[ edit: ifcfg ]

#! /bin/bash

CheckForwarding () {
  local sbase fwd
  sbase=/proc/sys/net/ipv4/conf
  fwd=0
  if [ -d $sbase ]; then
    for dir in $sbase/*/forwarding; do
      fwd=$[$fwd + `cat $dir`]
    done
  else
    fwd=2
  fi
  return $fwd
}

RestartRDISC () {
  killall -HUP rdisc || rdisc -fs
}

ABCMaskLen () {
  local class;

class=${1%%.*}
  if [ "$1" = "" -o $class -eq 0 -o $class -ge 224 ]; then return 0
  elif [ $class -ge 224 ]; then return 0
  elif [ $class -ge 192 ]; then return 24
  elif [ $class -ge 128 ]; then return 16
  else return 8; fi
}

label="label $1"
ldev="$1"
dev=${1%:*}
if [ "$dev" = "" -o "$1" = "help" ]; then
  echo "Usage: ifcfg DEV [[add|del [ADDR[/LEN]] [PEER] | stop]" 1>&2
  echo "       add - add new address" 1>&2
  echo "       del - delete address" 1>&2
  echo "       stop - completely disable IP" 1>&2
  exit 1
fi
shift

CheckForwarding
fwd=$?
if [ $fwd -ne 0 ]; then
  echo "Forwarding is ON or its state is unknown ($fwd). OK, No RDISC." 1>&2
fi

deleting=0
case "$1" in
add) shift ;;
stop)
  if [ "$ldev" != "$dev" ]; then
    echo "Cannot stop alias $ldev" 1>&2
    exit 1;
  fi
  ip -4 addr flush dev $dev $label || exit 1
  if [ $fwd -eq 0 ]; then RestartRDISC; fi
  exit 0 ;;
del*)
  deleting=1; shift ;;
*)
esac

ipaddr=
pfxlen=
if [ "$1" != "" ]; then
  ipaddr=${1%/*}
  if [ "$1" != "$ipaddr" ]; then
    pfxlen=${1#*/}
  fi
  if [ "$ipaddr" = "" ]; then
    echo "$1 is bad IP address." 1>&2
    exit 1
  fi
fi
shift

peer=$1
if [ "$peer" != "" ]; then
  if [ "$pfxlen" != "" -a "$pfxlen" != "32" ]; then
    echo "Peer address with non-trivial netmask." 1>&2
    exit 1
  fi
  pfx="$ipaddr peer $peer"
else
  if [ "$ipaddr" = "" ]; then
    echo "Missing IP address argument." 1>&2
    exit 1
  fi
  if [ "$pfxlen" = "" ]; then
    ABCMaskLen $ipaddr
    pfxlen=$?
  fi
  pfx="$ipaddr/$pfxlen"
fi

if [ "$ldev" = "$dev" -a "$ipaddr" != "" ]; then
  label=
fi

if [ $deleting -ne 0 ]; then
  ip addr del $pfx dev $dev $label || exit 1
  if [ $fwd -eq 0 ]; then RestartRDISC; fi
  exit 0
fi

if ! ip link set up dev $dev ; then
  echo "Error: cannot enable interface $dev." 1>&2
  exit 1
fi
if [ "$ipaddr" = "" ]; then exit 0; fi

if ! arping -q -c 2 -w 3 -D -I $dev $ipaddr ; then
  echo "Error: some host already uses address $ipaddr on $dev." 1>&2
  exit 1
fi

if ! ip address add $pfx brd + dev $dev $label; then
  echo "Error: failed to add $pfx on $dev." 1>&2
  exit 1
fi

arping -q -A -c 1 -I $dev $ipaddr
noarp=$?
( sleep 2 ;
  arping -q -U -c 1 -I $dev $ipaddr ) >& /dev/null </dev/null &

ip route add unreachable 224.0.0.0/24 >& /dev/null
ip route add unreachable 255.255.255.255 >& /dev/null
if [ `ip link ls $dev | grep -c MULTICAST` -ge 1 ]; then
  ip route add 224.0.0.0/4 dev $dev scope global >& /dev/null
fi

if [ $fwd -eq 0 ]; then
  if [ $noarp -eq 0 ]; then
    ip ro append default dev $dev metric 30000 scope global
  elif [ "$peer" != "" ]; then
    if ping -q -c 2 -w 4 $peer ; then
      ip ro append default via $peer dev $dev metric 30001
    fi
  fi
  RestartRDISC
fi

exit 0

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