SFL Prof. Dr. C. Rossow / S. Hausotte TU Dortmund WS 2021/2022 Exercise 3 (Network Security)
3.1 Firewalls
Given an internal network with all hosts in the range 192.168.0.0/24, write firewall rules according to the given textual descriptions. Remember to include default policies.
(a) HTTP/HTTPS only: Allow all internal hosts to act as HTTP/HTTPS clients, i.e. connect to TCP ports 80 and 443. Forbid all other communication.
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Action Source Source port Destination Dest. port Protocol State
(b) Blocklisting: Allow all outgoing connections, but block traffic to and from servers 6.6.6.6/32 and 66.66.66.66/32.
Action Source Source port Destination Dest. port Protocol State
(a) Which root CAs does your browser trust? How many are there?
Hints: For Chrome and Chromium derivatives, go to chrome://settings/certificates, and
look at the ¡°Authorities¡± tab. For Firefox, go to about:preferences#privacy, scroll down to ¡°Security¡±, and click ¡°View Certificates¡± under the ¡°Certificates¡± heading.
(b) Which and how many certificates are there in the certificate chain for github.com?
Hints: To view the certificate (chain) of a site, start by clicking the lock in the address bar. Then, in Chromium and derivatives, click ¡°Certificate¡±. For Firefox, click the sideways arrow next to ¡°Connection secure¡±, then click ¡°More Information¡±. In the newly opened window, click
¡°View Certificate¡±.
Until when is their certificate valid?
What cryptographic algorithm does the key underlying the certificate for google.com use? NB: This can be different from the algorithm used to sign the certificate.
Compare the certificates of google.com and github.com with regards to the key¡¯s algorithm and size.
3.3 Denial of Service
(a) What types of DoS attacks do you know? What countermeasures are there?
(b) Why is it more difficult to fend off DDoS attacks than centralized DoS attacks?
SFL Prof. Dr. C. Rossow / S. Hausotte TU Dortmund WS 2021/2022 Exercise 3 (Network Security)
3.4 Onion Routing
(a) Explain how onion routing (or TOR in particular, if you prefer) achieves the goal of anonymity. (b) Explain for each type of node (entry, middle, exit) which information they have access to.
(c) Assume an evil party operates a single node within a TOR network. Will they be able to cause damage? If so, how? What if they control multiple nodes?
(d) What are ways deanonymization could occur in a TOR network? Are they feasible?
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