#!/bin/bash

# goal:
#      change default route if two target hosts cannot be reached
# recommendation:
#      call this via cron each minute

start() { # called first after function defintions
    preparation "$@"

    if no_input; then
        help
        check_only
    else
        center
    fi
}

center() {
    if gateway_not_working ; then
        change_gateway
        exit
    fi

    if second_gateway_used ; then
        debug "preferred gateway not used."
        debug "can we switch back to the preferred gateway?"
        if first_gateway_works ; then
            debug "switching back to the preferred gateway"
            use_first_gateway
        else
            debug "switching to default gateway not yet possible"
        fi
    else
        debug "preferred gateway used."
    fi
}

no_input(){
    [ "$GATEWAY1" ] && [ "$GATEWAY2" ] && return 1
    return 0
}

help(){ echo \
"
usage:  $MYNAME GATEWAY1 GATEWAY2 [TEST_TARGET1 [TEST_TARGET2]]

usage2: $MYNAME    (only tests internet connection and shows help on stderr)

GATEWAY1        IP address of preferred gateway (or \"dsl\")
GATEWAY2        IP address of fallback gateway
TEST_TARGET1    IP address of a host only reachable through the two gateways
TEST_TARGET2    IP address of another host only reachable through the two gateways
" >&2
}

check_only() {
    if gateway_not_working ; then
        return 1
    else
        return 0
    fi
}

gateway_not_working() {
    GATEWAY=`get_gateway`
    if [ -z "$GATEWAY" ] ; then
        $OUTPUT "discovered: no gateway used!"
        return 0
    fi

    if $ISUP $TEST_TARGET2 || $ISUP $TEST_TARGET1 ; then
        debug "could reach $TEST_TARGET1 or $TEST_TARGET2, gateway seems to work"
        return 1
    fi

    if [ "$DOUBLE_TEST" = 1 ] ; then
        sleep 5
        $OUTPUT "doing second test, line may be busy/flakey:"
        if isup_debug $TEST_TARGET2 || isup_debug $TEST_TARGET1 ; then
            $OUTPUT "could reach $TEST_TARGET1 or $TEST_TARGET2 on second try, gateway seems to work"
            return 1
        fi
    fi

    $OUTPUT "could not be reached: $TEST_TARGET1 and $TEST_TARGET2, assuming that gateway is not working"
    return 0
}

isup_debug(){
    $ISUPDEBUG $1 2>&1 | grep -vE '^Before recvfrom|^After recvfrom' | sed 's/^/    /'
}

first_gateway_works()  { test_gateway $GATEWAY1 ;}

second_gateway_works() { test_gateway $GATEWAY2 ;}

test_gateway() { TEST_GATEWAY=$1
    ORIGINAL_GATEWAY=`get_gateway`

    debug " first test if the gateway $TEST_GATEWAY itself is reachable..."
    if ! $ISUP $TEST_GATEWAY ; then
        debug "gateway $TEST_GATEWAY not even reachable itself! I'm not even trying to use it."
        return 1
    fi
    debug " ... gateway $TEST_GATEWAY itself is reachable."

    debug " TEST: starting test for $TEST_GATEWAY ..."
    set_gateway $TEST_GATEWAY || return 1
    if $ISUP $TEST_TARGET2 || $ISUP $TEST_TARGET1 ; then
        debug "  TEST SUCCESSFULL"
        debug "  trying to set gateway that was present before test..."
        set_gateway $ORIGINAL_GATEWAY
        debug " TEST finished"
        return 0
    else
        debug "  TEST FAILED"
        debug "  trying to set gateway that was present before test..."
        set_gateway $ORIGINAL_GATEWAY
        debug " TEST finished"
        return 1
    fi
}

set_gateway() { NEW_GATEWAY=$1
    OLD_GATEWAY=`get_gateway`
    if [ "$OLD_GATEWAY" = "$NEW_GATEWAY" ] ; then
        debug "  set_gateway $NEW_GATEWAY: it is already gateway"
        return 0
    fi
    if [ -z "$NEW_GATEWAY" ] ; then
        debug "  no valid gateway given: \"${NEW_GATEWAY}\""
        return 1
    fi
    if [ ! -z "$OLD_GATEWAY" ] ; then
        debug "  unsetting $OLD_GATEWAY as gateway"
        $ROUTE del default gw $OLD_GATEWAY
        debug "  gateway now: \"`get_gateway`\""
    fi

    debug "  setting $NEW_GATEWAY as gateway"
    if [ "$DEBUG" = 1 ] ; then
        $ROUTE add default gw $NEW_GATEWAY
        RETURNVALUE=$?
    else
        $ROUTE add default gw $NEW_GATEWAY >/dev/null 2>&1
        RETURNVALUE=$?
    fi
    GATEWAY=`get_gateway`
    if [ -z "$GATEWAY" ] || [ $RETURNVALUE -gt 0 ] ; then
        debug "  gateway now: \"\""
        debug "  setting gateway FAILED, resetting to old gateway: $OLD_GATEWAY"
        $ROUTE add default gw $OLD_GATEWAY
        debug "  gateway now: \"`get_gateway`\""
        return 1
    fi
}

use_first_gateway() {
    $OUTPUT "setting preferred gateway $GATEWAY1"
    set_gateway $GATEWAY1
}

use_second_gateway() {
    $OUTPUT "setting fallback gateway $GATEWAY2"
    set_gateway $GATEWAY2
}

second_gateway_used() {
    GATEWAY=`get_gateway`
    debug " gateway used: $GATEWAY"
    if [ "$GATEWAY" = "$GATEWAY2" ] ; then
        return 0
    else
        return 1
    fi
}

get_gateway() {
 # the three separators (/):     /search pattern/replacement/
 #                  _____________|              |________   |
 #                 |                                     |  |
 route -n|sed -n 's/^0.0.0.0[^0-9]*\([0-9\.]*\)[^0-9\.].*/\1/p'
 #                  ^^^^^^^^^^^^^^^  ^^^^^^^^  ^^^^^^^^^^ ^^
 #                   gateway line    gateway   the      show only what
 #                   starts with     address   rest     is matched between
 #                   "0.0.0.0 "                         the first \(and\)
}

change_gateway() {
    debug "changing the gateway..."
    GATEWAY=`get_gateway`
    if [ -z "$GATEWAY" ] ; then
        $OUTPUT "no gateway set yet, setting gateway now..."
        if first_gateway_works; then
            use_first_gateway
        elif second_gateway_works; then
            use_second_gateway
        else
            $OUTPUT "none of the gateways work: $GATEWAY1 $GATEWAY2, NO internet connection!"
            use_first_gateway
        fi
    elif [ $GATEWAY = $GATEWAY1 ] ; then
        if second_gateway_works; then
            use_second_gateway
        else
            $OUTPUT "tried to use gateway $GATEWAY2, but it does not work, $GATEWAY1 remains as gateway"
        fi
    elif [ $GATEWAY = $GATEWAY2 ] ; then
        if first_gateway_works; then
            use_first_gateway
        else
            $OUTPUT "tried to use gateway $GATEWAY1, but it does not work, $GATEWAY2 remains as gateway"
        fi
    else
        $OUTPUT "unknown gateway discovered: $GATEWAY, trying known gateways..."
        if first_gateway_works; then
            use_first_gateway
        elif second_gateway_works; then
            use_second_gateway
        else
            $OUTPUT "none of the gateways work: $GATEWAY1 $GATEWAY2, NO internet connection!"
        fi
    fi
}

update_dsl_gateway() {
    DSLGWIP=`ifconfig|grep -A 1 '^ppp'|grep 'P-[-a-zA-Z]*P:'|head -n 1|sed 's/^.*P-[-a-zA-Z]*P:[ ]*\([0-9.]*\)[^0-9.].*$/\1/'`
    if [ -z "$DSLGWIP" ] ; then
        debug "no IP address of DSL gateway found with ifconfig"
        if [ -e "$DSLGWIP_CACHE_PATH" ] ; then
            debug "found file for last known IP address of DSL gateway: $DSLGWIP_CACHE_PATH"
            DSLGWIP=`cat "$DSLGWIP_CACHE_PATH"`
        else
            debug "did not find file for last known IP address of DSL gateway $DSLGWIP_CACHE_PATH"
        fi
        if [ -z "$DSLGWIP" ] ; then
            debug "no IP address of DSL gateway found in the file"
        else
            debug "taking $DSLGWIP as IP address of DSL gateway"
        fi
    else
        if [ -e "$DSLGWIP_CACHE_PATH" ] && [ `<"$DSLGWIP_CACHE_PATH"` = "$DSLGWIP" ] ; then
            debug "already in $DSLGWIP_CACHE_PATH : $DSLGWIP, not overwriting"
        else
            debug "storing $DSLGWIP in file for last known IP address of DSL gateway $DSLGWIP_CACHE_PATH"
            echo -n "$DSLGWIP" > "$DSLGWIP_CACHE_PATH"
        fi
    fi
    echo "$DSLGWIP"
}

syslog() { logger -t "$MYNAME" -s -p user.warning -- "$@" ;}

debug()  { [ "$DEBUG" -gt 1 ] && echo "DEBUG: $@" >&2     ;}

default_config(){
    # two test targets that can be pinged to decide whether "the internet" is reachable
    TEST_TARGET1=193.99.144.80  # default: heise.de, override with $3
    TEST_TARGET2=85.10.241.60   # default: merlinux.de, override with $4

    DSL=0   # by default do not assume that primary line is adsl

    # store IP address of (sometimes changing) DSL provider gateway
    DSLGWIP_CACHE_PATH="/var/cache/last_dsl_gateway_ip"

    # try to ping test destinations again after failure before changing anything
    DOUBLE_TEST=1

    ISUP="isup -t 3 -r 3"  # wait for returning pong for three seconds (-t 3), make two retries (-r 3 makes two)
    ISUPDEBUG="isup -t 3 -r 3 -v+ -d"  #verbose and debug output

    DEBUG=0                     # 1: extra output, 0: normal output
    OUTPUT=syslog               # where non-debugging messages go

    ROUTE=route
    #ROUTE="echo route"         # example configuration to not change anything, for tests
}

local_config(){
    [ -e /etc/backup_gateway.conf ] && . /etc/backup_gateway.conf && debug "using local configuration /etc/backup_gateway.conf"
}

command_line_config(){ MYPATH="$0"    ; MYNAME="${MYPATH##*/}"
                      GATEWAY1=$1     # preferred gateway
                      GATEWAY2=$2     # fallback gateway
      [ "$3" ] && TEST_TARGET1=$3     # two test targets that can be pinged
      [ "$4" ] && TEST_TARGET2=$4     #  to decide whether "the internet" is reachable
}

preparation(){
    default_config
    local_config
    command_line_config "$@"

    [ "$DEBUG" = 1 ] && OUTPUT=echo     # OUTPUT: where non-debugging messages go

    PATH="$PATH":/sbin:/usr/bin
    which route >/dev/null || exit 1
    which isup >/dev/null || exit 1
    # http://www.trillke.net/~fifi/download/isup-2.8.2.tar.gz

    # DSL special handling
    if [ "$GATEWAY1" = "dsl" ] || [ "$GATEWAY1" = "DSL" ] ; then
        DSL=1
        GATEWAY1=`update_dsl_gateway` # DYNAMIC ip address of GATEWAY
    fi
}

start "$@"

# contact to author:    klaus@trillke.net