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Grundlagen wozu?

27 09 2006 Networking

Viele Leute die im Bereich der Netzwerktechnik Anwendungen programmieren, stossen sobald diese sich intensiever mit der Materie befassen auf das Problem, das Ihnen Grundlagen im Bereich der Netzwerktechnik fehlen. Genauso verhält es sich mit der Netzwerksicherheit. Schnell kommen bei Experimenten mit tcpdump oder Ethereal fragen auf wie z.B. :

"Ich habe einen ADSL-Router und zwei PCs daran separat angeschlossen. Mit tcpdump kann ich aber nur den Verkehr mitschneiden, der vom PC1 ausgeht, auf dem tcpdump läuft.Den verkehr von PC2 kriege ich nicht mit, ausser ich schliesse einen Hub zwischen die PCs und den Router, dann hab ich den gesamten Verkehr. Nun stehe ich vor folgender frage: Kann ich den gesamten Netzverkehr der über meinen ADSL-Router führt mittels tcpdump abfangen ohne dass ich einen Hub dazwischen schliesse? Oder beseht hier das Problem, dass aufgrund der IP-Adressierungwirklich nur der PC die Pakete erhält, die von ihm aus angefordert wurden?"

Doch wenn man sich schon einwenig mit den Grundlagen von CSMA/CD und Ethernet mit all seinen wichtigen Komponentenvertraut gemacht hätte, wäre einem schnell klar wie die Fragen zu beantworten sind. So hätte manch einer auf Wikipedia verlinkt und gesagt wonach zu suchen ist. Allerdings möchte man ja nicht so sein und gestattet sich eine umfassende Erklärung abzugeben.

Hierzu mal ein Ausschnitt:

"Das Problem ist ein bischen tiefgründiger. Ich probiere es mal in einfachen Worten darzulegen.

Ein Hub sendet alle Daten (Frames) an alle angeschlossenen Geräte - genauer an alle Ports. Nun
vergleicht die NIC (Network Interface Card; Netzwerkkarte) von jedem Geräte welches sich am Hub befindet,
die eigene MAC Adresse mit der Ziel MAC Adresse des Frames.
Jetzt fragt man sich natürlich, wieso tcpdump wenn ein Switch angeschlossen und trotz faslcher MAC Adresse alles mithört.
Der Grund liegt darin das die NIC im Promiscuous mode (zu deutsch: Promiskmodus), arbeitet, d.h.
sie nimmt auch Frames an bei denen die Ziel-MAC Adresse nicht mit der
der NIC übereinstimmt.Im Non-Promiscuous mode hingegen werden nur Frames mit der Ziel-MAC-Adresse der eigenen NIC und die mit der MAC-Adresse FF:FF:FF:FF:FF:FF angenommen, welche bei
Ethernet-Broadcasts verwendet wird (z.B. bei einem ARP-Request).
Der Switch der im ISO OSI-Referenzmodell eine Schicht weiter oben arbeitet (Sicherungsschicht; Schicht 2), schickt diese Daten nur an den Teilnehmer der auch die entsprechende MAC Adresse besitzt. Das bedeutet er übernimmt schon vorher die Aufgabe wohin die einzelnen Frames geschickt werden. Dies macht der Switch anhand einer Tabelle
(CAM-Tabelle) in der MAC-Informationen für die einzelnen Ports enthalten sind.
Nun kann man aber über bestimmte Mittel z.B. ARP-Spoofing oder
MAC-Flooding den Switch dazu bringen sich wie ein Hub zuverhalten.
Der Router ist ein noch intelligenteres Gerät es arbeitet auf Schicht
3 (Vermittlunsschicht) des OSI-Referenzmodells. Dieser arbeitet noch zusätzlich anhand von IP Adressen. ...."

Somit sollte einem schnell klar sein, wenn man sich intensiever mit einer Sache beschäftigen will, sollte man sic hzunächst über die Grundlagen im Klaren sein und auch verinnerlicht haben, ansonsten wird man früher oder später (wahrscheinlich früher) große Schwierigkeiten haben.Als Analogie hierzu könnte man die Mathematik nehmen. Wenn jmd. die Grundlagen wie Bruchrechnung, Potenzgesetz, Distributivgesetz, Kommutativgesetz oder Assoziativgesetz nicht beherrscht, so wird er bei Themen in der Mathematik wie Differential- oder Integral-Rechnung erhebliche Probleme haben.

Damit sollte klar sein, dass die Grundlagen das Fundament für höhere Stockwerke im jeweiligen Gebiet darstellen.





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