, struct sockaddr *to, const char *answer)
{
	uint8_t buffer[256];
	struct blob_attr *attr;
	struct dns_header h = { 0 };
	struct iovec *iov;
	int answer_len, rem;
	int n_iov = 0;

	if (!dns_answer_cnt)
		return;

	h.answers = cpu_to_be16(dns_answer_cnt);
	h.flags = cpu_to_be16(0x8400);

	iov = alloca(sizeof(struct iovec) * ((dns_answer_cnt * 2) + 1));

	iov[n_iov].iov_base = &h;
	iov[n_iov].iov_len = sizeof(struct dns_header);
	n_iov++;

	answer_len = dn_comp(answer, buffer, sizeof(buffer), NULL, NULL);
	if (answer_len < 1)
		return;

	blob_for_each_attr(attr, ans_buf.head, rem) {
		struct dns_answer *a = blob_data(attr);

		iov[n_iov].iov_base = buffer;
		iov[n_iov].iov_len = answer_len;
		n_iov++;

		iov[n_iov].iov_base = blob_data(attr);
		iov[n_iov].iov_len = blob_len(attr);
		n_iov++;

		DBG(1, "A <- %s %s\n", dns_type_string(be16_to_cpu(a->type)), answer);
	}

	if (interface_send_packet(iface, to, iov, n_iov) < 0)
		perror("failed to send answer");
}

void
dns_reply_a(struct interface *iface, struct sockaddr *to, int ttl, const char *hostname)
{
	struct ifaddrs *ifap, *ifa;
	struct sockaddr_in *sa;
	struct sockaddr_in6 *sa6;

	getifaddrs(&ifap);

	dns_init_answer();
	for (ifa = ifap; ifa; ifa = ifa->ifa_next) {
		if (strcmp(ifa->ifa_name, iface->name))
			continue;
		if (ifa->ifa_addr->sa_family == AF_INET) {
			sa = (struct sockaddr_in *) ifa->ifa_addr;
			dns_add_answer(TYPE_A, (uint8_t *) &sa->sin_addr, 4, ttl);
		}
		if (ifa->ifa_addr->sa_family == AF_INET6) {
			sa6 = (struct sockaddr_in6 *) ifa->ifa_addr;
			dns_add_answer(TYPE_AAAA, (uint8_t *) &sa6->sin6_addr, 16, ttl);
		}
	}
	dns_send_answer(iface, to, hostname ? hostname : mdns_hostname_local);

	freeifaddrs(ifap);
}

void
dns_reply_a_additional(struct interface *iface, struct sockaddr *to, int ttl)
{
	struct hostname *h;

	vlist_for_each_element(&hostnames, h, node)
		dns_reply_a(iface, to, ttl, h->hostname);
}

static int
scan_name(const uint8_t *buffer, int len)
{
	int offset = 0;

	while (len && (*buffer != '\0')) {
		int l = *buffer;

		if (IS_COMPRESSED(l))
			return offset + 2;

		if (l + 1 > len) return -1;
		len -= l + 1;
		offset += l + 1;
		buffer += l + 1;
	}

	if (!len || !offset || (*buffer != '\0'))
		return -1;

	return offset + 1;
}

static struct dns_header*
dns_consume_header(uint8_t **data, int *len)
{
	struct dns_header *h = (struct dns_header *) *data;

	if (*len < sizeof(struct dns_header))
		return NULL;

	h->id = be16_to_cpu(h->id);
	h->flags = be16_to_cpu(h->flags);
	h->questions = be16_to_cpu(h->questions);
	h->answers = be16_to_cpu(h->answers);
	h->authority = be16_to_cpu(h->authority);
	h->additional = be16_to_cpu(h->additional);

	*len -= sizeof(struct dns_header);
	*data += sizeof(struct dns_header);

	return h;
}

static struct dns_question*
dns_consume_question(uint8_t **data, int *len)
{
	struct dns_question *q = (struct dns_question *) *data;

	if (*len < sizeof(struct dns_question))
		return NULL;

	q->type = be16_to_cpu(q->type);
	q->class = be16_to_cpu(q->class);

	*len -= sizeof(struct dns_question);
	*data += sizeof(struct dns_question);

	return q;
}

static struct dns_answer*
dns_consume_answer(uint8_t **data, int *len)
{
	struct dns_answer *a = (struct dns_answer *) *data;

	if (*len < sizeof(struct dns_answer))
		return NULL;

	a->type = be16_to_cpu(a->type);
	a->class = be16_to_cpu(a->class);
	a->ttl = be32_to_cpu(a->ttl);
	a->rdlength = be16_to_cpu(a->rdlength);

	*len -= sizeof(struct dns_answer);
	*data += sizeof(struct dns_answer);

	return a;
}

static char *
dns_consume_name(const uint8_t *base, int blen, uint8_t **data, int *len)
{
	int nlen = scan_name(*data, *len);

	if (nlen < 1)
		return NULL;

	if (dn_expand(base, base + blen, *data, name_buffer, MAX_NAME_LEN) < 0) {
		perror("dns_consume_name/dn_expand");
		return NULL;
	}

	*len -= nlen;
	*data += nlen;

	return name_buffer;
}

static int parse_answer(struct interface *iface, struct sockaddr *from,
			uint8_t *buffer, int len, uint8_t **b, int *rlen,
			int cache)
{
	char *name = dns_consume_name(buffer, len, b, rlen);
	struct dns_answer *a;
	uint8_t *rdata;

	if (!name || *rlen < 0) {
		fprintf(stderr, "dropping: bad question\n");
		return -1;
	}

	a = dns_consume_answer(b, rlen);
	if (!a) {
		fprintf(stderr, "dropping: bad question\n");
		return -1;
	}

	if ((a->class & ~CLASS_FLUSH) != CLASS_IN)
		return -1;

	rdata = *b;
	if (a->rdlength > *rlen) {
		fprintf(stderr, "dropping: bad question\n");
		return -1;
	}

	*rlen -= a->rdlength;
	*b += a->rdlength;

	if (cache)
		cache_answer(iface, from, buffer, len, name, a, rdata, a->class & CLASS_FLUSH);

	return 0;
}

static void
parse_question(struct interface *iface, struct sockaddr *from, char *name, struct dns_question *q)
{
	int is_unicast = (q->class & CLASS_UNICAST) != 0;
	struct sockaddr *to = NULL;
	struct hostname *h;
	char *host;

	/* TODO: Multicast if more than one quarter of TTL has passed */
	if (is_unicast) {
		to = from;
		if (interface_multicast(iface))
			iface = interface_get(iface->name, iface->type | SOCKTYPE_BIT_UNICAST);
	}

	DBG(1, "Q -> %s %s\n", dns_type_string(q->type), name);

	switch (q->type) {
	case TYPE_ANY:
		if (!strcmp(name, mdns_hostname_local)) {
			dns_reply_a(iface, to, announce_ttl, NULL);
			dns_reply_a_additional(iface, to, announce_ttl);
			service_reply(iface, to, NULL, NULL, announce_ttl, is_unicast);
		}
		break;

	case TYPE_PTR:
		if (!strcmp(name, C_DNS_SD)) {
			service_announce_services(iface, to, announce_ttl);
		} else {
			if (name[0] == '_') {
				service_reply(iface, to, NULL, name, announce_ttl, is_unicast);
			} else {
				/* First dot separates instance name from the rest */
				char *dot = strchr(name, '.');

				if (dot) {
					*dot = '\0';
					service_reply(iface, to, name, dot + 1, announce_ttl, is_unicast);
					*dot = '.';
				}
			}
		}
		break;

	case TYPE_AAAA:
	case TYPE_A:
		host = strstr(name, ".local");
		if (host)
			*host = '\0';
		if (!strcmp(umdns_host_label, name)) {
			dns_reply_a(iface, to, announce_ttl, NULL);
		} else {
			if (host)
				*host = '.';
			vlist_for_each_element(&hostnames, h, node)
				if (!strcmp(h->hostname, name))
					dns_reply_a(iface, to, announce_ttl, h->hostname);
		}
		break;
	};
}

void
dns_handle_packet(struct interface *iface, struct sockaddr *from, uint16_t port, uint8_t *buffer, int len)
{
	struct dns_header *h;
	uint8_t *b = buffer;
	int rlen = len;

	h = dns_consume_header(&b, &rlen);
	if (!h) {
		fprintf(stderr, "dropping: bad header\n");
		return;
	}

	if (h->questions && !interface_multicast(iface) && port != MCAST_PORT)
		/* silently drop unicast questions that dont originate from port 5353 */
		return;

	while (h->questions-- > 0) {
		char *name = dns_consume_name(buffer, len, &b, &rlen);
		struct dns_question *q;

		if (!name || rlen < 0) {
			fprintf(stderr, "dropping: bad name\n");
			return;
		}

		q = dns_consume_question(&b, &rlen);
		if (!q) {
			fprintf(stderr, "dropping: bad question\n");
			return;
		}

		if (!(h->flags & FLAG_RESPONSE))
			parse_question(iface, from, name, q);
	}

	if (!(h->flags & FLAG_RESPONSE))
		return;

	while (h->answers-- > 0)
		if (parse_answer(iface, from, buffer, len, &b, &rlen, 1))
			return;

	while (h->authority-- > 0)
		if (parse_answer(iface, from, buffer, len, &b, &rlen, 1))
			return;

	while (h->additional-- > 0)
		if (parse_answer(iface, from, buffer, len, &b, &rlen, 1))
			return;

}