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![IPv6] Address
Check/Test IPs
{{{
> ifconfig
> ip a | grep inet6
> ip -4 addr show
> ip -6 -o addr show dev eth0 scope global
> ping -6 2a02:5a0:4110:f590:fdd3:3e99:1234:9876
> ping -6 www.google.com
> nslookup -6 www.google.com
}}}

The raspberry may have a random ipv6 to hide target.
This can happen with "privacy extensions" enabled via dhcpcd (Dynamic Host Configuration Protocol Client Daemon).
But this is not enabled by default, check with \\
> service dhcpcd status \\
If "Unit dhcpcd.service could not be found." then the random IP is set from Fritz!Box, \\
If found, you can disable (not recommended):
{{{
> sudo vi /etc/dhcpcd.conf
> slaac private  # change to "slaac hwaddr"
> sudo service networking restart
}}}
see Heimnetz > Netzwerk > YourDevice > Heimnetz > "IPv6-GUA-Temporary" \\
The official IPv6-GUA (Global Unicast-Address) is another one.\\
But with dyndns we can use the random/temporary one.

Windows:
{{{
> ping -6 pi5backup.v6.rocks
> ping -6 2a02:5a0:410f:46f0:51ef:ea06:9875:1234
}}}

IP lifetime
{{{
> ip addr show eth0
set lifetime to forever:
> sudo ip addr change 2a02:5a0:4110:f590:6547:1234:9876:ee dev eth0 valid_lft forever preferred_lft forever
}}}

! [Network Manager|https://wiki.debian.org/NetworkConfiguration#DNS_configuration_for_NetworkManager]
I use a custom DNS server setting, because the fritzbox does not resolve my dynv6 domains (?).\\
Do not modify file "/etc/resolv.conf". Use network manager instead with "nmcli".

{{{
> systemctl status NetworkManager
> sudo vi /etc/NetworkManager/NetworkManager.conf            # config
> sudo vi /etc/NetworkManager/system-connections/*Wired*     # config
> sudo systemctl status NetworkManager.service 
> sudo systemctl restart NetworkManager.service 	# log in > journalctl -u NetworkManager.service -b
> sudo nmcli device reapply eth0
> sudo nmcli general
> sudo nmcli general reload dns-full    # flush dns cache
> sudo nmcli -o device show	        # show interfaces settings
> sudo nmcli -p connection show	        # show interface status

#THIS IS NECESSARY
> sudo nmtui	                                                                    # visual tool for network manager
> sudo nmcli con mod "Wired connection 1" ipv4.dns "1.1.1.1,8.8.8.8"                # add cloudflare and google dns
> sudo nmcli con mod "Wired connection 1" ipv4.ignore-auto-dns no                   # this adds "search fritz.box" to get ips at all from frit.box
> sudo nmcli con mod "Wired connection 1" ipv6.ignore-auto-dns yes                  # we skip ipv6 dns
> sudo nmcli c down "Wired connection 1" && sudo nmcli c up "Wired connection 1"    # restart
> sudo vi /etc/resolv.conf    # validate settings
}}} 

DHCP
{{{
> service --status-all    # check all services
}}}
If you see "networking", this seems do be dhclient
{{{
> sudo vi /etc/default/networking    # config
> sudo vi /etc/dhcp/dhclient.conf    # config
> sudo dhclient -v eth0              # renew
> sudo dhclient -6 -r -v eth0        # force renew
> sudo systemctl restart networking  # restart
> sudo ifconfig eth0 down && sudo ifconfig eth0 up
}}}

!! DynDNS

!DynDNS with [ddclient|https://github.com/ddclient/ddclient/releases]
{{{
> sudo apt-get update                               # update repositories
> sudo apt-get install ddclient                     # install
> sudo vi /etc/ddclient.conf                        # config file
> sudo service ddclient restart                     # restart daemon
> sudo rm /var/cache/ddclient/ddclient.cache        # clear IP cache uploaded before (or use -force)
> systemctl status ddclient                         # status check
> journalctl -xeu ddclient                          # log 
> sudo ddclient -query                              # try all common dyndns services
> sudo ddclient -daemon=0 -debug -verbose -noquiet  # start directly with verbose output to analyse

> sudo vi /etc/default/ddclient
> sudo vi /usr/bin/ddclient
}}}

With current Raspberry OS (Debian GNU/Linux 12, bookworm) you get ddclient __v3.10.0. This could not determine the ipv6 address__ ! \\
The latest v3.11.2 worked fine, see next.

!Install ddclient on your own
Based on [najigram.com|https://najigram.com/2024/02/how-to-install-the-latest-ddclient/]
{{{
> sudo apt purge ddclient
> sudo apt install libjson-any-perl libdata-validate-ip-perl libio-socket-ssl-perl
> sudo apt install libtool        # this is missing at najigram.com and solves issues if autogen does not work
> wget https://github.com/ddclient/ddclient/archive/refs/tags/v3.11.2.zip
> unzip v3.11.2.zip
> cd ddclient-3.11.2
> sudo ./autogen
> sudo ./configure --prefix=/usr --sysconfdir=/etc/ddclient --localstatedir=/var
> sudo make
> sudo make VERBOSE=1 check
> sudo make install	
> sudp cp sample-etc_systemd.service /etc/systemd/system/ddclient.service
}}}

> sudo vi /etc/ddclient/ddclient.conf

Add below "ssl=no":
{{{
protocol=dyndns2
#use=if, if=eth0
usev6=ipv6, ipv6=eth0
#use=web, web=checkip.dyndns.org,  web-skip=‘Current IP Address’
verbose=yes
daemon=900
#pid=/var/run/ddclient.pid
#mail-failure=msebel20@gmail.com
server=dynv6.com
login=XUsernameX
password=XPasswordX
<your dyndns server name>
}}}

! Test DynDNS / Error DNS_PROBE_FINISHED_NXDOMAIN
Note: it is sufficient to register just an ipv6 address with your dyndns provider (AAAA record). You do neither need an ip4 address (A record) nor any other DNS zone entry. \\
# enable a port to forward to your device in your router
# test the port forwarding without DNS first, you can call an ipv6 address like this https://~[2a02:x205:..~]:1234 (https://~[ipv6~]:port)
# test domain name resolution (DNS)
** Firefox uses its own DNS server (not the OS one) and those worked from scratch, see Settings > Security > DNS over HTTPS > Anbieter (Cloudflare)
** Windows used the FritzBox to resolve and that did not work (?). I entered a dedicated DNS server at Windows + R > ncpa.cpl > Ethernet > Porperties > Internetprotokoll, Version 4+ 6 > DNS (I used cloudflare and google as alternative, you can search for "DNS server list").
[{Table style='width:auto'

|Cloudflare|1.1.1.1|2606:4700:4700::1111
|Google|8.8.8.8 or 8.8.4.4|2001:4860:4860::8888 or 2001:4860:4860::8844
|Digitalcourage e.V.|46.182.19.48|2a02:2970:1002::18
|Digitale Gesellschaft|185.95.218.42|2a05:fc84::42
|dismail.de|80.241.218.68|2a02:c205:3001:4558::1
}]
{{{
> ipconfig
> ping -6 2a02:x205:... or serverName
> netsh int ip show address
> route print -6
> arp -a
}}} 
** Chrome uses the OS one and this did not work. You can either update Windows or use a dedicated for Chrome directly too at chrome://settings/security > Erweitert > Sicheres DNS verwenden = Enabled > DNS Anbieter auswählen > Cloudflare (1.1.1.1)
** FritzBox: see [here|https://www.heise.de/tipps-tricks/Fritzbox-DNS-Server-aendern-5054919.html], > Internet > Zugangsdaten > DNS-Server

!DNS Cache Lookup / Purge
* Firefox: about:networking#dns
* Chrome: chrome://net-internals/#dns
* Edge: edge://net-internals/#dns 
* Windows: 
** > ipconfig /release + ipconfig /flushdns + ipconfig /renew
** > ipconfig /displaydns|findstr pi5
* Linux:
** > sudo nmcli general reload dns-full