ec2_multicast.sh

#!/bin/sh

[[ -n "$1" && -n "$2" ]] || { echo "Usage: $0 <interface to grab multicast packets from> <interface to send modified packets to> [target MAC address 1] [target MAC address 2] ..."; exit 0 ; }

IIF=$1
OIF=$2
shift
shift
SRC_MACADDR=`ip link show $OIF | awk '/ether/ {print $2}' | tr -d :`

HANDLE=10
TC=tc
BASE_PRIO=100

# File which stores the priority of the tc filter that is to be registered 
PRIO_FILE=/var/run/ec2_broadcastd_tc_prio

# Load the priority that the previous filter is registered with
if [ -r $PRIO_FILE ]; then
  BASE_PRIO=`cat $PRIO_FILE`
fi

# Determine the priority that the new filter will be registered with 
# (Any number which is different from the previous one is fine unless 
# other filters exist.)
if [ `expr $BASE_PRIO % 2` -eq 1 ]; then
  PRIO=`expr $BASE_PRIO + 1`
else 
  
PRIO=`expr $BASE_PRIO - 1`
fi

# Create queue discipline for the origin network interface 
$TC qdisc add dev $IIF root handle ${HANDLE}: prio >& /dev/null

# Calculate bit mask for modifying packet ID. (We will send duplicate IP packets. 
# In order not to break underlying packet fragmentation and reconstruction, 
# each IP packet needs to have different ID)
RETAIN_MASK=0xfff0

# Create the list of actions to apply to outgoing multicast packets
i=0
for DST_MACADDR in $*
do
  DST_MACADDR=`echo $DST_MACADDR | tr -d :`
  if [ "${actions}X" != "X" ]; then
    # Action to mirror the filtered packet to the outgoing interface
    actions="$actions action mirred egress mirror dev $OIF "
  fi
  
  # Actions to modify Ethernet header
  # Offset 0 is the beginning of the IP header
  # Offset has to be factor of 4
  # Last a few bits of the packet ID are modified to make each packet unique 
  actions="$actions action pedit  
    munge offset -16 u32 set 0x0000`echo $DST_MACADDR | cut -c 1-4`  
    munge offset -12 u32 set 0x`echo $DST_MACADDR | cut -c 5-12` 
    munge offset  -8 u32 set 0x`echo $SRC_MACADDR | cut -c 1-8` 
    munge offset   4 u16 set 0x`printf "%04x" $i` retain $RETAIN_MASK pipe "
  
  # Recalculate IP header checksum because we have modified the packet ID field
  actions="$actions action csum ip4h pipe "

  i=`expr $i + 1`
done
# Action to redirect the filtered packet to the outgoing interface 
# (Note that this suppresses to send out the original frame with 
# the broadcast address.)
actions="$actions action mirred egress redirect dev $OIF"

# Register the filter that matches ethernet broadcast frames which 
# carry IP packets addressed to class D (IP multicast)
$TC filter add dev $IIF parent ${HANDLE}: protocol ip prio $PRIO u32 \
 match u8 0x01 0x01 at 0 match u8 0xe0 0xe0 at 16 $actions

# Delete the old filter if exists
if [ -r $PRIO_FILE ]; then
  $TC filter del dev $IIF parent ${HANDLE}: prio $BASE_PRIO
fi

# Store the priority used for the filter for future reference
echo $PRIO > $PRIO_FILE