Why do some apps break with Tor over Tor (Whonix, TAILS)?

Question

Some apps (which have a built-in Tor daemon) will not work on TAILS or Whonix (ie systems that force all their traffic though the Tor network) because this creates a Tor-over-Tor connection. Why?

Examples of such apps:

1. OnionShare
2. Feather Wallet

I understand that OnionShare works by creating an Onion Service, which is more complicated than apps that just connect as a client to Tor (eg Tor Browser).

And I also understand that running Tor-over-Tor is not advised. Sure, it will have performance (and privacy?) degradation. I understand that. But I'd expect that these apps would just run slightly slower. The reality is they completely break. Why?

Please explain [a] "like I'm 5" and also explain [b] "like I'm a Tor developer" why some apps with built-in Tor daemons fail to run on a system that forces all its traffic through Tor.

Answer 1

Relays on the Tor network do not allow Tor-over-Tor. This is to avoid circuit amplification attacks on the network. I would guess that this is designed to prevent malicious clients from wasting a lot of the network's bandwidth by having traffic loop through the network many times before exiting.

You can read some discussion on the tickets:

• "Exits should block reentry into the tor network"
• "Clients should predict that their stream will be refused for trying to reenter, and decline to try"