Introduction to Simple Local Peer Discovery Protocol (SLPD)

Full functionality of tinc mesh relays on having at least one node,
accessible, with known address to which all other nodes must connect
in order to exchange information about other peers.

Sometimes, however, in smaller networks or if two or more peers are
located in the same LAN segment without access to any of the nodes with
known address, there is no way of establishing a functional mesh
without manually changing the configuration.

SLPD addresses this problem utilizing multicast groups and autoconnect.
- Node sends periodically simple message to multicast group
  (default 224.0.42.23 port 1655) in this format:
   "sLPD 0 1 nodename port publickey"

   "0 1" is the "major minior" version of the protocol

- Node listens to the multicast group for messages on all interfaces:
  - if the nodename is known and the publickey matches the
    node's public key the source address of the packet
    will be stored as learned ip address

- at this point setup_outgoing_connection() will be able to
  choose the learned ip for connect

Configarion example:
* Roadwarriors: SLPDInterval = 30
* Router on your home network or in your hackerspace:
 - It should broadcast only in the direction of the LAN thus you should
   set SLPDInterface = eth0 and SLPDInterval = 10
* Defaults:
 SLPDGroup = "224.0.42.23"
 SLPDPort = 1655
 SLPDInterval = 0 (means SLPD is disabled)

The check of the publickey is not implemented yet. IPv6 support
must be implemented. This is the first commit - highly experimental.

src/conf.h

@@ -25,6 +25,9 @@
 #include "splay_tree.h"
 #include "subnet.h"
 
+#define DEFAULT_SLPD_GROUP "224.0.42.23"
+#define DEFAULT_SLPD_PORT "1655"
+
 typedef struct config_t {
 	char *variable;
 	char *value;

src/net.h

@@ -153,6 +153,7 @@ extern int listen_sockets;
 extern io_t unix_socket;
 extern int keylifetime;
 extern int edgeupdateinterval;
+extern int slpdinterval;
 extern int udp_rcvbuf;
 extern int udp_sndbuf;
 extern int max_connection_burst;
@@ -188,6 +189,7 @@ extern char *scriptextension;
 
 extern void retry_outgoing(outgoing_t *);
 extern void handle_incoming_vpn_data(void *, int);
+extern void handle_incoming_slpd_data(void *, int);
 extern void finish_connecting(struct connection_t *);
 extern bool do_outgoing_connection(struct outgoing_t *);
 extern void handle_new_meta_connection(void *, int);
@@ -215,8 +217,10 @@ extern bool read_ecdsa_public_key(struct connection_t *);
 extern bool read_rsa_public_key(struct connection_t *);
 extern void handle_device_data(void *, int);
 extern void handle_meta_connection_data(struct connection_t *);
+extern void send_slpd_broadcast(void);
 extern void regenerate_key(void);
 extern void update_edge_weight(void);
+extern int setup_slpd_in_socket(void);
 extern void purge(void);
 extern void retry(void);
 extern int reload_configuration(void);

src/net_packet.c

@@ -59,6 +59,7 @@
 
 int keylifetime = 0;
 int edgeupdateinterval = 0;
+int slpdinterval = 0;
 #ifdef HAVE_LZO
 static char lzo_wrkmem[LZO1X_999_MEM_COMPRESS > LZO1X_1_MEM_COMPRESS ? LZO1X_999_MEM_COMPRESS : LZO1X_1_MEM_COMPRESS];
 #endif
@@ -1514,6 +1515,51 @@ skip_harder:
 		send_mtu_info(myself, n, MTU);
 }
 
+static void handle_incoming_slpd_packet(listen_socket_t *ls, void *pkt, sockaddr_t *addr) {
+
+	int mav, miv, port;
+	char nodename[MAXSIZE], fng[MAXSIZE];
+
+	int i = sscanf(pkt, "sLPD %d %d %s %d %s", &mav, &miv, &nodename[0], &port, &fng[0]);
+	if (i != 5) {
+		logger(DEBUG_ALWAYS, LOG_ERR, "cant not parse packet... %d",i);
+		return;
+	}
+
+	if (mav == 0 && miv == 1) {
+
+		logger(DEBUG_TRAFFIC, LOG_ERR, "Got SLPD packet node:%s port:%d %d.%d <%s> from %s", nodename, port, mav, miv, fng, inet_ntoa(addr->in.sin_addr));
+
+		node_t *n = lookup_node(nodename);
+		if (!n) {
+			logger(DEBUG_ALWAYS, LOG_ERR, "unknown node: %s", nodename);
+			return;
+		}
+
+		if (!strncmp(n->name, myself->name, strlen(myself->name))) {
+			logger(DEBUG_SCARY_THINGS, LOG_NOTICE, "Ignore SLPD for myself: %s", nodename);
+			return;
+		}
+
+		config_t *cfg = NULL;
+		if (!n->slpd_address) {
+			cfg = new_config();
+			cfg->variable = xstrdup("Address");
+			cfg->value = xstrdup(inet_ntoa(addr->in.sin_addr));
+			cfg->file = NULL;
+			cfg->line = -1;
+
+			logger(DEBUG_ALWAYS, LOG_ERR, "Discovered %s on %s", nodename, inet_ntoa(addr->in.sin_addr));
+
+			n->slpd_address = cfg;
+			n->status.has_address = true;
+		}
+	} else {
+		logger(DEBUG_ALWAYS, LOG_ERR, "Got SLPD packet with wrong version %d.%d", mav, miv);
+	}
+	return;
+}
+
 void handle_incoming_vpn_data(void *data, int flags) {
 	listen_socket_t *ls = data;
 
@@ -1575,6 +1621,27 @@ void handle_incoming_vpn_data(void *data, int flags) {
 #endif
 }
 
+void handle_incoming_slpd_data(void *data, int flags) {
+	listen_socket_t *ls = data;
+
+	void *pkt;
+	sockaddr_t addr = {};
+	socklen_t addrlen = sizeof addr;
+
+	logger(DEBUG_SCARY_THINGS, LOG_INFO, "Receiving LPD packet");
+
+	int len = recvfrom(ls->udp.fd, (void *)&pkt, MAXSIZE, 0, &addr.sa, &addrlen);
+
+	if(len <= 0 || len > MAXSIZE) {
+		if(!sockwouldblock(sockerrno))
+			logger(DEBUG_ALWAYS, LOG_ERR, "Receiving SLPD packet failed: %s", sockstrerror(sockerrno));
+		return;
+	}
+
+	handle_incoming_slpd_packet(ls, &pkt, &addr);
+}
+
+
 void handle_device_data(void *data, int flags) {
 	vpn_packet_t packet;
 	UNUSED(data);

src/net_setup.c

@@ -49,6 +49,8 @@
 #endif
 
 char *myport;
+char *my_slpd_port;
+char *my_slpd_group;
 static char *myname;
 static io_t device_io;
 devops_t devops;
@@ -313,6 +315,13 @@ static bool read_rsa_private_key(void) {
 
 static timeout_t keyexpire_timeout;
 static timeout_t edgeupdate_timeout;
+/* Local Peer Discovery */
+static timeout_t slpdupdate_timeout;
+
+static void slpdupdate_handler(void *data) {
+	send_slpd_broadcast();
+	timeout_set(data, &(struct timeval){slpdinterval + (rand() % 10), rand() % 100000});
+}
 
 static void keyexpire_handler(void *data) {
 	regenerate_key();
@@ -324,6 +333,56 @@ static void edgeupdate_handler(void *data) {
 	timeout_set(data, &(struct timeval){edgeupdateinterval + (rand() % 10), rand() % 100000});
 }
 
+void send_slpd_broadcast(void) {
+	int sd;
+	struct sockaddr_in address;
+	char slpd_msg[MAXSIZE] = "";
+	char *iface;
+	struct in_addr lif;
+	struct ifreq ifr;
+	int fd;
+
+	sd = socket (AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+	if (sd < 0) {
+		logger(DEBUG_ALWAYS, LOG_INFO, "socket() error: [%s:%d]", strerror(errno), errno);
+		return;
+	}
+
+	/* Send SLPD only on this Interface */
+	if(get_config_string (lookup_config (config_tree, "SLPDInterface"), &iface)) {
+		fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+		ifr.ifr_addr.sa_family = AF_INET;
+		strncpy(ifr.ifr_name, iface, IFNAMSIZ-1);
+		ioctl(fd, SIOCGIFADDR, &ifr);
+		close(fd);
+
+		/* display result */
+		logger(DEBUG_ALWAYS, LOG_ERR, "%s has the ip: %s\n", iface,
+					 inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr));
+
+		lif.s_addr = inet_addr(inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr));
+		if(setsockopt(sd, IPPROTO_IP, IP_MULTICAST_IF, (char *)&lif, sizeof(lif)) < 0) {
+			logger(DEBUG_ALWAYS, LOG_ERR, "can not bind multicast to %s\n", iface);
+			return;
+		}
+	}
+
+	memset (&address, 0, sizeof (address));
+	address.sin_family = AF_INET;
+	address.sin_addr.s_addr = inet_addr (DEFAULT_SLPD_GROUP);
+	address.sin_port = htons(atoi(my_slpd_port));
+
+	snprintf(slpd_msg, MAXSIZE, "sLPD 0 1 %s %d none ", myname, atoi(myport));
+	slpd_msg[MAXSIZE-1] = '\00';
+	//ecdsa_sign(myself->sptps.mykey, msg, strlen(msg), sig);
+
+	if (sendto( sd, slpd_msg, strlen(slpd_msg), 0, (struct sockaddr *) &address, sizeof (address)) < 0) {
+		logger(DEBUG_ALWAYS, LOG_ERR, "SLPD send() error: [%s:%d]", strerror(errno), errno);
+	}
+	close(sd);
+	return;
+}
+
 void regenerate_key(void) {
 	logger(DEBUG_STATUS, LOG_INFO, "Expiring symmetric keys");
 	send_key_changed();
@@ -642,6 +701,9 @@ bool setup_myself_reloadable(void) {
 	if(!get_config_int(lookup_config(config_tree, "EdgeUpdateInterval"), &edgeupdateinterval))
 		edgeupdateinterval = 0;
 
+	if(!get_config_int(lookup_config(config_tree, "SLPDInterval"), &slpdinterval))
+		slpdinterval = 0;
+
 	config_t *cfg = lookup_config(config_tree, "AutoConnect");
 	if(cfg) {
 		if(!get_config_bool(cfg, &autoconnect)) {
@@ -803,6 +865,12 @@ static bool setup_myself(void) {
 	else
 		port_specified = true;
 
+	if(!get_config_string(lookup_config(config_tree, "SLPDPort"), &my_slpd_port))
+		my_slpd_port = xstrdup(DEFAULT_SLPD_PORT);
+
+	if(!get_config_string(lookup_config(config_tree, "SLPDGroup"), &my_slpd_group))
+		my_slpd_group = xstrdup(DEFAULT_SLPD_GROUP);
+
 	myself->connection->options = 0;
 	myself->connection->protocol_major = PROT_MAJOR;
 	myself->connection->protocol_minor = PROT_MINOR;
@@ -942,6 +1010,8 @@ static bool setup_myself(void) {
 		timeout_add(&edgeupdate_timeout, edgeupdate_handler, &edgeupdate_timeout, &(struct timeval){edgeupdateinterval, rand() % 100000});
 
 	if (slpdinterval)
+		timeout_add(&slpdupdate_timeout, slpdupdate_handler, &slpdupdate_timeout, &(struct timeval){slpdinterval, rand() % 100000});
+
 	/* Compression */
 
 	if(get_config_int(lookup_config(config_tree, "Compression"), &myself->incompression)) {
@@ -1062,6 +1132,16 @@ static bool setup_myself(void) {
 				return false;
 	}
 
+	if (slpdinterval) {
+		int slpd_fd = setup_slpd_in_socket();
+		if (slpd_fd < 0) {
+			close(slpd_fd);
+		} else {
+			logger(DEBUG_CONNECTIONS, LOG_NOTICE, "Listening on multicast group");
+			io_add(&listen_socket[listen_sockets].udp, handle_incoming_slpd_data, &listen_socket[listen_sockets], slpd_fd, IO_READ);
+		}
+	}
+
 	if(!listen_sockets) {
 		logger(DEBUG_ALWAYS, LOG_ERR, "Unable to create any listening socket!");
 		return false;

src/net_socket.c

@@ -301,6 +301,58 @@ int setup_vpn_in_socket(const sockaddr_t *sa) {
 	return nfd;
 } /* int setup_vpn_in_socket */
 
+
+int setup_slpd_in_socket(void) {
+	int nfd;
+	char *my_slpd_port;
+	char *my_slpd_group;
+	struct sockaddr_in lsock;
+	struct ip_mreq group;
+	struct ip_mreq mreq;
+
+	if(!get_config_string(lookup_config(config_tree, "SLPDPort"), &my_slpd_port))
+		my_slpd_port = xstrdup(DEFAULT_SLPD_PORT);
+
+	if(!get_config_string(lookup_config(config_tree, "SLPDGroup"), &my_slpd_group))
+		my_slpd_group = xstrdup(DEFAULT_SLPD_GROUP);
+
+	nfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+
+	if(nfd < 0) {
+		logger(DEBUG_ALWAYS, LOG_ERR, "Can not create socket for SLPD %s", sockstrerror(sockerrno));
+		return -1;
+	}
+
+	int reuse = 1;
+	if(setsockopt(nfd, SOL_SOCKET, SO_REUSEADDR, (char *)&reuse, sizeof(reuse)) < 0) {
+		logger(DEBUG_ALWAYS, LOG_ERR, "Can not set SO_REUSEADDR for SLPD %s", sockstrerror(sockerrno));
+		closesocket(nfd);
+		return -1;
+	}
+
+	memset((char *) &lsock, 0, sizeof(lsock));
+	lsock.sin_family = AF_INET;
+	lsock.sin_port = htons(atoi(my_slpd_port));
+	lsock.sin_addr.s_addr = htonl(INADDR_ANY);
+
+	if(bind(nfd, (struct sockaddr*)&lsock, sizeof(lsock)) < 0) {
+		logger(DEBUG_ALWAYS, LOG_ERR, "Can not bind() socket for SLPD %s", sockstrerror(sockerrno));
+		closesocket(nfd);
+		return -1;
+	}
+
+	group.imr_multiaddr.s_addr = inet_addr(my_slpd_group);
+	group.imr_interface.s_addr = htonl(INADDR_ANY);
+	if(setsockopt(nfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *)&group, sizeof(group)) < 0) {
+		logger(DEBUG_ALWAYS, LOG_ERR, "Can not join group for SLPD %s", sockstrerror(sockerrno));
+		closesocket(nfd);
+		return -1;
+	}
+	logger(DEBUG_STATUS, LOG_INFO, "SLPD socket ready (%d)", nfd);
+
+	return nfd;
+} /* int setup_slpd_in_socket */
+
 static void retry_outgoing_handler(void *data) {
 	setup_outgoing_connection(data);
 }
@@ -619,6 +671,9 @@ void setup_outgoing_connection(outgoing_t *outgoing) {
 		outgoing->cfg = lookup_config(outgoing->config_tree, "Address");
 	}
 
+	if (slpdinterval && !outgoing->cfg && n->slpd_address)
+		outgoing->cfg = n->slpd_address;
+
 	if(!outgoing->cfg) {
 		if(n)
 			outgoing->aip = outgoing->ai = get_known_addresses(n);

src/node.h

@@ -23,6 +23,7 @@
 
 #include "splay_tree.h"
 #include "cipher.h"
+#include "conf.h"
 #include "connection.h"
 #include "digest.h"
 #include "event.h"
@@ -81,7 +82,7 @@ typedef struct node_t {
 	splay_tree_t *edge_tree;                /* Edges with this node as one of the endpoints */
 
 	struct connection_t *connection;        /* Connection associated with this node (if a direct connection exists) */
-
+	struct config_t *slpd_address;          /* Address we learned via SLPD */
 	uint32_t sent_seqno;                    /* Sequence number last sent to this node */
 	uint32_t received_seqno;                /* Sequence number last received from this node */
 	uint32_t received;                      /* Total valid packets received from this node */