Support haproxy's proxy protocol (Tor's HiddenServiceExportCircuitID)

[Mikaela]

Tor 0.3.5 is introducing a new option HiddenServiceExportCircuitID which sends the circuit ID with Haproxy's proxy protocol to the service (Charybdis) using unused IPv6 address space.

I think supporting it could be desirable even if users can just request a new identity from Tor as it won't show them as connecting from 127.0.0.1 anymore and I think it might make one user making multiple connections from the same host more obvious.

• Tor's current documentation for HiddenServiceExportCircuitID
• Tor ML post saying 0.3.5
• haproxy version 1 proxy protocol documentation

When I asked if it's supported on IRC, I got these responses that may be helpful:

2018-10-08T21:26:43+0300 <mr_flea> Mikaela: I don't think there's support for it right now. I looked into writing it a while back but never ended up implementing it as it was unneeded at the time. however, it seems pretty easy to write a module for; it can be implemented as a command during registration
2018-10-08T21:27:14+0300 <mr_flea> the WEBIRC module does something very similar and could be modified to support the (human-readable, v1 variant) of proxy protocol
2018-10-08T21:27:38+0300 <mr_flea> the binary variant is more difficult to support (but it doesn't look like that's what Tor is implementing)