eventdns.c

关于tor匿名通信的源代码

/* Parse a raw request (packet,length) sent to a nameserver port (port) from */
/* a DNS client (addr,addrlen), and if it's well-formed, call the corresponding */
/* callback. */
static int
request_parse(u8 *packet, ssize_t length, struct evdns_server_port *port, struct sockaddr *addr, socklen_t addrlen)
{
	int j = 0;	/* index into packet */
	u16 _t;	 /* used by the macros */
	char tmp_name[256]; /* used by the macros */

	int i;
	u16 trans_id, flags, questions, answers, authority, additional;
	struct server_request *server_req = NULL;

	/* Get the header fields */
	GET16(trans_id);
	GET16(flags);
	GET16(questions);
	GET16(answers);
	GET16(authority);
	GET16(additional);

	if (flags & 0x8000) return -1; /* Must not be an answer. */
	flags &= 0x0110; /* Only RD and CD get preserved. */

    if (length > INT_MAX)
        return -1;

	server_req = malloc(sizeof(struct server_request));
	if (server_req == NULL) return -1;
	memset(server_req, 0, sizeof(struct server_request));

	server_req->trans_id = trans_id;
	memcpy(&server_req->addr, addr, addrlen);
	server_req->addrlen = addrlen;

	server_req->base.flags = flags;
	server_req->base.nquestions = 0;
	server_req->base.questions = malloc(sizeof(struct evdns_server_question *) * questions);
	if (server_req->base.questions == NULL)
		goto err;

	for (i = 0; i < questions; ++i) {
		u16 type, class;
		struct evdns_server_question *q;
		size_t namelen;
		if (name_parse(packet, (int)length, &j, tmp_name, sizeof(tmp_name))<0)
			goto err;
		GET16(type);
		GET16(class);
		namelen = strlen(tmp_name);
		q = malloc(sizeof(struct evdns_server_question) + namelen);
		if (!q)
			goto err;
		q->type = type;
		q->class = class;
		memcpy(q->name, tmp_name, namelen+1);
		server_req->base.questions[server_req->base.nquestions++] = q;
	}

	/* Ignore answers, authority, and additional. */

	server_req->port = port;
	port->refcnt++;

	/* Only standard queries are supported. */
	if (flags & 0x7800) {
		evdns_server_request_respond(&(server_req->base), DNS_ERR_NOTIMPL);
		return -1;
	}

	port->user_callback(&(server_req->base), port->user_data);

	return 0;
err:
	if (server_req) {
		if (server_req->base.questions) {
			for (i = 0; i < server_req->base.nquestions; ++i)
				free(server_req->base.questions[i]);
			free(server_req->base.questions);
		}
		CLEAR(server_req);
		free(server_req);
	}
	return -1;

#undef SKIP_NAME
#undef GET32
#undef GET16
#undef GET8
}

static uint16_t
default_transaction_id_fn(void)
{
	u16 trans_id;
#ifdef DNS_USE_CPU_CLOCK_FOR_ID
	struct timespec ts;
#ifdef CLOCK_MONOTONIC
	if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1)
#else
	if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
#endif
	event_err(1, "clock_gettime");
	trans_id = ts.tv_nsec & 0xffff;
#endif

#ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
	struct timeval tv;
	gettimeofday(&tv, NULL);
	trans_id = tv.tv_usec & 0xffff;
#endif

#ifdef DNS_USE_OPENSSL_FOR_ID
	if (RAND_pseudo_bytes((u8 *) &trans_id, 2) == -1) {
		/* in the case that the RAND call fails we back */
		/* down to using gettimeofday. */
		/*
		  struct timeval tv;
		  gettimeofday(&tv, NULL);
		  trans_id = tv.tv_usec & 0xffff;
		*/
		abort();
	}
#endif
	return (unsigned short) trans_id;
}

static uint16_t (*trans_id_function)(void) = default_transaction_id_fn;

void
evdns_set_transaction_id_fn(uint16_t (*fn)(void))
{
	if (fn)
		trans_id_function = fn;
	else
		trans_id_function = default_transaction_id_fn;
}

/* Try to choose a strong transaction id which isn't already in flight */
static u16
transaction_id_pick(void) {
	for (;;) {
		const struct request *req = req_head, *started_at;
		u16 trans_id = trans_id_function();

		if (trans_id == 0xffff) continue;
		/* now check to see if that id is already inflight */
		req = started_at = req_head;
		if (req) {
			do {
				if (req->trans_id == trans_id) break;
				req = req->next;
			} while (req != started_at);
		}
		/* we didn't find it, so this is a good id */
		if (req == started_at) return trans_id;
	}
}

/* choose a namesever to use. This function will try to ignore */
/* nameservers which we think are down and load balance across the rest */
/* by updating the server_head global each time. */
static struct nameserver *
nameserver_pick(void) {
	struct nameserver *started_at = server_head, *picked;
	if (!server_head) return NULL;

	/* if we don't have any good nameservers then there's no */
	/* point in trying to find one. */
	if (!global_good_nameservers) {
		server_head = server_head->next;
		return server_head;
	}

	/* remember that nameservers are in a circular list */
	for (;;) {
		if (server_head->state) {
			/* we think this server is currently good */
			picked = server_head;
			server_head = server_head->next;
			return picked;
		}

		server_head = server_head->next;
		if (server_head == started_at) {
			/* all the nameservers seem to be down */
			/* so we just return this one and hope for the */
			/* best */
			assert(global_good_nameservers == 0);
			picked = server_head;
			server_head = server_head->next;
			return picked;
		}
	}
}

/* this is called when a namesever socket is ready for reading */
static void
nameserver_read(struct nameserver *ns) {
	u8 packet[1500];

	for (;;) {
		const int r =
            (int)recv(ns->socket, packet,(socklen_t)sizeof(packet), 0);
		if (r < 0) {
			int err = last_error(ns->socket);
			if (error_is_eagain(err)) return;
			nameserver_failed(ns, strerror(err));
			return;
		}
		ns->timedout = 0;
		reply_parse(packet, r);
	}
}

/* Read a packet from a DNS client on a server port s, parse it, and */
/* act accordingly. */
static void
server_port_read(struct evdns_server_port *s) {
	u8 packet[1500];
	struct sockaddr_storage addr;
	socklen_t addrlen;
	ssize_t r;

	for (;;) {
		addrlen = (socklen_t)sizeof(struct sockaddr_storage);
		r = recvfrom(s->socket, packet, sizeof(packet), 0,
					 (struct sockaddr*) &addr, &addrlen);
		if (r < 0) {
			int err = last_error(s->socket);
			if (error_is_eagain(err)) return;
			log(EVDNS_LOG_WARN, "Error %s (%d) while reading request.",
				strerror(err), err);
			return;
		}
		request_parse(packet, r, s, (struct sockaddr*) &addr, addrlen);
	}
}

/* Try to write all pending replies on a given DNS server port. */
static void
server_port_flush(struct evdns_server_port *port)
{
	while (port->pending_replies) {
		struct server_request *req = port->pending_replies;
		ssize_t r = sendto(port->socket, req->response, req->response_len, 0,
                       (struct sockaddr*) &req->addr, (socklen_t)req->addrlen);
		if (r < 0) {
			int err = last_error(port->socket);
			if (error_is_eagain(err))
				return;
			log(EVDNS_LOG_WARN, "Error %s (%d) while writing response to port; dropping", strerror(err), err);
		}
		if (server_request_free(req)) {
			/* we released the last reference to req->port. */
			return;
		}
	}

	/* We have no more pending requests; stop listening for 'writeable' events. */
	(void) event_del(&port->event);
	CLEAR(&port->event);
	event_set(&port->event, port->socket, EV_READ | EV_PERSIST,
			  server_port_ready_callback, port);
	if (event_add(&port->event, NULL) < 0) {
		log(EVDNS_LOG_WARN, "Error from libevent when adding event for DNS server.");
		/* ???? Do more? */
	}
}

/* set if we are waiting for the ability to write to this server. */
/* if waiting is true then we ask libevent for EV_WRITE events, otherwise */
/* we stop these events. */
static void
nameserver_write_waiting(struct nameserver *ns, char waiting) {
	if (ns->write_waiting == waiting) return;

	ns->write_waiting = waiting;
	(void) event_del(&ns->event);
	CLEAR(&ns->event);
	event_set(&ns->event, ns->socket, EV_READ | (waiting ? EV_WRITE : 0) | EV_PERSIST,
			  nameserver_ready_callback, ns);
	if (event_add(&ns->event, NULL) < 0) {
		log(EVDNS_LOG_WARN, "Error from libevent when adding event for %s",
			debug_ntoa(ns->address));
		/* ???? Do more? */
	}
}

/* a callback function. Called by libevent when the kernel says that */
/* a nameserver socket is ready for writing or reading */
static void
nameserver_ready_callback(int fd, short events, void *arg) {
	struct nameserver *ns = (struct nameserver *) arg;
	(void)fd;

	if (events & EV_WRITE) {
		ns->choked = 0;
		if (!evdns_transmit()) {
			nameserver_write_waiting(ns, 0);
		}
	}
	if (events & EV_READ) {
		nameserver_read(ns);
	}
}

/* a callback function. Called by libevent when the kernel says that */
/* a server socket is ready for writing or reading. */
static void
server_port_ready_callback(int fd, short events, void *arg) {
	struct evdns_server_port *port = (struct evdns_server_port *) arg;
	(void) fd;

	if (events & EV_WRITE) {
		port->choked = 0;
		server_port_flush(port);
	}
	if (events & EV_READ) {
		server_port_read(port);
	}
}

/* This is an inefficient representation; only use it via the dnslabel_table_*
 * functions, so that is can be safely replaced with something smarter later. */
#define MAX_LABELS 128
/* Structures used to implement name compression */
struct dnslabel_entry { char *v; off_t pos; };
struct dnslabel_table {
	int n_labels; /* number of current entries */
	/* map from name to position in message */
	struct dnslabel_entry labels[MAX_LABELS];
};

/* Initialize dnslabel_table. */
static void
dnslabel_table_init(struct dnslabel_table *table)
{
	table->n_labels = 0;
}

/* Free all storage held by table, but not the table itself. */
static void
dnslabel_clear(struct dnslabel_table *table)
{
	int i;
	for (i = 0; i < table->n_labels; ++i)
		free(table->labels[i].v);
	table->n_labels = 0;
}

/* return the position of the label in the current message, or -1 if the label */
/* hasn't been used yet. */
static int
dnslabel_table_get_pos(const struct dnslabel_table *table, const char *label)
{
	int i;
	for (i = 0; i < table->n_labels; ++i) {
		if (!strcmp(label, table->labels[i].v)) {
			off_t pos = table->labels[i].pos;
            if (pos > 65535)
                return -1;
            return (int)pos;
        }
	}
	return -1;
}

/* remember that we've used the label at position pos */
static int
dnslabel_table_add(struct dnslabel_table *table, const char *label, off_t pos)
{
	char *v;
	int p;
	if (table->n_labels == MAX_LABELS)
		return (-1);
	v = strdup(label);
	if (v == NULL)
		return (-1);
	p = table->n_labels++;
	table->labels[p].v = v;
	table->labels[p].pos = pos;

	return (0);
}

/* Converts a string to a length-prefixed set of DNS labels, starting */
/* at buf[j]. name and buf must not overlap. name_len should be the length */
/* of name.	 table is optional, and is used for compression. */
/* */
/* Input: abc.def */
/* Output: <3>abc<3>def<0> */
/* */
/* Returns the first index after the encoded name, or negative on error. */
/* -1	 label was > 63 bytes */
/* -2	 name too long to fit in buffer. */
/* */
static off_t
dnsname_to_labels(u8 *const buf, size_t buf_len, off_t j,
				  const char *name, const size_t name_len,
				  struct dnslabel_table *table) {
	const char *end = name + name_len;
	int ref = 0;
	u16 _t;

#define APPEND16(x) do {						   \
		if (j + 2 > (off_t)buf_len)				   \
			goto overflow;						   \
		_t = htons(x);							   \
		memcpy(buf + j, &_t, 2);				   \
		j += 2;									   \
	} while (0)
#define APPEND32(x) do {						   \
		if (j + 4 > (off_t)buf_len)				   \
			goto overflow;						   \
		_t32 = htonl(x);						   \
		memcpy(buf + j, &_t32, 4);				   \
		j += 4;									   \
	} while (0)

	if (name_len > 255) return -2;

	for (;;) {
		const char *const start = name;
		if (table && (ref = dnslabel_table_get_pos(table, name)) >= 0) {
			APPEND16(ref | 0xc000);
			return j;
		}
		name = strchr(name, '.');
		if (!name) {
			const size_t label_len = end - start;
			if (label_len > 63) return -1;
			if ((size_t)(j+label_len+1) > buf_len) return -2;
			if (table) dnslabel_table_add(table, start, j);
			buf[j++] = (uint8_t)label_len;

			memcpy(buf + j, start, label_len);
			j += end - start;
			break;
		} else {
			/* append length of the label. */
			const size_t label_len = name - start;
			if (label_len > 63) return -1;
			if ((size_t)(j+label_len+1) > buf_len) return -2;
			if (table) dnslabel_table_add(table, start, j);
			buf[j++] = (uint8_t)label_len;

			memcpy(buf + j, start, name - start);
			j += name - start;
			/* hop over the '.' */
			name++;
		}
	}

	/* the labels must be terminated by a 0. */
	/* It's possible that the name ended in a . */
	/* in which case the zero is already there */
	if (!j || buf[j-1]) buf[j++] = 0;
	return j;
 overflow:
	return (-2);
}

/* Finds the length of a dns request for a DNS name of the given */
/* length. The actual request may be smaller than the value returned */
/* here */
static size_t
evdns_request_len(const size_t name_len) {
	return 96 + /* length of the DNS standard header */
		name_len + 2 +