Route priorities - metric values
Brief
It's not an uncommon scenario that a Linux system has several network interfaces that are all up and routeable. For example, consider a laptop with both Ethernet and WiFi.
But how does the system determine which route to use when trying to reach another host?
I was up to setup a system with both a 4G modem and a WiFi connection. My use case was that when the WiFi is available, that interface should be prioritized over 4G. This achieved by adjusting the route metric values for those interfaces.
Metric values
The metric value is one of many fields in the routing table and indicates the cost of the route. This become useful if multiple routes exists to a given destination and the system has to make a decision on which route to use. With that said, the lower metric value (lower cost) a route have, the highter priority i gets.
It's up to you or your network manager to set proper metric values for your routes. The actual value could be determine based on several different factors depending on what is important for your setup. E.g:
- Hop count - The number of routes (hops) in a path to reach a certein network. This is a common metric.
- Delay - Some interfaces have higher delays than others. Compare a 4G modem with a fiber connecton.
- Throughput - The expected throughput of the route.
- Reliability - If some links are more prone på link failures than others, prefer to use other interfaces.
The ip route command will show you all the routes that your system currently have, the last number in the output is the metric value:
1$ ip route
2default via 192.168.20.1 dev enp0s13f0u1u4 proto dhcp src 192.168.20.173 metric 100
3default via 192.168.20.1 dev wlp0s20f3 proto dhcp src 192.168.20.197 metric 600I have two routes that both is routed via 192.168.20.1.
As you can see, my wlp0s20f3 (Wireless) interface has a higher metric value than my enp0s13f0u1u4 (Ethernet) interface, which will cause the system to choose the ethernet interface over WiFi. In my case, these values are chosen by NetworkManager.
Set metric value
If you want to set specific metric values for your routes, the way will differ depending on how your routes are created.
iproute2
The ip command could be handy to manually create or change the metric value for a certain route:
1$ ip route replace default via {IP} dev {DEVICE} metric {METRIC}ifmetric
ifmetric is a tool for setting the metric value for IPv4 routes attached to a given network interface. Compared to the raw ip command above, ifmetric works on interfaces rather than routes.
1$ ifmetric INTERFACE [METRIC]dhcpcd
Metric values could be set in /etc/dhcpcd.conf according to the manual [1]:
metric metric Metrics are used to prefer an interface over another one, lowest wins.
e.g.:
interface wlan0 metric 200
If no metric value is given, the default metric is calculated by 200 + if_nametoindex(3). An extra 100 will be added for wireless interfaces.
NetworkManager
Add ipv4.route-metric METRIC to your /etc/NetworkManager/system-connections/<connection>.nmconnection file.
You could also use the command line tool:
1 $ nmcli connection edit tuxnet
2
3 ===| nmcli interactive connection editor | ===
4
5 Editing existing '802-11-wireless' connection: 'tuxnet'
6
7 Type 'help' or '?' for available commands.
8 Type 'print' to show all the connection properties.
9 Type 'describe [<setting>.<prop>]' for detailed property description.
10
11 You may edit the following settings: connection, 802-11-wireless (wifi), 802-11-wireless-security (wifi-sec), 802-1x, ethtool, match, ipv4, ipv6, tc, proxy
12 nmcli> set ipv4.route-metric 600
13 nmcli> save
14 nmcli> quitPPPD
PPP is a protocol used for establishing internet links over dial-up modems. These links is usually not the preferred link when the device has other more reliable and/or cheaper connections.
The pppd daemon has a few options as specified in the manual [2] for creating a default route and set the metric value:
defaultroute
Add a default route to the system routing tables, using
the peer as the gateway, when IPCP negotiation is
successfully completed. This entry is removed when the
PPP connection is broken. This option is privileged if
the nodefaultroute option has been specified.
defaultroute-metric
Define the metric of the defaultroute and only add it if
there is no other default route with the same metric.
With the default value of -1, the route is only added if
there is no default route at all.
replacedefaultroute
This option is a flag to the defaultroute option. If
defaultroute is set and this flag is also set, pppd
replaces an existing default route with the new default
route. This option is privileged.
E.g.
replacedefaultroute defaultroute-metric 900
Summary
It's not that often you actually have to set the metric value yourself. The network manager usually does a great job.
In my system, the NetworkManager did not manage the PPP interface so its metric-logic did not apply to that interface. Therefor I had to let pppd create a default route with a fixed metric.