[PATCH iproute2-next] ip nexthop: support fdb destination port

[PATCH iproute2-next] ip nexthop: support fdb destination port

[Jack Ma]

The kernel gained NHA_FDB_PORT, an optional per-nexthop UDP destination
port for fdb nexthops.  VXLAN uses it to reach several receivers that
share an underlay IP but listen on different ports, load-balancing a
flow across (IP, port) legs of one nexthop group.

Add the matching uapi value and wire it into `ip nexthop`:

  ip nexthop add id 1 via 192.0.2.1 fdb port 4790

The port is parsed on add (sent as NHA_FDB_PORT, __be16, network order)
and printed back on dump/get.

Signed-off-by: Jack Ma <jack4it@gmail.com>
---
Kernel side (net-next), which adds NHA_FDB_PORT and is the reason for
this change:

  [PATCH net-next v3 0/3] net: nexthop: per-nexthop UDP dst port for fdb
  (VXLAN) nexthops
  https://lore.kernel.org/netdev/20260721-b4-vxlan-fdb-port-v3-0-9aa0a543a78a@gmail.com/
---
 include/uapi/linux/nexthop.h |  2 ++
 ip/ipnexthop.c               | 16 +++++++++++++++-
 ip/nh_common.h               |  1 +
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/nexthop.h b/include/uapi/linux/nexthop.h
index afbd064..e1e1a66 100644
--- a/include/uapi/linux/nexthop.h
+++ b/include/uapi/linux/nexthop.h
@@ -83,6 +83,8 @@ enum {
 	/* u32; read-only; whether any driver collects HW stats */
 	NHA_HW_STATS_USED,
 
+	NHA_FDB_PORT,	/* be16; UDP dst port for an fdb nexthop (VXLAN) */
+
 	__NHA_MAX,
 };
 
diff --git a/ip/ipnexthop.c b/ip/ipnexthop.c
index 14b525a..4c33361 100644
--- a/ip/ipnexthop.c
+++ b/ip/ipnexthop.c
@@ -52,7 +52,7 @@ static void usage(void)
 		"SELECTOR := [ id ID ] [ dev DEV ] [ vrf NAME ] [ master DEV ]\n"
 		"            [ groups ] [ fdb ]\n"
 		"BUCKET_SELECTOR := SELECTOR | [ nhid ID ]\n"
-		"NH := { blackhole | [ via ADDRESS ] [ dev DEV ] [ onlink ]\n"
+		"NH := { blackhole | [ via ADDRESS ] [ dev DEV ] [ onlink ] [ fdb [ port PORT ] ]\n"
 		"        [ encap ENCAPTYPE ENCAPHDR ] |\n"
 		"        group GROUP [ fdb ] [ type TYPE [ TYPE_ARGS ] ] }\n"
 		"GROUP := [ <id[,weight]>/<id[,weight]>/... ]\n"
@@ -531,6 +531,8 @@ static int ipnh_parse_nhmsg(FILE *fp, const struct nhmsg *nhm, int len,
 
 	nhe->nh_blackhole = !!tb[NHA_BLACKHOLE];
 	nhe->nh_fdb = !!tb[NHA_FDB];
+	if (tb[NHA_FDB_PORT])
+		nhe->nh_fdb_port = rta_getattr_be16(tb[NHA_FDB_PORT]);
 
 	nhe->nh_family = nhm->nh_family;
 	nhe->nh_protocol = nhm->nh_protocol;
@@ -595,6 +597,10 @@ static void __print_nexthop_entry(FILE *fp, const char *jsobj,
 	if (nhe->nh_fdb)
 		print_null(PRINT_ANY, "fdb", "fdb", NULL);
 
+	if (nhe->nh_fdb_port)
+		print_uint(PRINT_ANY, "port", " port %u",
+			   nhe->nh_fdb_port);
+
 	if ((show_details > 0 || show_stats) && nhe->nh_hw_stats_supported) {
 		open_json_object("hw_stats");
 		print_on_off(PRINT_ANY, "enabled", "hw_stats %s ",
@@ -1112,6 +1118,14 @@ static int ipnh_modify(int cmd, unsigned int flags, int argc, char **argv)
 				req.nhm.nh_family = AF_INET;
 		} else if (!strcmp(*argv, "fdb")) {
 			addattr_l(&req.n, sizeof(req), NHA_FDB, NULL, 0);
+		} else if (!strcmp(*argv, "port")) {
+			__u16 port;
+
+			NEXT_ARG();
+			if (get_u16(&port, *argv, 0))
+				invarg("\"port\" value is invalid\n", *argv);
+			addattr16(&req.n, sizeof(req), NHA_FDB_PORT,
+				  htons(port));
 		} else if (!strcmp(*argv, "onlink")) {
 			nh_flags |= RTNH_F_ONLINK;
 		} else if (!strcmp(*argv, "group")) {
diff --git a/ip/nh_common.h b/ip/nh_common.h
index afbd16b..759c0c4 100644
--- a/ip/nh_common.h
+++ b/ip/nh_common.h
@@ -33,6 +33,7 @@ struct nh_entry {
 
 	bool			nh_blackhole;
 	bool			nh_fdb;
+	__u16			nh_fdb_port;
 
 	bool			nh_hw_stats_supported;
 	bool			nh_hw_stats_enabled;

---
base-commit: 35a237091c24858cac6c6679175bb6a628cbe400
change-id: 20260722-b4-vxlan-fdb-port-iproute2-13165877ba46

Best regards,
--  
Jack Ma <jack4it@gmail.com>

Re: [PATCH iproute2-next] ip nexthop: support fdb destination port

[Ido Schimmel]

On Wed, Jul 22, 2026 at 03:21:33AM +0000, Jack Ma wrote:
> The kernel gained NHA_FDB_PORT, an optional per-nexthop UDP destination
> port for fdb nexthops.  VXLAN uses it to reach several receivers that
> share an underlay IP but listen on different ports, load-balancing a
> flow across (IP, port) legs of one nexthop group.
> 
> Add the matching uapi value and wire it into `ip nexthop`:
> 
>   ip nexthop add id 1 via 192.0.2.1 fdb port 4790
> 
> The port is parsed on add (sent as NHA_FDB_PORT, __be16, network order)
> and printed back on dump/get.

FWIW, I don't post iproute2 changes before the kernel changes are
accepted. I put a link in the kernel cover letter to the repo where I
have the iproute2 patches.

> 
> Signed-off-by: Jack Ma <jack4it@gmail.com>
> ---
> Kernel side (net-next), which adds NHA_FDB_PORT and is the reason for
> this change:
> 
>   [PATCH net-next v3 0/3] net: nexthop: per-nexthop UDP dst port for fdb
>   (VXLAN) nexthops
>   https://lore.kernel.org/netdev/20260721-b4-vxlan-fdb-port-v3-0-9aa0a543a78a@gmail.com/
> ---
>  include/uapi/linux/nexthop.h |  2 ++

You need to perform this change in a separate patch. Example:

https://lore.kernel.org/netdev/20260320140847.1730633-2-justin.iurman@6wind.com/

>  ip/ipnexthop.c               | 16 +++++++++++++++-
>  ip/nh_common.h               |  1 +
>  3 files changed, 18 insertions(+), 1 deletion(-)

Missing update to the man page

> 
> diff --git a/include/uapi/linux/nexthop.h b/include/uapi/linux/nexthop.h
> index afbd064..e1e1a66 100644
> --- a/include/uapi/linux/nexthop.h
> +++ b/include/uapi/linux/nexthop.h
> @@ -83,6 +83,8 @@ enum {
>  	/* u32; read-only; whether any driver collects HW stats */
>  	NHA_HW_STATS_USED,
>  
> +	NHA_FDB_PORT,	/* be16; UDP dst port for an fdb nexthop (VXLAN) */
> +
>  	__NHA_MAX,
>  };
>  
> diff --git a/ip/ipnexthop.c b/ip/ipnexthop.c
> index 14b525a..4c33361 100644
> --- a/ip/ipnexthop.c
> +++ b/ip/ipnexthop.c
> @@ -52,7 +52,7 @@ static void usage(void)
>  		"SELECTOR := [ id ID ] [ dev DEV ] [ vrf NAME ] [ master DEV ]\n"
>  		"            [ groups ] [ fdb ]\n"
>  		"BUCKET_SELECTOR := SELECTOR | [ nhid ID ]\n"
> -		"NH := { blackhole | [ via ADDRESS ] [ dev DEV ] [ onlink ]\n"
> +		"NH := { blackhole | [ via ADDRESS ] [ dev DEV ] [ onlink ] [ fdb [ port PORT ] ]\n"

Assuming we go with "dst_port", you can drop "fdb". Only the kernel will
validate that "dst_port" is provided together with "fdb".

>  		"        [ encap ENCAPTYPE ENCAPHDR ] |\n"
>  		"        group GROUP [ fdb ] [ type TYPE [ TYPE_ARGS ] ] }\n"
>  		"GROUP := [ <id[,weight]>/<id[,weight]>/... ]\n"
> @@ -531,6 +531,8 @@ static int ipnh_parse_nhmsg(FILE *fp, const struct nhmsg *nhm, int len,
>  
>  	nhe->nh_blackhole = !!tb[NHA_BLACKHOLE];
>  	nhe->nh_fdb = !!tb[NHA_FDB];
> +	if (tb[NHA_FDB_PORT])
> +		nhe->nh_fdb_port = rta_getattr_be16(tb[NHA_FDB_PORT]);
>  
>  	nhe->nh_family = nhm->nh_family;
>  	nhe->nh_protocol = nhm->nh_protocol;
> @@ -595,6 +597,10 @@ static void __print_nexthop_entry(FILE *fp, const char *jsobj,
>  	if (nhe->nh_fdb)
>  		print_null(PRINT_ANY, "fdb", "fdb", NULL);
>  
> +	if (nhe->nh_fdb_port)
> +		print_uint(PRINT_ANY, "port", " port %u",

The convention here is "port %u ", not " port %u". I think you added it
like that because there's a missing space in "fdb" above. Fix it while
you are at it.

> +			   nhe->nh_fdb_port);
> +
>  	if ((show_details > 0 || show_stats) && nhe->nh_hw_stats_supported) {
>  		open_json_object("hw_stats");
>  		print_on_off(PRINT_ANY, "enabled", "hw_stats %s ",