Running an obfsproxy bridge on port X

First, find out which port obfs3 is currently listening on by looking at Tor's log. For the examples below, obfs3 is running on port 59003 and our IP address is 203.0.113.1.

Example torrc

DataDirectory /path/to/tor_data  # location to store runtime data
Log notice file /path/to/tor_log # location to log notices, warnings, and errors
RunAsDaemon 1                 # runs as a background process
User $USERNAME                # lowers our permissions to this user

PublishServerDescriptor 0       # do not publish server descriptor
ServerTransportPlugin obfs3 exec /usr/bin/obfsproxy managed # enable obfs3 transport
BridgeRelay 1                   # be a bridge

Nickname $RELAYNAME           # name for this relay
SocksPort 0                   # prevents tor from being used as a client
ExitPolicy reject *:*         # prevents us from connecting to non-relays

Example log output

[notice] Registered server transport 'obfs3' at '0.0.0.0:59003'

iptables rules

Mirror on port 80:

-A PREROUTING -d 203.0.113.1/32 -p tcp -m tcp --dport 80 -j DNAT --to-destination 203.0.113.1:59003

Mirror on port 443:

-A PREROUTING -d 203.0.113.1/32 -p tcp -m tcp --dport 443 -j DNAT --to-destination 203.0.113.1:59003

PF rules

For this example, our interface is msk0.