CSE2410 Fall 2014 Bounce

=pod

=head1 NAME

s_client - SSL/TLS client program

=head1 SYNOPSIS

B<openssl> B<s_client>

[B<-connect host:port>]

[B<-servername name>]

[B<-verify depth>]

[B<-verify_return_error>]

[B<-cert filename>]

[B<-certform DER|PEM>]

[B<-key filename>]

[B<-keyform DER|PEM>]

[B<-pass arg>]

[B<-CApath directory>]

[B<-CAfile filename>]

[B<-reconnect>]

[B<-pause>]

[B<-showcerts>]

[B<-debug>]

[B<-msg>]

[B<-nbio_test>]

[B<-state>]

[B<-nbio>]

[B<-crlf>]

[B<-ign_eof>]

[B<-no_ign_eof>]

[B<-quiet>]

[B<-ssl2>]

[B<-ssl3>]

[B<-tls1>]

[B<-no_ssl2>]

[B<-no_ssl3>]

[B<-no_tls1>]

[B<-bugs>]

[B<-cipher cipherlist>]

[B<-serverpref>]

[B<-starttls protocol>]

[B<-engine id>]

[B<-tlsextdebug>]

[B<-no_ticket>]

[B<-sess_out filename>]

[B<-sess_in filename>]

[B<-rand file(s)>]

[B<-status>]

[B<-nextprotoneg protocols>]

=head1 DESCRIPTION

The B<s_client> command implements a generic SSL/TLS client which connects

to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for

SSL servers.

=head1 OPTIONS

=over 4

=item B<-connect host:port>

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.

=item B<-servername name>

Set the TLS SNI (Server Name Indication) extension in the ClientHello message.

=item B<-cert certname>

The certificate to use, if one is requested by the server. The default is

not to use a certificate.

=item B<-certform format>

The certificate format to use: DER or PEM. PEM is the default.

=item B<-key keyfile>

The private key to use. If not specified then the certificate file will

be used.

=item B<-keyform format>

The private format to use: DER or PEM. PEM is the default.

=item B<-pass arg>

the private key password source. For more information about the format of B<arg>

see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)|openssl(1)>.

=item B<-verify depth>

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 verify failure.

=item B<-verify_return_error>

Return verification errors instead of continuing. This will typically

abort the handshake with a fatal error.

=item B<-CApath directory>

The directory to use for server certificate verification. This directory

must be in "hash format", see B<verify> for more information. These are

also used when building the client certificate chain.

=item B<-CAfile file>

A file containing trusted certificates to use during server authentication

and to use when attempting to build the client certificate chain.

=item B<-purpose, -ignore_critical, -issuer_checks, -crl_check, -crl_check_all, -policy_check, -extended_crl, -x509_strict, -policy -check_ss_sig>

Set various certificate chain valiadition option. See the

L<B<verify>|verify(1)> manual page for details.

=item B<-reconnect>

reconnects to the same server 5 times using the same session ID, this can

be used as a test that session caching is working.

=item B<-pause>

pauses 1 second between each read and write call.

=item B<-showcerts>

display the whole server certificate chain: normally only the server

certificate itself is displayed.

=item B<-prexit>

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.

=item B<-state>

prints out the SSL session states.

=item B<-debug>

print extensive debugging information including a hex dump of all traffic.

=item B<-msg>

show all protocol messages with hex dump.

=item B<-nbio_test>

tests non-blocking I/O

=item B<-nbio>

turns on non-blocking I/O

=item B<-crlf>

this option translated a line feed from the terminal into CR+LF as required

by some servers.

=item B<-ign_eof>

inhibit shutting down the connection when end of file is reached in the

input.

=item B<-quiet>

inhibit printing of session and certificate information.  This implicitly

turns on B<-ign_eof> as well.

=item B<-no_ign_eof>

shut down the connection when end of file is reached in the input.

Can be used to override the implicit B<-ign_eof> after B<-quiet>.

=item B<-psk_identity identity>

Use the PSK identity B<identity> when using a PSK cipher suite.

=item B<-psk key>

Use the PSK key B<key> when using a PSK cipher suite. The key is

given as a hexadecimal number without leading 0x, for example -psk

1a2b3c4d.

=item B<-ssl2>, B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1>

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 B<-no_tls> option others will only

support SSL v2 and may need the B<-ssl2> option.

=item B<-bugs>

there are several known bug in SSL and TLS implementations. Adding this

option enables various workarounds.

=item B<-cipher cipherlist>

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 B<ciphers>

command for more information.

=item B<-serverpref>

use the server's cipher preferences; only used for SSLV2.

=item B<-starttls protocol>

send the protocol-specific message(s) to switch to TLS for communication.

B<protocol> is a keyword for the intended protocol.  Currently, the only

supported keywords are "smtp", "pop3", "imap", and "ftp".

=item B<-tlsextdebug>

print out a hex dump of any TLS extensions received from the server.

=item B<-no_ticket>

disable RFC4507bis session ticket support. 

=item B<-sess_out filename>

output SSL session to B<filename>

=item B<-sess_in sess.pem>

load SSL session from B<filename>. The client will attempt to resume a

connection from this session.

=item B<-engine id>

specifying an engine (by its unique B<id> string) will cause B<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.

=item B<-rand file(s)>

a file or files containing random data used to seed the random number

generator, or an EGD socket (see L<RAND_egd(3)|RAND_egd(3)>).

Multiple files can be specified separated by a OS-dependent character.

The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for

all others.

=item B<-status>

sends a certificate status request to the server (OCSP stapling). The server

response (if any) is printed out.

=item B<-nextprotoneg protocols>

enable Next Protocol Negotiation TLS extension and provide a list of

comma-separated protocol names that the client should advertise

support for. The list should contain most wanted protocols first.

Protocol names are printable ASCII strings, for example "http/1.1" or

"spdy/3".

Empty list of protocols is treated specially and will cause the client to

advertise support for the TLS extension but disconnect just after

reciving ServerHello with a list of server supported protocols.

=back

=head1 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 B<-quiet> nor B<-ign_eof>

have been given), the session will be renegotiated if the line begins with an

B<R>, and if the line begins with a B<Q> or if end of file is reached, the

connection will be closed down.

=head1 NOTES

B<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 B<-bugs>, B<-ssl2>,

B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1> options can be tried

in case it is a buggy server. In particular you should play with these

options B<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 B<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 B<-prexit> option and send an HTTP request

for an appropriate page.

If a certificate is specified on the command line using the B<-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

B<-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

B<-no_sslv2> option.

The B<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 B<not> do this as it makes them vulnerable to a MITM

attack. This behaviour can be changed by with the B<-verify_return_error>

option: any verify errors are then returned aborting the handshake.

=head1 BUGS

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 B<-prexit> option is a bit of a hack. We should really report

information whenever a session is renegotiated.

=head1 SEE ALSO

L<sess_id(1)|sess_id(1)>, L<s_server(1)|s_server(1)>, L<ciphers(1)|ciphers(1)>

=cut