Linux "s_server" Command Line Options and Examples
SSL/TLS server program

The s_server command implements a generic SSL/TLS server which listens for connections on a given port using SSL/TLS..


Usage:

openssl s_server [-help] [-port port] [-accept val] [-naccept count] [-unix val] [-unlink] [-4] [-6] [-context
id] [-verify depth] [-Verify depth] [-crl_check] [-crl_check_all] [-cert filename] [-certform DER|PEM] [-key
keyfile] [-keyform DER|PEM] [-pass arg] [-dcert filename] [-dcertform DER|PEM] [-dkey keyfile] [-dkeyform
DER|PEM] [-dpass arg] [-dhparam filename] [-nbio] [-nbio_test] [-crlf] [-debug] [-msg] [-state] [-CApath
directory] [-CAfile filename] [-no-CAfile] [-no-CApath] [-attime timestamp] [-check_ss_sig] [-explicit_policy]
[-extended_crl] [-ignore_critical] [-inhibit_any] [-inhibit_map] [-no_check_time] [-partial_chain] [-policy
arg] [-policy_check] [-policy_print] [-purpose purpose] [-suiteB_128] [-suiteB_128_only] [-suiteB_192]
[-trusted_first] [-no_alt_chains] [-use_deltas] [-auth_level num] [-verify_depth num] [-verify_return_error]
[-verify_email email] [-verify_hostname hostname] [-verify_ip ip] [-verify_name name] [-x509_strict] [-nocert]
[-client_sigalgs sigalglist] [-named_curve curve] [-cipher cipherlist] [-serverpref] [-quiet] [-ssl3] [-tls1]
[-tls1_1] [-tls1_2] [-dtls] [-dtls1] [-dtls1_2] [-listen] [-async] [-split_send_frag] [-max_pipelines]
[-read_buf] [-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2] [-no_dhe] [-bugs] [-comp] [-no_comp] [-brief]
[-www] [-WWW] [-HTTP] [-engine id] [-tlsextdebug] [-no_ticket] [-id_prefix arg] [-rand file(s)] [-serverinfo
file] [-no_resumption_on_reneg] [-status] [-status_verbose] [-status_timeout nsec] [-status_url url] [-alpn
protocols] [-nextprotoneg protocols]






Command Line Options:

-help
Print out a usage message.
s_server -help ...
-port
The TCP port to listen on for connections. If not specified 4433 is used.
s_server -port ...
-accept
The optional TCP host and port to listen on for connections. If not specified, *:4433 is used.
s_server -accept ...
-naccept
The server will exit after receiving number connections, default unlimited.
s_server -naccept ...
-unix
Unix domain socket to accept on.
s_server -unix ...
-unlink
For -unix, unlink existing socket first.
s_server -unlink ...
-context
Sets the SSL context id. It can be given any string value. If this option is not present a default valuewill be used.
s_server -context ...
-cert
The certificate to use, most servers cipher suites require the use of a certificate and some require acertificate with a certain public key type: for example the DSS cipher suites require a certificatecontaining a DSS (DSA) key. If not specified then the filename "server.pem" will be used.
s_server -cert ...
-certform
The certificate format to use: DER or PEM. PEM is the default.
s_server -certform ...
-key
The private key to use. If not specified then the certificate file will be used.
s_server -key ...
-keyform
The private format to use: DER or PEM. PEM is the default.
s_server -keyform ...
-pass
The private key password source. For more information about the format of arg see the PASS PHRASEARGUMENTS section in openssl(1).
s_server -pass ...
-dcert
Specify an additional certificate and private key, these behave in the same manner as the -cert and -keyoptions except there is no default if they are not specified (no additional certificate and key is used).As noted above some cipher suites require a certificate containing a key of a certain type. Some ciphersuites need a certificate carrying an RSA key and some a DSS (DSA) key. By using RSA and DSS certificatesand keys a server can support clients which only support RSA or DSS cipher suites by using an appropriatecertificate.
s_server -dcert ...
-dcertform
Additional certificate and private key format and passphrase respectively.
s_server -dcertform ...
-nocert
If this option is set then no certificate is used. This restricts the cipher suites available to theanonymous ones (currently just anonymous DH).
s_server -nocert ...
-dhparam
The DH parameter file to use. The ephemeral DH cipher suites generate keys using a set of DH parameters.If not specified then an attempt is made to load the parameters from the server certificate file. If thisfails then a static set of parameters hard coded into the s_server program will be used.
s_server -dhparam ...
-no_dhe
If this option is set then no DH parameters will be loaded effectively disabling the ephemeral DH ciphersuites.
s_server -no_dhe ...
-crl_check
Check the peer certificate has not been revoked by its CA. The CRL(s) are appended to the certificatefile. With the -crl_check_all option all CRLs of all CAs in the chain are checked.
s_server -crl_check ...
-CApath
The directory to use for client certificate verification. This directory must be in "hash format", seeverify for more information. These are also used when building the server certificate chain.
s_server -CApath ...
-CAfile
A file containing trusted certificates to use during client authentication and to use when attempting tobuild the server certificate chain. The list is also used in the list of acceptable client CAs passed tothe client when a certificate is requested.
s_server -CAfile ...
-no-CAfile
Do not load the trusted CA certificates from the default file location
s_server -no-CAfile ...
-no-CApath
Do not load the trusted CA certificates from the default directory location
s_server -no-CApath ...
-verify
The verify depth to use. This specifies the maximum length of the client certificate chain and makes theserver request a certificate from the client. With the -verify option a certificate is requested but theclient does not have to send one, with the -Verify option the client must supply a certificate or an erroroccurs.If the ciphersuite cannot request a client certificate (for example an anonymous ciphersuite or PSK) thisoption has no effect.
s_server -verify ...
-attime
-check_ss_sig -crl_check -crl_check_all -explicit_policy -extended_crl -ignore_critical
s_server -attime ...
-inhibit_any
-inhibit_map -no_alt_chains -no_check_time -partial_chain -policy -policy_check
s_server -inhibit_any ...
-policy_print
-purpose -suiteB_128 -suiteB_128_only -suiteB_192 -trusted_first -use_deltas -auth_level
s_server -policy_print ...
-verify_depth
Set different peer certificate verification options. See the verify(1) manual page for details.
s_server -verify_depth ...
-verify_return_error
Verification errors normally just print a message but allow the connection to continue, for debuggingpurposes. If this option is used, then verification errors close the connection.
s_server -verify_return_error ...
-state
Prints the SSL session states.
s_server -state ...
-debug
Print extensive debugging information including a hex dump of all traffic.
s_server -debug ...
-msg
Show all protocol messages with hex dump.
s_server -msg ...
-trace
Show verbose trace output of protocol messages. OpenSSL needs to be compiled with enable-ssl-trace forthis option to work.
s_server -trace ...
-msgfile
File to send output of -msg or -trace to, default standard output.
s_server -msgfile ...
-nbio_test
Tests non blocking I/O
s_server -nbio_test ...
-nbio
Turns on non blocking I/O
s_server -nbio ...
-crlf
This option translated a line feed from the terminal into CR+LF.
s_server -crlf ...
-quiet
Inhibit printing of session and certificate information.
s_server -quiet ...
-psk_hint
Use the PSK identity hint hint when using a PSK cipher suite.
s_server -psk_hint ...
-psk
Use the PSK key key when using a PSK cipher suite. The key is given as a hexadecimal number withoutleading 0x, for example -psk 1a2b3c4d. This option must be provided in order to use a PSK cipher.
s_server -psk ...
-ssl2
These options require or disable the use of the specified SSL or TLS protocols. By default s_server willnegotiate the highest mutually supported protocol version. When a specific TLS version is required, onlythat version will be accepted from the client.
s_server -ssl2 ...
-dtls
These options make s_server use DTLS protocols instead of TLS. With -dtls, s_server will negotiate anysupported DTLS protocol version, whilst -dtls1 and -dtls1_2 will only support DTLSv1.0 and DTLSv1.2respectively.
s_server -dtls ...
-listen
This option can only be used in conjunction with one of the DTLS options above. With this option s_serverwill listen on a UDP port for incoming connections. Any ClientHellos that arrive will be checked to seeif they have a cookie in them or not. Any without a cookie will be responded to with aHelloVerifyRequest. If a ClientHello with a cookie is received then s_server will connect to that peerand complete the handshake.
s_server -listen ...
-async
Switch on asynchronous mode. Cryptographic operations will be performed asynchronously. This will onlyhave an effect if an asynchronous capable engine is also used via the -engine option. For test purposesthe dummy async engine (dasync) can be used (if available).
s_server -async ...
-split_send_frag
The size used to split data for encrypt pipelines. If more data is written in one go than this value thenit will be split into multiple pipelines, up to the maximum number of pipelines defined by max_pipelines.This only has an effect if a suitable ciphersuite has been negotiated, an engine that supports pipelininghas been loaded, and max_pipelines is greater than 1. See SSL_CTX_set_split_send_fragment(3) for furtherinformation.
s_server -split_send_frag ...
-max_pipelines
The maximum number of encrypt/decrypt pipelines to be used. This will only have an effect if an engine hasbeen loaded that supports pipelining (e.g. the dasync engine) and a suitable ciphersuite has beennegotiated. The default value is 1. See SSL_CTX_set_max_pipelines(3) for further information.
s_server -max_pipelines ...
-read_buf
The default read buffer size to be used for connections. This will only have an effect if the buffer sizeis larger than the size that would otherwise be used and pipelining is in use (seeSSL_CTX_set_default_read_buffer_len(3) for further information).
s_server -read_buf ...
-bugs
There are several known bug in SSL and TLS implementations. Adding this option enables variousworkarounds.
s_server -bugs ...
-comp
Enable negotiation of TLS compression. This option was introduced in OpenSSL 1.1.0. TLS compression isnot recommended and is off by default as of OpenSSL 1.1.0.
s_server -comp ...
-no_comp
Disable negotiation of TLS compression. TLS compression is not recommended and is off by default as ofOpenSSL 1.1.0.
s_server -no_comp ...
-brief
Provide a brief summary of connection parameters instead of the normal verbose output.
s_server -brief ...
-client_sigalgs
Signature algorithms to support for client certificate authentication (colon-separated list)
s_server -client_sigalgs ...
-named_curve
Specifies the elliptic curve to use. NOTE: this is single curve, not a list. For a list of all possiblecurves, use:$ openssl ecparam -list_curves
s_server -named_curve ...
-cipher
This allows the cipher list used by the server to be modified. When the client sends a list of supportedciphers the first client cipher also included in the server list is used. Because the client specifies thepreference order, the order of the server cipherlist irrelevant. See the ciphers command for moreinformation.
s_server -cipher ...
-serverpref
Use the server's cipher preferences, rather than the client's preferences.
s_server -serverpref ...
-tlsextdebug
Print a hex dump of any TLS extensions received from the server.
s_server -tlsextdebug ...
-no_ticket
Disable RFC4507bis session ticket support.
s_server -no_ticket ...
-www
Sends a status message back to the client when it connects. This includes information about the ciphersused and various session parameters. The output is in HTML format so this option will normally be usedwith a web browser.
s_server -www ...
-WWW
Emulates a simple web server. Pages will be resolved relative to the current directory, for example if theURL https://myhost/page.html is requested the file ./page.html will be loaded.
s_server -WWW ...
-HTTP
Emulates a simple web server. Pages will be resolved relative to the current directory, for example if theURL https://myhost/page.html is requested the file ./page.html will be loaded. The files loaded areassumed to contain a complete and correct HTTP response (lines that are part of the HTTP response line andheaders must end with CRLF).
s_server -HTTP ...
-rev
Simple test server which just reverses the text received from the client and sends it back to the server.Also sets -brief.
s_server -rev ...
-engine
Specifying an engine (by its unique id string) will cause s_server to attempt to obtain a functionalreference to the specified engine, thus initialising it if needed. The engine will then be set as thedefault for all available algorithms.
s_server -engine ...
-id_prefix
Generate SSL/TLS session IDs prefixed by arg. This is mostly useful for testing any SSL/TLS code (eg.proxies) that wish to deal with multiple servers, when each of which might be generating a unique range ofsession IDs (eg. with a certain prefix).
s_server -id_prefix ...
-rand
A file or files containing random data used to seed the random number generator, or an EGD socket (seeRAND_egd(3)). Multiple files can be specified separated by an OS-dependent character. The separator is ;for MS-Windows, , for OpenVMS, and : for all others.
s_server -rand ...
-serverinfo
A file containing one or more blocks of PEM data. Each PEM block must encode a TLS ServerHello extension(2 bytes type, 2 bytes length, followed by "length" bytes of extension data). If the client sends anempty TLS ClientHello extension matching the type, the corresponding ServerHello extension will bereturned.
s_server -serverinfo ...
-no_resumption_on_reneg
Set the SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION option.
s_server -no_resumption_on_reneg ...
-status
Enables certificate status request support (aka OCSP stapling).
s_server -status ...
-status_verbose
Enables certificate status request support (aka OCSP stapling) and gives a verbose printout of the OCSPresponse.
s_server -status_verbose ...
-status_timeout
Sets the timeout for OCSP response to nsec seconds.
s_server -status_timeout ...
-status_url
Sets a fallback responder URL to use if no responder URL is present in the server certificate. Withoutthis option an error is returned if the server certificate does not contain a responder address.
s_server -status_url ...
-alpn
these flags enable the Enable the Application-Layer Protocol Negotiation or Next Protocol Negotiationextension, respectively. ALPN is the IETF standard and replaces NPN. The protocols list is a comma-separated list of supported protocol names. The list should contain most wanted protocols first.Protocol names are printable ASCII strings, for example "http/1.1" or "spdy/3".CONNECTED COMMANDSIf a connection request is established with an SSL client and neither the -www nor the -WWW option has beenused then normally any data received from the client is displayed and any key presses will be sent to theclient.Certain single letter commands are also recognized which perform special operations: these are listed below.q end the current SSL connection but still accept new connections.Q end the current SSL connection and exit.r renegotiate the SSL session.R renegotiate the SSL session and request a client certificate.P send some plain text down the underlying TCP connection: this should cause the client to disconnect due toa protocol violation.S print out some session cache status information.NOTESs_server can be used to debug SSL clients. To accept connections from a web browser the command:openssl s_server -accept 443 -wwwcan be used for example.Although specifying an empty list of CAs when requesting a client certificate is strictly speaking a protocolviolation, some SSL clients interpret this to mean any CA is acceptable. This is useful for debuggingpurposes.The session parameters can printed out using the sess_id program.BUGSBecause this program has a lot of options and also because some of the techniques used are rather old, the Csource of s_server is rather hard to read and not a model of how things should be done. A typical SSL serverprogram would be much simpler.The output of common ciphers is wrong: it just gives the list of ciphers that OpenSSL recognizes and theclient supports.There should be a way for the s_server program to print out details of any unknown cipher suites a client saysit supports.
s_server -alpn ...