3. Arp2wol, small hack to send wake on lan frames based on arp requests.

Arp2wol, small hack to send wake on lan frames based on arp requests.


  • EDIT: second version below.
    Hi
    In a homesetup i have a server which can boot up very fast (~11 seconds).
    So i descided to shut it down whenever its ARP table is empty long enough (for now 20 minutes).
    To boot it up i have a small shellscript on the pfSense running which sends wake on lan packets if it detects an ARP lookup for the IP of this server.

    I thought someone might have some use for this:

    ~~#! /bin/sh

    This listens for arp lookups and triggers something (usually wol?)

    INTERFACE="vr2"
    TARGET_BROADCAST="10.0.8.255"
    TARGET_IP="10.0.8.13"
    TARGET_MAC=“00🇩🇪ad:be:ef:99”

    stop="" #this is a hack to prevent tcpdump from looping unnecessarily in the while read (and executing the trigger a lot…)
    while true
    do
           tcpdump -S -l -n -i $INTERFACE arp dst $TARGET_IP | while read data
           do
                   if [ -z “$stop” ]; then
                           fping -c 1 $TARGET_IP
                           if [ “$?” -ne “0” ]; then
                                   logger -t arp2wol “$TARGET_IP is down, starting it.”
                                   wol -i $TARGET_BROADCAST $TARGET_MAC
                           fi
                           killall tcpdump
                           stop=“1”
                   fi
           done;
           sleep 60
           stop=""
    done;

    Something that just came to mind:
    This will probably break a running packet-capture in the webinterface if the script happens to trigger in the wrong moment.
    Just a warning ^^"~~

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  • This is the second version.
    It takes the destination IP, destination subnet and destination mac as parameter now.

    This version allow one to run it multiple times without interfering with other running tcpdumps.
    Also tcpdump runs without putting the interface into promiscuous mode.

    example to start it:
    arp2wol.sh 10.0.8.33 10.0.8.255 01🇩🇪ad:be:ef:33 &

    
#! /bin/sh
# This program listens for arp lookups and triggers something
INTERFACE=vr2                   # Listen on this interface.
RESTART="1200"                   # After the trigger, wait this long to start listening again.
TARGET_IP=$1                    # Listen for this IP.
TARGET_BROADCAST=$2             # Broadcast to send the wol frame to.
TARGET_MAC=$3                   # MAC address to send the wol frame to.

# Create path to initial pid file. add some randomness in case you want to run it multiple times
pid="/var/run/dumper$(cat /dev/urandom | tr -dc '0-9' | head -c 10).pid"

while true
do
        # Check if not another process is using the same pidfile.
        while [ -f $pid ]
        do
                pid="/var/run/dumper$(cat /dev/urandom | tr -dc '0-9' | head -c 10).pid"
        done;

        # Actually run tcpdump
        (tcpdump -p -S -l -n -i $INTERFACE arp dst $TARGET_IP & echo $! >&3 ) 3>$pid | while read data
        do
                if [ -f $pid ]; then
                        fping -c 1 $TARGET_IP
                        if [ "$?" -ne "0" ]; then
                                logger -t arp2wol "$TARGET_IP is down, starting it."
                                wol -i $TARGET_BROADCAST $TARGET_MAC
                        fi
                        kill -9 $(cat $pid)
                        rm $pid
                fi
        done;
        sleep $RESTART
done;
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  • D
    Banned

    Interesting idea, thanks…

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  • K

    Thank you very much.
    In addition with the help from https://doc.pfsense.org/index.php/Executing_commands_at_boot_time
    it is the answer for my question 🙂

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  • B

    First wanted to thank GruensFroeschli,

    i used his script on ASUS 68U router then switched to PFSENSE, both works fine.
    However, in pfsense the “cat /dev/urandom” causes a loop when the script is used at shellcmd and observed a high CPU use.

    i modified the script slightly to fix this problem, any suggestion to enhance it would be great

    
#! /bin/sh
# This program listens for arp lookups and triggers something
INTERFACE=re1                   # Listen on this interface.
RESTART="120"                   # After the trigger, wait this long to start listening again.
TARGET_IP=$1                    # Listen for this IP.
TARGET_BROADCAST=$2             # Broadcast to send the wol frame to.
TARGET_MAC=$3                   # MAC address to send the wol frame to.

# Create path to initial pid file. add some randomness in case you want to run it multiple times
#pid="/var/run/dumper$(cat /dev/urandom | tr -dc '0-9' | head -c 10).pid"
pid="/var/run/dumper$$.pid"
trap 'rm -f $pid; exit' EXIT SIGHUP SIGINT SIGTERM

while true
do
        # check if not another process is using the same pidfile.
        #while [ -f $pid ]
        #do
                #pid="/var/run/dumper$(cat /dev/urandom | tr -dc '0-9' | head -c 10).pid"
                #pid="/var/run/dumper$$.pid"
        #done;

        # Actually run tcpdump
        (/usr/sbin/tcpdump -p -S -l -n -v -i $INTERFACE arp dst $TARGET_IP & echo $! >&3 ) 3>$pid | while read data
        do
                if [ -f $pid ]; then
                        /usr/local/sbin/fping -c 1 $TARGET_IP
                        if [ "$?" -ne "0" ]; then
                                /usr/bin/logger -t arp2wol "$TARGET_IP is down, starting it."
                                /usr/local/bin/wol -i $TARGET_BROADCAST $TARGET_MAC
                        fi
                        kill -9 $(cat $pid)
                        rm $pid
                fi
        done;
        sleep $RESTART
done;
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  • hmmm.
    I’m running this still myself but i haven’t seen a loop at all.
    What happens if you execute

    
cat /dev/urandom | tr -dc '0-9' | head -c 10

    on a shell directly?

    Your change will not work if you start multiple instances from the same shell since $$ returns the pid of the shell where you ask for $$.
    –> Running it multiple time will reuse the same file and break.

    On pfSense you could use jot to get a random number and then remove the \n.
    e.g.
    jot -r 1 0 9999999999 | tr -d ‘\n’

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  • B

    it works fine if i run it from the shell directly, it loops only when i run it on boot using shell comand or *sh file under /usr/local/etc/rc.d.

    The $$ should have a different value(PID) every time you start the script, no? not sure this will work properly with scripts running in the background.

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  • @bgarwin:

    The $$ should have a different value(PID) every time you start the script, no? not sure this will work properly with scripts running in the background.

    No $$ returns the pid of the current shell.
    If you fork multiple instance of the arp2wol.sh then all of them will have the same parent (the init script which executes it).
    –> All of them will use the same pid-file.

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  • B

    thank you let me test jot.

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  • B

    Many thanks for this script  😄

    Does this log the source of the arp request?

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  • Not really, but you can easily extend it to do this.
    $data contains the line which triggered the event.
    –> the arp responses look something like

    13:22:24.457157 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 80.219.3.42 tell 80.219.2.1, length 46

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  • B

    Can this script to be modified to work across subnets ie the device I want to wake up in this instance is on a different subnet to everything that is waking it connected to lan interface em2, those devices waking it are connected to lan em1?

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  • I don’t know, but this really has nothing to do with this script.

    This script exists to trigger something based on ARP lookups.
    What you trigger is your own thing.

    Try reading the documentation of the /usr/local/bin/wol which is used on pfsense to send magic packet.

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  • B

    I am not 100% sure but I think all I need to do is remove “arp” from the tcpdump command and ensure $TARGET_BROADCAST $TARGET_MAC are those of the machine to be woken and $INTERFACE is the lan on the source machines trying to access the target?

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  • K

    A client trying access a server on a different network segment (subnet) will never issue an ARP broadcast for the destination address because MAC addresses are local to the connected segment and access to the outside of the local segment is done via a gateway (the router in between the segments).  In other words, the MAC address of the destination host won’t show up in the traffic of the client’s network. You have to monitor the destination IP instead and then trigger the WOL action on the other network segment where the to be woken up hosts are.

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  • B

    Yep, removing the arp and it works great, I am actually doing it via port , thanks again for the script.

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  • If you want to change what the tcpdump is looking for:
    Here is some documentation to that: http://www.tcpdump.org/manpages/pcap-filter.7.html

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  • O

    I am using the original script posted by @GruensFroeschli with shellcmd. The cpu usage jumps to 100% when I use shellcmd through gui. When I run it as a sh script in ssh, it runs fine and cpu usage stays at 5% but need to login each time after reboot to do that. I am a newbie. Any help is greatly appreciated. I am running 2.4.2-RELEASE-p1 (amd64). Thanks !

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  • O

    Anyone have any suggestions … TIA

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