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

On Wed, Aug 28, 2024 at 12:22:18PM +0200, Laurent Vivier wrote:
...
  On 28/08/2024 07:56, David Gibson wrote:
  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(a)gibson.dropbear.id.au>
 ---
   conf.c |  1 +
   fwd.c  | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
   fwd.h  |  1 +
   3 files changed, 57 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..40f556e9 100644
 --- a/fwd.c
 +++ b/fwd.c
 @@ -28,8 +28,61 @@
   #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); 
 
 Why O_CLOEXEC? 
AIUI current security best practices recommend using O_CLOEXEC
basically always.  clang-tidy complains if it's not there.

...
  There is no close() in the function, do you rely on it
to close the file descriptor? 
No, just a very dumb oversight.

...
 
  +	if (fd <
0)
 +		warn_perror("Unable to open %s", PORT_RANGE_SYSCTL); 
 
 goto parse_error ? 
No, this is a different error, but there should be a return.  Added.

...
  or if you add the close() in parse_error, we need a
return.
 
  +
 +	lineread_init(&lr, fd);
 +	len = lineread_get(&lr, &line);
 +	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; 
 
 As /proc files are well formated, why don't you use fscanf()?
 Something like:
 
         FILE *f;
 
         f = fopen(PORT_RANGE_SYSCTL, "r");
 	if (f == NULL) {
 		warn("Unable to parse %s", PORT_RANGE_SYSCTL);
 		return;
 	}
         ret = fscanf(f, "%d %d", &min, &max);
         fclose(f);
         if (ret != 2)
                 goto parse_error; 
Hm, maybe.  I never feel like I know exactly what the parse rules for
scanf() are, so I tend to avoid it.  Stefano, any thoughts?

...
  
 Thanks,
 Laurent
  +
 +	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 { 
  
-- 
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