nc: make connecting to IPv4 from IPv6-enabled hosts easier

(was requiring -s <local addr>)

include/libbb.h

@@ -304,9 +304,12 @@ enum {
 		}
 	)
 };
-/* Create stream socket, and allocated suitable lsa
+/* Create stream socket, and allocate suitable lsa.
- * (lsa of correct size and lsa->sa.sa_family (AF_INET/AF_INET6)) */
+ * (lsa of correct size and lsa->sa.sa_family (AF_INET/AF_INET6))
-int xsocket_type(len_and_sockaddr **lsap, int sock_type);
+ * af == AF_UNSPEC will result in trying to create IPv6, and
+ * if kernel doesn't support it, IPv4.
+ */
+int xsocket_type(len_and_sockaddr **lsap, USE_FEATURE_IPV6(int af,) int sock_type);
 int xsocket_stream(len_and_sockaddr **lsap);
 /* Create server socket bound to bindaddr:port. bindaddr can be NULL,
  * numeric IP ("N.N.N.N") or numeric IPv6 address,

libbb/xconnect.c

@@ -208,23 +208,31 @@ len_and_sockaddr* xdotted2sockaddr(const char *host, int port)
 	return str2sockaddr(host, port, AF_UNSPEC, AI_NUMERICHOST | DIE_ON_ERROR);
 }
 
-int xsocket_type(len_and_sockaddr **lsap, int sock_type)
+int xsocket_type(len_and_sockaddr **lsap, USE_FEATURE_IPV6(int family,) int sock_type)
 {
+	SKIP_FEATURE_IPV6(enum { family = AF_INET };)
 	len_and_sockaddr *lsa;
 	int fd;
-	int len = sizeof(struct sockaddr_in);
+	int len;
-	int family = AF_INET;
 
 #if ENABLE_FEATURE_IPV6
-	fd = socket(AF_INET6, sock_type, 0);
+	if (family == AF_UNSPEC) {
-	if (fd >= 0) {
+		fd = socket(AF_INET6, sock_type, 0);
-		len = sizeof(struct sockaddr_in6);
+		if (fd >= 0) {
-		family = AF_INET6;
+			family = AF_INET6;
-	} else
+			goto done;
-#endif
+		}
-	{
+		family = AF_INET;
-		fd = xsocket(AF_INET, sock_type, 0);
 	}
+#endif
+	fd = xsocket(family, sock_type, 0);
+	len = sizeof(struct sockaddr_in);
+#if ENABLE_FEATURE_IPV6
+	if (family == AF_INET6) {
+ done:
+		len = sizeof(struct sockaddr_in6);
+	}
+#endif
 	lsa = xzalloc(offsetof(len_and_sockaddr, sa) + len);
 	lsa->len = len;
 	lsa->sa.sa_family = family;
@@ -234,7 +242,7 @@ int xsocket_type(len_and_sockaddr **lsap, int sock_type)
 
 int xsocket_stream(len_and_sockaddr **lsap)
 {
-	return xsocket_type(lsap, SOCK_STREAM);
+	return xsocket_type(lsap, USE_FEATURE_IPV6(AF_UNSPEC,) SOCK_STREAM);
 }
 
 static int create_and_bind_or_die(const char *bindaddr, int port, int sock_type)
@@ -247,7 +255,7 @@ static int create_and_bind_or_die(const char *bindaddr, int port, int sock_type)
 		/* user specified bind addr dictates family */
 		fd = xsocket(lsa->sa.sa_family, sock_type, 0);
 	} else {
-		fd = xsocket_type(&lsa, sock_type);
+		fd = xsocket_type(&lsa, USE_FEATURE_IPV6(AF_UNSPEC,) sock_type);
 		set_nport(lsa, htons(port));
 	}
 	setsockopt_reuseaddr(fd);

networking/nc_bloaty.c

@@ -751,6 +751,13 @@ int nc_main(int argc, char **argv)
 	/* We manage our fd's so that they are never 0,1,2 */
 	/*bb_sanitize_stdio(); - not needed */
 
+	if (argv[0]) {
+		themaddr = xhost2sockaddr(argv[0],
+			argv[1]
+			? bb_lookup_port(argv[1], o_udpmode ? "udp" : "tcp", 0)
+			: 0);
+	}
+
 	/* create & bind network socket */
 	x = (o_udpmode ? SOCK_DGRAM : SOCK_STREAM);
 	if (option_mask32 & OPT_s) { /* local address */
@@ -758,7 +765,11 @@ int nc_main(int argc, char **argv)
 		ouraddr = xhost2sockaddr(str_s, o_lport);
 		x = xsocket(ouraddr->sa.sa_family, x, 0);
 	} else {
-		x = xsocket_type(&ouraddr, x);
+		/* We try IPv6, then IPv4, unless addr family is
+		 * implicitly set by way of remote addr/port spec */
+		x = xsocket_type(&ouraddr,
+				USE_FEATURE_IPV6((themaddr ? themaddr->sa.sa_family : AF_UNSPEC),)
+				x);
 		if (o_lport)
 			set_nport(ouraddr, htons(o_lport));
 	}
@@ -789,14 +800,6 @@ int nc_main(int argc, char **argv)
 		xmove_fd(xopen(str_o, O_WRONLY|O_CREAT|O_TRUNC), ofd);
 #endif
 
-	if (argv[0]) {
-		themaddr = xhost2sockaddr(argv[0],
-			argv[1]
-			? bb_lookup_port(argv[1], o_udpmode ? "udp" : "tcp", 0)
-			: 0);
-///what if sa_family won't match??
-	}
-
 	if (o_listen) {
 		dolisten();
 		/* dolisten does its own connect reporting */