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

[ann0see]

Discussed in https://github.com/orgs/jamulussoftware/discussions/3621

^{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

qos.pcap.gz

tcpdump capture args: tcpdump -i lo -w qos.pcap -c 1000 -v 'port 22124'
jamulus server connect attempt to [::1]:22124

[ann0see]

This was provided for reference:

https://elixir.bootlin.com/linux/v6.18.6/source/net/core/filter.c#L5558