KNX unwanted reply

[krishy2]

PROBLEM DESCRIPTION

A clear and concise description of what the problem is.
An ESP8266 based device (ESP32 not) with enabled KNX communication will answer to a KNX query which wasn't addressed to it with a 0 length telegram to IP address 0.0.0.0, but port 3671 according to wireshark.

REQUESTED INFORMATION

Make sure your have performed every step and checked the applicable boxes before submitting your issue. Thank you!

• [x ] Read the Contributing Guide and Policy and the Code of Conduct
• [x ] Searched the problem in issues
• [x ] Searched the problem in discussions
• [x ] Searched the problem in the docs
• [x ] Searched the problem in the chat
• [x ] Problem is not scripter related, in this case open a discussion and tag gemu2015
• [x ] Device used (e.g., Sonoff Basic): Sonoff D1, YTF IR blaster
• [x ] Tasmota binary firmware version number used: 15.1.0
  • [x ] Pre-compiled
  • Self-compiled
• [x ] Flashing tools used: Chrome
• Provide the output of command: Backlog Template; Module; GPIO 255:

  Configuration output here:
15:07:52.580 RSL: RESULT = {"NAME":"Sonoff D1","GPIO":[1,3200,0,3232,0,0,0,0,0,320,0,0,0,0],"FLAG":0,"BASE":74}
15:07:52.808 RSL: RESULT = {"Module":{"0":"Sonoff D1"}}
15:07:53.063 RSL: RESULT = {"GPIO0":{"None":0},"GPIO1":{"Serial Tx":3200},"GPIO2":{"None":0},"GPIO3":{"Serial Rx":3232},"GPIO4":{"None":0},"GPIO5":{"None":0},"GPIO9":{"None":0},"GPIO10":{"None":0},"GPIO12":{"None":0},"GPIO13":{"Led_i1":320},"GPIO14":{"None":0},"GPIO15":{"None":0},"GPIO16":{"None":0},"GPIO17":{"None":0}}

• If using rules, provide the output of this command: Backlog Rule1; Rule2; Rule3:

  Rules output here:

• [x ] Provide the output of this command: Status 0:

  STATUS 0 output here:
15:08:43.124 RSL: STATUS = {"Status":{"Module":0,"DeviceName":"Ledfal-Dimmer","FriendlyName":["Tasmota"],"Topic":"tasmota_0F13F4","ButtonTopic":"0","Power":"1","PowerLock":"0","PowerOnState":3,"LedState":1,"LedMask":"FFFF","SaveData":1,"SaveState":1,"SwitchTopic":"0","SwitchMode":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"ButtonRetain":0,"SwitchRetain":0,"SensorRetain":0,"PowerRetain":0,"InfoRetain":0,"StateRetain":0,"StatusRetain":0}}
15:08:43.136 RSL: STATUS1 = {"StatusPRM":{"Baudrate":9600,"SerialConfig":"8N1","GroupTopic":"tasmotas","OtaUrl":"http://ota.tasmota.com/tasmota/release/tasmota-knx.bin.gz","RestartReason":"Software/System restart","Uptime":"1T20:17:13","StartupUTC":"2025-12-25T17:51:30","Sleep":50,"CfgHolder":4617,"BootCount":8,"BCResetTime":"2025-12-11T18:39:19","SaveCount":383,"SaveAddress":"FA000"}}
15:08:43.146 RSL: STATUS2 = {"StatusFWR":{"Version":"15.1.0(release-knx)","BuildDateTime":"2025-10-11T13:00:03","Boot":31,"Core":"2_7_8","SDK":"2.2.2-dev(38a443e)","CpuFrequency":80,"Hardware":"ESP8285N08","CR":"359/699"}}
15:08:43.155 RSL: STATUS3 = {"StatusLOG":{"SerialLog":0,"WebLog":2,"MqttLog":0,"SysLog":0,"LogHost":"","LogPort":514,"SSId":["KrisiNET",""],"TelePeriod":300,"Resolution":"558180C0","SetOption":["02008009","2805C80001000600003C5A0A192800000000","00000080","00006000","00004000","00000000"]}}
15:08:43.172 RSL: STATUS4 = {"StatusMEM":{"ProgramSize":652,"Free":348,"Heap":20,"ProgramFlashSize":1024,"FlashSize":1024,"FlashChipId":"144051","FlashFrequency":40,"FlashMode":"DOUT","Features":["0809","9F9AC587","04268011","000000CF","010013C0","8000F181","00004004","00001000","54000020","00000080","00000000"],"Drivers":"1,2,!3,4,!5,!6,7,!8,9,10,11,12,!16,!18,!19,!22,!24,26,!27,!30,!35,37,!45,62,!68","Sensors":"1,2,3,4,5,6","I2CDriver":"7"}}
15:08:43.185 RSL: STATUS5 = {"StatusNET":{"Hostname":"Ledfal-dimmer","IPAddress":"x.x.x.205","Gateway":"x.x.x.1","Subnetmask":"255.255.255.0","DNSServer1":"x.x.x.x","DNSServer2":"0.0.0.0","Mac":"24:A1:60:0F:13:F4","Webserver":2,"HTTP_API":1,"WifiConfig":4,"WifiPower":17.0}}
15:08:43.194 RSL: STATUS6 = {"StatusMQT":{"MqttHost":"","MqttPort":1883,"MqttClientMask":"DVES_%06X","MqttClient":"DVES_0F13F4","MqttUser":"DVES_USER","MqttCount":0,"MqttTLS":0,"MAX_PACKET_SIZE":1200,"KEEPALIVE":30,"SOCKET_TIMEOUT":4}}
15:08:43.206 RSL: STATUS7 = {"StatusTIM":{"UTC":"2025-12-27T14:08:43Z","Local":"2025-12-27T15:08:43","StartDST":"2025-03-30T02:00:00","EndDST":"2025-10-26T03:00:00","Timezone":"+01:00","Sunrise":"08:43","Sunset":"16:59"}}
15:08:43.216 RSL: STATUS10 = {"StatusSNS":{"Time":"2025-12-27T15:08:43"}}
15:08:43.227 RSL: STATUS11 = {"StatusSTS":{"Time":"2025-12-27T15:08:43","Uptime":"1T20:17:13","UptimeSec":159433,"Heap":20,"SleepMode":"Dynamic","Sleep":50,"LoadAvg":19,"MqttCount":0,"POWER":"ON","Dimmer":22,"Fade":"OFF","Speed":40,"LedTable":"ON","Wifi":{"AP":1,"SSId":"KrisiNET","BSSId":"9C:05:D6:0E:34:91","Channel":6,"Mode":"11n","RSSI":84,"Signal":-58,"LinkCount":4,"Downtime":"0T00:00:31"},"Hostname":"Ledfal-dimmer","IPAddress":"x.x.x.205"}}

• [x ] Set weblog to 4 and then, when you experience your issue, provide the output of the Console log:

  Console output here:

TO REPRODUCE

Steps to reproduce the behavior:
Switch another KNX device and capture the packet using wireguard.

EXPECTED BEHAVIOUR

A clear and concise description of what you expected to happen.
No transmission should occur.

SCREENSHOTS

If applicable, add screenshots to help explain your problem.

ADDITIONAL CONTEXT

Add any other context about the problem here.

(Please, remember to close the issue when the problem has been addressed)

[arendst]

A quick scan over the KNX code reveals the difference in UDP driver between ESP32 and ESP8266 likely the cause of your observation.

I don't use KNX but I'll try to set up a test env to see how and if needed fix this.

I need to set up:

• Wireshark on a router env (I have a spare MicroTik which should be able to do this)
• KNX test env. Perhaps you can provide some simple config.

[arendst]

Progress:

[arendst]

I know why it acts "strange" for ESP8266.

When an UDP message is received the code executes a flush which used to be a flush of the input buffers. Some semi-clever noobs decided to change the UDP driver code to flush the output buffers too. This is what you see; the empty output buffer is flushed on the ESP8266. The ESP32 doesn't do this.

For your reference, change line 561 in file /lib/lib_div/esp-knx-ip.cpp to get rid of the empty messages.

#ifndef ESP8266
  udp.flush();
#endif

I'll dive into the ESP8266 driver and see how to circumvent the output buffer flush on a receive.

[arendst]

Fixed: