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- Special Hostnames in Tor
- Nick Mathewson
- 1. Overview
- Most of the time, Tor treats user-specified hostnames as opaque: When
- the user connects to www.torproject.org, Tor picks an exit node and uses
- that node to connect to "www.torproject.org". Some hostnames, however,
- can be used to override Tor's default behavior and circuit-building
- rules.
- These hostnames can be passed to Tor as the address part of a SOCKS4a or
- SOCKS5 request. If the application is connected to Tor using an IP-only
- method (such as SOCKS4, TransPort, or NATDPort), these hostnames can be
- substituted for certain IP addresses using the MapAddress configuration
- option or the MAPADDRESS control command.
- 2. .exit
- SYNTAX: [hostname].[name-or-digest].exit
- [name-or-digest].exit
- Hostname is a valid hostname; [name-or-digest] is either the nickname of a
- Tor node or the hex-encoded digest of that node's public key.
- When Tor sees an address in this format, it uses the specified hostname as
- the exit node. If no "hostname" component is given, Tor defaults to the
- published IPv4 address of the exit node.
- It is valid to try to resolve hostnames, and in fact upon success Tor
- will cache an internal mapaddress of the form
- "www.google.com.foo.exit=64.233.161.99.foo.exit" to speed subsequent
- lookups.
- The .exit notation is disabled by default as of Tor 0.2.2.1-alpha, due
- to potential application-level attacks.
- EXAMPLES:
- www.example.com.exampletornode.exit
- Connect to www.example.com from the node called "exampletornode".
- exampletornode.exit
- Connect to the published IP address of "exampletornode" using
- "exampletornode" as the exit.
- 3. .onion
- SYNTAX: [digest].onion
- The digest is the first eighty bits of a SHA1 hash of the identity key for
- a hidden service, encoded in base32.
- When Tor sees an address in this format, it tries to look up and connect to
- the specified hidden service. See rend-spec.txt for full details.
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