In order to setup DSCP rules for Quality of Service under linux, the firewall needs to be configured.
These commands are for firewalld (CentOS 7.x) but since the --direct syntax is used, they can easily be adapted to iptables (CentOS 6.x).
The following examples show how this can be done, for port 3306 (the MySQL server connection), this concept can be easily extended to other ports and protocols.
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