Gratis Internet über DNS

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Man sitzt im Starbucks oder im Zug, und das einzige Internet, dass es dort gibt, ist ein kostenpflichtiger Wifi Hotspot. Der macht das Internet erst auf, wenn man sich anmeldet und zahlt. Pfeiffendeckel! Hier eine Anleitung, wie man darüber surfen kann, ohne sich anzumelden und etwas dafür zu bezahlen.

Bei einem öffentlichen WLAN sind die DNS-Abfragen offen.

Die müssen auch offen sein, damit überhaupt der Server für die Anmeldeseite gefunden wird und diese angezeigt werden kann.

DNS ist das Domain Name System und wandelt einen Domainnamen zu einer IP Adresse um, vereinfacht gesagt.

Beispiel: google.ch zeigt 173.194.44.183 und zwei weitere Adressen an:

Im Normalfall™ werden damit IP Adressen abgefragt, wie schon erwähnt. Diese Abfragen haben den Typ „A“ (A-Records).

Es gibt aber noch weitere Eintragstypen.

AAAA: IPv6 Adresse
TXT: Text
NULL: Irgendwas, bis zu 64k gross

Eine Abfrage nach einem TXT-Record sieht z.b. so aus:

Hier wird der kurze Beschreibungstext der Wikipedia als TXT-Record bereitgestellt (ist ein zusätzlicher Dienst/Hack, nix offizielles :))

Aber vom Prinzip her: Ich frage nach TXT (QUESTION SECTION) und erhalte eine Antwort (ANSWER SECTION).

Was wäre, wenn ich mir irgendwo im Netz einen eigenen DNS-Server bereitstelle, der mir Internet über diese TXT-Einträge durchschleust? Der mir einen VPN-Tunnel aufbaut, und als Übertragungsmedium verwendet er diese TXT-Records? Genau das machen wir nun. IP over DNS.

Voraussetzungen

Unser DNS-Server, der das kann, heisst iodine.

Iodine is a chemical element with symbol I and atomic number 53.

Dann brauchen wir irgendwo im Netz einen Server, wo man das laufen lassen kann.

Und dann brauchen wir zwei DNS-Einträge (foo.com mit der eigenen Domain ersetzen):

ns.foo.com NS –> a.foo.com
a.foo.com A –> [IP iodine-Server]

Das bedeutet:
Für Anfragen an ns.foo.com ist der a.foo.com zuständig,
a.foo.com hat die IP [IP iodine-Server].

Man kann das bei seinem eigenen DNS-Server einrichten; oder viele Hoster bieten eine Administrationsoberfläche dafür an wie z.B. Plesk. Bis die Änderungen aktiv sind, dauert es halt solange, wie das definiert wurde. Meistens einen Tag.

Iodine Server

Sind die Einträge erstellt, kann man den iodine Server einrichten und starten. Der @Tiefpunkt hat dazu zwei tolle Skripts erstellt: Wrapper shell scripts for iodine.

Beim iodine-server.sh Domäne (ns.foo.com) und Passwort eintragen und starten.

Auf dieser Seite kann man überprüfen, ob der Server läuft und alles ordnungsgemäss eingerichtet ist.

Well done, your iodine setup seems fine!

Iodine Client

iodine-client.sh mit den gleichen Einstellungen wie oben auf dem eigenen Notebook einrichten und starten – voilà, wir haben Internet!

Was im Hintergrund so passiert

Iodine richtet ein neues Interface dns0 ein:

iodine-client.sh setzt die Default-Route auf dieses Interface, behält aber die aktuellen DNS-Server.
iodine-server.sh erstellt ein kleines NAT, damit die Antworten sauber durchkommen, aktiviert IP-Forwarding im Kernel und startet iodined.

Hier ein Auszug aus der Kommunikation zwischen iodine Client und Server: