Debian/PowerBook G4: Network configuration

gistfile1.md

This was originally posted on 2009-08-10 to http://andrewho.co.uk/weblog/debian-powerbook-g4-network-configuration

Debian's network configuration is pretty straightforward (make sure you have the package wireless-tools installed). The first thing to note is that on the PowerBook G4, you'll need to extract the Broadcom firmware. You can do this by installing the b43-fwcutter package (it's in the contrib repository) which automatically extracts the firmware in its post-installation "configuration".

I have three scenarios that I need to account for:

• Wired ethernet port with a fixed IP on my home network
• Wireless on a defined access point (HO1) with a fixed IP on my home network
• Wireless roaming via DHCP

Debian stores all of the pertinent network configuration in /etc/network/interfaces. Wireless networks are usually defined in /etc/wpa_supplicant/wpa_supplicant.conf. I'll explain how these files are laid out before going ahead and showing my configuration. This explanation should cover a decent proportion of use cases, but for more information see interfaces(5), wpa_supplicant.conf(5), wpa_action(8) etc.

interfaces consists of stanzas defining logicial interfaces, starting with the word iface (there are also mapping, auto and allow- stanzas). What happens is that when you call:

% ifup <physical-interface>=<logical-interface>

Debian brings up the physical interface with the configuration defined for the logical interface in interfaces. If you don't specify the logical interface, Debian looks for a logical interface with the same name as the physical interface. In other words, when you issue ifup eth0, Debian thinks of this as ifup eth0=eth0. Thus, for very simple (e.g. single-location) configurations, you don't really need to think about the separation of physical and logical interfaces.

The first line of each stanza has this syntax:

iface <logical-interface> inet <method>

Of course, inet just defines TCP/IP networking; if you want IPv6 or something else then you probably don't need this article to help you with network configuration. <method>, for our sake, can be dhcp, static, or manual. dhcp is self-explanatory; static is for static IP configuration; manual is for completely manual configuration (as in the case of wireless).

So, to give an example, the wired configuration stanza for my home network looks like this:

iface eth0 inet static
    address 192.168.1.31
    netmask 255.255.255.0
    broadcast 192.168.1.0
    gateway 192.168.1.1

Wireless configuration is a bit more complicated, and is handled by the wpasupplicant package. There are two ways to configure a wireless network. The first is to manual describe it in interfaces, such as the following two examples:

iface ap1 inet dhcp
    wpa-ssid ap1
    wpa-psk <hex-key>

iface ap2 inet static
    wpa-ssid ap2
    wpa-key-mgmt NONE
    address 192.168.1.31
    netmask 255.255.255.0
    broadcast 192.168.1.0
    gateway 192.168.1.1

Whilst this is good for known configurations, it doesn't really offer much flexibility in terms of wireless roaming. The second option is to hand off deciding which wireless network to connect to (and associated configuration) to wpa_supplicant.conf. You do this with the following stanza:

iface wlan0 inet manual
    wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf

Then, define the wireless networks in wpa_supplicant.conf. One interesting trick that you can use is to set id_str in wpa_supplicant.conf for each network. This then invokes a logical interface of that name in interfaces, which is useful for IP configuration.

My configuration files

Hopefully this will bring it all together. Here are the two files:

[ /etc/network/interfaces ]

# Loopback
auto lo
iface lo inet loopback

# Wired interface with a static IP
iface eth0 inet static
    address 192.168.1.31
    netmask 255.255.255.0
    broadcast 192.168.1.0
    gateway 192.168.1.1
    up rm /etc/resolv.conf && ln -s /path/to/etc.git/resolv.conf /etc/resolv.conf

# Use wpa_supplicant to determine which wireless network to associate to
iface wlan0 inet manual
    wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf

# Configuration for id_str="wlan0-ho1"
iface wlan0-ho1 inet static
    address 192.168.1.31
    netmask 255.255.255.0
    broadcast 192.168.1.0
    gateway 192.168.1.1
    up rm /etc/resolv.conf && ln -s /path/to/etc.git/resolv.conf /etc/resolv.conf

# All other wireless networks should use DHCP
iface wlan0-dhcp inet dhcp

 

[ /etc/wpa_supplicant/wpa_supplicant.conf ]

# For wpa_cli
ctrl_interface=DIR=/var/run/wpa_supplicant

# HO1 network
network={
    ssid="HO1"
    priority=10
    key_mgmt=NONE
    id_str="wlan0-ho1"
}

# Default: associate with any open network
network={
    key_mgmt=NONE
    id_str="wlan0-dhcp"
}

The eth0 configuration should be pretty self explanatory. I bring it up with ifup eth0 and down with the corresponding command. The wireless interface is a bit more interesting. In all cases, I bring it up with ifup wlan0 and down with wpa_action wlan0 stop. This uses the wlan0 stanza which tells Debian to look at wpa_supplicant.conf. Here, we decide which network to associate to. If HO1 is available, it associates to that, otherwise it associates to any open network. In both cases, id_str causes the invokation of a wlan-* logical interface which results in either a static IP or DHCP assignment.

The final thing to note is those up commands. They just restore my home DNS configuration (the master resolv.conf is stored in a git repository) because DHCP overwrites resolv.conf.