Cryptography and SSL/TLS Toolkit



openssl-s_server, s_server - SSL/TLS server program


openssl s_server [-help] [-port +int] [-accept val] [-unix val] [-4] [-6] [-unlink] [-context val] [-verify int] [-Verify int] [-cert infile] [-nameopt val] [-naccept +int] [-serverinfo val] [-certform PEM|DER] [-key infile] [-keyform format] [-pass val] [-dcert infile] [-dcertform PEM|DER] [-dkey infile] [-dkeyform PEM|DER] [-dpass val] [-nbio_test] [-crlf] [-debug] [-msg] [-msgfile outfile] [-state] [-CAfile infile] [-CApath dir] [-no-CAfile] [-no-CApath] [-nocert] [-quiet] [-no_resume_ephemeral] [-www] [-WWW] [-servername] [-servername_fatal] [-cert2 infile] [-key2 infile] [-tlsextdebug] [-HTTP] [-id_prefix val] [-rand file...] [-writerand file] [-keymatexport val] [-keymatexportlen +int] [-CRL infile] [-crl_download] [-cert_chain infile] [-dcert_chain infile] [-chainCApath dir] [-verifyCApath dir] [-no_cache] [-ext_cache] [-CRLform PEM|DER] [-verify_return_error] [-verify_quiet] [-build_chain] [-chainCAfile infile] [-verifyCAfile infile] [-ign_eof] [-no_ign_eof] [-status] [-status_verbose] [-status_timeout int] [-status_url val] [-status_file infile] [-trace] [-security_debug] [-security_debug_verbose] [-brief] [-rev] [-async] [-ssl_config val] [-max_send_frag +int] [-split_send_frag +int] [-max_pipelines +int] [-read_buf +int] [-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2] [-no_tls1_3] [-bugs] [-no_comp] [-comp] [-no_ticket] [-serverpref] [-legacy_renegotiation] [-no_renegotiation] [-legacy_server_connect] [-no_resumption_on_reneg] [-no_legacy_server_connect] [-allow_no_dhe_kex] [-prioritize_chacha] [-strict] [-sigalgs val] [-client_sigalgs val] [-groups val] [-curves val] [-named_curve val] [-cipher val] [-dhparam infile] [-record_padding val] [-debug_broken_protocol] [-policy val] [-purpose val] [-verify_name val] [-verify_depth int] [-auth_level int] [-attime intmax] [-verify_hostname val] [-verify_email val] [-verify_ip] [-ignore_critical] [-issuer_checks] [-crl_check] [-crl_check_all] [-policy_check] [-explicit_policy] [-inhibit_any] [-inhibit_map] [-x509_strict] [-extended_crl] [-use_deltas] [-policy_print] [-check_ss_sig] [-trusted_first] [-suiteB_128_only] [-suiteB_128] [-suiteB_192] [-partial_chain] [-no_alt_chains] [-no_check_time] [-allow_proxy_certs] [-xkey] [-xcert] [-xchain] [-xchain_build] [-xcertform PEM|DER] [-xkeyform PEM|DER] [-nbio] [-psk_identity val] [-psk_hint val] [-psk val] [-srpvfile infile] [-srpuserseed val] [-ssl3] [-tls1] [-tls1_1] [-tls1_2] [-tls1_3] [-dtls] [-timeout] [-mtu +int] [-listen] [-dtls1] [-dtls1_2] [-sctp] [-no_dhe] [-nextprotoneg val] [-use_srtp val] [-alpn val] [-engine val] [-keylogfile outfile] [-max_early_data int] [-early_data]


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


In addition to the options below the s_server utility also supports the common and server only options documented in the in the "Supported Command Line Commands" section of the SSL_CONF_cmd(3) manual page.


Print out a usage message.

-port +int

The TCP port to listen on for connections. If not specified 4433 is used.

-accept val

The optional TCP host and port to listen on for connections. If not specified, *:4433 is used.

-unix val

Unix domain socket to accept on.


Use IPv4 only.


Use IPv6 only.


For -unix, unlink any existing socket first.

-context val

Sets the SSL context id. It can be given any string value. If this option is not present a default value will be used.

-verify int, -Verify int

The verify depth to use. This specifies the maximum length of the client certificate chain and makes the server request a certificate from the client. With the -verify option a certificate is requested but the client does not have to send one, with the -Verify option the client must supply a certificate or an error occurs.

If the cipher suite cannot request a client certificate (for example an anonymous cipher suite or PSK) this option has no effect.

-cert infile

The certificate to use, most servers cipher suites require the use of a certificate and some require a certificate with a certain public key type: for example the DSS cipher suites require a certificate containing a DSS (DSA) key. If not specified then the filename "server.pem" will be used.

-nameopt val

Option which determines how the subject or issuer names are displayed. The val argument can be a single option or multiple options separated by commas. Alternatively the -nameopt switch may be used more than once to set multiple options. See the x509(1) manual page for details.

-naccept +int

The server will exit after receiving the specified number of connections, default unlimited.

-serverinfo val

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 an empty TLS ClientHello extension matching the type, the corresponding ServerHello extension will be returned.

-certform PEM|DER

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

-key infile

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

-keyform format

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

-pass val

The private key password source. For more information about the format of val see the PASS PHRASE ARGUMENTS section in openssl(1).

-dcert infile, -dkey infile

Specify an additional certificate and private key, these behave in the same manner as the -cert and -key options 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 cipher suites need a certificate carrying an RSA key and some a DSS (DSA) key. By using RSA and DSS certificates and keys a server can support clients which only support RSA or DSS cipher suites by using an appropriate certificate.

-dcertform PEM|DER, -dkeyform PEM|DER, -dpass val

Additional certificate and private key format and passphrase respectively.


Tests non blocking I/O.


This option translated a line feed from the terminal into CR+LF.


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


Show all protocol messages with hex dump.

-msgfile outfile

File to send output of -msg or -trace to, default standard output.


Prints the SSL session states.

-CAfile infile

A file containing trusted certificates to use during client authentication and to use when attempting to build the server certificate chain. The list is also used in the list of acceptable client CAs passed to the client when a certificate is requested.

-CApath dir

The directory to use for client certificate verification. This directory must be in "hash format", see verify for more information. These are also used when building the server certificate chain.


Do not load the trusted CA certificates from the default file location.


Do not load the trusted CA certificates from the default directory location.


If this option is set then no certificate is used. This restricts the cipher suites available to the anonymous ones (currently just anonymous DH).


Inhibit printing of session and certificate information.


Sends a status message back to the client when it connects. This includes information about the ciphers used and various session parameters. The output is in HTML format so this option will normally be used with a web browser.


Emulates a simple web server. Pages will be resolved relative to the current directory, for example if the URL https://myhost/page.html is requested the file ./page.html will be loaded.


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


Emulates a simple web server. Pages will be resolved relative to the current directory, for example if the URL https://myhost/page.html is requested the file ./page.html will be loaded. The files loaded are assumed to contain a complete and correct HTTP response (lines that are part of the HTTP response line and headers must end with CRLF).

-id_prefix val

Generate SSL/TLS session IDs prefixed by val. 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 of session IDs (eg. with a certain prefix).

-rand file...

A file or files containing random data used to seed the random number generator. Multiple files can be specified separated by an OS-dependent character. The separator is ; for MS-Windows, , for OpenVMS, and : for all others.

[-writerand file]

Writes random data to the specified file upon exit. This can be used with a subsequent -rand flag.


Verification errors normally just print a message but allow the connection to continue, for debugging purposes. If this option is used, then verification errors close the connection.


Enables certificate status request support (aka OCSP stapling).


Enables certificate status request support (aka OCSP stapling) and gives a verbose printout of the OCSP response.

-status_timeout int

Sets the timeout for OCSP response to int seconds.

-status_url val

Sets a fallback responder URL to use if no responder URL is present in the server certificate. Without this option an error is returned if the server certificate does not contain a responder address.

-status_file infile

Overrides any OCSP responder URLs from the certificate and always provides the OCSP Response stored in the file. The file must be in DER format.


Show verbose trace output of protocol messages. OpenSSL needs to be compiled with enable-ssl-trace for this option to work.


Provide a brief summary of connection parameters instead of the normal verbose output.


Simple test server which just reverses the text received from the client and sends it back to the server. Also sets -brief.


Switch on asynchronous mode. Cryptographic operations will be performed asynchronously. This will only have an effect if an asynchronous capable engine is also used via the -engine option. For test purposes the dummy async engine (dasync) can be used (if available).

-max_send_frag +int

The maximum size of data fragment to send. See SSL_CTX_set_max_send_fragment(3) for further information.

-split_send_frag +int

The size used to split data for encrypt pipelines. If more data is written in one go than this value then it 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 cipher suite has been negotiated, an engine that supports pipelining has been loaded, and max_pipelines is greater than 1. See SSL_CTX_set_split_send_fragment(3) for further information.

-max_pipelines +int

The maximum number of encrypt/decrypt pipelines to be used. This will only have an effect if an engine has been loaded that supports pipelining (e.g. the dasync engine) and a suitable cipher suite has been negotiated. The default value is 1. See SSL_CTX_set_max_pipelines(3) for further information.

-read_buf +int

The default read buffer size to be used for connections. This will only have an effect if the buffer size is larger than the size that would otherwise be used and pipelining is in use (see SSL_CTX_set_default_read_buffer_len(3) for further information).

-ssl2, -ssl3, -tls1, -tls1_1, -tls1_2, -tls1_3, -no_ssl2, -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2, -no_tls1_3

These options require or disable the use of the specified SSL or TLS protocols. By default s_server will negotiate the highest mutually supported protocol version. When a specific TLS version is required, only that version will be accepted from the client.


There are several known bug in SSL and TLS implementations. Adding this option enables various workarounds.


Disable negotiation of TLS compression. TLS compression is not recommended and is off by default as of OpenSSL 1.1.0.


Enable negotiation of TLS compression. This option was introduced in OpenSSL 1.1.0. TLS compression is not recommended and is off by default as of OpenSSL 1.1.0.


Disable RFC4507bis session ticket support.


Use the server's cipher preferences, rather than the client's preferences.


Prioritize ChaCha ciphers when preferred by clients. Requires -serverpref.



-client_sigalgs val

Signature algorithms to support for client certificate authentication (colon-separated list).

-named_curve val

Specifies the elliptic curve to use. NOTE: this is single curve, not a list. For a list of all possible curves, use:

    $ openssl ecparam -list_curves
-cipher val

This allows the cipher list used by the server to be modified. When the client sends a list of supported ciphers the first client cipher also included in the server list is used. Because the client specifies the preference order, the order of the server cipherlist irrelevant. See the ciphers command for more information.

-dhparam infile

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 this fails then a static set of parameters hard coded into the s_server program will be used.

-attime, -check_ss_sig, -crl_check, -crl_check_all, -explicit_policy, -extended_crl, -ignore_critical, -inhibit_any, -inhibit_map, -no_alt_chains, -no_check_time, -partial_chain, -policy, -policy_check, -policy_print, -purpose, -suiteB_128, -suiteB_128_only, -suiteB_192, -trusted_first, -use_deltas, -auth_level, -verify_depth, -verify_email, -verify_hostname, -verify_ip, -verify_name, -x509_strict

Set different peer certificate verification options. See the verify(1) manual page for details.

-crl_check, -crl_check_all

Check the peer certificate has not been revoked by its CA. The CRL(s) are appended to the certificate file. With the -crl_check_all option all CRLs of all CAs in the chain are checked.


Turns on non blocking I/O.

-psk_identity val

Expect the client to send PSK identity val when using a PSK cipher suite, and warn if they do not. By default, the expected PSK identity is the string "Client_identity".

-psk_hint val

Use the PSK identity hint val when using a PSK cipher suite.

-psk val

Use the PSK key val when using a PSK cipher suite. The key is given as a hexadecimal number without leading 0x, for example -psk 1a2b3c4d. This option must be provided in order to use a PSK cipher.


This option can only be used in conjunction with one of the DTLS options above. With this option s_server will listen on a UDP port for incoming connections. Any ClientHellos that arrive will be checked to see if they have a cookie in them or not. Any without a cookie will be responded to with a HelloVerifyRequest. If a ClientHello with a cookie is received then s_server will connect to that peer and complete the handshake.

-dtls, -dtls1, -dtls1_2

These options make s_server use DTLS protocols instead of TLS. With -dtls, s_server will negotiate any supported DTLS protocol version, whilst -dtls1 and -dtls1_2 will only support DTLSv1.0 and DTLSv1.2 respectively.


Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in conjunction with -dtls, -dtls1 or -dtls1_2. This option is only available where OpenSSL has support for SCTP enabled.


If this option is set then no DH parameters will be loaded effectively disabling the ephemeral DH cipher suites.

-alpn val, -nextprotoneg val

These flags enable the Enable the Application-Layer Protocol Negotiation or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the IETF standard and replaces NPN. The val list is a comma-separated list of supported protocol names. The list should contain the most desirable protocols first. Protocol names are printable ASCII strings, for example "http/1.1" or "spdy/3". The flag -nextprotoneg cannot be specified if -tls1_3 is used.

-engine val

Specifying an engine (by its unique id string in val) will cause s_server 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.

-keylogfile outfile

Appends TLS secrets to the specified keylog file such that external programs (like Wireshark) can decrypt TLS connections.

-max_early_data int

Change the default maximum early data bytes that are specified for new sessions and any incoming early data (when used in conjunction with the -early_data flag). The default value is approximately 16k. The argument must be an integer greater than or equal to 0.


Accept early data where possible.


If a connection request is established with an SSL client and neither the -www nor the -WWW option has been used then normally any data received from the client is displayed and any key presses will be sent to the client.

Certain single letter commands are also recognized which perform special operations: these are listed below.


End the current SSL connection but still accept new connections.


End the current SSL connection and exit.


Renegotiate the SSL session.


Renegotiate the SSL session and request a client certificate.


Send some plain text down the underlying TCP connection: this should cause the client to disconnect due to a protocol violation.


Print out some session cache status information.


s_server can be used to debug SSL clients. To accept connections from a web browser the command:

 openssl s_server -accept 443 -www

can be used for example.

Although specifying an empty list of CAs when requesting a client certificate is strictly speaking a protocol violation, some SSL clients interpret this to mean any CA is acceptable. This is useful for debugging purposes.

The session parameters can printed out using the sess_id program.


Because this program has a lot of options and also because some of the techniques used are rather old, the C source of s_server is rather hard to read and not a model of how things should be done. A typical SSL server program would be much simpler.

The output of common ciphers is wrong: it just gives the list of ciphers that OpenSSL recognizes and the client supports.

There should be a way for the s_server program to print out details of any unknown cipher suites a client says it supports.


SSL_CONF_cmd(3), sess_id(1), s_client(1), ciphers(1) SSL_CTX_set_max_send_fragment(3), SSL_CTX_set_split_send_fragment(3) SSL_CTX_set_max_pipelines(3)


The -no_alt_chains option was first added to OpenSSL 1.1.0.

The -allow-no-dhe-kex and -prioritize_chacha options were first added to OpenSSL 1.1.1.

Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at