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As you take your laptop from one network to the next, from home to work or from the coffee shop to the conference, how do you tell it about each new network?
Modern Linux systems come with nifty GUI programs such as NetworkManager -- but they have some drawbacks. They're sometimes tricky to use if you switch among several different networks, and NetworkManager in particular is rather heavyweight: it doesn't work unless you have a desktop with a panel and are running hald and all its associated daemons, and they can get confused if you have multiple network interfaces. On a laptop with modest hardware, it all seems like overkill. Isn't there a lighter-weight way to manage networks?
This HOWTO covers easy ways of setting up schemes, to make it very easy to move among different types of networks. I started developing this back around 2002 under Redhat 7.3 and 8.0, but the modern version (circa 2009) is based around Ubuntu/Debian systems and hasn't been tested on non-Debian systems.
Some years after I first wrote this, a package called netenv appeared on Debian and Ubuntu which appeared to do exactly what my network schemes had been doing all along (even to using some of the same code). Unfortunately it was missing a few important elements and didn't actually work. So I've stuck to my own scripts.
Here's how my network schemes work:
You always have a net scheme which defines which network you'll
try to connect to. For instance, you start out at home on your
wired network, so you start with the scheme named "home".
Now you carry your laptop to work and want to connect to the wireless network there. You have to tell your laptop to disable the wired network, enable the wireless one, and connect with the appropriate passwords. You do that by typing:
netscheme work
After work, you go to your favorite coffee shop. They have a wireless net too, but it's open, so using the password from work doesn't work there. So you type:
netscheme cafe
And so on. If you go to a new place, you just make a new scheme.
On Debian-based systems (including derivatives like Ubuntu),
networking is handled through a directory called
/etc/network; in particular, there's a file called
/etc/network/interfaces that controls how the machine will
attempt to connect to either wired or wireless networks.
That's where you set all sorts of values like the IP address or
DHCP, the wireless essid and key, and so forth.
What we'll do is create a bunch of interfaces files suitable
for different conditions, and give them appropriate names.
Let's start with the simplest possible interfaces file:
a wired network, on interface eth0, using DHCP.
That looks like this:
Compare that with your own /etc/network/interfaces. Use your
own version for the next step if it's different from this one.
Now set up a schemes directory. I'm assuming that you have a
currently working setup in /etc/network/interfaces and you're going
to call this initial scheme "home".
You're almost done; now all you need is the scheme-switching script.
It's a very simple shell script which you can get here:
One more thing: you have to be root to run the netscheme script.
If you use sudo, you might want to add it in the sudoers file to the
list of programs you're allowed to run without a password.
If you can't remember what scheme you're in,
running
If you finished the steps in the previous section, you're all set
with network schemes. But one network scheme isn't very useful!
You need to make some more schemes so you can really use the system.
For each scheme, you need to create a file called
So how do you figure out what to put in the file?
Obviously, it depends on your setup, but here are some typical
interfaces files:
Obviously, change the network numbers appropriately.
If your wireless is on wlan0 instead of eth1, use that instead.
WPA used to be really complicated, but under modern Debian and
Ubuntu systems it's gotten simple. It looks like this:
For the password you can just use the user-readable password --
you don't have to convert it to a hex code.
If your network card isn't always present on the system, you'll want
to add this line right under the "auto" line (using eth0, eth1, wlan0
or whatever is appropriate for the interface):
I think this applies to USB wi-fi as well, though I've never
actually used one.
There's not much documentation on what allow-hotplug actually
does.
It's important to have only one real network interface (not
including lo) specified in the interface-schemename file,
(if you're deliberately routing from one network interface
to the other, then you probablyknow enough to be able to ignore
that advice safely).
At least on my current system (Ubuntu Hardy), if I simultaneously
leave up both eth0 (the built-in wired networking) and eth1 (the
built-in wireless), then routing will get horribly confused
even if one of the two is completely disconnected.
The only way I've found to fix this is
This also means that if you want a scheme that specifies no
networking at all, just make a scheme containing only the lo entry,
and all other interfaces will be
You may have noticed that all the schemes start with the entry for lo,
the loopback device. I could make the netscheme script put that in
automatically, but it seems easy enough to leave it in each scheme
so I haven't bothered.
Sometimes you go to a cafe or a conference that doesn't have any
complicated key or other information, but it does require you to
specify the essid (the name of the wireless net). netscheme has a
special case for that: if you type
It doesn't currently save that as a new scheme, but you can always
run it again if you return to the same network.
Linux specifies DNS settings in the file /etc/resolv.conf
If you use a static IP address, then you won't get DNS information
from the DHCP server. In that case, you'll probably want to create
a resolv.conf for your setup, and save it as
Occasionally, you may find a network that works okay except that it
doesn't have proper DNS information. (This is fairly common in
wrongly-configured motel wireless networks.)
If you can find some DNS settings that work (hint: sometimes your
home or work settings will work, or sometimes the network number
with a host id of 1 will work), save it as
That's really all you need to know - go forth and network!
The rest of this howto is older
information concerning details that were important to older systems.
For ages I wanted a simple way to set up PPP and ethernet at the
same time, so that on
trips, my husband and I could share a single PPP connection between our
two laptops. We finally have it, and it turns out to be fairly
simple! It doesn't matter whether the second connection is a
network card, wireless, or ad-hoc wireless; they're interchangeable and
all follow the principles outlined in the rest of this document.Initial setup
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
auto eth0
iface eth0 inet dhcp
mkdir /etc/network/schemes
cp /etc/network/interfaces /etc/network/schemes/interfaces-home
netscheme.py
There's also a simpler, older shell script version, for comparison:
netscheme.
Take that script and name it /etc/network/schemes/netscheme and
be sure that it's executable:
chmod 755 /etc/network/schemes/netscheme
netscheme with no scheme name will
tell you.
Writing new schemes
/etc/network/schemes/interfaces-schemename
... don't forget the interfaces- part.
Static IP on eth0
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address 192.168.1.24
netmask 255.255.255.0
broadcast 192.168.1.255
gateway 192.168.1.1
Wireless access on eth1 with an essid and a WEP key
auto lo
iface lo inet loopback
auto eth1
iface eth1 inet dhcp
wireless-essid network name
wireless-key 6BF278D208
Wireless with WPA
auto lo
iface lo inet loopback
auto eth1
iface eth1 inet dhcp
wpa-ssid the-essid
wpa-psk the-password
If your network card is PCMIA, Cardbus or other hot-pluggable design
allow-hotplug wlan0
man interfaces (a fairly good man page, BTW)
implies that ifup will refuse to bring up an interface unless
allow-hotplug is set; but if the network is being started via
/etc/init.d/networking start either way, why do different
things happen depending on whether allow-hotplug is present?
It's all very confusing, but take my word for it, you need
allow-hotplug if you're using PCMCIA or Cardbus (or
probably USB).
Notes on scheme writing
ifconfig eth0 up
ifconfig eth1 down
to bring one interface up and one down. You'd think
ifup and ifdown would do it, based on
their very vague man pages, but in fact they don't do anything
as far as I can tell. So the netscheme script
starts out by marking all interfaces down, then bringing up
any interfaces specified in the file.
ifconfiged
down.
Automatic schemes
netscheme essid
and there's no scheme already named essid, it will ask you
if you want to make a new scheme. If you answer anything but n,
it will change the scheme to the name you specified.
Special DNS for specific schemes
/etc/network/schemes/resolv.conf-schemename.
It will be copied into place every time you switch to that scheme.
resolv.conf-schemename.
That's it!
PPP + Local Net
I'm not going to write up a full howto here, but there are a few
important keys you need to know:
First, on the gateway machine (the one that is going to run the modem
connection), you need a network scheme that uses static IP addresses
(we use the 192.168.1 network) and no
gateway entry. No gateway is very important; if you leave
in your gateway line, then as soon as you activate the network, it will
take over and leave your PPP connection dead.
On the slave machine (the one not running the modem), set the gateway
to be the local IP address (on the 192.168.1 net or whatever local
network you're using) of the machine with the modem, and that should be
all you need to do on that machine.
Finally, on the gateway machine, you need iptables installed and
configured.
Once all these are in place, do these steps:
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