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tq-032 Attempt upgrade to H2

When a transparent HTTP proxy is in the path between the probe and the target server, an attempt to upgrade to H2 may reveal its presence in one of the following ways:

  1. The connection fails due to an unknown header being present
  2. The header is passed successfully to the server but the connection fails because the reply using h2c was not understood by the proxy
  3. A server that is expected to negotiate h2 or h2c does not, or a server that is not expected to negotiate h2 or h2c does

When a transparent TLS proxy is in the path, it may reveal its presence in one of the following ways:

  1. Stripping the ALPN option which would cause h2 to not be negotiated where expected
  2. The connection fails because the ALPN option is unknown


  1. Fetch a webpage from a target server using either HTTP or HTTPS, with an H2 upgrade mechanism
  2. If the connection fails, retry the request without the use of the upgrade mechanism to confirm that this was responsible for the connectivity failure, and not just that the host was down

Implementation issues

  • It would be possible to attempt the use of multiple protocol features on the first connection, falling back to trying each individually only if the first connection fails. In some rare cases the use of a protocol feature can brick the CPE or upstream middleboxes but this should either happen immediately or never happen.


  • None yet



  • An independent implementation exists for this technique in PATHspider's H2 plugin