[openssl-users] SSL_connect returns SSL_ERROR_SYSCALL and errno == EWOULDBLOCK

Jahn, Gerhard gerhard.jahn at atos.net
Mon Sep 10 08:05:24 UTC 2018

Ad:  The "correct" answer is that if you get SSL_ERROR_SYSCALL then the connection has failed and you shouldn't use that connection any more.

This somehow contradicts the description of returncode <0 on SSL_connect which says that


    The TLS/SSL handshake was not successful, because a fatal error occurred either at the protocol level or a connection failure occurred.
The shutdown was not clean. It can also occur of action is need to continue the operation for non-blocking BIOs.
Call SSL_get_error() with the return value ret to find out the reason.

If SSL_ERROR_SYSCAL would always mean connection failure, why then any action to continue the operation.....
So we're getting SSL_connect() = -1 and we call SSL_get_error() returning 5  as advised
Then as SSL_get_error() says


      Some non-recoverable I/O error occurred. The OpenSSL error queue may contain more information on the error.
      For socket I/O on Unix systems, consult errno for details

We call  ERR_print_errors_fp(stderr) which gives no output.
We inspect errno which indicates EWOULDBLOCK or EAGAIN
This seems to happen rarely (once per hundreds of SSL_Connect) and as we're currently treating any SSL_ERROR_SYSCALL
as bogus and terminate the connection (SSL_shutdown+Socketclose)
As our server runs "forever" and has high load we see a lot of such "SSL_Connect errors in our Logs"
Additionally it seems to happen more frequently when connecting to a remote host rather than when connecting to a server co-located....
I have experienced the same behavior with SSL_read/SSL_write where we also get SSL_ERROR_SYSCALL and find that errno is EWOULDBLOCK
But in these cases we "know" what to do (wait for readable when it appears in SSL_read and wait for writeable when in SSL_write)
Therefore we have the feeling that same blocking happens during SSL_connect........?

-----Original Message-----
From: openssl-users [mailto:openssl-users-bounces at openssl.org] On Behalf Of Matt Caswell
Sent: Friday, September 07, 2018 11:24 AM
To: openssl-users at openssl.org
Subject: Re: [openssl-users] SSL_connect returns SSL_ERROR_SYSCALL and errno == EWOULDBLOCK

On 07/09/18 09:16, Jahn, Gerhard wrote:
> Hi,
> We are using OpenSSl 1.0.2n in our server running on LINUX.
> We call SSL_connect() on async socket (after TCP connect completion)
> to establish a secure connection.
> According to DOC SSL_get_error(() has to be called if SSL_connect()
> returns <=0
> We do not understand what to do if SSL_get_error(() returns
> SSL_ERROR_SYSCALLand errno is EWOULDBLOCK If SSL_get_error returns
> SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE it pretty clear what to
> do... (we set the socket descriptor either in the readfds or writefds
> and call select to wait until the socket becomes readable or writeable
> (or times-out) But when EWOULDBLOCK is indicated, we do not know
> whether to wait for readable/writeable...... (setting both might be an
> idea but could easily lead to a live-loop as we suppose that the
> handshake either waits for a read or for a write but not both...

That's quite a surprising result. Possibly intervening code somewhere between the sys call and where you check errno has changed its value?

The "correct" answer is that if you get SSL_ERROR_SYSCALL then the connection has failed and you shouldn't use that connection any more.
Have you checked the openssl error stack for any reported issues?


> Any ideas?
> Thanks
> Mit freundlichen Grüßen/Best regards,
> *____________
> **Gerhard Jahn*
> Identity and Access Management
> Phone:  +49 (211) 399-33276
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