IfIsEvil

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<table id="fuck-ie"> <tr> <td width="30%"> <div id="userlogin"> <a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=Special:UserLogin&returnto=IfIsEvil" title="Log in / create account">Log in / create account</a> </div> </td> <td width="60%" align="right"> <form name="searchform" action="/web/20121216232625/http://wiki.nginx.org/Special:Search" id="searchform" method="get"> <input id="searchInput" name="search" type="text" accesskey="f" value=""/>  <input type="submit" name="fulltext" class="searchButton" value="Search"/> </form> </td> <td width="4em" align="left" style="padding: 0 0 0 1em;"><a href="Faq" id="faqlink">FAQ</a></td> </tr> </table> </div>  <a class="portlet" id="p-logo" href="/web/20121216232625/http://wiki.nginx.org/Main" title="Main"> <img src="/web/20121216232625im_/http://wiki.nginx.org/local/nginx-logo.png"/> </a> <div style="float: right;"> <ul id="icon-nav"> <li><a href="/web/20121216232625/http://wiki.nginx.org/Install"><img 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src="/web/20121216232625im_/http://wiki.nginx.org/skins/bluwiki_skin/Edit-small.png" class="" alt="Edit"/></a> <a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=history" title="History"><img src="/web/20121216232625im_/http://wiki.nginx.org/skins/bluwiki_skin/History-small.png" class="" alt="History"/></a> </div> <h1 class="firstHeading">IfIsEvil</h1> <div id="bodyContent"> <div id="contentSub"></div>  <div id="mw-content-text" lang="en" dir="ltr" class="mw-content-ltr"><table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div> <ul> <li class="toclevel-1 tocsection-1"><a href="#Introduction"><span class="tocnumber">1</span> <span class="toctext">Introduction</span></a></li> <li class="toclevel-1 tocsection-2"><a href="#What_to_do_instead"><span class="tocnumber">2</span> <span class="toctext">What to do instead</span></a></li> <li class="toclevel-1 tocsection-3"><a href="#Examples"><span class="tocnumber">3</span> <span class="toctext">Examples</span></a></li> <li class="toclevel-1 tocsection-4"><a href="#Why_this_happens_and_still_not_fixed"><span class="tocnumber">4</span> <span class="toctext">Why this happens and still not fixed</span></a></li> <li class="toclevel-1 tocsection-5"><a href="#If_you_still_want_to_use_if"><span class="tocnumber">5</span> <span class="toctext">If you still want to use if</span></a></li> </ul> </td></tr></table> <h1><span class="editsection">[<a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=edit&section=1" title="Edit section: Introduction">edit</a>]</span> <span class="mw-headline" id="Introduction"> Introduction </span></h1> <p>Directive <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if" title="NginxHttpRewriteModule" class="mw-redirect">if</a> has problems when used in location context, in some cases it doesn't do what you expect but something completely different instead. In some cases it even segfaults. It's generally a good idea to avoid it if possible. </p><p>The only 100% safe things which may be done inside if in location context are: </p> <ul><li> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#return" title="NginxHttpRewriteModule" class="mw-redirect">return</a> ...; </li><li> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#rewrite" title="NginxHttpRewriteModule" class="mw-redirect">rewrite</a> ... last; </li></ul> <p>Anything else may possibly cause unpredictable behaviour, including potential SIGSEGV. </p><p>It is important to note that the behaviour of if is not inconsistent, given two identical requests it will not randomly fail on one and work on the other, with proper testing and understanding ifs <b>can</b> be used. The advice to use other directives where available still very much apply, though. </p><p>There are cases where you simply cannot avoid using an if, for example if you need to test a variable which has no equivalent directive. </p> <div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="nginx source-nginx"><pre class="de1"><a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$request_method</span> <span class="sy0">=</span> POST <span class="br0">)</span> <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#return"><span class="kw22">return</span></a> <span class="nu0">405</span><span class="sy0">;</span> <span class="br0">}</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$args</span> <span class="sy0">~</span> post<span class="sy0">=</span><span class="nu0">140</span><span class="br0">)</span><span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#rewrite"><span class="kw22">rewrite</span></a> ^ <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#http"><span class="kw3">http</span></a>://example.com/ permanent<span class="sy0">;</span> <span class="br0">}</span></pre></div></div> <h1><span class="editsection">[<a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=edit&section=2" title="Edit section: What to do instead">edit</a>]</span> <span class="mw-headline" id="What_to_do_instead"> What to do instead </span></h1> <p>Use <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#try_files" title="NginxHttpCoreModule" class="mw-redirect">try_files</a> if it suits your needs. Use the "return ..." or "rewrite ... last" in other cases. In some cases it's also possible to move ifs to server level (where it's safe as only other rewrite module directives are allowed within it). </p><p>E.g. the following may be used to safely change location which will be used to process request: </p> <div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="nginx source-nginx"><pre class="de1"> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#location"><span class="kw3">location</span></a> / <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#error_page"><span class="kw3">error_page</span></a> <span class="nu0">418</span> <span class="sy0">=</span> <span class="re0">@other</span><span class="sy0">;</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#recursive_error_pages"><span class="kw3">recursive_error_pages</span></a> on<span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$something</span><span class="br0">)</span> <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#return"><span class="kw22">return</span></a> <span class="nu0">418</span><span class="sy0">;</span> <span class="br0">}</span>   <span class="co1"># some configuration</span> ... <span class="br0">}</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#location"><span class="kw3">location</span></a> <span class="re0">@other</span> <span class="br0">{</span> <span class="co1"># some other configuration</span> ... <span class="br0">}</span></pre></div></div> <p>In some cases it may be good idea to use embedded scripting modules (<a href="/web/20121216232625/http://wiki.nginx.org/EmbeddedPerlModule" title="EmbeddedPerlModule" class="mw-redirect">embedded perl</a>, or various <a href="/web/20121216232625/http://wiki.nginx.org/3rdPartyModules" title="3rdPartyModules">3rd party modules</a>) to do the scripting. </p> <h1><span class="editsection">[<a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=edit&section=3" title="Edit section: Examples">edit</a>]</span> <span class="mw-headline" id="Examples"> Examples </span></h1> <p>Here are some examples which explain why if is evil. Don't try this at home. You were warned. </p> <div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="nginx source-nginx"><pre class="de1"> <span class="co1"># Here is collection of unexpectedly buggy configurations to show that</span> <span class="co1"># if inside location is evil.</span>   <span class="co1"># only second header will be present in response</span> <span class="co1"># not really bug, just how it works</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#location"><span class="kw3">location</span></a> /only-one-<a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#set"><span class="kw22">set</span></a> <span class="re0">$true</span> <span class="nu0">1</span><span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$true</span><span class="br0">)</span> <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpHeadersModule#add_header"><span class="kw14">add_header</span></a> X-First <span class="nu0">1</span><span class="sy0">;</span> <span class="br0">}</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$true</span><span class="br0">)</span> <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpHeadersModule#add_header"><span class="kw14">add_header</span></a> X-Second <span class="nu0">2</span><span class="sy0">;</span> <span class="br0">}</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#return"><span class="kw22">return</span></a> <span class="nu0">204</span><span class="sy0">;</span> <span class="br0">}</span>   <span class="co1"># request will be sent to backend without uri changed</span> <span class="co1"># to '/' due to if</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#location"><span class="kw3">location</span></a> /proxy-pass-uri <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpProxyModule#proxy_pass"><span class="kw21">proxy_pass</span></a> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#http"><span class="kw3">http</span></a>://127.0.0.1:<span class="nu0">8080</span>/<span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#set"><span class="kw22">set</span></a> <span class="re0">$true</span> <span class="nu0">1</span><span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$true</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1"># nothing</span> <span class="br0">}</span> <span class="br0">}</span>   <span class="co1"># try_files wont work due to if</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#location"><span class="kw3">location</span></a> /if-try-files <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#try_files"><span class="kw3">try_files</span></a> /file <span class="re0">@fallback</span><span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#set"><span class="kw22">set</span></a> <span class="re0">$true</span> <span class="nu0">1</span><span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$true</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1"># nothing</span> <span class="br0">}</span> <span class="br0">}</span>   <span class="co1"># nginx will SIGSEGV</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#location"><span class="kw3">location</span></a> /crash <span class="br0">{</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#set"><span class="kw22">set</span></a> <span class="re0">$true</span> <span class="nu0">1</span><span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$true</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1"># fastcgi_pass here</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpFcgiModule#fastcgi_pass"><span class="kw11">fastcgi_pass</span></a> 127.0.0.1:<span class="nu0">9000</span><span class="sy0">;</span> <span class="br0">}</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$true</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1"># no handler here</span> <span class="br0">}</span> <span class="br0">}</span>   <span class="co1"># alias with captures isn't correcly inherited into implicit nested</span> <span class="co1"># location created by if</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#location"><span class="kw3">location</span></a> <span class="sy0">~</span>* ^/if-and-<a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#alias"><span class="kw3">alias</span></a>/<span class="br0">(</span>?<span class="re4"><file></span>.*<span class="br0">)</span> <span class="br0">{</span> <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpCoreModule#alias"><span class="kw3">alias</span></a> /tmp/<span class="re0">$file</span><span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#set"><span class="kw22">set</span></a> <span class="re0">$true</span> <span class="nu0">1</span><span class="sy0">;</span>   <a href="/web/20121216232625/http://wiki.nginx.org/NginxHttpRewriteModule#if"><span class="kw22">if</span></a> <span class="br0">(</span><span class="re0">$true</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1"># nothing</span> <span class="br0">}</span> <span class="br0">}</span></pre></div></div> <p>In case you think you found an example which isn't listed here - it's a good idea to report it to <a href="/web/20121216232625/http://wiki.nginx.org/User:MaximDounin" title="User:MaximDounin">MaximDounin</a>. </p> <h1><span class="editsection">[<a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=edit&section=4" title="Edit section: Why this happens and still not fixed">edit</a>]</span> <span class="mw-headline" id="Why_this_happens_and_still_not_fixed"> Why this happens and still not fixed </span></h1> <p>Directive "if" is part of rewrite module which evaluates instructions imperatively. On the other hand, nginx configuration in general is declarative. At some point due to users demand an attempt was made to enable some non-rewrite directives inside "if", and this lead to situation we have now. It mostly works, but... see above. </p><p>Looks like the only correct fix would be to disable non-rewrite directives inside if completely. It would break many configuration out there though, so wasn't done yet. </p> <h1><span class="editsection">[<a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=edit&section=5" title="Edit section: If you still want to use if">edit</a>]</span> <span class="mw-headline" id="If_you_still_want_to_use_if"> If you still want to use if </span></h1> <p>If you read all of the above and still want to use if: </p> <ul><li> Please make sure you actually do understand how it works. Some basic idea may be found e.g. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20121216232625/http://agentzh.blogspot.com/2011/03/how-nginx-location-if-works.html">here</a>. </li><li> Do proper testing. </li></ul> <p>You were warned. </p>   </div><div class="printfooter"> Retrieved from "<a href="https://web.archive.org/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&oldid=466">http://wiki.nginx.org/index.php?title=IfIsEvil&oldid=466</a>"</div>   <div class="visualClear"></div> </div> </div> </div>  <div id="dropdownpanel"> <div class="contentdiv"> <div id="toolbox">  <div id="user-tools"> <div id="p-cactions" class="portlet"> <h5>Views</h5> <ul> <li id="ca-nstab-main" class="selected"> <a href="/web/20121216232625/http://wiki.nginx.org/IfIsEvil"> Page</a></li> <li id="ca-talk" class="new"> <a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=Talk:IfIsEvil&action=edit&redlink=1"> Discussion</a></li> <li id="ca-edit"> <a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=edit"> Edit</a></li> <li id="ca-history"> <a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&action=history"> History</a></li> </ul> </div> <div class="portlet" id="p-tb"> <h5>Toolbox</h5> <div class="pBody"> <ul> <li id="t-whatlinkshere"><a href="/web/20121216232625/http://wiki.nginx.org/Special:WhatLinksHere/IfIsEvil">What links here</a></li> <li id="t-recentchangeslinked"><a href="/web/20121216232625/http://wiki.nginx.org/Special:RecentChangesLinked/IfIsEvil">Related changes</a></li> <li id="t-specialpages"><a href="/web/20121216232625/http://wiki.nginx.org/Special:SpecialPages">Special pages</a></li> <li id="t-print"><a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&printable=yes">Printable version</a></li> <li id="t-permalink"><a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=IfIsEvil&oldid=466">Permanent link</a></li> </ul> </div> </div> <div class="portlet" id="p-personal"> <h5>Personal tools</h5> <div class="pBody"> <ul> <li id="pt-login"><a href="/web/20121216232625/http://wiki.nginx.org/index.php?title=Special:UserLogin&returnto=IfIsEvil">Log in / create account</a></li> </ul> </div> </div>   <script type="text/javascript"> var _gaq = _gaq || []; _gaq.push(['_setAccount', 'UA-22238880-1']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 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