s_client — SSL/TLS client program
openssl s_client [-connect host:port>] [-verify depth] [-cert filename] [-key filename] [-CApath directory] [-CAfile filename] [-reconnect] [-pause] [-showcerts] [-debug] [-msg] [-nbio_test] [-state] [-nbio] [-crlf] [-ign_eof] [-quiet] [-ssl2] [-ssl3] [-tls1] [-no_ssl2] [-no_ssl3] [-no_tls1] [-bugs] [-cipher cipherlist] [-starttls protocol] [-engine id] [-rand file(s)]
The s_client command implements a generic SSL/TLS client which
connects to a remote host using SSL/TLS. It is a very useful diagnostic tool for SSL servers.
This specifies the host and optional port to connect to. If
not specified then an attempt is made to connect to the local host
on port 4433.
The certificate to use, if one is requested by the server.
The default is not to use a certificate.
The private key to use. If not specified then the certificate
file will be used.
The verify depth to use. This specifies the maximum length
of the server certificate chain and turns on server certificate
verification. Currently the verify operation continues after errors
so all the problems with a certificate chain can be seen. As a side
effect the connection will never fail due to a server certificate
The directory to use for server certificate verification.
This directory must be in "hash format", see verify for more information.
These are also used when building the client certificate chain.
A file containing trusted certificates to use during server
authentication and to use when attempting to build the client certificate
reconnects to the same server 5 times using the same session
ID, this can be used as a test that session caching is working.
pauses 1 second between each read and write call.
display the whole server certificate chain: normally only
the server certificate itself is displayed.
print session information when the program exits. This will
always attempt to print out information even if the connection fails.
Normally information will only be printed out once if the connection
succeeds. This option is useful because the cipher in use may be
renegotiated or the connection may fail because a client certificate
is required or is requested only after an attempt is made to access
a certain URL. Note: the output produced by this option is not always
accurate because a connection might never have been established.
prints out the SSL session states.
print extensive debugging information including a hex dump
of all traffic.
show all protocol messages with hex dump.
tests non-blocking I/O
turns on non-blocking I/O
this option translated a line feed from the terminal into
CR+LF as required by some servers.
inhibit shutting down the connection when end of file is reached
in the input.
inhibit printing of session and certificate information. This
implicitly turns on -ign_eof as well.
-ssl2, -ssl3, -tls1, -no_ssl2, -no_ssl3, -no_tlsl
these options disable the use of certain SSL or TLS protocols.
By default the initial handshake uses a method which should be compatible
with all servers and permit them to use SSL v3, SSL v2 or TLS as appropriate.
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.
there are several known bug in SSL and TLS implementations.
Adding this option enables various workarounds.
this allows the cipher list sent by the client to be modified.
Although the server determines which cipher suite is used it should
take the first supported cipher in the list sent by the client.
See the ciphers command for more information.
send the protocol-specific message(s) to switch to TLS for
communication. protocol is a keyword for the intended protocol.
Currently, the only supported keywords are "smtp" and "pop3".
specifying an engine (by it's unique id string) will cause
s_client to attempt to obtain a functional reference to the specified
engine, thus initialising it if needed. The engine will then be
set as the default for all available algorithms.
a file or files containing random data used to seed the random
number generator, or an EGD socket (see RAND_egd(3)).
Multiple files can be specified separated by a OS-dependent character.
The separator is ; for MS-Windows, , for OpenVMS, and : for all
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_tlsl 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 command 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
If there are problems verifying a server certificate then
the -showcerts option can be used to show the whole chain.
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.
The -verify option should really exit if the server verification
The -prexit option is a bit of a hack. We should really report
information whenever a session is renegotiated.
sess_id(1), s_server(1), ciphers(1)