Test whether the network connection to the local gateway is up, and if not, reset the network interface, and if necessary also avahi

network-watch.sh

#!/bin/bash

# Tests whether the network connection to the local router is up. If not,
# resets the network interface that was supposed to be connected. After
# that tests whether multicast DNS resolution of the localdomain hostname
# is still working, and if not, resets the avahi daemon as well.
#
# Hilmar Lapp <hlapp@drycafe.net>
#
# To the extent possible under law, I have waived all copyright and related
# or neighboring rights to this work under the CC0 1.0 Universal Public Domai
# Dedication. See https://creativecommons.org/publicdomain/zero/1.0

# log message level
#
# Log to standard output if in a terminal, and to syslog otherwise. Level
# defaults to info.
function log() {
    level=$2
    if [ -z "$level" -a ! -t 1 ] ; then
        level=info
    fi
    if [ -t 1 ] ; then
        if [ -n "$level" ] ; then
            level="${level}: "
        fi
        echo "$level$1"
    else
        logger -p local0.$level -t "network-watch" "$1"
    fi
}

# determine the default gateway router for internet traffic
gw=$(ip route list scope global | cut -d" " -f3)

# if there's no gateway then we're not online
if [ -z "$gw" ] ; then
    online=1
else
    # most routers have an HTTP server, see whether we can connect to the port
    nc -z -w 3 $gw 80  >/dev/null 2>&1
    # success (exit status zero) means we're online
    online=$?
fi

if [ $online -ne 0 ] ; then
    # determine the network interface that was supposed to be connected
    dev=$(ip route list scope global | cut -d" " -f5)

    # if there's no device then there's no point in continuing
    if [ -z "$dev" ] ; then
        log "No network device for global route. Exiting." error
        exit 1
    fi

    log "Failed to reach gateway $gw through interface $dev, resetting it" error
    # cycle the network interface
    ifdown --force $dev
    sleep 15
    ifup $dev

    # Sometimes, mDNS names continue to hang around after cycling the interface,
    # resulting in a false hostname conflict when avahi tries to advertise the
    # zeroconf name again. Therefore, after a minute test whether our IP address
    # resolves correctly through multicast DNS, and if not, restart avahi.
    sleep 60
    addr=$(ip -4 addr show $dev | grep inet | perl -n -e '/inet\s([0-9.]+)/ && print $1;')
    mname=$(dig +short +time=2 -x $addr @224.0.0.251 -p 5353)
    hname=$(hostname)
    if [ "$mname" = "${hname}.local." ] ; then
        log "mDNS correctly resolves ${hname}.local"
    else
        log "mDNS fails to correctly resolve ${hname}.local, restarting avahi" error
        service avahi-daemon stop
        sleep 30
        service avahi-daemon start
    fi
fi