#! /bin/bash
# or /bin/ksh
#**********************************************************************
#
# File:         gwswitch
#
# Description:  Monitors a primary and secondary gateway and
#               switches to the secondary should the primary
#               fail.  This should be run every minute out
#               of cron.
#
# Author:       Wil Cooley <wcooley@nakedape.cc>
#               Naked Ape Consulting <http://nakedape.cc>
#
# Version:      1.0
#
# Summary Changelog:
#   o 1.0 - Initial implementation
#   o 1.1 - Allow for checking of hosts other than the
#           gateways themselves; use 'route' and 'netstat' instead
#           of 'iproute' for portability.  This version
#           should be portable across SysV and BSD systems;
#           if not, please let me know.  Added a "dry-run"
#           option.  Verify that route changing happens
#           correctly, and if not, roll back to the previous
#           gateway.
#
# $Id: gwswitch,v 1.4 2003/03/03 23:43:02 wcooley Exp $
#
# Copyright (C) 2003, Naked Ape Consulting
#
# 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
#
#**********************************************************************

VERSION=1.1
#**********************************************************************
# Configuration Variables
#**********************************************************************
netstat="/bin/netstat"
netstat_opts=" -rn"
logger="/usr/bin/logger"
loglevel="daemon.warning"
myname=$(basename $0)
route="/sbin/route"
fping="/usr/sbin/fping"
fping_opts=" -q -a"

#**********************************************************************
# Shell functions
#**********************************************************************

# Log a message to syslog and stderr
do_log () {
    [ "${debug}" == 0 ] && logopt="-s"
    ${logger} ${logopt} -p ${loglevel} -t ${myname} "$1"
}

# Prints messages on stderr, if debug is set
debug_msg() {
    if [ "${debug}" == 0 ]; then
        printf "$@" 1>&2
    fi
}

# Show usage
usage () {
    printf "Copyright (C) 2003, Naked Ape Consulting\n"
    printf "gwswitch %s - 2 March 2003\n" ${VERSION}
    printf "Usage: %s [-h|[-d] [-n] -p host [-P host]" ${myname}
    printf " -s host [-S host]]\n"
    printf "Options:\n"
    printf "  -h\tHelp\n"
    printf "  -n\tDry run (test, but don't change)\n"
    printf "  -p\tPrimary gateway\n"
    printf "  -P\tPrimary test address\n"
    printf "  -s\tSecondary gateway\n"
    printf "  -S\tSecondary test address\n"
    printf "  -d\tPrint some debugging statements to stderr\n"
    printf "\nThe -P and -S options allow you specify hosts in addition to\n"
    printf "the actual gateways, to cover the case where the gateway\n"
    printf "itself is on the local side of the WAN connection, as may\n"
    printf "be the case with DSL that's not bridged.  Note that the\n"
    printf "gateway itself is still tested first.\n"

    exit 1
}

# Get current default route
# Note that this only works if there is one default.  I'm
# not sure how to handle the case of multiple configured
# default routes.
current_gw() {
    local gw="$(${netstat} ${netstat_opts} |awk '/^0.0.0.0/ {print $2}')"
    
    debug_msg "Current gateway: %s\n" "$gw"
    printf "%s" $gw
}

# This replaces the previous function.  We'll use fping so
# we can ping a bunch of hosts.  Sets
# {pri,sec}_{gw,test}_alive to '0' (true) or '1' (false).
get_alive_hosts() {
    alive_hosts=$(${fping} ${fping_opts} "$@" 2>/dev/null)

    pri_gw_alive=1
    pri_test_alive=1
    sec_gw_alive=1
    sec_test_alive=1

    debug_msg "Tested host: %s\n" "$@"
    debug_msg "Alive host: %s\n" $(echo "${alive_hosts}"|sed -e 's/\n/ /')

    for host in ${alive_hosts}; do
        [ "${host}" = "${pri_gw}" ] && pri_gw_alive=0
        [ "${host}" = "${pri_test}" ] && pri_test_alive=0
        [ "${host}" = "${sec_gw}" ] && sec_gw_alive=0
        [ "${host}" = "${sec_test}" ] && sec_test_alive=0
    done
}

# Set the default route
set_default_gw() {
    local gw=$1
    local old_gw=${curr_gw}
    local route_del_out
    local route_add_out
    local ret

    do_log "Setting default gateway to $gw"

    if [ -z "${old_gw}" ]; then
        do_log "No current gateway."
    else
        # 'route delete' is not documented in the Linux man
        # page, but works and is consistent with BSD and
        # SysV AFAICT
        route_del_out=$(${route} delete default 2>&1)
        if [ $? != 0 ]; then
            do_log "Error deleting route: $?"
            do_log "Error was: ${route_del_out}"
        fi
    fi

    route_add_out=$(${route} add default gw ${gw} 2>&1)
    ret=$?
    if [ ${ret} != 0 ]; then
        ${route} add default gw ${old_gw}
        do_log "Error adding route: $ret"
        do_log "Error was: ${route_add_out}"
        do_log "Restoring old gateway ${old_gw}"
    fi
}

# 
#**********************************************************************
# Main
#**********************************************************************

# Process arguments
while getopts 'p:P:s:S:hdn' Opt; do
    case "$Opt" in
        p)
            typeset -r pri_gw=$OPTARG
            ;;
        P)
            typeset -r pri_test=$OPTARG
            ;;
        s)
            typeset -r sec_gw=$OPTARG
            ;;
        S)
            typeset -r sec_test=$OPTARG
            ;;
        h)
            typeset -ri showusage=0
            ;;
        d)
            typeset -ri debug=0
            ;;
        n)
            typeset -ri dryrun=0
            ;;
    esac
done

if [ "${pri_gw}" = "" ] \
    || [ "${sec_gw}" = "" ] \
    || [ "${showusage}" == 0 ]; then
    usage
fi

[ "${pri_test}" = "" ] && typeset -r pri_test=${pri_gw}
[ "${sec_test}" = "" ] && typeset -r sec_test=${sec_gw}

curr_gw=$(current_gw)

get_alive_hosts "${pri_gw}" "${sec_gw}" "${pri_test}" "${sec_test}"

if [ "${pri_gw_alive}" == 0 ] && [ "${pri_test_alive}" == 0 ]; then
    # Primary is current gw, and is alive, and primary
    # test is alive, so we can stop here.  This is first 
    # because it should be the commonest case.
    if [ "${curr_gw}" = "${pri_gw}" ]; then
        exit 0
    else
        if [ "${dryrun}" != 0 ]; then
            set_default_gw "${pri_gw}"
            exit 0
        fi
    fi
# Primary failed; try the secondary
elif [ "${sec_gw_alive}" == 0 ] && [ "${sec_test_alive}" == 0 ]; then
    if [ "${curr_gw}" = "${sec_gw}" ]; then
        exit 0
    else
        if [ "${dryrun}" != 0 ]; then
            set_default_gw "${sec_gw}"
            exit 0
        fi
    fi
else
    # We've been unable to find either the primary
    # or secondary alive, so give up and complain
    # loudly
    loglevel="daemon.crit"
    do_log "Unable to find a live default route!"
    exit 1
fi

# EOF gwswitch