An Exit Enclave is a term used to reference a Tor relay that allows exit to another, non-tor service on that same IP address. An example is https://archive.torproject.org/ has IP Address 184.108.40.206 and is reachable on tcp port 443. The Tor Exit Enclave is named archivetpo with ExitPolicy accept 220.127.116.11:443. The Tor relay archivetpo becomes the preferred path to https://archive.torproject.org/ through the Tor Network.
This feature is useful for people who run services and wish to ensure Tor users have access though their nodes.
The current stable version of Tor (0.2.2.x) still supports Exit enclaves. The new versions of Tor (> 0.2.3.x) use a new descriptor format (microdescriptors) allow relays to specify an Exit Enclave policy, but clients will not use it, therefore voiding the purpose of exit enclaving.
Since microdescriptors don't include the addresses listed in the exit policy there is no way for the client to understand that a certain Tor exit is an enclave for a certain destination address.
Even with Tor 0.2.2.x Exit Enclaves are not used for the first connection since the first connection is usually done by hostname and not IP address. This means that if the client does not have the hostname lookup result in cache they would not be using the enclave on the first connection, but only on subsequent ones.
Another reason why enclaves are not going to work properly with 0.2.3.x is that it is not possible to prevent a relay from being overloaded by setting a low "MaxAdvertisedBandwidth". This means that if the enclave is receiving a lot of Tor relay traffic the connection to the enclaved service end up being very slow.
The general feeling is that something like exit enclaves is needed, but the current design of enclaves does not fit even the most lax threat models.
The only case in which Exit Enclaves make sense is for censorship circumvention on a IP basis, though even in this case if the client were to exit through a censoring exit on first connection they would not be able to access the content. All subsequent connection would be enclaved, but it requires the user to make another connection attempt.
Overall the use of enclaves is not advised for these two main reasons:
They will not be supported in future versions of Tor (> 0.2.3.x)
They do not fit any particular threat model
If you would still like to be and Exit Enclave this is an example torrc of an Exit Enclave that only allows connections out of the Tor network services offered on the same system. The following configuration is used for the Tor exit enclave archivetpo:
SocksPort 9050 # what port to open for local application connections SocksListenAddress 127.0.0.1 # accept connections only from localhost ORPort 9001 Nickname archivetpo ExitPolicyRejectPrivate 0 ExitPolicy accept 18.104.22.168:443 ExitPolicy reject *:*