Check to see if you can ping the Gateway IP address, and if not, reset the interface.

reset_ethernet.sh

#!/bin/bash
# Check to see if you can ping the Gateway IP address, and if not, reset the interface.
# Author:       Nathan Felton
# Date:         2013-07-09
 
if [ $EUID -ne 0 ]; then
        echo "Run as root."
        exit 1
fi
 
GATEWAY=$(netstat -rn | grep "default" | awk '{print $2}')
 
if [[ $GATEWAY =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
        INTERFACE=$(netstat -rn | grep "default" | awk '{print $6}')
        PING=$(ping -c 1 $GATEWAY)
        if [ $? -eq 2 ]; then
                echo "Gateway ping failed. Resetting $INTERFACE"
                ifconfig $INTERFACE down
                ifconfig $INTERFACE up
        fi
else
        echo "Network not active or is misconfigured."
fi