nsupdate $(ip neigh list)

neighsupdate.sh

#!/bin/bash
# Tiny tool to run as a cronjob, which will update
# (using nsupdate) your zone files to include dhcp-
# and statelessly-assigned addresses.

# Space-separated list of address-to-name resolution methods to try.
# They'll be tried in the order listed here.
METHODS="hostfile avahi localdns"

# Zone to update for each type of IP address.  In my home network,
# ipv4 is natted but ipv6 is globally reachable.  So I only publish
# records for ipv6 addresses.
ZONE4=
ZONE4KEY=
ZONE6=lab.xrtc.net.
ZONE6KEY=/config/Klab.xrtc.net.private

# Subnets to restrict query to.  In my home network, 192.168.0.0/24 is
# available for guest use, while 22.2.3.0/24 is hosts that I actually
# care about.
#
# Restricting NET6 to 2000::/3 will only publish globally reachable
# IPv6 addresses.
NET4=22.2.3.0/24
NET6=2000::/3

IF4=br0
IF6=br0

zeroize_macaddrs ()
{
    sed -e 's/\(\s\)\([0-9a-f]\):/\10\2:/' -e 's/\([^0-9a-f]\)\([0-9a-f]\):/\10\2:/g' -e 's/:\([0-9a-f]\)$/:0\1/'
}
lowercase ()
{
    tr A-Z a-z
}

# Some DHCP servers can be configured to update the /etc/hosts file
# with the MAC address.  If yours does this, the records it puts in
# will be useful for other address families too!
#
# If your DHCP server does this but treats leading zeros in MAC
# addresses differently than 'ip', then, well, you're intermittently
# fucked.
resolve_hostfile ()
{
    local ip mac name_ip_raw name_ip name_mac_raw name_mac
    ip="$1"
    mac="$2"

    name_ip_raw=$( fgrep -i "$ip" /etc/hosts | lowercase | cut -f2 )
    # Remove domain name, if present
    name_ip=${name_ip_raw%%.*}

    name_mac_raw=$( cat /etc/hosts | lowercase | zeroize_macaddrs | fgrep -i "$mac" | cut -f2 )
    # Remove domain name, if present
    name_mac=${name_mac_raw%%.*}

    # Prefer the IP-matched name, but also accept MAC-matched name.
    echo ${name_ip:-$name_mac}
}

# nope
resolve_avahi ()
{
    echo
}

# Maybe localhost has a DNS server that knows things!
resolve_localdns ()
{
    local ip name_raw name
    ip="$1"

    name_raw=$( host "$ip" localhost | tail -n 1 | cut -d\  -f5 )
    name=${name_raw%%.*}
    echo $name
}

list4 ()
{
    ip neigh show ${IF4:+dev $IF4} to ${NET4:-0/0} |\
        lowercase |\
        grep lladdr |\
        zeroize_macaddrs |\
        cut -d\  -f1,3
}

list6 ()
{
    ip neigh show ${IF6:+dev $IF6} to ${NET6:-::/0} |\
        lowercase |\
        grep lladdr |\
        zeroize_macaddrs |\
        cut -d\  -f1,3
}

do_resolve ()
{
    local method name addr mac
    addr="$1"
    mac="$2"
    unset name
    # try, in order, all the configured resolution methods
    for method in $METHODS ; do
        name=$( resolve_$method $addr $mac )
        # if we got a name that's nonempty, that's a winner!
        if [ x"$name" != "x" ] ; then
            #echo found $addr as $name using $method >&2
            break
        fi
    done
    echo "$name"
}

do_update ()
{
    # Remove any old record and add the new one
    echo update delete $2 $3
    echo update add $2 30 $3 $1
}

do_entry ()
{
    local addr mac zone name
    addr="$1"
    mac="$2"
    zone="$3"
    type="$4"

    name=$( do_resolve $addr $mac )
    # no name for this address, sorry bub
    if [ x"$name" == "x" ] ; then return ; fi
    if [ x"$name" == "x3(NXDOMAIN)" ] ; then return ; fi

    #echo do_update addr =$addr name =$name zone =$zone
    do_update $addr $name.$zone $type
}

if [ x"$ZONE4" != "x" ] ; then
    ( echo zone $ZONE4
      list4 | while read addr mac ; do
                  do_entry $addr $mac $ZONE4 A
              done
      echo send
      echo show
    ) | nsupdate -k "$ZONE4KEY"
fi

if [ x"$ZONE6" != "x" ] ; then
    ( echo zone $ZONE6
      #echo update delete
      list6 | while read addr mac ; do
                  do_entry $addr $mac $ZONE6 AAAA
              done
      echo show
      echo send
    ) | nsupdate -k "$ZONE6KEY"
fi

# Now, after we've added all these, flush the cache so that only new
# addresses are added in the future.  This is sub-optimal because it
# will cause requests for neighbors to be reissued before necessary,
# but ... whatever.  Computers can deal and I'm not running a very big
# network.
ip neigh flush dev $IF4
ip neigh flush dev $IF6