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Difference between revisions of "Pitfalls"

(Passing Uncontrolled Requests to PHP)
(Passing Uncontrolled Requests to PHP)
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<geshi lang="nginx">
 
<geshi lang="nginx">
 
location /uploaddir {
 
location /uploaddir {
if $request_uri ~ \.php$ {
+
if $request_uri ~ \.php$ { deny all;}
deny all;
+
}
+
 
...
 
...
 
}
 
}
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<geshi lang="nginx">
 
<geshi lang="nginx">
 
location ~* \.php$ {
 
location ~* \.php$ {
location ~ \..*/.*\.php$ {  
+
location ~ \..*/.*\.php$ {deny all;}
                deny all;  
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        }
+
 
fastcgi_pass backend;
 
fastcgi_pass backend;
 
...
 
...
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<geshi lang="nginx">
 
<geshi lang="nginx">
 
location ~* \.php$ {
 
location ~* \.php$ {
if $request_uri ~ \..*/.*\.php$ {  
+
if $request_uri ~ \..*/.*\.php$ {deny all;}
                deny all;  
+
        }
+
 
fastcgi_pass backend;
 
fastcgi_pass backend;
 
...
 
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Revision as of 15:21, 7 January 2012

Contents

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 the if. That's what we care about now. Why is this bad? Did you read If is Evil? When nginx receives a request no matter what is the subdomain being requested, be it www.domain.com or just the plain domain.com this if directive is always evaluated. Since you're requesting nginx to check for the Host header for every request. It's extremely inefficient. You should avoid it. Instead use two server directives like the example below.

GOOD:

server {
  server_name www.domain.com;
  rewrite ^ $scheme://domain.com$request_uri redirect;
}
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 pass every URI ending in .php to the PHP parser. Note that this presents a serious security issue on most PHP setups by arbitrary PHP 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/index.php and the file does not exist but /forum/avatar/1232.jpg does, PHP will instead execute /forum/avatar/1232.jpg. The 1232.jpg however, might contain embedded PHP code which will be executed in this instance.

Options for avoiding this are:

  • Set cgi.fix_pathinfo=0 in php.ini. This causes PHP to only try the literal path given.
  • 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 {
	if $request_uri ~ \.php$ {	deny all;}
	...
}
  • 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$ {deny all;}
	fastcgi_pass backend;
	...
}
  • Use a conditional statement to test for the problem condition
location ~* \.php$ {
	if $request_uri ~ \..*/.*\.php$ {deny all;}
	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;

GOOD:

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

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.

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 ^ domain.com permanent;

GOOD:

    rewrite ^ http://domain.com 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 _;
    root /var/www/site;
    location / {
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_pass unix:/tmp/phpcgi.socket;
    }
}

Yucky. In this instance, you pass EVERYTHING to PHP. Why? Apache might do this, you don't need to. Let me put it this way... The try_files directive exists for an amazing reason. It tries files in a specific order. This means that Nginx can first try to server the static content. If it can't, then it moves on. This means PHP doesn't get involved at all. MUCH faster. 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 _;
    root /var/www/site;
    location / {
        try_files $uri $uri/ @proxy;
    }
    location @proxy {
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_pass unix:/tmp/phpcgi.socket;
    }
}

It's easy, right? You see if the requested URI exists and can be served by Nginx. If not, is it a directory that can be served. If not, then you pass it to your proxy. Only when Nginx can't serve that requested URI directly does your proxy overhead get involved.

Now.. consider how much 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.
[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.