[wifi] capture: test each channel

Try to capture on each channel, in case there is no traffic on the first
channel.

wifi/capture.sh

@@ -32,12 +32,48 @@
 #set -x
 set -e
 
+# Displays the message in argument and exits
 error()
 {
 	echo "$1"
 	exit 1
 }
 
+# Switches the capture interface on the channel in argument
+set_channel()
+{
+	echo -n "Setting interface $IFACE on channel $1... "
+	case $OS in
+		Linux)
+			[ -x /sbin/iwconfig ] \
+				|| error "iwconfig is required to run this program on Linux!"
+			iwconfig $IFACE channel $1
+			;;
+		NetBSD)
+			ifconfig $IFACE chan $1
+			;;
+		*)
+			error "Your OS is not supported."
+			;;
+	esac \
+		&& echo "OK." \
+		|| error "Cannot set the channel!"
+}
+
+# Invokes tcpdump and returns the number of packets captured
+invoke_tcpdump()
+{
+	return $(tcpdump -i $IFACE -c $NB_PKT -w "$FILE" 2>&1 \
+		| sed -n 's/ packets captured$//p')
+}
+
+# Waits for a number of seconds, then kills any tcpdump process
+wait_tcpdump()
+{
+	sleep $1
+	pkill tcpdump
+}
+
 [ $# -eq 1 ] \
 	|| error "Usage: $0 <wifi_interface>"
 
@@ -47,11 +83,13 @@ which tcpdump >/dev/null \
 ## Parameters (tune them if you want) ##
 # Number of packet to capture:
 NB_PKT=10
+# Maximal time of a capture on a given channel:
+TIMEOUT=15
 # Temporary directory:
 TMP=/tmp
 # Name of the capture file:
 FILE=capture.pcap
-# Channel to scan
+# First channel to scan:
 CHANNEL=1
 
 ## Generated parameters ##
@@ -73,10 +111,10 @@ case $OS in
 	Linux)
 		[ -x /sbin/iwconfig ] \
 			|| error "iwconfig is required to run this program on Linux!"
-		iwconfig $IFACE mode monitor channel $CHANNEL
+		iwconfig $IFACE mode monitor
 		;;
 	NetBSD)
-		ifconfig $IFACE media autoselect mediaopt monitor chan $CHANNEL
+		ifconfig $IFACE media autoselect mediaopt monitor
 		;;
 	*)
 		error "Your OS is not supported."
@@ -89,12 +127,24 @@ echo -n "Turning up interface $IFACE... "
 ifconfig $IFACE up && echo "OK." \
 	|| error "Cannot turn the interface up!"
 
-echo "Capturing $NB_PKT packets..."
+echo "Trying to capture $NB_PKT packets..."
 DIR=$(mktemp -d \
 	"$TMP"/capture_${OS}-${OS_RELEASE}_${IFACE}_${HOSTNAME}_${DATE}_XX)
 FILE="$DIR/$FILE"
-tcpdump -i $IFACE -c $NB_PKT -w "$FILE"
-echo "Capture file \"$FILE\" created."
+CAPTURED=0
+while [ $CAPTURED -eq 0 -a $CHANNEL -le 14 ] ; do
+	set_channel $CHANNEL
+	wait_tcpdump $TIMEOUT &
+	invoke_tcpdump
+	CAPTURED=$?
+	[ $CAPTURED -eq 0 ] \
+		&& echo "No packet captured on channel $CHANNEL."
+	CHANNEL=`expr $CHANNEL + 1`
+done
+
+[ $CAPTURED -gt 0 ] \
+	&& echo "Capture file \"$FILE\" created with $CAPTURED packets." \
+	|| error "Failed to capture any packet!"
 
 echo -n "Gathering system information... "
 # Kernel & other information: