Wireless AP auto-selection with `/etc/network/interfaces`

The legacy /etc/network/interfaces contains a mapping section that can be used to automatically select logical interface when we are bringing up an interface. This is useful for mobile users who regularly connect to different wireless access points at different places. (note 1)

I will use my laptop as an example; you can adapt this example to suit your own need. This laptop has two interfaces (eth0 = wired, eth1 = ipw2200 wireless). I am using wired interface whenever available, but it is enabled and disabled manually through ifup and ifdown command. The eth1 interface is brought up automatically, and it can detect the wireless network (which ESSID, etc.) automatically via the mapping section with the help of a custom script (given below).

The stuff below requires a wireless hardware that works with wireless-tools package. Some older wireless cards seem to not use wireless-tools at all, thus they are at disadvantage.

Custom script /etc/network/custom/wireless-select

# The loopback network interface
auto lo
auto eth1

iface lo inet loopback

# The primary network interfaces: eth0 and eth1

iface eth0 inet static
  address 192.168.0.176
  netmask 255.255.255.0
  network 192.168.0.0
  broadcast 192.168.0.255
  gateway 192.168.0.1

mapping eth1
  script /etc/network/custom/wireless-select
  map  eth1-home    AP_MAC      NN:NN:NN:NN:NN:NN
  map  eth1-office  AP_ESSID    Office_ESSID
# Fallback interface: uncomment the following line to enable it
# This must be the last one
  map  eth1-NOT-FOUND AP_NOT_FOUND -

# Default/fallback eth1 interface is my home interface
iface eth1 inet static
  wireless-essid THIS_IS_MY_ESSID
  wireless-key THIS_IS_MY_WEP_KEY
  wireless-keymode open
  address 192.168.0.177
  netmask 255.255.255.0
  gateway 192.168.0.1

# eth1 wireless connection at home: the ESSID is hidden
iface eth1-home inet static
  wireless-essid THIS_IS_MY_ESSID
  wireless-key THIS_IS_MY_WEP_KEY
  wireless-keymode open
  address 192.168.0.177
  netmask 255.255.255.0
  gateway 192.168.0.1

iface eth1-office inet dhcp
  wireless-essid Office_ESSID

iface eth1-NOT-FOUND inet static
  # EXPERIMENTAL [20081204]
  # Use this if we want to fail the eth1 setup when no right AP is found.
  pre-up  /etc/network/custom/wireless-select-not-found
  address 192.168.0.2
  netmask 255.255.255.0
  gateway 192.168.0.1

Custom script /etc/network/custom/wireless-select

#
# Helper script to determine which wireless configuration to use
# For use with debian systems for the "script" subsection in the "mapping" section of
# the /etc/network/interfaces file.
#
# Created: 20080920
# Wirawan Purwanto
#

set -e
export LANG=C
IFACE="$1"
export PATH=/bin:/sbin:/usr/bin:/usr/sbin

AP_SCAN_TEXT=
AP_SCANNED=

# Some debug dump: for diagnosis

scan_ap()
{
  if [ ! "$AP_SCANNED" ]; then
    AP_SCANNED=yes
    # Disable the tee-ing if you don't want debugging
    AP_SCAN_TEXT=$(/sbin/iwlist "$IFACE" scanning | tee /tmp/iwlist-$IFACE-scanning)
  fi
  echo "$AP_SCAN_TEXT"
}

match_ap_mac()
{
  local wanted_mac="$1"
  scan_ap | grep -qiE "^.*Cell *[0-9]+ *- *Address: *$wanted_mac"
}

match_ap_essid()
{
  local wanted_essid="$1"
  scan_ap \
  | awk -v Wanted_ESSID="$wanted_essid"  '
    BEGIN { notfound=1 }
    /^[\t ]*ESSID: *"/ {
      sub(/^[^"]+"/, "");
      sub(/"[^"]*$/, "");
      if ($0 == Wanted_ESSID) {
        notfound=0
        exit
      }
    }
    END { exit(notfound) }
    '
}

WHICH=
while read ETH_SCHEME KIND VALUE JUNKIE; do
  if [ "$WHICH" ]; then continue; fi
  case "$KIND" in
  AP_MAC)
    if match_ap_mac "$VALUE"; then
      WHICH=$ETH_SCHEME
    fi
    ;;
  AP_ESSID)
    if match_ap_essid "$VALUE"; then
      WHICH=$ETH_SCHEME
    fi
    ;;
  AP_NOT_FOUND|AP_DEFAULT)
    # Fallback scheme.
    WHICH=$ETH_SCHEME
    ;;
  esac
done

if [ "$WHICH" ]; then 
  echo "$WHICH"
  exit 0
else
  exit 1
fi

Custom script /etc/network/custom/wireless-select-not-found

#
# Helper script to fail ifup when no known wireless AP is found.
# This is a rather dirty hack, since we employ the combination of a "fake"
# wireless configuration and the pre-up section to accomplish the trick.
#
# Created: 20081204
# Wirawan Purwanto
#

#set
echo "Cannot bring up the network interface $IFACE." >&2
echo "Reason: no appropriate wireless AP found." >&2
echo "Bailing out." >&2
exit 1

You'll need to change the MAC, ESSID, and WEP key. Using WPA/WPA2 is a different beast. Look at man 5 wpa_supplicant.conf as a starting point.

Further references

I use the following references to get to the abovementioned setup:

* Manual pages, e.g. man 5 interfaces

* example files in /usr/share/doc/ifupdown/examples/

Footnotes

1. Some users, like me, want to continue using /etc/network/interfaces rather than the newly packaged NetworkManager. That package was okay, actually, but somehow it is too dependent on GUI for configuration. AFAIK there is no command-line tools to access NetworkManager facility. For machines that is occasionally accessed remotely via ssh, the use of GUI tools is not necessarily the best. I want a minimalist approach with as little dependence as possible. Or perhaps you are just a control freak, like I am. Back in the Feisty days, it seems that GNOME network management utility (network-admin) modifies /etc/network/interfaces to whatever configuration set up there.

2. Please send me feedback if you find this article useful or you have an improvement idea. For reuse: All the scripts above are GPL2+ licensed. Use as you see appropriate.

CategoryNetworking CategoryNetworking