Why no SSL ?ΒΆ

This is turning into a bit of a FAQ, but the answer is too big to fit in the margin we use for those.

There are a number of reasons why there are no plans in sight that will grow SSL support in Varnish.

First, I have yet to see a SSL library where the source code is not a nightmare.

As I am writing this, the varnish source-code tree contains 82.595 lines of .c and .h files, including JEmalloc (12.236 lines) and Zlib (12.344 lines).

OpenSSL, as imported into FreeBSD, is 340.722 lines of code, nine times larger than the Varnish source code, 27 times larger than each of Zlib or JEmalloc.

This should give you some indication of how insanely complex the canonical implementation of SSL is.

Second, it is not exactly the best source-code in the world. Even if I have no idea what it does, there are many aspect of it that scares me.

Take this example in a comment, randomly found in s3-srvr.c:

/* Throw away what we have done so far in the current handshake,
 * which will now be aborted. (A full SSL_clear would be too much.)
 * I hope that tmp.dh is the only thing that may need to be cleared
 * when a handshake is not completed ... */

I hope they know what they are doing, but this comment doesn't exactly carry that point home, does it ?

But let us assume that a good SSL library can be found, what would Varnish do with it ?

We would terminate SSL sessions, and we would burn CPU cycles doing that. You can kiss the highly optimized delivery path in Varnish goodbye for SSL, we cannot simply tell the kernel to put the bytes on the socket, rather, we have to corkscrew the data through the SSL library and then write it to the socket.

Will that be significantly different, performance wise, from running a SSL proxy in separate process ?

No, it will not, because the way varnish would have to do it would be to ... start a separate process to do the SSL handling.

There is no other way we can guarantee that secret krypto-bits do not leak anywhere they should not, than by fencing in the code that deals with them in a child process, so the bulk of varnish never gets anywhere near the certificates, not even during a core-dump.

Would I be able to write a better stand-alone SSL proxy process than the many which already exists ?

Probably not, unless I also write my own SSL implementation library, including support for hardware crypto engines and the works.

That is not one of the things I dreamt about doing as a kid and if I dream about it now I call it a nightmare.

So the balance sheet, as far as I can see it, lists "It would be a bit easier to configure" on the plus side, and everything else piles up on the minus side, making it a huge waste of time and effort to even think about it..

Poul-Henning, 2011-02-15