It is speculated that Google and Amazon blocked domain fronting upon request from the Russian government since domain fronting allows circumventing censorship. What is domain fronting, and how to use it in Nginx?
Today, almost every connection on the web is encrypted using TLS, making it impossible for eavesdroppers to see what you do online. However, since many websites are hosted on the same machine, the server needs to know ahead of time which certificate it should present to you. So, before the encrypted communication channel is set up, your client sends the unencrypted Server Name Indication to the server. Any eavesdropper (or a government that can only exist if the free exchange of information is prohibited) is able to read the SNI and knows which website you want to visit. If the SNI violates censorship regulations, the request might be blocked.
There are developments to overcome these privacy issues, for example, by
the initial Hello message from the client.
A much simple method is domain fronting by supplying a benign SNI.
A client establishes a secured
communcation channel to the server using some of the domains hosted on the
machine. In the case of large CDNs, there are possibly millions of valid domains,
for example, including
Eavesdroppers see the SNI and must assume you visit
Inside the encrypted communication channel, your client can send arbitrary HTTP
For example, it can send HTTP requests,
asking for content from a different domain, e.g.,
wikipedia.org that is
GET /wiki/Вторжение_России_на_Украину_(2022) HTTP/1.1 Host: ru.wikipedia.org ...
If the server supports domain fronting, it will respond with the content specified in the HTTP request circumventing censorship.
In practice with Nginx
$ cat <<EOF | gnutls-cli vinogreets.com -p 443 GET / HTTP/1.1 Host: howlargeisthelhc.com EOF
The server will respond with
HTTP/1.1 200 OK Server: nginx Date: Tue, 22 Mar 2022 21:31:32 GMT Content-Type: text/html Content-Length: 4665 Last-Modified: Wed, 24 Jun 2020 21:09:43 GMT Connection: keep-alive ETag: "5ef3c117-1239" Strict-Transport-Security: max-age=31536000 Accept-Ranges: bytes <!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8" /> <title>How large is the LHC?</title> ...
clearly sending you content for howlargeisthelhc.com even though the encrypted channel used vinogreets.com. Domain fronting is enabled by default in Nginx. You would need to disable it explicitly in Nginx.
What about client verification?
What happens if a domain uses TLS client verification?
With client verification, clients need to present a valid certificate themself
in order to establish the communication channel.
This can be seen as an
additional layer of authentification and authorization.
Suppose we have
secure.example.com protected using TLS client verification in
Nginx. The same instance of Nginx also powers
What happens if we establish the TLS connection with
then request content from the secured endpoint?
$ cat <<EOF | gnutls-cli public.example.com -p 443 GET / HTTP/1.1 Host: secure.example.com EOF
The server will respond with
GET / HTTP/1.1 Host: secure.example.com HTTP/1.1 421 Misdirected Request Server: nginx Date: Tue, 22 Mar 2022 21:35:44 GMT Content-Type: text/html Content-Length: 166 Connection: close <html> <head><title>421 Misdirected Request</title></head> <body> <center><h1>421 Misdirected Request</h1></center> <hr><center>nginx</center> </body> </html> ...
denying our attempt to access the secured endpoint. This line in the Nginx source code is responsible for stopping these kinds of attacks.
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