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Add info about rendezvous methods to client README.

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David Fifield 2021-07-19 09:01:17 -06:00
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@ -52,3 +52,59 @@ To bootstrap Tor, run:
tor -f torrc
```
This should start the client plugin, bootstrapping to 100% using WebRTC.
### Registration methods
The Snowflake client supports a few different ways of communicating with the broker.
This initial step is sometimes called rendezvous.
#### Domain fronting HTTPS
For domain fronting rendezvous, use the `-url` and `-front` command-line options together.
[Domain fronting](https://www.bamsoftware.com/papers/fronting/)
hides the externally visible domain name from an external observer,
making it appear that the Snowflake client is communicating with some server
other than the Snowflake broker.
* `-url` is the HTTPS URL of a forwarder to the broker, on some service that supports domain fronting, such as a CDN.
* `-front` is the domain name to show externally. It must be another domain on the same service.
Example:
```
-url https://snowflake-broker.torproject.net.global.prod.fastly.net/ \
-front cdn.sstatic.net \
```
#### AMP cache
For AMP cache rendezvous, use the `-url`, `-ampcache`, and `-front` command-line options together.
[AMP](https://amp.dev/documentation/) is a standard for web pages for mobile computers.
An [AMP cache](https://amp.dev/documentation/guides-and-tutorials/learn/amp-caches-and-cors/how_amp_pages_are_cached/)
is a cache and proxy specialized for AMP pages.
The Snowflake broker has the ability to make its client registration responses look like AMP pages,
so it can be accessed through an AMP cache.
When you use AMP cache rendezvous, it appears to an observer that the Snowflake client
is accessing an AMP cache, or some other domain operated by the same organization.
You still need to use the `-front` command-line option, because the
[format of AMP cache URLs](https://amp.dev/documentation/guides-and-tutorials/learn/amp-caches-and-cors/amp-cache-urls/)
would otherwise reveal the domain name of the broker.
There is only one AMP cache that works with this option,
the Google AMP cache at https://cdn.ampproject.org/.
* `-url` is the HTTPS URL of the broker.
* `-ampcache` is `https://cdn.ampproject.org/`.
* `-front` is any Google domain, such as `www.google.com`.
Example:
```
-url https://snowflake-broker.torproject.net/ \
-ampcache https://cdn.ampproject.org/ \
-front www.google.com \
```
#### Direct access
It is also possible to access the broker directly using HTTPS, without domain fronting,
for testing purposes. This mode is not suitable for circumvention, because the
broker is easily blocked by its address.