FirewallD の RichRule や Direct を使用して、細かい設定(TOS 値設定) などを行ってみます。

SSH への通信を “Minimize-Delay” に設定して最優先させる。

root@plum:~# firewall-cmd --direct --permanent --add-rule ipv4 mangle OUTPUT 0 -p tcp -m tcp --dport 22 -j TOS --set-tos Minimize-Delay

root@plum:~# iptables-save 
# Generated by iptables-save v1.6.1 on Tue May 21 14:49:51 2019
*mangle
:PREROUTING ACCEPT [582:53027]
:INPUT ACCEPT [580:52371]
... 中略 ...
:PRE_trusted_log - [0:0]
-A PREROUTING -j PREROUTING_direct
-A PREROUTING -j PREROUTING_ZONES_SOURCE
-A PREROUTING -j PREROUTING_ZONES
-A INPUT -j INPUT_direct
-A FORWARD -j FORWARD_direct
-A OUTPUT -j OUTPUT_direct
-A POSTROUTING -j POSTROUTING_direct
-A OUTPUT_direct -p tcp -m tcp --dport 22 -j TOS --set-tos 0x10/0x3f  <----