conf, fwd: Make ephemeral port logic more flexible

"Ephemeral" ports are those which the kernel may allocate as local
port numbers for outgoing connections or datagrams.  Because of that,
they're generally not good choices for listening servers to bind to.

Thefore when using -t all, -u all or exclude-only ranges, we map only
non-ephemeral ports.  Our logic for this is a bit rigid though: we
assume the ephemeral ports are always a fixed range at the top of the
port number space.  We also assume PORT_EPHEMERAL_MIN is a multiple of
8, or we won't set the forward bitmap correctly.

Make the logic in conf.c more flexible, using a helper moved into
fwd.[ch], although we don't change which ports we consider ephemeral
(yet).

The new handling is undoubtedly more computationally expensive, but
since it's a once-off operation at start off, I don't think it really
matters.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

4 files changed, 27 insertions, 7 deletions

diff --git a/conf.c b/conf.c
index e29b6a9..6b3dafd 100644
--- a/conf.c
+++ b/conf.c
@@ -156,9 +156,12 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg,
 			die("'all' port forwarding is only allowed for passt");
 
 		fwd->mode = FWD_ALL;
-		memset(fwd->map, 0xff, PORT_EPHEMERAL_MIN / 8);
 
-		for (i = 0; i < PORT_EPHEMERAL_MIN; i++) {
+		for (i = 0; i < NUM_PORTS; i++) {
+			if (fwd_port_is_ephemeral(i))
+				continue;
+
+			bitmap_set(fwd->map, i);
 			if (optname == 't') {
 				ret = tcp_sock_init(c, AF_UNSPEC, NULL, NULL,
 						    i);
@@ -259,8 +262,9 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg,
 	} while ((p = next_chunk(p, ',')));
 
 	if (exclude_only) {
-		for (i = 0; i < PORT_EPHEMERAL_MIN; i++) {
-			if (bitmap_isset(exclude, i))
+		for (i = 0; i < NUM_PORTS; i++) {
+			if (fwd_port_is_ephemeral(i) ||
+			    bitmap_isset(exclude, i))
 				continue;
 
 			bitmap_set(fwd->map, i);
diff --git a/fwd.c b/fwd.c
index 2a0452f..8fa312a 100644
--- a/fwd.c
+++ b/fwd.c
@@ -27,6 +27,23 @@
 #include "lineread.h"
 #include "flow_table.h"
 
+/* Empheral port range: values from RFC 6335 */
+static const in_port_t fwd_ephemeral_min = (1 << 15) + (1 << 14);
+static const in_port_t fwd_ephemeral_max = NUM_PORTS - 1;
+
+/**
+ * fwd_port_is_ephemeral() - Is port number ephemeral?
+ * @port:	Port number
+ *
+ * Return: true if @port is ephemeral, that is may be allocated by the kernel as
+ *         a local port for outgoing connections or datagrams, but should not be
+ *         used for binding services to.
+ */
+bool fwd_port_is_ephemeral(in_port_t port)
+{
+	return (port >= fwd_ephemeral_min) && (port <= fwd_ephemeral_max);
+}
+
 /* See enum in kernel's include/net/tcp_states.h */
 #define UDP_LISTEN	0x07
 #define TCP_LISTEN	0x0a
diff --git a/fwd.h b/fwd.h
index b4aa8d5..99dd66c 100644
--- a/fwd.h
+++ b/fwd.h
@@ -12,6 +12,8 @@ struct flowside;
 /* Number of ports for both TCP and UDP */
 #define	NUM_PORTS	(1U << 16)
 
+bool fwd_port_is_ephemeral(in_port_t port);
+
 enum fwd_ports_mode {
 	FWD_UNSET = 0,
 	FWD_SPEC = 1,
diff --git a/util.h b/util.h
index 1463c92..c7a59d5 100644
--- a/util.h
+++ b/util.h
@@ -95,9 +95,6 @@
 #define FD_PROTO(x, proto)						\
 	(IN_INTERVAL(c->proto.fd_min, c->proto.fd_max, (x)))
 
-#define PORT_EPHEMERAL_MIN	((1 << 15) + (1 << 14))		/* RFC 6335 */
-#define PORT_IS_EPHEMERAL(port) ((port) >= PORT_EPHEMERAL_MIN)
-
 #define MAC_ZERO		((uint8_t [ETH_ALEN]){ 0 })
 #define MAC_IS_ZERO(addr)	(!memcmp((addr), MAC_ZERO, ETH_ALEN))