/etc/network/if-up.d/30check-gateway

#!/bin/sh
# Check if the (default) gateway configured for the interface is 
# present in our network
#
# This script should be installed in /etc/network/if-up.d/
# It can also be used as a standalone script by setting up
# its environment:
#    IFACE=eth0 IF_GATEWAY=192.168.0.1 check-gateway
#
# NOTE: If IF_GATEWAY is not provided the script will try to test
# the default gateway.
#
# TODO:
#
#   - Support non-default gateways that might have been set at the 
#     same time the interface was enabled.
#
# ------------------------------------------------------------------------
#
#   This program is free software; you can redistribute it and/or modify
#   it under the terms of the GNU General Public License as published by
#   the Free Software Foundation; either version 2 of the License, or
#   (at your option) any later version.
#
#   This program is distributed in the hope that it will be useful,
#   but WITHOUT ANY WARRANTY; without even the implied warranty of
#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#   GNU General Public License for more details.
#
#   You should have received a copy of the GNU General Public License
#   along with this program; if not, write to the Free Software
#   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
#  
# You can also find a copy of the GNU General Public License at
# http://www.gnu.org/licenses/licenses.html#TOCLGPL
# Check if an IP we are going to assign to an Ethernet interface
# is already in use by another system.
#

# Read system default file
[ -r /etc/default/network-test ] && . /etc/default/network-test

# Defaults
ARPING=/usr/bin/arping
ETHTOOL=/sbin/ethtool
[ ! -x "$ETHTOOL" ] && [ -x "/usr/sbin/ethtool" ] && ETHTOOL=/usr/sbin/ethtool
ARP_COUNT=${ARP_COUNT:-2}
ARP_TIMEOUT=${ARP_TIMEOUT:-3}
DO_SYSLOG=${DO_SYSLOG:-yes}
VERBOSITY=${VERBOSITY:-0}

# Do not continue if ETHTOOL is not available
[ ! -x "$ARPING" ] && exit 0
# or if the user has told us to not do arpings
[ "$DO_ARPING" = "no" ]  && exit 0

# Break out if we don't have an interface to work with
[ -z "$IFACE" ] && exit 0

if [ "$DO_SYSLOG" = "yes" ] ; then
	OUTPUT="logger -i -p daemon.err -s"
else
	OUTPUT="echo"
fi

# Try to obtain the IP address of our gateway (DHCP case)
if [ -z "$IF_GATEWAY" ] ; then
       IF_GATEWAY=$(ip route list | grep "^default " | grep "dev $IFACE" | awk '{print $3}')
        # Warn if there are multiple gateways
        echo $IF_GATEWAY | grep -q " " && [ "$VERBOSITY" -eq 1 ] && $OUTPUT "Found multiple gateways as default routes for $IFACE"
fi
# Still no IP? Bail out
[ -z "$IF_GATEWAY" ] && exit 0


# Set up our environment
LC_ALL=C
export LC_ALL

do_arping() {
# Send ARP pings to detect if the default gateway is "out there"
# Curiously enough, the script will return faster if there *is* a system
# with the same IP address and will take ${ARP_TIMEOUT}*${ARP_COUNT} seconds
# to return if there is none.

# Do not do the check if ethtool (if installed) tells us the interface
# does not have link, notice that ARPING will try to send the ARP requests
# even if there is no link so we use this to speed things up

        local GATEWAY=$1
        local ARPING_OPTIONS="-q -c $ARP_COUNT -w $ARP_TIMEOUT -f -I $IFACE"
        local GATEWAY_FOUND=1

        if [ "`id -u`" = 0 ] ; then
        # Only do this if we are root, otherwise assume the interface is
        # up
            	if [ -x "$ETHTOOL" ] ; then
                    LINK=$($ETHTOOL "$IFACE" 2>&1| grep "Link detected")
                    if ! $ETHTOOL "$IFACE" | grep -q "Link detected: yes" ; then
                        return 0
                    fi
                fi
        fi

	[ "$VERBOSITY" -eq 1 ] && $OUTPUT "DEBUG: Sending arp pings through $IFACE to detect if the gateway $GATEWAY is present"
        if [ "`id -u`" = 0 ] ; then
            if $ARPING $ARPING_OPTIONS $GATEWAY ; then
                GATEWAY_FOUND=0
            fi
        else
        # If we are not root we can only use arping in DAD mode
        # in this case we negate the check as it will return 1
        # if there is an answer
            if ! $ARPING $ARPING_OPTIONS -D  $GATEWAY ; then
                GATEWAY_FOUND=0
            fi
        fi

        if [ "$GATEWAY_FOUND" = 1 ] ; then
                $OUTPUT "ERROR: Cannot find default gateway $GATEWAY in the network where $IFACE is connected to"
	fi
}

# Check our IFACE name, if it does not start with eth, bail out
case "$IFACE" in 
	eth*) for gateway in $IF_GATEWAY ; do do_arping $gateway; done ;;
	*) ;;
esac
exit 0
ifupdown-extra 0.28 / etc / network / if-up.d / 30check-gateway
