Provided by: openssl_1.1.1f-1ubuntu2.24_amd64 
NAME
openssl-s_server, s_server - SSL/TLS server program
SYNOPSIS
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] [-num_tickets] [-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] [-ciphersuites 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] [-psk_session file] [-srpvfile infile] [-srpuserseed val] [-ssl3] [-tls1] [-tls1_1]
[-tls1_2] [-tls1_3] [-dtls] [-timeout] [-mtu +int] [-listen] [-dtls1] [-dtls1_2] [-sctp]
[-sctp_label_bug] [-no_dhe] [-nextprotoneg val] [-use_srtp val] [-alpn val] [-engine val] [-keylogfile
outfile] [-max_early_data int] [-early_data] [-anti_replay] [-no_anti_replay]
DESCRIPTION
The s_server command implements a generic SSL/TLS server which listens for connections on a given port
using SSL/TLS.
OPTIONS
In addition to the options below the s_server utility also supports the common and server only options
documented in the "Supported Command Line Commands" section of the SSL_CONF_cmd(3) manual page.
-help
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.
-4 Use IPv4 only.
-6 Use IPv6 only.
-unlink
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.
-cert_chain
A file containing trusted certificates to use when attempting to build the client/server certificate
chain related to the certificate specified via the -cert option.
-build_chain
Specify whether the application should build the certificate chain to be provided to the client.
-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.
-dcert_chain
A file containing trusted certificates to use when attempting to build the server certificate chain
when a certificate specified via the -dcert option is in use.
-dcertform PEM|DER, -dkeyform PEM|DER, -dpass val
Additional certificate and private key format and passphrase respectively.
-xkey infile, -xcert infile, -xchain
Specify an extra certificate, private key and certificate chain. These behave in the same manner as
the -cert, -key and -cert_chain options. When specified, the callback returning the first valid
chain will be in use by the server.
-xchain_build
Specify whether the application should build the certificate chain to be provided to the client for
the extra certificates provided via -xkey infile, -xcert infile, -xchain options.
-xcertform PEM|DER, -xkeyform PEM|DER
Extra certificate and private key format respectively.
-nbio_test
Tests non blocking I/O.
-crlf
This option translated a line feed from the terminal into CR+LF.
-debug
Print extensive debugging information including a hex dump of all traffic.
-msg
Show all protocol messages with hex dump.
-msgfile outfile
File to send output of -msg or -trace to, default standard output.
-state
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(1) for more information. These are also used when building the server certificate chain.
-chainCApath dir
The directory to use for building the chain provided to the client. This directory must be in "hash
format", see verify(1) for more information.
-chainCAfile file
A file containing trusted certificates to use when attempting to build the server certificate chain.
-no-CAfile
Do not load the trusted CA certificates from the default file location.
-no-CApath
Do not load the trusted CA certificates from the default directory location.
-nocert
If this option is set then no certificate is used. This restricts the cipher suites available to the
anonymous ones (currently just anonymous DH).
-quiet
Inhibit printing of session and certificate information.
-www
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. Cannot be used in conjunction with -early_data.
-WWW
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. Cannot be used
in conjunction with -early_data.
-tlsextdebug
Print a hex dump of any TLS extensions received from the server.
-HTTP
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). Cannot be used in conjunction with -early_data.
-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.
-verify_return_error
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.
-status
Enables certificate status request support (aka OCSP stapling).
-status_verbose
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.
-trace
Show verbose trace output of protocol messages. OpenSSL needs to be compiled with enable-ssl-trace
for this option to work.
-brief
Provide a brief summary of connection parameters instead of the normal verbose output.
-rev
Simple test server which just reverses the text received from the client and sends it back to the
server. Also sets -brief. Cannot be used in conjunction with -early_data.
-async
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. Note that not all protocols and flags
may be available, depending on how OpenSSL was built.
-bugs
There are several known bugs in SSL and TLS implementations. Adding this option enables various
workarounds.
-no_comp
Disable negotiation of TLS compression. TLS compression is not recommended and is off by default as
of OpenSSL 1.1.0.
-comp
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.
-no_ticket
Disable RFC4507bis session ticket support. This option has no effect if TLSv1.3 is negotiated. See
-num_tickets.
-num_tickets
Control the number of tickets that will be sent to the client after a full handshake in TLSv1.3. The
default number of tickets is 2. This option does not affect the number of tickets sent after a
resumption handshake.
-serverpref
Use the server's cipher preferences, rather than the client's preferences.
-prioritize_chacha
Prioritize ChaCha ciphers when preferred by clients. Requires -serverpref.
-no_resumption_on_reneg
Set the SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION option.
-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 list of TLSv1.2 and below ciphersuites used by the server to be modified. This list
is combined with any TLSv1.3 ciphersuites that have been configured. 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 is irrelevant. See the
ciphers command for more information.
-ciphersuites val
This allows the list of TLSv1.3 ciphersuites used by the server to be modified. This list is
combined with any TLSv1.2 and below ciphersuites that have been configured. 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 is irrelevant. See the
ciphers command for more information. The format for this list is a simple colon (":") separated list
of TLSv1.3 ciphersuite names.
-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.
-nbio
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.
-psk_session file
Use the pem encoded SSL_SESSION data stored in file as the basis of a PSK. Note that this will only
work if TLSv1.3 is negotiated.
-listen
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.
-sctp
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.
-sctp_label_bug
Use the incorrect behaviour of older OpenSSL implementations when computing endpoint-pair shared
secrets for DTLS/SCTP. This allows communication with older broken implementations but breaks
interoperability with correct implementations. Must be used in conjunction with -sctp. This option is
only available where OpenSSL has support for SCTP enabled.
-no_dhe
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.
-early_data
Accept early data where possible. Cannot be used in conjunction with -www, -WWW, -HTTP or -rev.
-anti_replay, -no_anti_replay
Switches replay protection on or off, respectively. Replay protection is on by default unless
overridden by a configuration file. When it is on, OpenSSL will automatically detect if a session
ticket has been used more than once, TLSv1.3 has been negotiated, and early data is enabled on the
server. A full handshake is forced if a session ticket is used a second or subsequent time. Any early
data that was sent will be rejected.
CONNECTED COMMANDS
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 commands are also recognized which perform special operations. These commands are a letter which
must appear at the start of a line. They 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 (TLSv1.2 and below only).
R Renegotiate the SSL session and request a client certificate (TLSv1.2 and below only).
P Send some plain text down the underlying TCP connection: this should cause the client to disconnect
due to a protocol violation.
S Print out some session cache status information.
B Send a heartbeat message to the client (DTLS only)
k Send a key update message to the client (TLSv1.3 only)
K Send a key update message to the client and request one back (TLSv1.3 only)
c Send a certificate request to the client (TLSv1.3 only)
NOTES
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.
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_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.
SEE ALSO
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)
HISTORY
The -no_alt_chains option was added in OpenSSL 1.1.0.
The -allow-no-dhe-kex and -prioritize_chacha options were added in OpenSSL 1.1.1.
COPYRIGHT
Copyright 2000-2019 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
<https://www.openssl.org/source/license.html>.
1.1.1f 2025-02-05 S_SERVER(1SSL)