I have a Laptop with debian jessie installed. At my home network, i could connect successfully to the internet (ping google.de successful). At the external network, i can't ping google.de (result: unknown host google.de), although i can ping 8.8.8.8 and 8.8.4.4 (those are the google public dns servers Source) I assume, /etc/resolv.conf was configured to use a private DNS resolver(nameserver 255.0.0.0) at home, therefore it doesn't work at the external network.  Changing that to nameserver 8.8.8.8 and 8.8.4.4 as suggested in 1 doesn't help, it is overwritten after the dhclient is run. chmod -w /etc/resolv.conf (it has now only read permissions) also doesn't help. It is still overwritten, when running dhclient.I use ifup and ifdown and those legacy tools to connect.my /etc/network/interfaces:source /etc/network/interfaces.d/*allow-hotplug wlan0auto wlan0iface wlan0 inet dhcp    wpa-ssid <...>    wpa-psk <...>Now my question:You need to make sure DHCP (if in use) is configured correctly, and  your local DHCP server (usually your cable / adsl modem) is handing  out the right details.How do i do that? I can't use that suggestion, because obviously i have no internet connection. 
can ping google dns, but not google.de -> unknown host // worked in another network
debian;networking;dns;resolv.conf
After you have setup your network device(s) with DHCP change the /etc/resolv.conf file manually, the file will only be overwritten when dhclient is restarted and not constantly. When you changed the /etc/resolv.conf file you should be able to perform hostname lookups and to be able to install the resolvconf package. After the package is installed you can follow the instructions in the referenced question debian, problem with DNS to finish your's.