qemu does not send data between udp and virtserialport

Host environment

  • Operating system: VoidLinux
  • OS/kernel version: Linux rootkit 6.14.9_1 #1 SMP PREEMPT_DYNAMIC Fri May 30 23:17:54 UTC 2025 x86_64 GNU/Linux
  • Architecture: x86_64
  • QEMU flavor: qemu-system-x86_64
  • QEMU version: QEMU emulator version 10.0.0
  • QEMU command line: 
    qemu-system-x86_64 -hda debian.qcow2 -enable-kvm -m 8192 -usbdevice tablet -vga virtio -display gtk,gl=on -device virtio-net-pci,netdev=net0 -netdev user,id=net0,hostfwd=tcp::2222-:22 -device virtio-serial -chardev udp,id=wgchar,host=127.0.0.1,port=53861, -device virtserialport,chardev=wgchar,name=wgchar

Emulated/Virtualized environment

  • Operating system: Debian

  • OS/kernel version: Linux debian 6.1.0-37-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.140-1 (2025-05-22) x86_64 GNU/Linux

  • Architecture: x86_64

Description of problem

qemu does not send and/or receive any data when using chardev udp with device virtserialport

Steps to reproduce

  1. Boot a new VM with a configured serial interface:
qemu-system-x86_64 -hda debian.qcow2 -enable-kvm -m 8192 -usbdevice tablet -vga virtio -display gtk,gl=on -device virtio-net-pci,netdev=net0 -netdev user,id=net0,hostfwd=tcp::2222-:22 -device virtio-serial -chardev udp,id=wgchar,host=127.0.0.1,port=53861, -device virtserialport,chardev=wgchar,name=wgchar
  1. Start netcat: gnetcat -u -l -p 53861
  2. Start tcpdump -n lo port 53861
  3. In the VM echo 123 > /dev/virtio-ports/wgchar

Expected:

  • See output 123 in the terminal running netcat
  • see packages flowing in tcpdump.
  • command in the vm terminating

Actual:

No output from tcp and netcat command hangs in the vm.

Additional information

qemu-system-x86_64 --version QEMU emulator version 10.0.0 Copyright (c) 2003-2025 Fabrice Bellard and the QEMU Project developers

Edited by Enno Boland