allowed edge to optionally auto-detetct local IP address ('-e auto') for advertising as preferred (#776)

edge.8

@@ -44,10 +44,11 @@ TOS for packets, e.g. 0x48 for SSH like priority
 enable PMTU discovery, it can reduce fragmentation but
 causes connections to stall if not properly supported
 .TP
-\fB\-e \fR<\fIlocal ip\fR>
+\fB\-e \fR<\fIlocal_ip_address\fR>
 advertises the provided local IP address as preferred,
 useful if multicast peer detection is not available, e.g.
-disabled on routers
+disabled on routers. \fB\-e auto\fR tries auto-detection of
+local IP address.
 .TP
 \fB\-S1\fR ... \fB\-S2\fR
 do not connect p2p, always use the supernode,

include/n2n_typedefs.h

@@ -652,6 +652,7 @@ typedef struct n2n_edge_conf {
     int                      register_ttl;           /**< TTL for registration packet when UDP NAT hole punching through supernode. */
     in_addr_t                bind_address;           /**< The address to bind to if provided (-b) */
     n2n_sock_t               preferred_sock;         /**< propagated local sock for better p2p in LAN (-e) */
+    uint8_t                  preferred_sock_auto;    /**< indicates desired auto detect for preferred sock */
     int                      local_port;
     int                      mgmt_port;
     uint8_t                  connect_tcp;            /** connection to supernode 0 = UDP; 1 = TCP */

src/edge.c

@@ -255,7 +255,8 @@ static void help (int level) {
                "                   | causes connections to stall if not properly supported\n");
 #endif
         printf(" -e <local ip>     | advertises the provided local IP address as preferred,\n"
-               "                   | useful if multicast peer detection is not available\n");
+               "                   | useful if multicast peer detection is not available,\n"
+               "                   | '-e auto' tries IP address auto-detection\n");
         printf(" -S1 ... -S2       | do not connect p2p, always use the supernode,\n"
                "                   | -S1 = via UDP"
 
@@ -615,8 +616,16 @@ static int setOption (int optkey, char *optargument, n2n_tuntap_priv_config_t *e
         }
 
         case 'e': {
+            in_addr_t address_tmp;
+
             if(optargument) {
-                in_addr_t address_tmp = inet_addr(optargument);
+
+                if(!strcmp(optargument, "auto")) {
+                    address_tmp = INADDR_ANY;
+                    conf->preferred_sock_auto = 1;
+                } else {
+                    address_tmp = inet_addr(optargument);
+                }
 
                 memcpy(&(conf->preferred_sock.addr.v4), &(address_tmp), IPV4_SIZE);
 
@@ -1098,7 +1107,6 @@ int main (int argc, char* argv[]) {
     eee->last_sup = 0; /* if it wasn't zero yet */
     eee->curr_sn = eee->conf.supernodes;
     supernode_connect(eee);
-
     while(runlevel < 5) {
 
         now = time(NULL);

src/edge_utils.c

@@ -203,8 +203,10 @@ void reset_sup_attempts (n2n_edge_t *eee) {
 int supernode_connect(n2n_edge_t *eee) {
 
     int sockopt;
-    struct sockaddr_in sock;
+    struct sockaddr_in sn_sock;
-    int sock_len = sizeof(sock);
+    struct sockaddr_in local_sock;
+    int sock_len = sizeof(local_sock);
+    SOCKET probe_sock;
 
     if((eee->conf.connect_tcp) && (eee->sock >= 0)) {
         closesocket(eee->sock);
@@ -227,18 +229,7 @@ int supernode_connect(n2n_edge_t *eee) {
             return -1;
         }
 
-        // detetct local port, even/especially if chosen by OS...
+        fill_sockaddr((struct sockaddr*)&sn_sock, sizeof(sn_sock), &eee->curr_sn->sock);
-        if((getsockname(eee->sock, (struct sockaddr *)&sock, &sock_len) == 0)
-        && (sock.sin_family == AF_INET)
-        && (sock_len == sizeof(sock))) {
-            // ... and write to local preferred socket -- no matter if used or not
-            eee->conf.preferred_sock.port = ntohs(sock.sin_port);
-        }
-
-        // variable 'sock' gets re-used from here on (for sn)
-        sock.sin_family = eee->curr_sn->sock.family;
-        sock.sin_port = htons(eee->curr_sn->sock.port);
-        memcpy(&(sock.sin_addr.s_addr), &(eee->curr_sn->sock.addr.v4), IPV4_SIZE);
 
         // set tcp socket to O_NONBLOCK so connect does not hang
         // requires checking the socket for readiness before sending and receving
@@ -249,7 +240,7 @@ int supernode_connect(n2n_edge_t *eee) {
 #else
             fcntl(eee->sock, F_SETFL, O_NONBLOCK);
 #endif
-            if((connect(eee->sock, (struct sockaddr*)&(sock), sizeof(struct sockaddr)) < 0)
+            if((connect(eee->sock, (struct sockaddr*)&(sn_sock), sizeof(struct sockaddr)) < 0)
                && (errno != EINPROGRESS)) {
                 eee->sock = -1;
                 return -1;
@@ -275,6 +266,28 @@ int supernode_connect(n2n_edge_t *eee) {
             traceEvent(TRACE_INFO, "PMTU discovery %s", (eee->conf.disable_pmtu_discovery) ? "disabled" : "enabled");
 #endif
 
+        // detetct local port even/especially if chosen by OS...
+        if((getsockname(eee->sock, (struct sockaddr *)&local_sock, &sock_len) == 0)
+        && (local_sock.sin_family == AF_INET)
+        && (sock_len == sizeof(local_sock))) {
+            // ... and write to local preferred socket -- no matter if used or not
+            eee->conf.preferred_sock.port = ntohs(local_sock.sin_port);
+            // probe for & overwrite local address only if auto-detection mode
+            if(eee->conf.preferred_sock_auto) {
+                probe_sock = socket(PF_INET, SOCK_DGRAM, 0);
+                // connecting the UDP socket makes getsockname read the local address it uses to connect (to the sn in this case)
+                // we cannot do it with the real (eee->sock) socket because socket does not accept any conenction from elsewhere then,
+                // e.g. from another edge instead of the supernode; hence, we use a temporary socket
+                connect(probe_sock, (struct sockaddr *)&sn_sock, sizeof(sn_sock));
+                if((getsockname(probe_sock, (struct sockaddr *)&local_sock, &sock_len) == 0)
+                && (local_sock.sin_family == AF_INET)
+                && (sock_len == sizeof(local_sock))) {
+                    memcpy(&(eee->conf.preferred_sock.addr.v4), &(local_sock.sin_addr.s_addr), IPV4_SIZE);
+                }
+                close(probe_sock);
+            }
+        }
+
         if(eee->cb.sock_opened)
             eee->cb.sock_opened(eee);
     }