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VyOS PLATFORM BLOG

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Using the "policy route" and packet marking for custom QoS matches

There is only that much you can do in a QoS rules to describe the traffic you want it to match. There's DCP, source/destination, and protocol, and that's enough to cover most of the use cases. Most, but not all. Fortunately, they can also match packet marks and that's what enables creating custom matches.

Packet marks are numeric values set by Netfilter rules that are local to the router and can be used as match criteria in other Netfilter rules and many other components of the Linux kernel (ip, tc, and so on).

Suppose you have a few phones in the office and you want to prioritize their VoIP traffic. You could create a QoS match for each of them, but it's quite some config duplication, which will only get worse when you add more phones. If you find a way to group those addresses in one match, wouldn't it be nice? Sadly, there's no such option in QoS. Firewall can use address groups though, so we can make the QoS rule match a packet mark (e.g. 100) and set that mark to traffic from the phones.

# show traffic-policy
priority-queue VoIP {
class 7 {
match SIP {
mark 100
}
queue-type drop-tail
}
default {
queue-type fair-queue
}
}

Now the confusing bit. Where do we set the mark? Around Vyatta 6.5, an unfortunate design decision was made: "firewall modify" was moved under overly narrow and not so obvious "policy route". Sadly we are stuck with it for the time being because it's not so easy to automatically convert the syntax for upgrades. But, its odd name notwithstanding, it still does the job.

Let's create an address group and a "policy route" instance that sets the mark 100:

# show firewall group
address-group Phones {
address 10.4.5.10
address 10.4.5.11
address 10.4.5.12
}
[edit]
# show policy route
route VoIP {
rule 10 {
set {
mark 100
}
source {
group {
address-group Phones
}
}
}
}

Now we need to assign the QoS ruleset to our WAN and the "policy route" instance to our LAN interface:

set interfaces ethernet eth0 policy route VoIP
set interfaces ethernet eth1 traffic-policy out VoIP

You can as well take advantage of "policy route" ruleset options for time-based filtering or matching related connections. Besides, you can use it to set DSCP values in case your QoS setup is on a different router:

set policy route Foo rule 10 set dscp 46
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