QoS argument not applying specified value to differentiated services field in packets when IPv6 enabled - linux clients

### Discussed in https://github.com/orgs/jamulussoftware/discussions/3621 <div type='discussions-op-text'> ^{Originally posted by **rdica** February 7, 2026} I thought I'd do some testing using the QoS argument. In reading the help output I see: ``` -Q, --qos set the QoS value. Default is 128. Disable with 0 (see the Jamulus website to enable QoS on Windows) ``` The default is applied when IPv6 is not enabled. When specifying a value using `./jamulus -j -6 -Q 128 -i /home/jamulus/.config/Jamulus/sa.ini` the programs outputs: ``` - disable auto Jack connections - IPv6 enabled - selected QoS value: 128 - initialization file name: /home/jamulus/.config/Jamulus/sa.ini - allocated port number: 22134 ``` but tcpdump captures still show differentiated services field as CS0 instead of the expected CS4 that the value of 128 would set. This only happens when enabling IPv6. I ran the above command with `strace -o ~/strace-qos.log -e trace=network ./jamulus -j -6 -Q 128 -i /home/jamulus/.config/Jamulus/sa.ini` This line from strace log seems to indicate a failure with `setsockopt()` trying to apply the QoS value: ``` setsockopt(15, SOL_IPV6, IPV6_TCLASS, "\200", 1) = -1 EINVAL (Invalid argument) ``` After speaking with some IPv6 networking associates it seems the issue may lie in IPV6_TCLASS should take an 'int', not 'char'. I've attached a pcap and strace log to help illustrate the issue. [strace-qos.log](https://github.com/user-attachments/files/25153511/strace-qos.log) [qos.pcap.gz](https://github.com/user-attachments/files/25153532/qos.pcap.gz) tcpdump capture args: `tcpdump -i lo -w qos.pcap -c 1000 -v 'port 22124'` jamulus server connect attempt to `[::1]:22124` </div>