inadyn is a small and simple dynamic dns update client. As the most distributions just provide a quite old version 1.9 or older this will handle just the version from troglobit.com/projects/inadyn/
They provide already a debian package which is easy to install. The sources are available here -> github.com/troglobit/inadyn

inadyn 2.7

/etc/inadyn.conf sample config

# /etc/inadyn.conf

period = 300
allow-ipv6 = true
secure-ssl = true

custom ipv4.myonlineportal {
    username = your-myonlineportal-user
    password = your-myonlineportal-password
    checkip-server = ipv4.myonlineportal.net
    checkip-path = /checkip
    checkip-ssl = false
    ddns-server = myonlineportal.net
    ddns-path = "/updateddns?hostname=%h&ip=%i"
    hostname = your-myonlineportal-domain
}

custom ipv6.myonlineportal {
    username = your-myonlineportal-user
    password = your-myonlineportal-password
    checkip-server = ipv6.myonlineportal.net
    checkip-path = /checkip
    checkip-ssl = false
    ddns-server = myonlineportal.net
    ddns-path = "/updateddns?hostname=%h&ip=%i"
    hostname = your-myonlineportal-domain
}