TOR Transparent Proxy - How does to the TCP redirecting work

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http://unix.stackexchange.com – I used an iptables ruleset based on the sample ruleset from https://trac.torproject.org/projects/tor/wiki/doc/TransparentProxy, Section Linux, Local Redirection Through Tor, to set up Tor as a transparent Proxy. I thought I had understood that ruleset, but theres one point I simply dont get: That TCP segments are only redirected to the port Tor listens on when the SYN flag is set. For example I want to visit www.example.com. My browser does a DNS request, gets the corresponding IP address and sends a TCP segment to that address, the SYN flag is set. According to the ruleset, the segment is (HowTos)