Re: [PATCH HACK] fwd, fwd_rule: Implement configurable destination address mapping

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, May 07, 2026 at 07:50:36AM +0200, Stefano Brivio wrote:
> This isn't complete, it's rather a quick hack to enable early
> integration testing.
> 
> Add a 'daddr' field to forwarding rules, and some rudimentary parsing.
> 
> Format (for either command line or pesto):
> 
>   -t 2222:192.0.2.1/2222
> 
> This should work along with all the other bits, that is, say:
> 
>   -t 192.0.2.1%eth0/2222-2225:192.0.2.2/22-25
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Very nice for a quick hack, and it gets surprisingly far with not much
code at all.


> ---
>  fwd.c      |  4 +++-
>  fwd_rule.c | 46 +++++++++++++++++++++++++++++++++++-----------
>  fwd_rule.h |  2 ++
>  3 files changed, 40 insertions(+), 12 deletions(-)
> 
> diff --git a/fwd.c b/fwd.c
> index d224c0a..75db350 100644
> --- a/fwd.c
> +++ b/fwd.c
> @@ -1095,7 +1095,9 @@ uint8_t fwd_nat_from_host(const struct ctx *c,
>  	}
>  	tgt->oport = ini->eport;
>  
> -	if (inany_v4(&tgt->oaddr)) {
> +	if (!inany_is_unspecified(&rule->daddr)) {
> +		tgt->eaddr = rule->daddr;

Longer term, there are at least two options for what we want to do
if the rule doesn't specify a specific destination address:
  * Use the observed guest address (what we do now)
  * Use the host side destination address (potentially useful if we
    have multiple containers each assigned different host addresses)

So we'll probably want to allow for some new rule flags to cover
options like this.

> +	} else if (inany_v4(&tgt->oaddr)) {
>  		tgt->eaddr = inany_from_v4(c->ip4.addr_seen);
>  	} else {
>  		if (inany_is_linklocal6(&tgt->oaddr))
> diff --git a/fwd_rule.c b/fwd_rule.c
> index 5fc04d7..5bce2fb 100644
> --- a/fwd_rule.c
> +++ b/fwd_rule.c
> @@ -465,6 +465,7 @@ static int parse_keyword(const char *s, const char **endptr, const char *kw)
>   */
>  static void fwd_rule_range_except(struct fwd_table *fwd, bool del,
>  				  uint8_t proto, const union inany_addr *addr,
> +				  const union inany_addr *daddr,
>  				  const char *ifname,
>  				  uint16_t first, uint16_t last,
>  				  const uint8_t *exclude, uint16_t to,
> @@ -472,6 +473,7 @@ static void fwd_rule_range_except(struct fwd_table *fwd, bool del,
>  {
>  	struct fwd_rule rule = {
>  		.addr = addr ? *addr : inany_any6,
> +		.daddr = daddr ? *daddr : inany_any6,
>  		.ifname = { 0 },
>  		.proto = proto,
>  		.flags = flags,
> @@ -544,13 +546,13 @@ fail:
>   */
>  static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  				 const union inany_addr *addr,
> -				 const char *ifname,
> -				 const char *spec)
> +				 const char *ifname, char *spec)
>  {
> +	union inany_addr daddr_buf = inany_any6, *daddr = &daddr_buf;
>  	uint8_t exclude[PORT_BITMAP_SIZE] = { 0 };
>  	bool exclude_only = true;
> -	const char *p, *ep;
>  	uint8_t flags = 0;
> +	char *p, *ep;
>  	unsigned i;
>  
>  	if (!strcmp(spec, "all")) {
> @@ -568,7 +570,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  			continue;
>  		}
>  
> -		if (parse_keyword(p, &p, "auto") == 0) {
> +		if (parse_keyword(p, (const char **)&p, "auto") == 0) {
>  			if (p != ep) /* Garbage after the keyword */
>  				goto bad;
>  
> @@ -586,7 +588,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  			goto bad;
>  		p++;
>  
> -		if (parse_port_range(p, &p, &xrange))
> +		if (parse_port_range(p, (const char **)&p, &xrange))
>  			goto bad;
>  		if (p != ep) /* Garbage after the range */
>  			goto bad;
> @@ -599,7 +601,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  		/* Exclude ephemeral ports */
>  		fwd_port_map_ephemeral(exclude);
>  
> -		fwd_rule_range_except(fwd, del, proto, addr, ifname,
> +		fwd_rule_range_except(fwd, del, proto, addr, NULL, ifname,
>  				      1, NUM_PORTS - 1, exclude,
>  				      1, flags | FWD_WEAK);
>  		return;
> @@ -613,11 +615,32 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  			/* Already parsed */
>  			continue;
>  
> -		if (parse_port_range(p, &p, &orig_range))
> +		if (parse_port_range(p, (const char **)&p, &orig_range))
>  			goto bad;
>  
> -		if (*p == ':') { /* There's a range to map to as well */
> -			if (parse_port_range(p + 1, &p, &mapped_range))
> +		if (*p == ':') {
> +			/* There's a range or address to map to as well */
> +			char *addr_end = strchr(p, '/');
> +
> +			if (addr_end) {
> +				*addr_end = '\0';
> +
> +				if (*p == '[' && p[strlen(p) - 1] == ']') {
> +					p[strlen(p) - 1] = '\0';
> +					p++;
> +				}
> +
> +				if (!inany_pton(p + 1, daddr))
> +					die("Bad forwarding address '%s'", p);
> +
> +				p = addr_end;
> +			} else {
> +				daddr = NULL;
> +			}

We probably want to factor some of this address parsing out into a
helper, since we're now doing it twice.

> +
> +
> +			if (parse_port_range(p + 1, (const char **)&p,
> +			    &mapped_range))
>  				goto bad;
>  			if ((mapped_range.last - mapped_range.first) !=
>  			    (orig_range.last - orig_range.first))
> @@ -629,7 +652,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  		if (p != ep) /* Garbage after the ranges */
>  			goto bad;
>  
> -		fwd_rule_range_except(fwd, del, proto, addr, ifname,
> +		fwd_rule_range_except(fwd, del, proto, addr, daddr, ifname,
>  				      orig_range.first, orig_range.last,
>  				      exclude,
>  				      mapped_range.first, flags);
> @@ -675,7 +698,8 @@ void fwd_rule_parse(char optname, bool del, const char *optarg,
>  
>  	strncpy(buf, optarg, sizeof(buf) - 1);
>  
> -	if ((spec = strchr(buf, '/'))) {
> +	if ((spec = strchr(buf, '/')) &&
> +	    strchr(spec, ':') == strchr(buf, ':')) {

strchr() for ':' needs a lot of caution, since it can appear within
IPv6 addresses.

>  		*spec = 0;
>  		spec++;
>  
> diff --git a/fwd_rule.h b/fwd_rule.h
> index ae9a3cb..3a2a809 100644
> --- a/fwd_rule.h
> +++ b/fwd_rule.h
> @@ -35,6 +35,7 @@
>  /**
>   * struct fwd_rule - Forwarding rule governing a range of ports
>   * @addr:	Address to forward from
> + * @daddr:	Optional address to set as destination when forwarding
>   * @ifname:	Interface to forward from
>   * @first:	First port number to forward
>   * @last:	Last port number to forward
> @@ -47,6 +48,7 @@
>   */
>  struct fwd_rule {
>  	union inany_addr addr;
> +	union inany_addr daddr;

We probably want to rethink the names here.  Both of these are
destination addresses, just one is host side the other is guest side
(or, in general, they're destination addresses for two different
pifs).  "initiating" vs. "target" are probably the terms to use, since
we already use that in the flow table.

At some point I'm pretty sure we'll also want to put a target pif in
the table.  Not sure if we want to introduce that initially, or
whether just an address makes sense for the first cut.

>  	char ifname[IFNAMSIZ];
>  	in_port_t first;
>  	in_port_t last;
> -- 
> 2.43.0
> 

-- 
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

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