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diff --git a/doc/s6-tlsc-io.html b/doc/s6-tlsc-io.html new file mode 100644 index 0000000..2f02841 --- /dev/null +++ b/doc/s6-tlsc-io.html @@ -0,0 +1,217 @@ +<html> + <head> + <meta name="viewport" content="width=device-width, initial-scale=1.0" /> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>s6-networking: the s6-tlsc-io program</title> + <meta name="Description" content="s6-networking: the s6-tlsc-io program" /> + <meta name="Keywords" content="s6-networking s6-tlsc-io tlsc tls ssl ucspi tcp inet network tcp/ip client" /> + <!-- <link rel="stylesheet" type="text/css" href="//skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">s6-networking</a><br /> +<a href="//skarnet.org/software/">Software</a><br /> +<a href="//skarnet.org/">skarnet.org</a> +</p> + +<h1> The <tt>s6-tlsc-io</tt> program </h1> + +<p> +<tt>s6-tlsc-io</tt> is a program that establishes a TLS or SSL +client connection over an existing TCP connection, then +communicates with an existing local program over already +established pipes. It is the only client-side program in +s6-networking that performs cryptography. +</p> + +<p> + <a href="index.html">s6-networking</a> does not include +cryptographic software. All the crypto used in <tt>s6-tlsc-io</tt> +is provided by the chosen SSL backend: +<a href="https://bearssl.org/">BearSSL</a> or +<a href="https://www.libressl.org/">LibreSSL</a>, depending on +the options given when configuring s6-networking. +</p> + +<h2> Interface </h2> + +<pre> + s6-tlsc-io [ -S | -s ] [ -Y | -y ] [ -v <em>verbosity</em> ] [ -K kimeout ] [ -k <em>servername</em> ] [ -d notif ] [ -- ] <em>fdr</em> <em>fdw</em> +</pre> + +<ul> + <li> s6-tlsc-io expects to have an open connection it +can talk to on its standard input and output. It also expects to read +cleartext data from file descriptor <em>fdr</em> and write cleartext +data to file descriptor <em>fdw</em>. </li> + <li> It initiates a TLS handshake over the network connection, +expecting a TLS server on the other side. </li> + <li> Then it acts as a full duplex tunnel, decrypting and transmitting +data from stdin to <em>fdw</em>, and encrypting and transmitting data +from <em>fdr</em> to stdout. </li> + <li> When it cannot transmit any more data from/to the local application +because connections have closed, <tt>s6-tlsc-io</tt> exits. </li> +</ul> + +<h2> Exit codes </h2> + +<ul> + <li> 0: the connection terminated normally. + <li> 96: error while configuring the TLS context - for instance, invalid trust anchor set. </li> + <li> 97: error while setting up the TLS client engine. </li> + <li> 98: TLS error while running the engine. </li> + <li> 100: wrong usage. </li> + <li> 111: system call failed. </li> +</ul> + +<h2> Protocol version and parameters </h2> + +<p> + During the TLS handshake, <tt>s6-tlsc-io</tt> tries +every version of the protocol that is supported by the +backend, with all supported algorithms and cipher suites; +the backend normally ensures that the most secure combination +is tried first, with slow degradation until the client and +the server agree. +</p> + +<ul> + <li> For BearSSL, this means use of the +<a href="https://bearssl.org/apidoc/bearssl__ssl_8h.html#aa386dd0b03a0123760bf63df5a41c1e0">br_ssl_client_init_full()</a> +function. The supported protocol versions are described +<a href="https://bearssl.org/support.html#supported-versions">here</a>. </li> + <li> For LibreSSL, this means use of the +<a href="https://man.openbsd.org/OpenBSD-current/man3/tls_config_set_protocols.3">tls_config_set_protocols(TLS_PROTOCOLS_ALL)</a> +call. </li> +</ul> + +<p> + As a client, it is better for <tt>s6-tlsc-io</tt> to adapt to as many servers +as possible, that's why it adopts a liberal approach to protocol +versions. +</p> + +<h2> Environment variables </h2> + +<p> + <tt>s6-tlsc-io</tt> expects to have one of the +<tt>CADIR</tt> or <tt>CAFILE</tt> environment variables set. +It will refuse to run if both are unset. If both are set, +<tt>CADIR</tt> has priority. The value of that variable is: +</p> + +<ul> + <li> for <tt>CADIR</tt>: a directory where trust anchors +(i.e. root or intermediate CA certificates) can be found, +one per file, DER- or PEM-encoded. </li> + <li> for <tt>CAFILE</tt>: a file containing the whole set +of trust anchors, PEM-encoded. </li> +</ul> + +<p> + If you are using client certificates, s6-tlsc-io also reads +two more environment variables: <tt>KEYFILE</tt> contains +the path to a file containing the private key, DER- or +PEM-encoded; and <tt>CERTFILE</tt> contains the path to +a file containing the client certificate, DER- or +PEM-encoded. +</p> + +<p> + If <tt>s6-tlsc-io</tt> is run as root, it can also read two +other environment variables, <tt>TLS_UID</tt> and <tt>TLS_GID</tt>, +which contain a numeric uid and a numeric gid; <tt>s6-tlsc-io</tt> +then drops its root privileges to this uid/gid after spawning +<em>prog...</em>. This ensures that the TLS/engine and the +application run with different privileges. Note that <em>prog...</em> +should drop its own root privileges by its own means: the +<a href="//skarnet.org/software/s6/s6-applyuidgid.html">s6-applyuidgid</a> +program is a chainloading way of doing it. +</p> + +<h2> Server name determination for SNI </h2> + +<p> + The <tt>-k <em>servername</em></tt> option is important to +<tt>s6-tlsc-io</tt>: it tells it to send <em>servername</em> +as the name to require a certificate for. +Not setting this option allows <tt>s6-tlsc-io</tt> to +proceed without SNI, <strong>which may be a security risk.</strong> +</p> + +<h2> SSL close handling </h2> + +<p> + If the local application initiates the end of the session by sending +EOF to <em>fdr</em>, there are two ways for the TLS layer to handle it. +</p> + +<ul> + <li> It can send a <tt>close_notify</tt> alert, and wait for +an acknowledgement from the peer, at which point the connection +is closed. The advantage of this setup is that it is secure +even when the application protocol is not auto-terminated, i.e. +when it does not know when its data stops. Old protocols such +as HTTP-0.9 are in this case. The drawback of this setup is +that it breaks full-duplex: once a peer has sent the +<tt>close_notify</tt>, it must discard all the incoming +records that are not a <tt>close_notify</tt> from the +other peer. So if a client sends EOF while it is still +receiving data from the server, the connection closes +immediately and the data can be truncated. </li> + <li> It can simply transmit the EOF, shutting down +half the TCP connection, and wait for the EOF back. +The advantage of this setup is that it maintains +full-duplex: a client can send EOF after its initial +request, and still receive a complete answer from the +server. The drawback is that it is insecure when the application +protocol is not auto-terminated. </li> +</ul> + +<p> + Nowadays (2020), most protocols are auto-terminated, so +it is not dangerous anymore to use EOF tranmission, and that +is the default for <tt>s6-tlsc-io</tt>. Nevertheless, by +using the <tt>-S</tt> option, you can +force it to use the <tt>close_notify</tt> method if your +application requires it to be secure. +</p> + +<h2> <tt>s6-tlsc-io</tt> options </h2> + +<ul> + <li> <tt>-v <em>verbosity</em></tt> : Be more or less +verbose. Default for <em>verbosity</em> is 1. 0 is quiet, 2 is +verbose, more than 2 is debug output. This option currently has +no effect. </li> + <li> <tt>-S</tt> : send a <tt>close_notify</tt> alert +and break the connection when receiving a local EOF. </li> + <li> <tt>-s</tt> : transmit EOF by half-closing the TCP +connection without using <tt>close_notify</tt>. This is the default. </li> + <li> <tt>-Y</tt> : Do not send a client certificate. This is the default. </li> + <li> <tt>-y</tt> : Send a client certificate. </li> + <li> <tt>-k <em>servername</em></tt> : use Server Name +Indication, and send <em>servername</em>. The default is not to +use SNI, which may be a security risk. </li> + <li> <tt>-K <em>kimeout</em></tt> : close the connection +if <em>kimeout</em> milliseconds elapse without any data being +received from either side. The default is 0, which means +infinite timeout (never kill the connection). </li> + <li> <tt>-d <em>notif</em></tt> : handshake notification. +<em>notif</em> must be a file descriptor open for writing. When the +TLS handshake has completed, some data (terminated by two null +characters) will be sent to file descriptor <em>notif</em>. The +data contains information about the TLS parameters of the connection; +its exact contents are left unspecified, but there's at least +a <tt>SSL_PROTOCOL=<em>protocol</em></tt> string +and a <tt>SSL_CIPHER=<em>cipher</em></tt> string, both null-terminated. +Sending this data serves a dual purpose: telling the <em>notif</em> +reader that the handshake has completed, and providing it with some +basic information about the connection. If this option is not given, +no such notification is performed. </li> +</ul> + +</body> +</html> |