update_ipv6.sh

#!/bin/bash
# in /etc/ddclient.conf:
# postscript=/opt/scripts/update_ipv6.sh

function update_domain() {
  DOMAIN=$1
  USERNAME=$2
  PASSWORD=$3
  SERVER=$4
  #DEVICE="dev eth0"
  URL=https://$SERVER/nic/update
  FORCE_GET="-X GET -G"
  DNS_SERVER=@2606:4700:4700::1111 # bypass DNS rebinding protection
  VERBOSITY="-s -S"
  LOGGER="logger -t $(basename $0 .sh)"

  ACTUAL_ADDRESS=$(dig $DOMAIN AAAA $DNS_SERVER +short)
  TARGET_ADDRESS=$(\
          /sbin/ip -6 -o addr show $DEVICE scope global dynamic -deprecated -temporary \
          | grep -v " inet6 f[cd]" | head -n 1 | cut -d " " -f 7 | cut -d / -f 1)
  [ "$ACTUAL_ADDRESS" = "$TARGET_ADDRESS" ] || \
    curl $VERBOSITY $FORCE_GET \
      --user $USERNAME:$PASSWORD \
      --data-urlencode hostname=$DOMAIN \
      --data-urlencode myip=$TARGET_ADDRESS $URL | \
    xargs echo updating $DOMAIN from $ACTUAL_ADDRESS to $TARGET_ADDRESS: | \
    $LOGGER
}


update_domain xxx.ath.cx myusername password members.dyndns.org
update_domain something.net otherusername otherpassword dyndns.strato.com

similar:

• https://gist.github.com/corny/7a07f5ac901844bd20c9
• https://www.krausmueller.de/en/2017/07/04/dynamic-dns-client-for-ipv6/