Re: [PATCH v3 04/14] conf, fwd: Record "auto" port forwards in forwarding table

[Stefano Brivio <sbrivio@redhat.com>]

On Thu,  8 Jan 2026 13:29:38 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> Currently the forwarding table records details of explicit port forwards,
> but nothing for -[tTuU] auto.  That looks a little odd on the debug output,
> and will be a problem for future changes.
> 
> Extend the forward table to have rules for auto-scanned forwards,
> using a new FWD_SCAN flag.  For now the mechanism of auto port
> forwarding isn't updated, and we will only create a single FWD_SCAN
> rule per protocol and direction.  We'll better integrate auto scanning
> with other forward table mechanics in future.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  conf.c | 31 ++++++++++++++++++++++++++-----
>  fwd.c  | 18 +++++++++++-------
>  fwd.h  |  2 ++
>  3 files changed, 39 insertions(+), 12 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index b486fefe..0bcf80d7 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -135,7 +135,7 @@ static int parse_port_range(const char *s, char **endptr,
>   * @ifname:	Listening interface
>   * @first:	First port to forward
>   * @last:	Last port to forward
> - * @exclude:	Bitmap of ports to exclude
> + * @exclude:	Bitmap of ports to exclude (may be NULL)
>   * @to:		Port to translate @first to when forwarding
>   * @flags:	Flags for forwarding entries
>   */
> @@ -168,11 +168,11 @@ static void conf_ports_range_except(const struct ctx *c, char optname,
>  	}
>  
>  	for (base = first; base <= last; base++) {
> -		if (bitmap_isset(exclude, base))
> +		if (exclude && bitmap_isset(exclude, base))
>  			continue;
>  
>  		for (i = base; i <= last; i++) {
> -			if (bitmap_isset(exclude, i))
> +			if (exclude && bitmap_isset(exclude, i))
>  				break;
>  
>  			if (bitmap_isset(fwd->map, i)) {
> @@ -180,9 +180,9 @@ static void conf_ports_range_except(const struct ctx *c, char optname,
>  "Altering mapping of already mapped port number: %s", optarg);
>  			}
>  
> -			if (optname == 't')
> +			if (!(flags & FWD_SCAN) && optname == 't')
>  				fd = tcp_listen(c, PIF_HOST, addr, ifname, i);
> -			else if (optname == 'u')
> +			else if (!(flags & FWD_SCAN) && optname == 'u')
>  				fd = udp_listen(c, PIF_HOST, addr, ifname, i);
>  			else
>  				/* No way to check in advance for -T and -U */
> @@ -2202,6 +2202,27 @@ void conf(struct ctx *c, int argc, char **argv)
>  	if (!c->udp.fwd_out.mode)
>  		c->udp.fwd_out.mode = fwd_default;
>  
> +	if (c->tcp.fwd_in.mode == FWD_AUTO) {
> +		conf_ports_range_except(c, 't', "auto", &c->tcp.fwd_in,
> +					NULL, NULL, 1, NUM_PORTS - 1,
> +					NULL, 1, FWD_SCAN);
> +	}
> +	if (c->tcp.fwd_out.mode == FWD_AUTO) {
> +		conf_ports_range_except(c, 'T', "auto", &c->tcp.fwd_out,
> +					NULL, "lo", 1, NUM_PORTS - 1,
> +					NULL, 1, FWD_SCAN);
> +	}
> +	if (c->udp.fwd_in.mode == FWD_AUTO) {
> +		conf_ports_range_except(c, 'u', "auto", &c->udp.fwd_in,
> +					NULL, NULL, 1, NUM_PORTS - 1,
> +					NULL, 1, FWD_SCAN);
> +	}
> +	if (c->udp.fwd_out.mode == FWD_AUTO) {
> +		conf_ports_range_except(c, 'U', "auto", &c->udp.fwd_out,
> +					NULL, "lo", 1, NUM_PORTS - 1,
> +					NULL, 1, FWD_SCAN);
> +	}
> +
>  	if (!c->quiet)
>  		conf_print(c);
>  }
> diff --git a/fwd.c b/fwd.c
> index ad398e54..69aca441 100644
> --- a/fwd.c
> +++ b/fwd.c
> @@ -344,7 +344,7 @@ void fwd_rule_add(struct fwd_ports *fwd, uint8_t flags,
>  		  in_port_t first, in_port_t last, in_port_t to)
>  {
>  	/* Flags which can be set from the caller */
> -	const uint8_t allowed_flags = FWD_WEAK;
> +	const uint8_t allowed_flags = FWD_WEAK | FWD_SCAN;
>  	struct fwd_rule *new;
>  	unsigned port;
>  
> @@ -375,7 +375,8 @@ void fwd_rule_add(struct fwd_ports *fwd, uint8_t flags,
>  
>  	for (port = new->first; port <= new->last; port++) {
>  		/* Fill in the legacy data structures to match the table */
> -		bitmap_set(fwd->map, port);
> +		if (!(new->flags & FWD_SCAN))
> +			bitmap_set(fwd->map, port);
>  		fwd->delta[port] = new->to - new->first;
>  	}
>  }
> @@ -391,19 +392,22 @@ void fwd_rules_print(const struct fwd_ports *fwd)
>  	for (i = 0; i < fwd->count; i++) {
>  		const struct fwd_rule *rule = &fwd->rules[i];
>  		const char *weak = rule->flags & FWD_WEAK ? " WEAK" : "";
> +		const char *scan = rule->flags & FWD_SCAN ? " AUTO" : "";
>  		const char *percent = *rule->ifname ? "%" : "";
>  		char addr[INANY_ADDRSTRLEN];
>  
>  		inany_ntop(fwd_rule_addr(rule), addr, sizeof(addr));
>  
>  		if (rule->first == rule->last) {
> -			info("    [%s]%s%s:%hu  =>  %hu %s",
> +			info("    [%s]%s%s:%hu  =>  %hu %s%s",
>  			     addr, percent, rule->ifname,
> -			     rule->first, rule->to, weak);
> +			     rule->first, rule->to, weak, scan);
>  		} else {
> -			info("    [%s]%s%s:%hu-%hu  =>  %hu-%hu %s",
> -			     addr, percent, rule->ifname, rule->first, rule->last,
> -			     rule->to, rule->last - rule->first + rule->to, weak);
> +			info("    [%s]%s%s:%hu-%hu  =>  %hu-%hu %s%s",
> +			     addr, percent, rule->ifname,
> +			     rule->first, rule->last,
> +			     rule->to, rule->last - rule->first + rule->to,
> +			     weak, scan);
>  		}
>  	}
>  }
> diff --git a/fwd.h b/fwd.h
> index 3dfc7959..94869c2a 100644
> --- a/fwd.h
> +++ b/fwd.h
> @@ -26,6 +26,7 @@ bool fwd_port_is_ephemeral(in_port_t port);
>   * @flags:	Flag mask
>   * 	FWD_DUAL_STACK_ANY - match any IPv4 or IPv6 address (@addr should be ::)
>   *	FWD_WEAK - Don't give an error if binds fail for some forwards
> + *	FWD_SCAN - Only forward if we scan a listener on the target

Nit: I guess this could be slightly more descriptive than "scan a
listener", mentioning for example "if the corresponding port is bound
on the target".

>   *
>   * FIXME: @addr and @ifname currently ignored for outbound tables
>   */
> @@ -35,6 +36,7 @@ struct fwd_rule {
>  	in_port_t first, last, to;
>  #define FWD_DUAL_STACK_ANY	BIT(0)
>  #define FWD_WEAK		BIT(1)
> +#define FWD_SCAN		BIT(2)
>  	uint8_t flags;
>  };
>  

-- 
Stefano