Did you ever have to switch between WiFi and network cable several times a day? If yes, than this blog post is for you.
DISCLAIMER: Check your security policy before trying out solutions from this blog post.
Actually, it does not have to be a WiFi and a network cable. It can be two WiFi connections or two network cable connections. The point is, that you have 2 (or more) network interfaces.
Usually, I have this situation when I need to use intranet resources, which are available for local network and this network has "issues" with connecting to outside world.
By default, Ubuntu allows you to have multiple network connections at the same time. The problem is, that it needs some help on deciding which one to use in certain circumstances.
On this screenshot you can see two active connections:
DISCLAIMER: Check your security policy before trying out solutions from this blog post.
Actually, it does not have to be a WiFi and a network cable. It can be two WiFi connections or two network cable connections. The point is, that you have 2 (or more) network interfaces.
Usually, I have this situation when I need to use intranet resources, which are available for local network and this network has "issues" with connecting to outside world.
By default, Ubuntu allows you to have multiple network connections at the same time. The problem is, that it needs some help on deciding which one to use in certain circumstances.
"Wired connection 1" - default name for wired network connections in Ubuntu. Let's assume that this is a local network that does not have access to internet.
"aleksz-wifi" - wireless connection. Has access to internet resources, like Skype.
How does Ubuntu know which one of those connections should be used to access public web? Simple, there is a routing table. You can see it with "route" command.
That's what I get. I did not configure anything yet. You can assume this routing table to be more or less random.
Probably, you would not be reading this post if you would understand how routing table works, so I will try to explain it a bit. In last column ("Iface"), you see interface name. eth0 is for "Wired connection 1" network and wlan0 is for "aleksz-wifi". First column ("Destination") together with third column ("Genmask") form the range of IP addresses for which this interface should be used. That's actually what we need. By changing routing table, we can say that one range of IP addresses should use one interface, and another range should use the other interface.
In given scenario, we need two ranges. On screenshot, you might have noticed that there is a "default" destination. This means, that we actually have to define rule for one range and all other addresses will be using "default" rule.
Let's check the routing table:
Here you go, default interface is wlan0. eth0 is still used for local connections.
That was easy. However, in real world it is highly possible that your local network structure will not be as plain as in my example. You might need access to resources on other subnets. This requires a bit more complex routing configuration:
How does Ubuntu know which one of those connections should be used to access public web? Simple, there is a routing table. You can see it with "route" command.
The Routing Table
That's what I get. I did not configure anything yet. You can assume this routing table to be more or less random.
Probably, you would not be reading this post if you would understand how routing table works, so I will try to explain it a bit. In last column ("Iface"), you see interface name. eth0 is for "Wired connection 1" network and wlan0 is for "aleksz-wifi". First column ("Destination") together with third column ("Genmask") form the range of IP addresses for which this interface should be used. That's actually what we need. By changing routing table, we can say that one range of IP addresses should use one interface, and another range should use the other interface.
In given scenario, we need two ranges. On screenshot, you might have noticed that there is a "default" destination. This means, that we actually have to define rule for one range and all other addresses will be using "default" rule.
Default interface
Now you should think what interface will be "default". In given scenario, there are two facts that make us choose wlan0 as a default interface:- We are going to use Skype on that interface. As Skype uses P2P network, it will be quite hard to identify destination IP range.
- If routing table hits some unknown IP, then most probably it will be an external resource.
If you pay attention, then you will notice that routing table on screenshot puts eth0 as default interface.
Changing configuration
There are many ways to configure routing table, but we are going to use the easiest one.
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| Open "Edit Connections..." dialog |
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| Open "Wired" tab and click "Add" button. We are going to create a new configuration that can be enabled manually |
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| Enter some "Connection name" and choose "Device MAC address" . Dropdown should contain MAC for eth0 interface. Uncheck "Connect automatically" check box if this is not your primary setting for wired connections. |
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| Next go to "IPv4 Settings" tab (or v6, depending on your network). Click "Routes..." button. |
| In case if you are going to use this connection to access only resources available on local subnet - check "Use this connection only for resources on its network". Save your new configuration. |
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| Now you can choose "my intranet" for wired connection |
Here you go, default interface is wlan0. eth0 is still used for local connections.
That was easy. However, in real world it is highly possible that your local network structure will not be as plain as in my example. You might need access to resources on other subnets. This requires a bit more complex routing configuration:
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| Except for local subnet addresses, this configuration resolves addresses in other subnets through gateway. |
