S_CLIENT(1) OpenSSL S_CLIENT(1)
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] [-engine id] [-rand
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
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
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 verifica-
tion. 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 verify
The directory to use for server certificate verification. This
directory must be in "hash format", see verify for more informa-
tion. These are also used when building the client certificate
A file containing trusted certificates to use during server authen-
tication 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 connec-
tion succeeds. This option is useful because the cipher in use may
be renegotiated or the connection may fail because a client cer-
tificate 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
prints out the SSL session states.
print extensive debugging information including a hex dump of all
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
inhibit printing of session and certificate information. This
implicitly turns on -ign_eof as well.
-ssl2, -ssl3, -tls1, -no_ssl2, -no_ssl3, -no_tls1
these options disable the use of certain SSL or TLS protocols. By
default the initial handshake uses a method which should be compat-
ible with all servers and permit them to use SSL v3, SSL v2 or TLS
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 oth-
ers 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.
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 num-
ber 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 others.
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 suc-
ceeds then an HTTP command can be given such as "GET /" to retrieve a
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 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 nec-
essary to use the -prexit command and send an HTTP request for an
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.
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 fails.
The -prexit option is a bit of a hack. We should really report informa-
tion whenever a session is renegotiated.
sess_id(1), s_server(1), ciphers(1)
0.9.7a 2001-11-09 S_CLIENT(1)