Fix DTLS asynchronous write handling.

Although the DTLS transport layer logic drops failed writes on the floor, it is
actually set up to work correctly. If an SSL_write fails at the transport,
dropping the buffer is fine. Arguably it works better than in TLS because we
don't have the weird "half-committed to data" behavior. Likewise, the handshake
keeps track of how far its gotten and resumes the message at the right point.

This broke when the buffering logic was rewritten because I didn't understand
what the DTLS code was doing. The one thing that doesn't work as one might
expect is non-fatal write errors during rexmit are not recoverable. The next
timeout must fire before we try again.

This code is quite badly sprinkled in here, so add tests to guard it against
future turbulence. Because of the rexmit issues, the tests need some hacks
around calls which may trigger them. It also changes the Go DTLS implementation
from being completely strict about sequence numbers to only requiring they be

The tests also revealed another bug. This one seems to be upstream's fault, not
mine. The logic to reset the handshake hash on the second ClientHello (in the
HelloVerifyRequest case) was a little overenthusiastic and breaks if the
ClientHello took multiple tries to send.

Change-Id: I9b38b93fff7ae62faf8e36c4beaf848850b3f4b9
Reviewed-by: Adam Langley <>
9 files changed
tree: fd223cf619949b51b8bd91bbb98e0d73d72a1b3f
  1. .clang-format
  2. .gitignore
  4. CMakeLists.txt
  9. codereview.settings
  10. crypto/
  11. decrepit/
  12. include/
  13. ssl/
  14. tool/
  15. util/


BoringSSL is a fork of OpenSSL that is designed to meet Google's needs.

Although BoringSSL is an open source project, it is not intended for general use, as OpenSSL is. We don't recommend that third parties depend upon it. Doing so is likely to be frustrating because there are no guarantees of API or ABI stability.

Programs ship their own copies of BoringSSL when they use it and we update everything as needed when deciding to make API changes. This allows us to mostly avoid compromises in the name of compatibility. It works for us, but it may not work for you.

BoringSSL arose because Google used OpenSSL for many years in various ways and, over time, built up a large number of patches that were maintained while tracking upstream OpenSSL. As Google's product portfolio became more complex, more copies of OpenSSL sprung up and the effort involved in maintaining all these patches in multiple places was growing steadily.

Currently BoringSSL is the SSL library in Chrome/Chromium, Android (but it's not part of the NDK) and a number of other apps/programs.

There are other files in this directory which might be helpful:

  • how to port OpenSSL-using code to BoringSSL.
  • how to build BoringSSL
  • rules and guidelines for coding style.
  • include/openssl: public headers with API documentation in comments. Also available online.