udp_sock_create creates a UDP socket and binds it according to
udp_port_cfg.

Add a freebind option to udp_port_cfg that allows a socket to be bound
as though IP_FREEBIND is set.

This change is required for binding vxlan sockets to their local address
when the outgoing interface is down.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
---
 include/net/udp_tunnel.h   | 3 ++-
 net/ipv4/udp_tunnel_core.c | 1 +
 net/ipv6/ip6_udp_tunnel.c  | 1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/net/udp_tunnel.h b/include/net/udp_tunnel.h
index 9acef2fbd2fd..6c1362aa3576 100644
--- a/include/net/udp_tunnel.h
+++ b/include/net/udp_tunnel.h
@@ -34,7 +34,8 @@ struct udp_port_cfg {
 	unsigned int		use_udp_checksums:1,
 				use_udp6_tx_checksums:1,
 				use_udp6_rx_checksums:1,
-				ipv6_v6only:1;
+				ipv6_v6only:1,
+				freebind:1;
 };
 
 int udp_sock_create4(struct net *net, struct udp_port_cfg *cfg,
diff --git a/net/ipv4/udp_tunnel_core.c b/net/ipv4/udp_tunnel_core.c
index fce945f23069..147fd8ff4f49 100644
--- a/net/ipv4/udp_tunnel_core.c
+++ b/net/ipv4/udp_tunnel_core.c
@@ -28,6 +28,7 @@ int udp_sock_create4(struct net *net, struct udp_port_cfg *cfg,
 	udp_addr.sin_family = AF_INET;
 	udp_addr.sin_addr = cfg->local_ip;
 	udp_addr.sin_port = cfg->local_udp_port;
+	inet_assign_bit(FREEBIND, sock->sk, cfg->freebind);
 	err = kernel_bind(sock, (struct sockaddr *)&udp_addr,
 			  sizeof(udp_addr));
 	if (err < 0)
diff --git a/net/ipv6/ip6_udp_tunnel.c b/net/ipv6/ip6_udp_tunnel.c
index 0ff547a4bff7..65ff44c274b8 100644
--- a/net/ipv6/ip6_udp_tunnel.c
+++ b/net/ipv6/ip6_udp_tunnel.c
@@ -40,6 +40,7 @@ int udp_sock_create6(struct net *net, struct udp_port_cfg *cfg,
 	memcpy(&udp6_addr.sin6_addr, &cfg->local_ip6,
 	       sizeof(udp6_addr.sin6_addr));
 	udp6_addr.sin6_port = cfg->local_udp_port;
+	inet_assign_bit(FREEBIND, sock->sk, cfg->freebind);
 	err = kernel_bind(sock, (struct sockaddr *)&udp6_addr,
 			  sizeof(udp6_addr));
 	if (err < 0)
-- 
2.36.1

On Tue, Aug 12, 2025 at 02:51:51PM +0200, Richard Gobert wrote:
> udp_sock_create creates a UDP socket and binds it according to
> udp_port_cfg.
> 
> Add a freebind option to udp_port_cfg that allows a socket to be bound
> as though IP_FREEBIND is set.
> 
> This change is required for binding vxlan sockets to their local address
> when the outgoing interface is down.

It's not necessarily the outgoing interface, but rather the interface to
which the address is assigned.

Anyway, I'm not sure this change is actually necessary. It was only
added in v4 because back then the default behavior was changed to bind
the VXLAN socket to the local address and existing selftests do not
necessarily configure the address before putting the VXLAN device up.

Given that in this version binding the VXLAN socket to the local address
is opt-in, it seems legitimate to prevent user space from putting the
VXLAN device up if the new option is enabled and the local address is
not present. It can also be documented in the man page so that users are
not surprised.

Ido Schimmel wrote:
> On Tue, Aug 12, 2025 at 02:51:51PM +0200, Richard Gobert wrote:
>> udp_sock_create creates a UDP socket and binds it according to
>> udp_port_cfg.
>>
>> Add a freebind option to udp_port_cfg that allows a socket to be bound
>> as though IP_FREEBIND is set.
>>
>> This change is required for binding vxlan sockets to their local address
>> when the outgoing interface is down.
> 
> It's not necessarily the outgoing interface, but rather the interface to
> which the address is assigned.
> 
> Anyway, I'm not sure this change is actually necessary. It was only
> added in v4 because back then the default behavior was changed to bind
> the VXLAN socket to the local address and existing selftests do not
> necessarily configure the address before putting the VXLAN device up.
> 
> Given that in this version binding the VXLAN socket to the local address
> is opt-in, it seems legitimate to prevent user space from putting the
> VXLAN device up if the new option is enabled and the local address is
> not present. It can also be documented in the man page so that users are
> not surprised.

Sounds good, will change in v6.