While setting up obfs3 bridges on ports lower than 1024 seem to be tricky, some firewalls only allow connections on these certain ports.
An in detail, step by step guide could help us here.
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This is kind of annoying at the moment and you give up running obfsproxy in managed mode, but you can do something like this.
Setup your OS firewall to redirect port 80 (or 443) to a higher port:
iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 8000
Note: just executing this command as root does not persist across reboots. See your firewall documentation for how to set this up correctly.
Run obfs3 in standalone mode. You need to do this separately from launching Tor:
obfsproxy obfs3 --dest 127.0.0.1:9001 server YOUR_EXTERNAL_IP:8000
Use the following line in the torrc to report that the obfsproxy instance is listening on port 80 to tor.
ServerTransportPlugin obfs3 proxy YOUR_EXTERNAL_IP:80
In an ideal world, we would come up with a solution to allow obfs3 to run in managed mode and support port forwarding (or to bind to ports < 1024), but till then this is probably the best that can be done. See #7875 for discussion on the proper path to take.
On Windows any user can make use of any port, as long as it is not in use already.
Adding the following lines to your torrc, with Obfsproxy already in place.[*]
ServerTransportPlugin obfs3 exec X:\$pathtoexecutable\obfsproxy.exe managed
Where X: is the drive the binary was placed on, what is most likely C:\
ServerTransportListenAddr obfs3 0.0.0.0:80
ServerTransportListenAddr obfs3 0.0.0.0:443
[*] Please note, that there is currently no Obfsproxy package for Windows, one way is to take the relevant files from the Pluggable Transport Bundle.