Re: [PATCH v2 1/2] fwd_rule: Parse target addresses for forwarding rules

[Stefano Brivio <sbrivio@redhat.com>]

On Thu,  2 Jul 2026 17:32:14 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> Extend the parsing of forwarding rules (-[tu]) to allow the destination
> address on the target side to be specified.  For now just parse them, and
> give an error if we try to create rules with a specified target address.
> We'll implement the actual forwarding logic in another patch.
> 
> 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
> 
> FIXME: Ban for -[TU] for now
> FIXME: Check interaction with splice handling
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> [dwg: Syntax from Stefano's earlier draft, largely rewritten on top of new
>  parsing helpers]
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  fwd_rule.c | 90 +++++++++++++++++++++++++++++++++++++++++++++---------
>  passt.1    | 31 +++++++++++++------
>  2 files changed, 98 insertions(+), 23 deletions(-)
> 
> diff --git a/fwd_rule.c b/fwd_rule.c
> index ef35e1b4..bed29ed9 100644
> --- a/fwd_rule.c
> +++ b/fwd_rule.c
> @@ -378,14 +378,17 @@ int fwd_rule_add(struct fwd_table *fwd, const struct fwd_rule *new)
>   * @first:	First port to forward
>   * @last:	Last port to forward
>   * @exclude:	Bitmap of ports to exclude (may be NULL)
> - * @to:		Port to translate @first to when forwarding
> + * @tgt_addr:	Destination address on the target side
> + * @tgt_first:	Destination port to use for @first on the target side
>   * @flags:	Flags for forwarding entries
>   */
>  static void fwd_rule_range_except(struct fwd_table *fwd, bool del,
>  				  uint8_t proto, const union inany_addr *addr,
>  				  const char *ifname,
>  				  uint16_t first, uint16_t last,
> -				  const uint8_t *exclude, uint16_t to,
> +				  const uint8_t *exclude,
> +				  const union inany_addr *tgt_addr,
> +				  uint16_t tgt_first,
>  				  uint8_t flags)
>  {
>  	struct fwd_rule rule = {
> @@ -394,10 +397,31 @@ static void fwd_rule_range_except(struct fwd_table *fwd, bool del,
>  		.proto = proto,
>  		.flags = flags,
>  	};
> +	unsigned delta = tgt_first - first;
>  	char rulestr[FWD_RULE_STRLEN];
> -	unsigned delta = to - first;
>  	unsigned base, i;
>  
> +	if (tgt_addr && !inany_is_unspecified(tgt_addr)) {
> +		char astr[INANY_ADDRSTRLEN];
> +
> +		if (!inany_is_unicast(tgt_addr)) {
> +			die("Target address %s is not unicast",
> +			    inany_ntop(tgt_addr, astr, sizeof(astr)));
> +		}

Nit: an extra newline here would help readability.

> +		if (!addr || !!inany_v4(addr) != !!inany_v4(tgt_addr)) {

I wonder: if !addr, shouldn't we replace it, for the moment, with
inany_any4 or inany_any6 depending on !!inany_v4(tgt_addr)?

I see that an empty address already works with -4 or -6, and I
understand that, by doing this, we'll have a change in behaviour once
forwarding between IP versions is implemented.

But it will take a while before we get there, and, meanwhile, I guess
almost all users will just want to do stuff like -t 8080:192.0.2.1/80,
just to hit:

  Forwarding between IP versions (* => 192.0.2.1) not implemented

which isn't obvious if you aren't familiar with the implementation. I
can also picture a constant flow of incoming tickets as a result.

The change in behaviour once we implement forwarding between IP
versions, by the way, looks a bit like an extension rather than a real
change that could reasonably cause trouble to anybody.

> +			char bstr[INANY_ADDRSTRLEN];
> +
> +			die(
> +"Forwarding between IP versions (%s => %s) not implemented",
> +			    inany_ntop(addr, bstr, sizeof(bstr)),
> +			    inany_ntop(tgt_addr, astr, sizeof(astr)));
> +		}

About both validations: shouldn't they live in fwd_rule_add(), where
we already have this kind of stuff? Or at least be duplicated there?

Now, I tried dropping those in pesto (#ifndef PESTO ...) to see what
happens, and I couldn't really spot anything really bad, not with
multicast and not with mixing IPv4 and IPv6:

   14  20.647728      fe80::1 → ::ffff:127.0.0.1 82 TCP 57812 → 5201 [SYN] Seq=0 Win=65535 Len=0 MSS=61440 WS=256 

but still I wonder if we shouldn't make it a bit more robust.

> +
> +		info("Target address: %s",
> +		     inany_ntop(tgt_addr, astr, sizeof(astr)));
> +		die("Target address remapping not yet implemented");
> +	}
> +
>  	if (!addr)
>  		rule.flags |= FWD_DUAL_STACK_ANY;
>  	if (ifname) {
> @@ -458,19 +482,31 @@ enum fwd_port_chunk_kind {
>   * @cursor:	Parsing point (see parse.c)
>   * @kindp:	Updated with kind of chunk we parsed
>   * @lrange:	Updated with listening port range (for INCLUDE & EXCLUDE)
> + * @taddr:	Updated with target address (for INCLUDE & ALL)
>   * @trange:	Updated with target port range (for INCLUDE)
>   */
>  static bool parse_port_chunk(const char **cursor,
>  			     enum fwd_port_chunk_kind *kindp,
>  			     struct port_range *lrange,
> +			     union inany_addr *taddr,
>  			     struct port_range *trange)
>  {
>  	struct port_range lr = { 0 }, tr = { 0 };
> +	union inany_addr taddr_tmp = inany_any6;
>  	enum fwd_port_chunk_kind kind;
>  	const char *p = *cursor;
>  
>  	if (parse_literal(&p, "all")) {
> +		const char *tgtspec = p;
> +
>  		kind = CHUNK_ALL;
> +		if (p = tgtspec,
> +		    parse_literal(&p, ":")		&&
> +		    parse_inany(&p, &taddr_tmp)) {
> +			/* Target address */
> +		} else {
> +			p = tgtspec;
> +		}
>  	} else if (parse_literal(&p, "auto")) {
>  		kind = CHUNK_AUTO;
>  	} else if (parse_literal(&p, "~")) {
> @@ -478,12 +514,29 @@ static bool parse_port_chunk(const char **cursor,
>  		if (!parse_port_range(&p, &lr))
>  			return false;
>  	} else if (parse_port_range(&p, &lr)) {
> -		kind = CHUNK_INCLUDE;
> +		const char *tgtspec = p;
>  
> -		if (parse_literal(&p, ":")) {
> -			if (!parse_port_range(&p, &tr))
> -				return false;
> +		kind = CHUNK_INCLUDE;
> +		if (p = tgtspec,
> +		    parse_literal(&p, ":")		&&
> +		    parse_inany(&p, &taddr_tmp)		&&
> +		    parse_literal(&p, "/")		&&
> +		    parse_port_range(&p, &tr)) {
> +			/* Target address & range */
> +		} else if (p = tgtspec,
> +			   parse_literal(&p, ":")	&&
> +			   parse_inany(&p, &taddr_tmp)) {
> +			/* Target address only */
> +			tr = lr;
> +		} else if (p = tgtspec,
> +			   parse_literal(&p, ":")	&&
> +			   parse_port_range(&p, &tr)) {
> +			/* Target range only */
> +			taddr_tmp = inany_any6;
>  		} else {
> +			p = tgtspec;
> +			/* No target specification */
> +			taddr_tmp = inany_any6;
>  			tr = lr;
>  		}
>  	} else {
> @@ -492,6 +545,8 @@ static bool parse_port_chunk(const char **cursor,
>  
>  	*kindp = kind;
>  	*lrange = lr;
> +	if (taddr)
> +		*taddr = taddr_tmp;
>  	if (trange)
>  		*trange = tr;
>  	*cursor = p;
> @@ -551,6 +606,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  				 const char *spec)
>  {
>  	uint8_t exclude[PORT_BITMAP_SIZE] = { 0 };
> +	union inany_addr all_taddr = inany_any6;
>  	enum fwd_port_chunk_kind kind;
>  	struct port_range lrange;
>  	bool exclude_only = true;
> @@ -561,7 +617,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  	/* Consider excluded ranges and "auto" in the first pass */
>  	p = spec;
>  	do {
> -		if (!parse_port_chunk(&p, &kind, &lrange, NULL))
> +		if (!parse_port_chunk(&p, &kind, &lrange, NULL, NULL))
>  			goto bad;
>  
>  		switch (kind) {
> @@ -586,14 +642,19 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  	p = spec;
>  	do {
>  		struct port_range trange;
> +		union inany_addr taddr;
>  
> -		if (!parse_port_chunk(&p, &kind, &lrange, &trange))
> +		if (!parse_port_chunk(&p, &kind, &lrange, &taddr, &trange))
>  			goto bad;
>  
>  		switch (kind) {
> -		case CHUNK_AUTO:	/* already handled */
> -		case CHUNK_EXCLUDE:	/* already handled */
> -		case CHUNK_ALL:		/* handled later */
> +		case CHUNK_AUTO:
> +		case CHUNK_EXCLUDE:
> +			continue; /* already handled */
> +
> +		case CHUNK_ALL:
> +			/* Save the address to use later */
> +			all_taddr = taddr;
>  			continue;
>  
>  		case CHUNK_INCLUDE:
> @@ -604,7 +665,8 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  
>  			fwd_rule_range_except(fwd, del, proto, addr, ifname,
>  					      lrange.first, lrange.last,
> -					      exclude, trange.first, flags);
> +					      exclude, &taddr, trange.first,
> +					      flags);
>  			break;
>  		default:
>  			goto bad;
> @@ -620,7 +682,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
>  
>  		fwd_rule_range_except(fwd, del, proto, addr, ifname,
>  				      1, NUM_PORTS - 1, exclude,
> -				      1, flags | FWD_WEAK);
> +				      &all_taddr, 1, flags | FWD_WEAK);
>  	}
>  	return;
>  bad:
> diff --git a/passt.1 b/passt.1
> index c3722ef9..9ece0e0c 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -449,12 +449,15 @@ interface name (since Linux 5.7) can be specified.
>  
>  \fIports\fR is a comma-separated list of entries which may be any of:
>  .TP
> -\fIfirst\fR[\fB-\fR\fIlast\fR][\fB:\fR\fItofirst\fR[\fB-\fR\fItolast\fR]]
> +\fIfirst\fR[\fB-\fR\fIlast\fR][\fB:\fR[\fItoaddr\fR\fB/\fR]\fItofirst\fR[\fB-\fR\fItolast\fR]]
> +.TP
> +\fIfirst\fR[\fB-\fR\fIlast\fR][\fB:\fR\fItoaddr\fR]
>  Include range. Forward port numbers between \fIfirst\fR and \fIlast\fR
> -(inclusive) to ports between \fItofirst\fR and \fItolast\fR.  If
> -\fItofirst\fR and \fItolast\fR are omitted, assume the same as
> -\fIfirst\fR and \fIlast\fR.  If \fIlast\fR is omitted, assume the same
> -as \fIfirst\fR.
> +(inclusive) to ports between \fItofirst\fR and \fItolast\fR to address
> +\fItoaddr\fR.  If \fItoaddr\fR is omitted, automatically determine the
> +guest or namespace address.  If \fItofirst\fR and \fItolast\fR are
> +omitted, assume the same as \fIfirst\fR and \fIlast\fR.  If \fIlast\fR
> +is omitted, assume the same as \fIfirst\fR.
>  
>  .TP
>  \fB~\fR\fIfirst\fR[\fB-\fR\fIlast\fR]
> @@ -462,11 +465,13 @@ Exclude range.  Don't forward port numbers between \fIfirst\fR and
>  \fIlast\fR.  This takes precedences over include ranges.
>  
>  .TP
> -.BR all
> +.BR all\fR[\fB:\fItoaddr\fR]
>  Forward all unbound, non-ephemeral ports, not covered by exclude
> -ranges above, as permitted by current capabilities.  For low (< 1024)
> -ports, see \fBNOTES\fR. No failures are reported for unavailable
> -ports, unless no ports could be forwarded at all.
> +ranges above, as permitted by current capabilities, to the
> +corresponding ports on address \fItoaddr\fR.  If \fItoaddr\fR is
> +omitted, automatically determine the guest or namespace address.  For
> +low (< 1024) ports, see \fBNOTES\fR. No failures are reported for
> +unavailable ports, unless no ports could be forwarded at all.
>  
>  .TP
>  .BR auto
> @@ -516,6 +521,14 @@ Forward local port 22, bound to 192.0.2.1 and interface eth0, to port 22
>  -t %eth0/22
>  Forward local port 22, bound to any address on interface eth0, to port 22
>  .TP
> +-t 0.0.0.0/5000:192.0.2.5/6000
> +Forward local port 5000, bound to any IPv4 address, to port 6000 on address 192.0.2.5.
> +.TP
> +-t 127.0.0.6/all:192.0.2.6
> +For the local address 127.0.0.6 forward all unbound, non-ephemeral

I think this is a bit difficult to follow without a comma, it should be:

  For the local address 127.0.0.6, forward all unbound, [...]

also for consistency with the existing:

  For the local address 192.0.2.1, forward ports between 20 and 24 [...]

> +ports as permitted by current capabilities to the corresponding port
> +on 192.0.2.6.
> +.TP
>  -t 2000-5000,~3000-3010
>  Forward local ports between 2000 and 5000, except for those between 3000 and
>  3010

the rest and 2/2 look good to me, no further comments.

-- 
Stefano