A while ago I wrote an OpenBSD guide to fairly share the Internet bandwidth to the LAN network, it was more or less working. Now I switched my router to Linux, I wanted to achieve the same. Unfortunately, it's not really documented as well as on OpenBSD.
The command needed for this job is "tc", acronym for Traffic Control, the Jack of all trades when it comes to manipulate your network traffic. It can add delays or packets lost (this is fun when you want to simulate poor conditions), but also traffic shaping and Quality of Service (QoS).
Fortunately, tc is not that complicated for what we will achieve in this how-to (fair share) and will give results way better than what I achieved with OpenBSD!
I don't want to explain how the whole stack involved works, but with tc we will define a queue on the interface we want to apply the QoS, it will create a number of flows assigned to each active network streams, each active flow will receive 1/total_active_flows shares of bandwidth. It mean if you have three connections downloading data (from the same computer or three different computers), they should in theory receive 1/3 of bandwidth each. In practice, you don't get exactly that, but it's quite close.
I made a script with variables to make it easy to reuse, it deletes any traffic control set on the interfaces and then creates the configuration. You are supposed to run it at boot.
It contains two variables, DOWNLOAD_LIMIT and UPLOAD_LIMIT that should be approximately 95% of each maximum speed, it can be defined in bits with kbits/mbits or in bytes with kbps/mbps, the reason to use 95% is to let the router some room for organizing the packets. It's like a "15 puzzle", you need one empty square to use it.
#!/bin/sh TC=$(which tc) # LAN interface on which you have NAT LAN_IF=br0 # WAN interface which connects to the Internet WAN_IF=eth0 # 95% of maximum download DOWNLOAD_LIMIT=13110kbit # 95% of maximum upload UPLOAD_LIMIT=840kbit $TC qdisc del dev $LAN_IF root $TC qdisc del dev $WAN_IF root $TC qdisc add dev $WAN_IF root handle 1: htb default 1 $TC class add dev $WAN_IF parent 1: classid 1:1 htb rate $UPLOAD_LIMIT $TC qdisc add dev $WAN_IF parent 1:1 fq_codel noecn $TC qdisc add dev $LAN_IF root handle 1: htb default 1 $TC class add dev $LAN_IF parent 1: classid 1:1 htb rate $DOWNLOAD_LIMIT $TC qdisc add dev $LAN_IF parent 1:1 fq_codel
tc is very effective but not really straightfoward to understand. What's cool is you can apply it on the fly without incidence.
It has been really effective for me, now if some device is downloading on the network, it doesn't affect much the other devices when they need to reach the Internet.
After lurking on the Internet looking for documentation about tc, I finally found someone who made a clear explanation about this tool. tc is documented, but it's too abstract for me.