Sending UDP packets when more than one network interface is up

[SamsonBox]

I have a esp32 with two network interfaces. The first one is the WIFI_AP interface an the second is the WIFI_STA interface. When the esp32 is only connected to my local wlan and no client is connected to the AP, then I can send and receive UPD packages succsessfully via the WIFI_AP interface (e.i. I can send a SNTP package to a NTP server and I receive an correct answer). But as soon as a client connects to the AP (and the IP adresses are configured) the UDP package is send only to the WIFI_AP interface. From the logs I can see, that the package is send via the WIFI_AP interface. So I never get an answer from the NTP server, which is not reachable though that interface. So my question is: What do I need to configure, that the UDP package is allways send to the WIFI_STA interface?

[jukkar]

What zephyr version is this?

Zephyr uses normal IP routing, based on destination IP address, to select which network interface to send data.
How is your networks setup, are the interfaces clearly in separate subnets?

[SamsonBox]

I use the Zephyr version v4.1.0-5166-g2c851d998caa. Here is the output from the "net iface" shell command:

uart:~$ net iface
Hostname: xxxxx
Default interface: 1

Interface wlan0 (0x3ffb3588) (WiFi) [1]
===============================
Link addr : 3C:8A:1F:A0:BB:95
MTU       : 1500
Flags     : AUTO_START,IPv4
Device    : wifi_ap (0x3f41f8c8)
Status    : oper=UP, admin=UP, carrier=ON
IPv4 TTL             : 64
IPv4 mcast TTL       : 1
Ethernet capabilities supported:
Ethernet PHY device: <none> (0)
IPv4 unicast addresses (max 1):
        192.168.4.1/255.255.255.0 manual preferred infinite
IPv4 multicast addresses (max 2):
        224.0.0.1
IPv4 gateway : 192.168.4.1
DHCPv4 state      : disabled

Interface wlan1 (0x3ffb36c0) (WiFi) [2]
===============================
Link addr : 3C:8A:1F:A0:BB:94
MTU       : 1500
Flags     : AUTO_START,IPv4
Device    : wifi (0x3f41f8ec)
Status    : oper=UP, admin=UP, carrier=ON
IPv4 TTL             : 64
IPv4 mcast TTL       : 1
Ethernet capabilities supported:
Ethernet PHY device: <none> (0)
IPv4 unicast addresses (max 1):
        10.76.67.149/255.255.255.0 DHCP preferred
IPv4 multicast addresses (max 2):
        224.0.0.1
IPv4 gateway : 10.76.67.2
DHCPv4 lease time : 864000
DHCPv4 renew time : 432000
DHCPv4 server     : 10.76.67.2
DHCPv4 requested  : 10.76.67.149

I think they are in separate subnets. I think the problem is, than I want to connect to server which is asneither in the subnet wlan0 nor in the subnet wlan1. But the server is reachable via wlan1 (via the interface the internet is reachable).
Thank you and BR

[jukkar]

The interfaces are in separate subnets and they are properly configured (netmask and gateway are set). What you would need is a proper IP routing where you could say where a separate subnet, that does not belong to either of those, is located. Currently we do not have that implemented. There is an enhancement request in #44394 created for this and I recently started to look into it but so far I do not have anything that would work.

[SamsonBox]

Yes this enhancement would help.
Is there any quick solution? Would it help to make the wlan1 interface to the default interface?

BR

[jukkar]

You can try to make the wlan1 the default, not sure if that will work, but if it does not work, we could add support for such thing (so that if there are multiple interfaces and no routing, then use default interface is used). Anyway, please try and report if it worked for you.