Protects Tor bridges from being scanned (and as a result blocked) by GFW probes.
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UPDATE: Rumour has it that brdgrd is no longer working because the GFW seems to do TCP stream reassembly now.
Brdgrd runs in user space and makes use of the libnetfilter_queue mechanism (and hence only runs on Linux) to move packets from kernel into user space. Only TCP SYN/ACK segments are passed into user space because brdgrd is only interested in TCP handshakes and not in established connections. Hence, there are virtually no performance implications.
Brdgrd intercepts the SYN/ACK segment that a Tor bridge sends to its client. It then rewrites the TCP window size announced in this segment. The window size is rewritten to a smaller, randomly chosen value. That way, the client "fragments" its cipher list inside the TLS client hello. The GFW will not recognize the cipher list and as a result will not scan the bridge.
Brdgrd needs iptables rules to feed it with data. The following script passes all Tor-related SYN/ACKs to brdgrd:
iptables -A OUTPUT -p tcp --tcp-flags SYN,ACK SYN,ACK --sport $TORPORT -j NFQUEUE --queue-num 0
You can compile brdgrd by running
makeand then start it by running
sudo ./brdgrd. Keep in mind that the above iptables rule tries to push SYN/ACKs to userspace. If brdgrd is not running, Tor cannot handle new connections because there is no userspace program to process the SYN/ACK segments.
It is possible to set the
CAP_NET_ADMINcapability for the brdgrd executable so that you do not need root privileges to run the binary:
sudo setcap cap_net_admin=ep ./brdgrd
Please send patches, suggestions, and comments to [email protected]
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