ddclient is an open source dynamic dns update client, based on Perl and available on the most available distributions.
The sources are available here -> github.com/ddclient/ddclient

ddclient 3.8.3

/etc/ddclient.conf sample config

# /etc/ddclient.conf

protocol=dyndns2
# use=web, web=ipv4.myonlineportal.net/checkip
use=if, if=eth0
server=myonlineportal.net
login="MyOnlinePortal Username"
password="MyOnlinePortal Password"
# Your complete MyOnlinePortal Domain
yourdomain.myonlineportal.net


With 'use=if' the ip from the specified interface from 'if=' will be used.
With 'use=web' ddclient will call a checkip script to identify the used ip for external communication. This is helpfull if the ddclient is running behind a router and don't have the external interface bounded.

ddclient 3.9.1

Version 3.9.1 is the latest available version from the homepage and includes a lot of fixes especially for IPv6 updates

/etc/ddclient/ddclient.conf sample config

# /etc/ddclient/ddclient.conf

protocol=dyndns2
ipv6=yes
use=web, web=ipv6.myonlineportal.net/checkip
server=myonlineportal.net
login="MyOnlinePortal Username"
password="MyOnlinePortal Password"
# Your complete MyOnlinePortal Domain
yourdomain.myonlineportal.net


With this version the IPv6 works fine. For this we provide an URL which will only return your IPv6 address if you call the checkip.