[PATCH v2 0/3] Probe host's ephemeral ports, rather than using RFC values

[PATCH v2 0/3] Probe host's ephemeral ports, rather than using RFC values

[David Gibson]

As discussed on our recent call, this implements basing which ports we
consider "ephemeral" on probing the host's settings, rather than just
assuming the RFC 6335 recommended values, which are not what Linux
uses by default.

I think this is more correct, but additionally using the Linux values
means we consider more ports ephemeral, reducing kernel memory
consumption for -t all -u all.

Changes in v2:
 * Add missing close() for the sysctl file

David Gibson (3):
  conf, fwd: Make ephemeral port logic more flexible
  conf, fwd: Don't attempt to forward port 0
  fwd, conf: Probe host's ephemeral ports

 conf.c | 19 +++++++++++----
 fwd.c  | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 fwd.h  |  3 +++
 util.h |  3 ---
 4 files changed, 92 insertions(+), 7 deletions(-)

-- 
2.46.0

[PATCH v2 1/3] conf, fwd: Make ephemeral port logic more flexible

[David Gibson]

"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>
---
 conf.c | 12 ++++++++----
 fwd.c  | 17 +++++++++++++++++
 fwd.h  |  2 ++
 util.h |  3 ---
 4 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/conf.c b/conf.c
index e29b6a92..6b3dafd5 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 2a0452fa..adf61cb5 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 uint16_t fwd_ephemeral_min = (1 << 15) + (1 << 14);
+static const uint16_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(uint16_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 b4aa8d57..42fe57eb 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(uint16_t port);
+
 enum fwd_ports_mode {
 	FWD_UNSET = 0,
 	FWD_SPEC = 1,
diff --git a/util.h b/util.h
index 1463c921..c7a59d5d 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))
 
-- 
@@ -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))
 
-- 
2.46.0

Re: [PATCH v2 1/3] conf, fwd: Make ephemeral port logic more flexible

[Stefano Brivio]

Nit:

On Thu, 29 Aug 2024 11:32:40 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> +/* Empheral port range: values from RFC 6335 */
> +static const uint16_t fwd_ephemeral_min = (1 << 15) + (1 << 14);
> +static const uint16_t fwd_ephemeral_max = NUM_PORTS - 1;

Maybe you could use in_port_t instead, just for consistency with
(almost) everything else, no advantages otherwise.

-- 
Stefano

Re: [PATCH v2 1/3] conf, fwd: Make ephemeral port logic more flexible

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 703 bytes --]

On Thu, Aug 29, 2024 at 06:59:05AM +0200, Stefano Brivio wrote:
> Nit:
> 
> On Thu, 29 Aug 2024 11:32:40 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > +/* Empheral port range: values from RFC 6335 */
> > +static const uint16_t fwd_ephemeral_min = (1 << 15) + (1 << 14);
> > +static const uint16_t fwd_ephemeral_max = NUM_PORTS - 1;
> 
> Maybe you could use in_port_t instead, just for consistency with
> (almost) everything else, no advantages otherwise.

Duh, of course.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

[PATCH v2 2/3] conf, fwd: Don't attempt to forward port 0

[David Gibson]

When using -t all, -u all or exclude-only ranges, we'll attempt to forward
all non-ephemeral port numbers, including port 0.  However, this won't work
as intended: bind() treats a zero port not as literal port 0, but as
"pick a port for me".  Because of the special meaning of port 0, we mostly
outright exclude it in our handling.

Do the same for setting up forwards, not attempting to forward for port 0.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
---
 conf.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/conf.c b/conf.c
index 6b3dafd5..3eb117ff 100644
--- a/conf.c
+++ b/conf.c
@@ -157,7 +157,10 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg,
 
 		fwd->mode = FWD_ALL;
 
-		for (i = 0; i < NUM_PORTS; i++) {
+		/* Skip port 0.  It has special meaning for many socket APIs, so
+		 * trying to bind it is not really safe.
+		 */
+		for (i = 1; i < NUM_PORTS; i++) {
 			if (fwd_port_is_ephemeral(i))
 				continue;
 
@@ -262,7 +265,10 @@ 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 < NUM_PORTS; i++) {
+		/* Skip port 0.  It has special meaning for many socket APIs, so
+		 * trying to bind it is not really safe.
+		 */
+		for (i = 1; i < NUM_PORTS; i++) {
 			if (fwd_port_is_ephemeral(i) ||
 			    bitmap_isset(exclude, i))
 				continue;
-- 
@@ -157,7 +157,10 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg,
 
 		fwd->mode = FWD_ALL;
 
-		for (i = 0; i < NUM_PORTS; i++) {
+		/* Skip port 0.  It has special meaning for many socket APIs, so
+		 * trying to bind it is not really safe.
+		 */
+		for (i = 1; i < NUM_PORTS; i++) {
 			if (fwd_port_is_ephemeral(i))
 				continue;
 
@@ -262,7 +265,10 @@ 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 < NUM_PORTS; i++) {
+		/* Skip port 0.  It has special meaning for many socket APIs, so
+		 * trying to bind it is not really safe.
+		 */
+		for (i = 1; i < NUM_PORTS; i++) {
 			if (fwd_port_is_ephemeral(i) ||
 			    bitmap_isset(exclude, i))
 				continue;
-- 
2.46.0

[PATCH v2 3/3] fwd, conf: Probe host's ephemeral ports

[David Gibson]

When we forward "all" ports (-t all or -u all), or use an exclude-only
range, we don't actually forward *all* ports - that wouln't leave local
ports to use for outgoing connections.  Rather we forward all non-ephemeral
ports - those that won't be used for outgoing connections or datagrams.

Currently we assume the range of ephemeral ports is that recommended by
RFC 6335, 49152-65535.  However, that's not the range used by default on
Linux, 32768-60999 but configurable with the net.ipv4.ip_local_port_range
sysctl.

We can't really know what range the guest will consider ephemeral, but if
it differs too much from the host it's likely to cause problems we can't
avoid anyway.  So, using the host's ephemeral range is a better guess than
using the RFC 6335 range.

Therefore, add logic to probe the host's ephemeral range, falling back to
the RFC 6335 range if that fails.  This has the bonus advantage of
reducing the number of ports bound by -t all -u all on most Linux machines
thereby reducing kernel memory usage.  Specifically this reduces kernel
memory usage with -t all -u all from ~380MiB to ~289MiB.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 conf.c |  1 +
 fwd.c  | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 fwd.h  |  1 +
 3 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/conf.c b/conf.c
index 3eb117ff..b2758864 100644
--- a/conf.c
+++ b/conf.c
@@ -1721,6 +1721,7 @@ void conf(struct ctx *c, int argc, char **argv)
 	/* Inbound port options & DNS can be parsed now (after IPv4/IPv6
 	 * settings)
 	 */
+	fwd_probe_ephemeral();
 	udp_portmap_clear();
 	optind = 0;
 	do {
diff --git a/fwd.c b/fwd.c
index adf61cb5..d9daa582 100644
--- a/fwd.c
+++ b/fwd.c
@@ -28,8 +28,65 @@
 #include "flow_table.h"
 
 /* Empheral port range: values from RFC 6335 */
-static const uint16_t fwd_ephemeral_min = (1 << 15) + (1 << 14);
-static const uint16_t fwd_ephemeral_max = NUM_PORTS - 1;
+static uint16_t fwd_ephemeral_min = (1 << 15) + (1 << 14);
+static uint16_t fwd_ephemeral_max = NUM_PORTS - 1;
+
+#define PORT_RANGE_SYSCTL	"/proc/sys/net/ipv4/ip_local_port_range"
+
+/** fwd_probe_ephemeral() - Determine what ports this host considers ephemeral
+ *
+ * Work out what ports the host thinks are emphemeral and record it for later
+ * use by fwd_port_is_ephemeral().  If we're unable to probe, assume the range
+ * recommended by RFC 6335.
+ */
+void fwd_probe_ephemeral(void)
+{
+	char *line, *tab, *end;
+	struct lineread lr;
+	long min, max;
+	ssize_t len;
+	int fd;
+
+	fd = open(PORT_RANGE_SYSCTL, O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		warn_perror("Unable to open %s", PORT_RANGE_SYSCTL);
+		return;
+	}
+
+	lineread_init(&lr, fd);
+	len = lineread_get(&lr, &line);
+	close(fd);
+
+	if (len < 0)
+		goto parse_err;
+
+	tab = strchr(line, '\t');
+	if (!tab)
+		goto parse_err;
+	*tab = '\0';
+
+	errno = 0;
+	min = strtol(line, &end, 10);
+	if (*end || errno)
+		goto parse_err;
+
+	errno = 0;
+	max = strtol(tab + 1, &end, 10);
+	if (*end || errno)
+		goto parse_err;
+
+	if (min < 0 || min >= NUM_PORTS ||
+	    max < 0 || max >= NUM_PORTS)
+		goto parse_err;
+
+	fwd_ephemeral_min = min;
+	fwd_ephemeral_max = max;
+
+	return;
+
+parse_err:
+	warn("Unable to parse %s", PORT_RANGE_SYSCTL);
+}
 
 /**
  * fwd_port_is_ephemeral() - Is port number ephemeral?
diff --git a/fwd.h b/fwd.h
index 42fe57eb..23aac5b2 100644
--- a/fwd.h
+++ b/fwd.h
@@ -12,6 +12,7 @@ struct flowside;
 /* Number of ports for both TCP and UDP */
 #define	NUM_PORTS	(1U << 16)
 
+void fwd_probe_ephemeral(void);
 bool fwd_port_is_ephemeral(uint16_t port);
 
 enum fwd_ports_mode {
-- 
@@ -12,6 +12,7 @@ struct flowside;
 /* Number of ports for both TCP and UDP */
 #define	NUM_PORTS	(1U << 16)
 
+void fwd_probe_ephemeral(void);
 bool fwd_port_is_ephemeral(uint16_t port);
 
 enum fwd_ports_mode {
-- 
2.46.0