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.
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=184.108.40.206.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.
SYNTAX: [digest].onion [ignored].[digest].onion
Version 2 addresses (deprecated since 0.4.6.1-alpha), the digest is the first eighty bits of a SHA1 hash of the identity key for a hidden service, encoded in base32.
Version 3 addresses, the digest is defined as:
onion_address = base32(PUBKEY | CHECKSUM | VERSION) CHECKSUM = H(".onion checksum" | PUBKEY | VERSION)[:2] where: - PUBKEY is the 32 bytes ed25519 master pubkey of the onion service. - VERSION is a one byte version field (default value '\x03') - ".onion checksum" is a constant string - H is SHA3-256 - CHECKSUM is truncated to two bytes before inserting it in onion_address
When Tor sees an address in this format, it tries to look up and connect to the specified onion service. See rend-spec-v3.txt for full details.
The "ignored" portion of the address is intended for use in vhosting, and is supported in Tor 0.2.4.10-alpha and later.
When Tor sees an address in this format, it immediately closes the connection without attaching it to any circuit. This is useful for controllers that want to test whether a given application is indeed using the same instance of Tor that they're controlling.
This feature was added in Tor 0.1.2.4-alpha, and taken out in Tor 0.2.2.1-alpha over fears that it provided another avenue for detecting Tor users via application-level web tricks.