Section: OpenSSL (1SSL)
openssl s_client [-connect host:port] [-servername name] [-verify depth] [-verify_return_error] [-cert filename] [-certform DER|PEM] [-key filename] [-keyform DER|PEM] [-pass arg] [-CApath directory] [-CAfile filename] [-reconnect] [-pause] [-showcerts] [-debug] [-msg] [-nbio_test] [-state] [-nbio] [-crlf] [-ign_eof] [-no_ign_eof] [-quiet] [-ssl2] [-ssl3] [-tls1] [-no_ssl2] [-no_ssl3] [-no_tls1] [-bugs] [-cipher cipherlist] [-serverpref] [-starttls protocol] [-engine id] [-tlsextdebug] [-no_ticket] [-sess_out filename] [-sess_in filename] [-rand file(s)] [-status] [-nextprotoneg protocols]
Unfortunately there are a lot of ancient and broken servers in use which cannot handle this technique and will fail to connect. Some servers only work if TLS is turned off with the -no_tls option others will only support SSL v2 and may need the -ssl2 option.
If a connection is established with an SSL server then any data received from the server is displayed and any key presses will be sent to the server. When used interactively (which means neither -quiet nor -ign_eof have been given), the session will be renegotiated if the line begins with an R, and if the line begins with a Q or if end of file is reached, the connection will be closed down.
s_client can be used to debug SSL servers. To connect to an SSL HTTP server the command:
openssl s_client -connect servername:443
would typically be used (https uses port 443). If the connection succeeds then an HTTP command can be given such as ``GET /'' to retrieve a web page.
If the handshake fails then there are several possible causes, if it is nothing obvious like no client certificate then the -bugs, -ssl2, -ssl3, -tls1, -no_ssl2, -no_ssl3, -no_tls1 options can be tried in case it is a buggy server. In particular you should play with these options before submitting a bug report to an OpenSSL mailing list.
A frequent problem when attempting to get client certificates working is that a web client complains it has no certificates or gives an empty list to choose from. This is normally because the server is not sending the clients certificate authority in its ``acceptable CA list'' when it requests a certificate. By using s_client the CA list can be viewed and checked. However some servers only request client authentication after a specific URL is requested. To obtain the list in this case it is necessary to use the -prexit option and send an HTTP request for an appropriate page.
If a certificate is specified on the command line using the -cert option it will not be used unless the server specifically requests a client certificate. Therefor merely including a client certificate on the command line is no guarantee that the certificate works.
If there are problems verifying a server certificate then the -showcerts option can be used to show the whole chain.
Since the SSLv23 client hello cannot include compression methods or extensions these will only be supported if its use is disabled, for example by using the -no_sslv2 option.
The s_client utility is a test tool and is designed to continue the handshake after any certificate verification errors. As a result it will accept any certificate chain (trusted or not) sent by the peer. None test applications should not do this as it makes them vulnerable to a MITM attack. This behaviour can be changed by with the -verify_return_error option: any verify errors are then returned aborting the handshake.
Because this program has a lot of options and also because some of the techniques used are rather old, the C source of s_client is rather hard to read and not a model of how things should be done. A typical SSL client program would be much simpler.
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