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56 lines
2.8 KiB
Plaintext
56 lines
2.8 KiB
Plaintext
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Part one: Overview and explanation
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Because tor is an application-level proxy, it needs client-side support
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from every client program that wants to use it. (This is different from
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systems like Freedom, which used a single client-side program to capture
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all packets and redirect them to the Freedom network.) Client applications
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need two general classes of modifications to be compatible with tor:
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1) Whenever they call connect(), they instead should connect() to the
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local onion proxy and tell it "address and port". The onion proxy will
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itself make a connection to "address and port", and then the client
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application can talk through that socket as if it's directly connected. To
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support as many applications as possible, tor uses the common "socks"
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protocol which does exactly the above. So applications with socks support
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will support tor without needing any modifications.
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2) Applications must not call gethostbyname() to resolve an address
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they intend to later connect() to via onion routing. gethostbyname()
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contacts the dns server of the target machine -- thus giving away the
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fact that you intend to make an anonymous connection to it.
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To clarify, I need to explain more about the socks protocol. Socks
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comes in three flavors: 4, 4a, and 5. The socks4 protocol basically
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uses IP and port -- so it is unsuitable because of the gethostbyname()
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issue above. Socks4a is a slight modification to the socks4 protocol,
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whereby you can specify an IP of 0.0.0.x to signal the socks server
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that you will instead be sending a hostname (fqdn). So applications with
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socks4a support are all set. Socks5, on the other hand, allows the client
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to specify "address type" and then an address -- so some applications
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choose to supply an IP and others choose to supply a hostname. If the
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application uses socks5 you must investigate further to decide whether
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it's leaking anonymity.
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Part two: using tsocks to transparently replace library calls
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tsocks (available from http://tsocks.sourceforge.net/ or from your
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favorite apt-get equivalent) allows you to run a program as normal,
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but it replaces the system calls for connect() to connect to the socks
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server first and then pass it your destination info. In our case the
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socks server is a tor process (running either locally or elsewhere).
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In general this works quite well for command-line processes like finger,
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ssh, etc. But there are a couple of catches: A) tsocks doesn't intercept
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calls to gethostbyname. So unless you specify an IP rather than hostname,
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you'll be giving yourself away. B) Programs which are suid root (or
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anybody else) don't let you intercept the system calls -- ssh falls into
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this category. But you can make a local copy of ssh and use that. C)
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Probably tsocks doesn't behave well for behemoths like Mozilla.
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Part three: applications which support tor correctly
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