Traffic Control (tc) in Linux
In this page, basic usage of the Linux networking tool tc is explored. The
primary reason for this article is to create a network configuration that will
re-order packets.
Physical Network Configuration
This setup requires three devices:
- Device from/to which to control traffic
- Device to control the traffic (with at least two network interfaces)
- Device that will communicate with the first device via the control device
Client Router
+---------------+ +---------------+
| 192.168.1.169 | | 192.168.1.226 | Server
| eth0 +------+ eth0 | +---------------+
| | | | | 192.168.1.72 |
+---------------+ | eth1 +------+ eth0 |
| 192.168.1.80 | | |
+---------------+ +---------------+
Operating System
It is recommended that you make use of a simple Linux distribution, at least for the Router as in modern Linux desktop distributions there are many complicating factors, for example NetworkManager, SELinux, systemd, firewalld to name a few. These daemons will interfere with the network configuration and so confusing things may happen if they are running.
I recommend using Alpine Linux (Extended).
Install required tools:
apk add iproute2 iptables tcpdump
Routing Configuration
The network layout above is troublesome since if the netmask were /24 all
hosts would be directly accessible. In routing however the match that is the
closest will be taken, a routing table containing these entries:
1 192.168.1.0/24 dev eth0
2 192.168.1.72/32 dev eth1
3 192.168.1.128/25 via 192.168.1.251
4 default via 192.168.1.1
A packet destined for 192.168.1.72 will match after being anded with the mask
for route 1 (192.168.1.72 & 255.255.255.0 == 192.168.1.0) but will be a
better match for route 2 (192.168.1.72 & 255.255.255.255 == 192.168.1.72),
the best match will be chosen and the packet will be sent on eth1.
Traffic Control Configuration
tc qdisc show dev eth0
# 16 1s :-) (this is the 4 bit ToS field in the IPv4 packet)
tc qdisc add dev eth0 root handle 1: prio priomap 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
tc qdisc add dev eth0 root handle 1: prio priomap 1 2 1 1 2 2 2 2 0 0 0 0 1 1 1 1
# now delay all TRANSMITTED packets by 100ms
tc qdisc add dev eth0 parent 1:2 handle 20: netem delay 100ms
# 0 -> to 1:1
# 1 -> to 1:2
# 2 -> to 1:3
# for the others 50ms and 10ms
tc qdisc add dev eth0 parent 1:1 handle 10: netem delay 50ms
tc qdisc add dev eth0 parent 1:3 handle 30: netem delay 10ms
TODO: more detail in this section
Bridge Mode
tc will also work when interfaces are part of a bridge... can you believe?
TODO: Write about bridge mode.
References
Network Configuration using a Single Host
TODO