TorBrowser should advertise Onion Networking capability
For many years, it's been expected that people who use Tor to surf the web, need to hide the fact that they are using Tor, and that the best way to achieve that is to attempt to mimic some other browser - typically Windows-Firefox or similar.
There is no argument that other protections such as protecting screen sizes and so forth, continue to be useful in protecting the user against fingerprinting and tracking, so those and other protections should certainly continue.
However, there can be no reasonable argument against the observation that:
the world treats any inbound TCP connection from a Tor exit node as "traffic from Tor"
there exist any number of "IP reputation" systems which track (with varying amounts of lag) the state of the Tor exit node cloud, and essentially assign Tor a "geography" and offer the user the opportunity to "block" it, etc
that there exist some number of sites which would like to do something "nice" for Tor users (there are people who would like to do something "nasty", too, but those people are already served by upstream geoblock providers) and those sites are typically hampered by having no concept of "How many of our users are legitimate people coming from a Tor Browser?"
So: people who use Tor to access a website are:
1/ immediately "outed" as coming from Tor, by virtue of their apparent IP address
2/ treated as a faceless group, worthy of (usually: trivial) blocking, by flicking a "Block Bad Sites" IP-reputation switch
3/ hard to identify at "Layer-7" (ie: web logs) as being legitimate users
4/ any Trolls amongst the Tor userbase who harass are investigated, determined as "being from Planet Tor" and thereby drag down the reputation of Tor even further.
Two personal observations are useful here:
a) When I was building the Facebook Onion, we had to run an experiment which was essentially "Are the people who access Facebook over Tor, generally bad people?" - because common sense suggested that we check this. We checked a sample of users who accessed Facebook over Tor, and found that overwhelmingly (> %2014 Tor Blog Replacement) they were just normal users doing normal things. This was a realisation, and combined with the scale of Tor usage, led to the creation of the Facebook onion.
b) Conversations with [people at Cloudflare] who noted to me that TorBrowser's attempts to "hide" meant that in site-protection "flows" (think: Captchas) it was hard to economically (think: experience latency from a geocheck hit) identify TorBrowser users so as to give them special consideration and care.
Therefore, I propose that TorBrowser be amended to include "OnionCapable" - or "TorCapable" or some other well-advertised, similar string - in the "User-Agent" header of the browser.
== Why the User-Agent?
Because if a special, magic header were created, it would probably be dropped en-route upstream.
Also: site owners already know how to act on User-Agent strings, whereas new headers would be considered "deep magic".
There is the possibility to go to a commercial CDN and ask them to serve certain content or return certain headers ("Alt-Svc: foofoofoofoofoof.onion" perhaps?) on the basis of "User-Agent", but asking them to detect and act upon the presence of "X-Tor-Browser-Special-Header:1" would be onerous and unlikely to succeed.
== But what about Logs?
Let's think about that threat model:
We want more people to use Tor because it's a better network.
We want sites to know that their users are using Tor
Tor usage can be inferred anyway, from IP addresses and time cross-checked against relay logs
So what's the concrete problem, here? Perhaps that some Stasi will in some case subpoena the logs of a service provider and then use that to prove that [person] was using Tor at the time? That's a pretty small risk compared to concretely enabling better Tor for everyone.
== What about #21952 (moved) ?
That's some interesting client-side work, and I am sure that it would benefit from telling the server "I can do Tor, please be nice to me", but otherwise it's essentially disjoint.
== But many many site owners will detect this new header and block Tor access!
If they are inclined then they already do, and it's probably better to enable them get it out of the way more easily, so that their behaviour and attitudes can be called-out.
If a site is so trivially tricked that their "OnionCapable" detection is their only protection, then this also screams for a TorBrowserExtension which tests whether that header is being detected and causing blocking, which will help enumerate hostile sites for public awareness.
Sites, if they care, can already determine whether someone is accessing them from Tor.
If such determination were made easier, other sites could be nicer to Tor users
Tor would become even more normal.
It's time for TorBrowser to come out of the closet.