netdev - Re: Bug report: broadcast address as incomplete entry in arp table, effectively a blackhole; reproducer included

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Date:	Wed, 19 Nov 2014 15:39:58 +0100
From:	Lars Ellenberg <lars.ellenberg@...bit.com>
To:	netdev@...r.kernel.org
Cc:	Olaf Kirch <okir@...e.com>
Subject: Re: Bug report: broadcast address as incomplete entry in arp table,
 effectively a blackhole; reproducer included

Ping ...
Any ideas?

	Lars

On Wed, Sep 24, 2014 at 06:18:47PM +0200, Lars Ellenberg wrote:
> 
> You have some interface you want to broadcast on,
> so you resolve its broadcast address (once),
> and keep sending (e.g. some continuous status updates, or "heartbeat").
> 
> For some reason that interface goes down.
> If you keep sending to the previously resolved address,
> that will create an incomplete arp entry.
> 
> Unfortunately, that entry *stays* there, even if the interface is then
> brought back up (with the same network and broadcast settings).
> Once the interface is up, you won't be able to delete that entry
> (because, its a broadcast address; that arp entry is not supposed to be
> there anyways; that deletion request will be filtered out early...)
> 
> Anyone trying to send to that broadcast address will now effectively
> send to a black hole: there is an incomplete arp entry.
> 
> 
> Fix is then to stop all processes sending to that address,
> bring down the device, delete the arp entry, bring it back up,
> and then continue all processes sending to that address.
> 
> 
> I can reproduce this easily with the script below, anywhere I tested,
> on a large variety of platforms and kernels.
> (you'll obviously have to adjust DEV, BROADCAST and possibly PORT).
> 
> 
> I suspect this is not intentional, but there is simply some
> neigh_flush_dev() or similar missing "somewhere".
> 
> If you want to point me in the right direction as to probable values of
> "somewhere", I'll likely be able to figure out a minimal patch myself.
> 
> If you know the right place to fix this from the top of your head,
> even better ;-)
> 
> Thanks,
> 	Lars
> 
> ------------------------------------------------------
> 
> #!/bin/bash
> 
> DEV=eth1
> BROADCAST=192.168.133.255
> PORT=6666
> 
> send_udp_broadcast()
> {
> exec python -c '
> from socket import *
> from time import sleep
> from struct import pack
> from datetime import datetime
> 
> 
> s = socket(AF_INET, SOCK_DGRAM)
> s.setsockopt(SOL_SOCKET, SO_BROADCAST, 1)
> s.setsockopt(SOL_SOCKET, SO_BINDTODEVICE, pack("'$(( 1+${#DEV} ))'s","'$DEV'"))
> 
> while 1:
>    s.sendto(datetime.now().strftime("hi there, it is now %T.%f"), ("'$BROADCAST'",'$PORT'))
>    sleep(0.1)
> '
> }
> 
> p() { printf "\n"; printf "::: %s\n" "$@"; printf "%s\n" "-----------"; }
> 
> arp -n | grep $BROADCAST && {
> 	echo >&2 "Sorry, fix the arp table first!"
> 	exit 1
> }
> 
> ( set +x ; send_udp_broadcast ) &
> kid=$!
> p "[$kid] Started to send udp broadcasts on $DEV to $BROADCAST:$PORT"
> 
> p "We should see the packets being sent on $DEV"
> tcpdump -n -i $DEV -c 2 -xX udp and port $PORT
> 
> p "We should not have any arp entries for $BROADCAST"
> arp -n | grep $BROADCAST
> 
> p "But we soon will, after we take down $DEV"
> ip link set down $DEV
> 
> sleep 2
> 
> p "Now we should have an incomplete arp entry for $BROADCAST"
> arp -n | grep $BROADCAST
> 
> p "It will still be there after we bring $DEV back up"
> ip link set up $DEV
> 
> p "There won't be any packets now, this should timeout:"
> while read -r -t 5 line ; do
> 	echo "$line"
> done < <(tcpdump -n -i $DEV -c 2 -xX udp and port $PORT 2>&1)
> 
> p "Because we still have that arp entry"
> arp -n | grep $BROADCAST
> 
> p "And we cannot get rid of it, either, as long as this $DEV is up"
> arp -d $BROADCAST
> arp -n | grep $BROADCAST
> 
> p "but we can stop the child," \
>   "take down the device," \
>   "remove the arp entry then," \
>   "and bring the device back up"
> 
> kill -STOP $kid
> ip link set down $DEV
> sleep 1
> arp -d $BROADCAST
> ip link set up $DEV
> arp -n | grep $BROADCAST
> 
> p "continue the child" \
>   "and now we should see outgoing packets again"
> 
> kill -CONT $kid
> tcpdump -n -i $DEV -c 2 -xX udp and port $PORT
> 
> p "and no more arp entry"
> arp -n | grep $BROADCAST
> 
> p "Done."
> kill $kid
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