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Nginx Pitfalls

If you're new or old to Nginx you will likely wind up in a pitfall situation. Below we outline these pitfalls and how to best avoid them. These are issues seen time and time again in the #nginx channel on Freenode. Please, just don't do this.

This Guide Says

Don't follow the guides out there unless you can understand what they're trying to do and can clean it up. So many of the configs out there are horribly written. The Pitfalls page was written entirely from poor configs found on the Internet. They weren't found by searching but were instead found by support requests asking why something doesn't work. The common response is to disagree with what we say because some random guide says to do it the wrong way. That's what this page is designed to address. If you're having issues, this is the place to look.

Root inside Location Block

BAD:

server {
  server_name www.domain.com;
  location / {
    root /var/www/nginx-default/;
    [...]
  }
  location /foo {
    root /var/www/nginx-default/;
    [...]
  }
  location /bar {
    root /var/www/nginx-default/;
    [...]
  }
}

This works. Putting root inside of a location block will work and it's perfectly valid. What's wrong is when you start adding location blocks. If you add a root to every location block then a location block that isn't matched will have no root. Let's look at a good configuration.

GOOD:

server {
  server_name www.domain.com;
  root /var/www/nginx-default/;
  location / {
    [...]
  }
  location /foo {
    [...]
  }
  location /bar {
    [...]
  }
}

Multiple Index Directives

BAD:

http {
  index index.php index.htm index.html;
  server {
    server_name www.domain.com;
    location / {
      index index.php index.htm index.html;
      [...]
    }
  }
  server {
    server_name domain.com;
    location / {
      index index.php index.htm index.html;
      [...]
    }
    location /foo {
      index index.php;
      [...]
    }
  }
}

Why repeat so many lines when not needed. Simply use the "index" directive one time. It only needs to occur in your http { } block and it will be inherited below.

GOOD:

http {
  index index.php index.htm index.html;
  server {
    server_name www.domain.com;
    location / {
      [...]
    }
  }
  server {
    server_name domain.com;
    location / {
      [...]
    }
    location /foo {
      [...]
    }
  }
}

Using If

There is a little page about using if statements. It's called IfIsEvil and you really should check it out. Let's take a look at a few uses of if that are bad.

Server Name

BAD:

server {
  server_name domain.com *.domain.com;
  if ($host ~* ^www\.(.+)) {
    set $raw_domain $1;
    rewrite ^/(.*)$ $raw_domain/$1 permanent;
    [...]
  }
}

There are actually three problems here. The first being if directives. That's what we care about now. Why is this bad? Did you read If is Evil?
With if directives Nginx is forced to evaluate every request for all domains. Evaluating every request against if directives is extremely inefficient.
Avoid using if directives and use two server directives as shown.


GOOD:

server {
  server_name www.domain.com;
  return 301 $scheme://domain.com$request_uri;
}
server {
  server_name domain.com;
  [...]
}

Besides making the configuration file easier to read. This approach decreases nginx processing requirements. We got rid of the spurious if. We're also using $scheme which doesn't hardcodes the URI scheme you're using, be it http or https.

Check IF File Exists

Using if to ensure a file exists is horrible. It's mean. If you have any recent version of Nginx you should look at try_files which just made life much easier.

BAD:

server {
  root /var/www/domain.com;
  location / {
    if (!-f $request_filename) {
      break;
    }
  }
}

GOOD:

server {
  root /var/www/domain.com;
  location / {
    try_files $uri $uri/ /index.html;
  }
}

What we changed is that we try to see if $uri exists without requiring an if. Using try_files mean that you can test a sequence. If $uri doesn't exist, try $uri/, if that doesn't exist try a fallback location.

In this case it will see if the $uri file exists. If it does then serve it. If it doesn't then tests if that directory exists. If not, then it will proceed to serve index.html which you make sure exists. It's loaded but oh so simple. This is another instance you can completely eliminate If.

Front Controller Pattern based packages

"Front Controller Pattern" designs are popular and used on the many of the most popular PHP software packages. A lot of examples are more complex than they need to be. To get Drupal, Joomla, etc. to work, just use this:

try_files $uri $uri/ /index.php?q=$uri&$args;

Note - the parameter names are different based on the package you're using. For example:

  • "q" is the parameter used by Drupal, Joomla, WordPress
  • "page" is used by CMS Made Simple

Some software doesn't even need the query string, and can read from REQUEST_URI (WordPress supports this, for example):

try_files $uri $uri/ /index.php;

Of course, your mileage may vary and you may need more complex things based on your needs, but for a basic sites, these will work perfectly. You should always start simple and build from there.

You can also decide to skip the directory check and remove "$uri/" from it as well, if you don't care about checking for the existence of directories.

Passing Uncontrolled Requests to PHP

Many example Nginx configurations for PHP on the web advocate passing every URI ending in .php to the PHP interpreter. Note that this presents a serious security issue on most PHP setups as it may allow arbitrary code execution by third parties.

The problem section usually looks like this:

location ~* \.php$ {
  fastcgi_pass backend;
  ...
}

Here, every request ending in .php will be passed to the FastCGI backend. The issue with this is that the default PHP configuration tries to guess which file you want to execute if the full path does not lead to an actual file on the filesystem.

For instance, if a request is made for /forum/avatar/1232.jpg/file.php which does not exist but if /forum/avatar/1232.jpg does, the PHP interpreter will process /forum/avatar/1232.jpg instead. If this contains embedded PHP code, this code will be executed accordingly.

Options for avoiding this are:

  • Set cgi.fix_pathinfo=0 in php.ini. This causes the PHP interpreter to only try the literal path given and to stop processing if the file is not found.
  • Ensure that Nginx only passes specific PHP files for execution.
location ~* (file_a|file_b|file_c)\.php$ {
  fastcgi_pass backend;
  ...
}
  • Specifically disable the execution of PHP files in any directory containing user uploads.
location /uploaddir {
  location ~ \.php$ {return 403;}
  ...
}
  • Use the try_files directive to filter out the problem condition
location ~* \.php$ {
  try_files $uri =404;
  fastcgi_pass backend;
  ...
}
  • Use a nested location to filter out the problem condition
location ~* \.php$ {
  location ~ \..*/.*\.php$ {return 404;}
  fastcgi_pass backend;
  ...
}

FastCGI Path in Script Filename

So many guides out there like to rely on absolute paths to get to your information. This is commonly seen in PHP blocks. When you install Nginx from a repository you'll usually wind up being able to toss "include fastcgi_params;" in your config. This is a file located in your Nginx root directory which is usually around /etc/nginx/.

GOOD:

fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;

BAD:

fastcgi_param  SCRIPT_FILENAME    /var/www/yoursite.com/$fastcgi_script_name;

Where is $document_root set? It's set by the root directive that should be in your server block. Is your root directive not there? See the first pitfall.

Taxing Rewrites

Don't feel bad here, it's easy to get confused with regular expressions. In fact, it's so easy to do that we should make an effort to keep them neat and clean. Quite simply, don't add cruft.

BAD:

    rewrite ^/(.*)$ http://domain.com/$1 permanent;

Also BAD:

    rewrite ^ http://domain.com$request_uri? permanent;

GOOD:

    return 301 http://domain.com$request_uri;

Look at the above. Then back here. Then up, and back here. OK. The first rewrite captures the full URI minus the first slash. By using the built-in variable $request_uri we can effectively avoid doing any capturing or matching at all, and by using the return directive we can completely avoid evaluation of regular expression.

Rewrite Missing http://

Very simply, rewrites are relative unless you tell nginx that they're not. Making a rewrite absolute is simple. Add a scheme.

BAD:

    rewrite ^/blog(/.*)$ blog.domain.com$1 permanent;

GOOD:

    rewrite ^/blog(/.*)$ http://blog.domain.com$1 permanent;

In the above you will see that all we did was add "http://" to the rewrite. It's simple, easy, and effective.

Proxy Everything

BAD:

server {
    server_name example.org;
    root /var/www/site;
 
    location / {
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_pass unix:/tmp/phpcgi.socket;
    }
}

In this example, you pass everything to PHP. Apache might do this, but with nginx you don't need to. The try_files directive tries files in a specific order. This means that nginx can first look for a number of static files to serve and if not found move on to a user defined fallback. This way PHP doesn't get involved unless an actual PHP file is requested and you save resources, especially if you're serving a 1MB image over PHP a few thousand times versus serving it directly. Let's take a look at how to do that.

GOOD:

server {
    server_name example.org;
    root /var/www/site;
 
    location / {
        try_files $uri $uri/ @proxy;
    }
 
    location @proxy {
        include fastcgi.conf;
        fastcgi_pass unix:/tmp/php-fpm.sock;
    }
}

Also GOOD:

server {
    server_name example.org;
    root /var/www/site;
 
    location / {
        try_files $uri $uri/ /index.php;
    }
 
    location ~ \.php$ {
        include fastcgi.conf;
        fastcgi_pass unix:/tmp/php-fpm.sock;
    }
}

If the requested URI exists and can be served by nginx do that. If not, is it a directory that can be indexed or do we have an index directive to apply? If not, then rewrite the request internally to /index.php and pass it to your backend. Only when nginx can't serve that requested URI directly does your backend get involved.

Consider how many of your requests are static content, such as images, css, javascript, etc. That's probably a lot of overhead you just saved.

Config Changes Not Reflected

Browser cache. Your configuration may be perfect but you'll sit there and beat your head against a cement wall for a month. What's wrong is your browser cache. When you download something, your browser stores it. It also stores how that file was served. If you are playing with a types{} block you'll encounter this.

The fix:
[Option 1] In Firefox press Ctrl+Shift+Delete, check Cache, click Clear Now. In any other browser just ask your favorite search engine. Do this after every change (unless you know it's not needed) and you'll save yourself a lot of headaches.

In Firefox, you can also choose a more permanent solution : in the URI bar type about:config, then search for browser.cache.check_doc_frequency and set its value to 1 to check for a new version every time a page is loaded.
[Option 2] Use curl.


VirtualBox

If this does not work, and you're running nginx on a virtual machine in VirtualBox, it may be sendfile() that is causing the trouble. Simply comment out the sendfile directive or set it to "off". The directive is most likely found in your nginx.conf file.


Missing (disappearing) HTTP headers

If you do not explicitly set underscores_in_headers on;, nginx will silently drop HTTP headers with underscores (which are perfectly valid according to the HTTP standard). This is done in order to prevent ambiguities when mapping headers to CGI variables, as both dashes and underscores are mapped to underscores during that process.