If a Ubuntu 11.04 machine is connected to WiFi and 3G simultaneously, how do I set the priority to let the applications (browser etc.) to use WiFi first? If that's not available, it should use the 3G.
Basically, I would like to set the order in which the network connections are used.
Edit: I am looking for an easier approach which would be useful for those who are just comfortable and not experts in Ubuntu/Linux.
I am surprised no one has mentioned the simplest command to do this: ifmetric. It can be installed using sudo apt install ifmetric. This command can be used to change the metric of any interface. The interface with lower metric is preferred for Internet.
To use this, first see the metrics using route command:
$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.42.0.1 0.0.0.0 UG 100 0 0 eth0
0.0.0.0 10.42.0.2 0.0.0.0 UG 600 0 0 wlan0
Here, eth0 has lower metric, so it will be preferred over wlan0. If you want to prefer wlan0, then lower its metric:
sudo ifmetric wlan0 50
Now, the routing table would look like:
$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.42.0.2 0.0.0.0 UG 50 0 0 wlan0
0.0.0.0 10.42.0.1 0.0.0.0 UG 100 0 0 eth0
Now Linux will be using wlan0 for Internet. The change will be reflected immediately.
crontab -e and then add the line at the end: @reboot sudo ifmetric wlan0 50.
Setting the metrics is how you change priorities. The higher metric is more "expensive" to use, so the OS will use the interfaces with the lowest metric if it needs to route traffic. In case the lower metric interface is shutdown it will use the higher metric interface since it is the only interface which can be used to route traffic towards that particular network/destination.
The metrics are specified in the file /etc/network/interfaces, link points to the documentation.
Use any text editor to edit the file, identify the networks, and just change the metric parameter and save. Reboot is the simplest way to reset all the values without getting into the geeky details of restarting the network services.
Prioritising interfaces for general traffic is done by manipulating the routing metrics. Each route has associated parameters such as hop-counts and bandwidths. See the "metric" option in the man-page for route command.
$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.1.254 255.255.255.255 U 0 0 0 eth0
192.168.1.0 0.0.0.0 255.255.255.0 U 256 0 0 eth0
Prioritising application access to network resources is often addressed by "traffic shaping" - I'd use a web search-engine to see if Ubuntu or the router can do that.
Footnote.
On MS Windows, but not on Linux, the netstat -nr command outputs the same information as route print. Including the routing metrics.
This is all because of route metrics. You want to delete the default route with lowest metric and then reinstate the old route with higher metric. Please follow commands below.
Lets say your routing table looks like this:
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.16.87.254 0.0.0.0 UG 100 0 0 ens38
0.0.0.0 192.168.151.2 0.0.0.0 UG 101 0 0 ens33
10.16.86.64 0.0.0.0 255.255.255.248 U 100 0 0 ens38
10.16.87.254 0.0.0.0 255.255.255.255 UH 100 0 0 ens38
10.16.88.6 10.16.87.254 255.255.255.255 UGH 100 0 0 ens38
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 ens38
192.168.151.0 0.0.0.0 255.255.255.0 U 100 0 0 ens33
Now delete the default gateway
# route delete default gateway
Now reinstate the older default gateway (note that metric in this case is higher 102 than current default route 101)
# route add default gw 10.16.87.254 metric 102 dev ens38
# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.151.2 0.0.0.0 UG 101 0 0 ens33
0.0.0.0 10.16.87.254 0.0.0.0 UG 102 0 0 ens38
10.16.86.64 0.0.0.0 255.255.255.248 U 100 0 0 ens38
10.16.87.254 0.0.0.0 255.255.255.255 UH 100 0 0 ens38
10.16.88.6 10.16.87.254 255.255.255.255 UGH 100 0 0 ens38
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 ens38
192.168.151.0 0.0.0.0 255.255.255.0 U 100 0 0 ens33
Gateway=10.16.87.254, which is the UG with the lowest Metric , and step 2 puts it back with a higher Metric so it is still available as backup in case the 192.168.151.2 is unavailable.
route del default gw 192.168.7.1 where the second network was 192.168.7.0/24.
[Update] As of Ubuntu 18.04 LTS (server), netplan is the default wrapper for network management. Configuring Netplan is done through a YAML file, by default /etc/netplan/01-netcfg.yaml (more details here).
Routing metric is defined by the "metric" option, which expects a positive integer (100 is the default value generally). Here's the example from the reference page:
network:
version: 2
renderer: networkd
ethernets:
eno1:
addresses:
- 10.0.0.10/24
- 11.0.0.11/24
nameservers:
addresses:
- 8.8.8.8
- 8.8.4.4
routes:
- to: 0.0.0.0/0
via: 10.0.0.1
metric: 100
- to: 0.0.0.0/0
via: 11.0.0.1
metric: 100
The route with the lowest metric (path length) becomes the "preferred" gateway. (Use: sudo netplan try to enable changes)
Note that in a roaming environment (multiple connections, going on and off), you might want to set the optional (boolean) parameter to true (default is false):
network:
version: 2
ethernets:
enred:
dhcp4: yes
dhcp4-overrides:
route-metric: 100
engreen:
dhcp4: yes
dhcp4-overrides:
route-metric: 200
# this is plugged into a test network that is often
# down - don't wait for it to come up during boot.
optional: true
Notice the slightly different syntax for route metric in the case of DHCP connections.
You can also use NetworkManager as a renderer, which I suppose (haven't tested myself yet) would let you see/edit that part of the configuration also via GUI tools.
renderer(scalar)Use the given networking backend for this definition. Currently supported are
networkdandNetworkManager. This property can be specified globally innetworks:, for a device type (in e. g.ethernets:) or for a particular device definition. Default isnetworkd.
(The very last 'big' example on the reference page shows such a hybrid use of both renderers).
See also this question (askubuntu).
I haven't really tried it out, but NCD (Network Configuration Daemon - 1) can be used for this purpose. The site claims to make the network configuration easy. The syntax seems to be simple.
#Wait for some network connection. Prefer eth1 by putting it in front of eth0.
list("NET-eth1", "NET-eth0") pnames;
pnames list is in fact just an argument to the multidepend() statement on the next line - this is the one which implements the priorities. The mechanism behind this is that when eth1 comes up when eth0 is already up, it executed multiprovide("NET-eth1");, because NET-eth1 is in front of NET-eth0, multidepend() will go down and come back up immediately, but exposing variables from the eth1 process instead of eth0.
Commented
Sep 11, 2011 at 13:47
nm-connection-editorEver since the (network) settings were redesigned in GNOME, some functionality cannot be accessed anymore from the new settings interface. To use the old one, which exposes more options, run nm-connection-editor.
Setting connection priority in nm-connection-editor
Credits to: https://askubuntu.com/a/1061981
Here, when both are connected, and the primary one does not have internet, is not using the second connection.
It is possible to use ifmetric but must be auto started.
If wi-fi internet is failing, to set the wi-fi connection without looking for internet, in Ubuntu:
Settings > Wi-Fi > Under "Visible Networks" use the wheel at the connection > IPv4 > Under "Routes" check "Use this connection only for resources on its network".
To test it is right: route -n and wi-fi interface (Iface, here is wlp3s0) will have one less rule (the Destination 0.0.0.0 was not added).
