s_client(1ssl)s_client(1ssl)NAMEs_client - SSL/TLS client program
SYNOPSIS
openssl s_client [-connect host:port>] [-verify depth] [-cert filename]
[-key filename] [-CApath directory] [-CAfile filename] [-reconnect]
[-pause] [-showcerts] [-debug] [-nbio_test] [-state] [-nbio] [-crlf]
[-ign_eof] [-quiet] [-ssl2] [-ssl3] [-tls1] [-no_ssl2] [-no_ssl3]
[-no_tls1] [-bugs] [-cipher cipherlist] [-rand filename] [-engine id]
OPTIONS
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 con‐
tinues 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 failure. The directory to use for server certifi‐
cate verification. This directory must be in hash format. See verify
for more information. These are also used when building the client cer‐
tificate chain. A file containing trusted certificates to use during
server authentication and to use when attempting to build the client
certificate chain. 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 one second between each read and write call. Displays
the whole server certificate chain. Normally only the server certifi‐
cate is displayed. Prints session information when the program exits.
This will always attempt to print out information even if the connec‐
tion 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 cer‐
tificate is required or is requested only after an attempt is made to
access a certain URL. The output produced by this option is not always
accurate because a connection might never have been established.
Prints out the SSL session states. Prints extensive debugging informa‐
tion including a hex dump of all traffic. Tests non-blocking I/O Turns
on non-blocking I/O Translates a line feed from the terminal into CR+LF
as required by some servers. Inhibits shutting down the connection
when end-of-file is reached in the input. Inhibits printing of session
and certificate information. This implicitely turns on -ign_eof as
well. These options disable the use of certain SSL or TLS protocols.
By default the initial handshake uses a method which should be compati‐
ble 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 bugs in SSL and TLS implementa‐
tions. Adding this option enables various workarounds. 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. A file or files contain‐
ing random data used to seed the random number generator, or an
EGD socket. (See RAND_egd(3).) Multiple files can be specified
separated by an OS-dependent character. The separator is a semi‐
colon (;) for MS-Windows, a comma (,) for OpenVMS, and a colon
(:) for all others. Specifying an engine (by its unique id
string) will cause the s_client command to attempt to obtain a
functional reference to the specified engine, thus initializing
it if needed. The engine will then be set as the default for
all available algorithms.
CONNECTED COMMANDS
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. If the line begins with a Q or if end-of-file is
reached, the connection will be closed down.
DESCRIPTION
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.
NOTES
The s_client command can be used to debug SSL servers. To connect to
an SSL HTTP server, the following command would typically be used
(https uses port 443): openssl s_client-connect servername: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, such as no client certificate, then the -bugs, -ssl2,
-ssl3, -tls1, -no_ssl2, -no_ssl3, -no_tls1 options can be tried. You
should try 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. Therefore 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.
RESTRICTIONS
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 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 exit if the server verification fails.
The -prexit option should report information whenever a session is
renegotiated.
SEE ALSO
Commands: sess_id(1ssl), s_server(1ssl), ciphers(1ssl)s_client(1ssl)