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<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>SOURCE_DATE_EPOCH specification</title><meta name="generator" content="DocBook XSL Stylesheets V1.79.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="article"><div class="titlepage"><div><div><h2 class="title"><a name="idm1"></a><code class="envar">SOURCE_DATE_EPOCH</code> specification</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Chris</span> <span class="surname">Lamb</span></h3><div class="affiliation"><div class="address"><p><code class="email"><<a class="email" href="mailto:lamby@debian.org">lamby@debian.org</a>></code></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Ximin</span> <span class="surname">Luo</span></h3><div class="affiliation"><div class="address"><p><code class="email"><<a class="email" href="mailto:infinity0@debian.org">infinity0@debian.org</a>></code></p></div></div></div></div><div><div class="revhistory"><table style="border-style:solid; width:100%;" summary="Revision History"><tr><th align="left" valign="top" colspan="2"><b>Revision History</b></th></tr><tr><td align="left">Revision 1.1</td><td align="left">27 November 2017</td></tr><tr><td align="left" colspan="2"> Document timestamp clamping. Drop prohibition of overwriting the variable, and instead specify more tightly how to set it. </td></tr><tr><td align="left">Revision 1.0</td><td align="left">01 September 2015</td></tr><tr><td align="left" colspan="2">Initial version.</td></tr></table></div></div></div><hr></div><div class="toc"><p><b>Table of Contents</b></p><dl class="toc"><dt><span class="sect1"><a href="#idm26">Introduction</a></span></dt><dt><span class="sect1"><a href="#idm32">Motivation</a></span></dt><dd><dl><dt><span class="sect2"><a href="#idm47">Build timestamps</a></span></dt></dl></dd><dt><span class="sect1"><a href="#idm53">Specification</a></span></dt><dd><dl><dt><span class="sect2"><a href="#idm55"><code class="envar">SOURCE_DATE_EPOCH</code></a></span></dt></dl></dd><dt><span class="sect1"><a href="#idm69">Examples</a></span></dt><dd><dl><dt><span class="sect2"><a href="#idm71">Setting the variable</a></span></dt><dt><span class="sect2"><a href="#idm80">Timestamp clamping</a></span></dt><dt><span class="sect2"><a href="#idm86">Other examples</a></span></dt></dl></dd><dt><span class="sect1"><a href="#idm90">Copyright</a></span></dt><dt><span class="sect1"><a href="#idm96">Contributors</a></span></dt></dl></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idm26"></a>Introduction</h2></div></div></div><p> This specification defines a distribution-agnostic standard for build systems to exchange a timestamp. </p><p> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. </p><p> The canonical URI for this document is: <a class="ulink" href="https://reproducible-builds.org/specs/source-date-epoch/" target="_top">https://reproducible-builds.org/specs/source-date-epoch/</a>. </p></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idm32"></a>Motivation</h2></div></div></div><p> Whilst anyone can inspect the source code of free software for malicious flaws, most distributions provide binary (or "compiled") packages to end users. The motivation behind "reproducible" builds is to allow verification that no flaws have been introduced during this compilation process by promising identical binary packages are always generated from a given source. </p><p> This prevents against the installation of backdoor-introducing malware on developers' machines as an attacker would need to simultaneously infect all the developers attempting to reproduce the build. In addition, a reproducible build has other technical advantages: </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> Requires the removal of any non-deterministic and/or unsafe behaviour, eg. connecting to the internet to download build-dependencies or reading from uninitialised memory </p></li><li class="listitem"><p> Detects corrupted or outdated build environments </p></li><li class="listitem"><p> Provides validation of packages built on foreign architectures </p></li><li class="listitem"><p> Reduces time-to-detection of a build host compromise </p></li><li class="listitem"><p> Can show that proposed changes have no impact on binaries </p></li></ul></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="idm47"></a>Build timestamps</h3></div></div></div><p> Software packages are often unreproducible because they embed compile-time timestamps into generated files. As the current time changes between builds, this results in the binaries containing different contents. Futhermore, these dates are unreliable indicators of the software's age given that software can be arbitrarily rebuilt. </p><p> An improvement is to use the last modification time of the source; if the source is then modified, the binaries will change by design. This timestamp is also more informative as it reflects the actual age of the software and not when it was last compiled. </p><p> However, in the context of a distribution, the last modification time is not a property of the upstream source, but rather of the packaging that encapsulates it. </p><p> This specification therefore defines a distribution-agnostic standard for upstream build processes to consume this timestamp from packaging systems. The intended result is a build where the output looks as if the build had happened instantly at the time specified in that timestamp. </p></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idm53"></a>Specification</h2></div></div></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="idm55"></a><code class="envar">SOURCE_DATE_EPOCH</code></h3></div></div></div><p> A UNIX timestamp, defined as the number of seconds, excluding leap seconds, since <code class="computeroutput">01 Jan 1970 00:00:00 UTC</code>. </p><p> The value MUST be exported through the operating system's usual environment mechanism. </p><p> The value MUST be an ASCII representation of an integer with no fractional component, identical to the output format of <span class="command"><strong>date +%s</strong></span>. </p><p> The value MUST be reproducible (deterministic) across different executions of the build, depending only on the source code. It SHOULD be set to the last modification time of the source, incorporating any packaging-specific modifications. </p><p> Build processes MUST use this variable for embedded timestamps in place of the "current" date and time. </p><p> Where build processes embed timestamps that are not "current", but are nevertheless still specific to one execution of the build process, they MUST use a timestamp no later than the value of this variable. This is often called "timestamp clamping". </p><p> Build processes MUST NOT unset this variable for child processes if it is already present. </p><p> Formatting MUST be deferred until runtime if an end user should observe the value in their own locale or timezone. </p><p> If the value is malformed, the build process SHOULD exit with a non-zero error code. </p></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idm69"></a>Examples</h2></div></div></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="idm71"></a>Setting the variable</h3></div></div></div><p> Distributions could set <code class="envar">SOURCE_DATE_EPOCH</code> by using the value of a changelog file. For instance, Debian packages can set it to the value of the latest entry of <code class="filename">debian/changelog</code>. </p><p> Developers could set <code class="envar">SOURCE_DATE_EPOCH</code> to the date of the latest commit in their version control system. In this case, it is recommended to also update all source file timestamps, which git does not do by itself, otherwise old timestamps specific to the developer's working tree may be embedded into the output. </p><p> Though it is not forbidden to set <code class="envar">SOURCE_DATE_EPOCH</code> several times during a build, such as for different child modules, build processes doing this should ensure that any differing values do not interfere with each other in a nondeterministic way. </p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="idm80"></a>Timestamp clamping</h3></div></div></div><p> One can reasonably assume that all source timestamps are before <code class="envar">SOURCE_DATE_EPOCH</code> and all builds take place after it. This means we can efficiently both preserve source-based timestamps and omit build-specific timestamps, by rewriting timestamps more recent than <code class="envar">SOURCE_DATE_EPOCH</code> back to the latter. See for example the <code class="option">--clamp-mtime</code> option to GNU tar. </p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="idm86"></a>Other examples</h3></div></div></div><p> Other examples are available at <a class="ulink" href="https://reproducible-builds.org/docs/source-date-epoch/" target="_top">https://reproducible-builds.org/docs/source-date-epoch/</a>. </p></div></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idm90"></a>Copyright</h2></div></div></div><p> Copyright 漏 2015-2017 See Contributors List </p><p> Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: </p><p> The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. </p><p> THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. </p></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="idm96"></a>Contributors</h2></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> Axel Beckert </p></li><li class="listitem"><p> Chris Lamb (lamby) </p></li><li class="listitem"><p> Chris West (Faux) </p></li><li class="listitem"><p> Daniel Kahn Gillmor (dkg) </p></li><li class="listitem"><p> Dmitry Shachnev </p></li><li class="listitem"><p> Eduard Sanou </p></li><li class="listitem"><p> Holger Levsen </p></li><li class="listitem"><p> J茅r茅my Bobbio (Lunar) </p></li><li class="listitem"><p> Mattia Rizzolo </p></li><li class="listitem"><p> Ximin Luo </p></li></ul></div></div></div></body></html>