resolv: Add support to local bind while connecting remote nameserver.

This feature is available via following VTY command in apn conf :
"nameserver-bind (A.B.C.D|X:X:X:X) port <1024-65535>"

This feature is useful when your local networking configuration
is based on loopback interfaces (dummy). In that situation
connected interface and service interface are different so you
need to bind to the proper interface for service.

glibc resolver interface (not documented), is providing context
structure in order to set remote nameserver and connection to
use. We are then, creating our nameserver resolv socket directly,
bind to the proper Address & Port, and pass it to glibc as
pre-init.

src/gtp_apn.c

@@ -535,6 +535,37 @@ DEFUN(apn_nameserver,
 	return CMD_SUCCESS;
 }
 
+DEFUN(apn_nameserver_bind,
+      apn_nameserver_bind_cmd,
+      "nameserver-bind (A.B.C.D|X:X:X:X) port <1024-65535>",
+      "Set Global PDN nameserver binding Address\n"
+      "IP Address\n"
+      "IPv4 Address\n"
+      "IPv6 Address\n"
+      "UDP Port\n"
+      "Number\n")
+{
+	gtp_apn_t *apn = vty->index;
+	struct sockaddr_storage *addr = &apn->nameserver_bind;
+	int ret, port;
+
+	if (argc < 2) {
+		vty_out(vty, "%% missing arguments%s", VTY_NEWLINE);
+		return CMD_WARNING;
+	}
+
+	VTY_GET_INTEGER_RANGE("UDP Port", port, argv[1], 1024, 65535);
+
+	ret = inet_stosockaddr(argv[0], port, addr);
+	if (ret < 0) {
+		vty_out(vty, "%% malformed IP address %s%s", argv[0], VTY_NEWLINE);
+		memset(addr, 0, sizeof(struct sockaddr_storage));
+		return CMD_WARNING;
+	}
+
+	return CMD_SUCCESS;
+}
+
 DEFUN(apn_nameserver_timeout,
       apn_nameserver_timeout_cmd,
       "nameserver-timeout INTEGER",
@@ -1250,6 +1281,11 @@ apn_config_write(vty_t *vty)
 			vty_out(vty, " nameserver %s%s"
 				   , inet_sockaddrtos(&apn->nameserver)
 				   , VTY_NEWLINE);
+		if (apn->nameserver_bind.ss_family)
+			vty_out(vty, " nameserver-bind %s port %d%s"
+				   , inet_sockaddrtos(&apn->nameserver_bind)
+				   , ntohs(inet_sockaddrport(&apn->nameserver_bind))
+				   , VTY_NEWLINE);
 		if (apn->nameserver_timeout)
 			vty_out(vty, " nameserver-timeout %d%s"
 				   , apn->nameserver_timeout
@@ -1310,6 +1346,7 @@ gtp_apn_vty_init(void)
 
 	install_default(APN_NODE);
 	install_element(APN_NODE, &apn_nameserver_cmd);
+	install_element(APN_NODE, &apn_nameserver_bind_cmd);
 	install_element(APN_NODE, &apn_nameserver_timeout_cmd);
 	install_element(APN_NODE, &apn_resolv_max_retry_cmd);
 	install_element(APN_NODE, &apn_resolv_cache_update_cmd);

src/gtp_resolv.c

@@ -103,13 +103,88 @@ ns_log_error(const char *dn, int error)
 	}
 }
 
+static int
+ns_bind_connect(gtp_apn_t *apn)
+{
+	struct sockaddr_storage *addr = &apn->nameserver_bind;
+	socklen_t addrlen;
+	int fd, err;
+
+	if (!apn->nameserver_bind.ss_family)
+		return -1;
+
+	/* Create UDP Client socket */
+	fd = socket(addr->ss_family, SOCK_DGRAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
+	fd = (fd < 0) ? fd : if_setsockopt_reuseaddr(fd, 1);
+	fd = (fd < 0) ? fd : if_setsockopt_rcvtimeo(fd, 2000);
+	fd = (fd < 0) ? fd : if_setsockopt_sndtimeo(fd, 2000);
+	if (fd < 0) {
+		log_message(LOG_INFO, "%s(): error creating UDP [%s]:%d socket"
+				    , __FUNCTION__
+				    , inet_sockaddrtos(addr)
+				    , ntohs(inet_sockaddrport(addr)));
+		return -1;
+	}
+
+	/* Bind listening channel */
+	addrlen = (addr->ss_family == AF_INET) ? sizeof(struct sockaddr_in) :
+						 sizeof(struct sockaddr_in6);
+	err = bind(fd, (struct sockaddr *) addr, addrlen);
+	if (err) {
+		log_message(LOG_INFO, "%s(): Error binding to [%s]:%d (%m)"
+				    , __FUNCTION__
+				    , inet_sockaddrtos(addr)
+				    , ntohs(inet_sockaddrport(addr)));
+		close(fd);
+		return -1;
+	}
+
+	err = connect(fd, (struct sockaddr *) &apn->nameserver, addrlen);
+	if (err) {
+		log_message(LOG_INFO, "%s(): Error connecting to [%s]:%d (%m)"
+				    , __FUNCTION__
+				    , inet_sockaddrtos(&apn->nameserver)
+				    , ntohs(inet_sockaddrport(&apn->nameserver)));
+		close(fd);
+		return -1;
+	}
+
+	return fd;
+}
+
+static int
+ns_ctx_init(gtp_resolv_ctx_t *ctx)
+{
+	gtp_apn_t *apn = ctx->apn;
+	struct sockaddr_storage *addr;
+	int fd;
+
+	fd = ns_bind_connect(apn);
+	if (fd < 0)
+		return -1;
+
+	addr = (apn->nameserver.ss_family) ? &apn->nameserver : &daemon_data->nameserver;
+
+	/* glibc resolver is providing extension to set remote nameserver.
+	 * We are using this facility to set pre-allocated/pre-initialized
+	 * socket connection to remote nameserver. Specially useful when you
+	 * want to bind the connection to a local IP Address. */
+	ctx->ns_rs._u._ext.nssocks[0] = fd;
+	ctx->ns_rs._u._ext.nsaddrs[0] = MALLOC(sizeof(struct sockaddr_in6));
+	*ctx->ns_rs._u._ext.nsaddrs[0] = *((struct sockaddr_in6 *) addr);
+	ctx->ns_rs._u._ext.nscount = 1;
+	return 0;
+}
+
+
 static int
 ns_res_nquery_retry(gtp_resolv_ctx_t *ctx, int class, int type)
 {
 	int retry_count = 0;
 	int ret;
 
 retry:
+	ns_ctx_init(ctx);
 	ret = res_nquery(&ctx->ns_rs, ctx->nsdisp, class, type, ctx->nsbuffer, GTP_RESOLV_BUFFER_LEN);
 	if (ret < 0) {
 		ns_log_error(ctx->nsdisp, h_errno);
@@ -375,6 +450,7 @@ gtp_resolv_naptr(gtp_resolv_ctx_t *ctx, list_head_t *l, const char *format, ...)
 	return 0;
 }
 
+
 gtp_resolv_ctx_t *
 gtp_resolv_ctx_alloc(gtp_apn_t *apn)
 {
@@ -385,6 +461,7 @@ gtp_resolv_ctx_alloc(gtp_apn_t *apn)
 	if (!ctx)
 		return NULL;
 
+	ctx->apn = apn;
 	ctx->max_retry = apn->resolv_max_retry;
 	addr = (apn->nameserver.ss_family) ? &apn->nameserver : &daemon_data->nameserver;
 	if (!addr->ss_family) {

src/gtp_server.c

@@ -90,25 +90,23 @@ gtp_server_udp_init(gtp_server_t *srv)
 				    , __FUNCTION__
 				    , inet_sockaddrtos(addr)
 				    , ntohs(inet_sockaddrport(addr)));
-		goto socket_error;
+		return -1;
 	}
 
 	/* Bind listening channel */
 	addrlen = (addr->ss_family == AF_INET) ? sizeof(struct sockaddr_in) :
 						 sizeof(struct sockaddr_in6);
 	err = bind(fd, (struct sockaddr *) addr, addrlen);
-	if (err < 0) {
+	if (err) {
 		log_message(LOG_INFO, "%s(): Error binding to [%s]:%d (%m)"
 				    , __FUNCTION__
 				    , inet_sockaddrtos(addr)
 				    , ntohs(inet_sockaddrport(addr)));
-		goto socket_error;
+		close(fd);
+		return -1;
 	}
 
 	return fd;
-
-  socket_error:
-	return -1;
 }
 
 static void *

src/include/gtp_apn.h

@@ -84,6 +84,7 @@ typedef struct _gtp_apn {
 	char			name[GTP_APN_MAX_LEN];
 	char			realm[GTP_REALM_LEN];
 	struct sockaddr_storage	nameserver;
+	struct sockaddr_storage	nameserver_bind;
 	int			nameserver_timeout;
 	uint8_t			resolv_max_retry;
 	int			resolv_cache_update;

src/include/gtp_resolv.h

@@ -39,7 +39,7 @@ typedef struct _gtp_pgw {
 	uint16_t		priority;
 	uint16_t		weight;
 	char			srv_name[GTP_DISPLAY_SRV_LEN];
-	struct _gtp_naptr	*naptr;	  /*Back-pointer */
+	struct _gtp_naptr	*naptr;	  /* Back-pointer */
 	struct sockaddr_storage	addr;
 	uint64_t		cnt;
 	time_t			last_resp;
@@ -71,6 +71,7 @@ typedef struct _gtp_service {
 } gtp_service_t;
 
 typedef struct _gtp_resolv_ctx {
+	gtp_apn_t		*apn;	/* Back-pointer */
 	char			*realm;
 	struct __res_state	ns_rs;
 	ns_msg			msg;