Image resizing on the fly

In almost any web application that uses images, there is a need to generate small copies of these images, and often there are several additional image formats.
It also causes some headaches to add new dimensions to an existing application. Hence the task:

Task

Let's designate the list of requirements:

• Generate additional images of any format on the fly without adding additional functionality to the application at any time during the existence of the application;
• Additional images should not be generated with every request;
• Close the possibility of generating additional images of undefined formats.

I will explain the last point, because it slightly contradicts the first point. If we open the formation of any images, then there is the possibility of attacking the site by generating a large number of requests for image resizing in an infinite number of formats, so this vulnerability needs to be closed.

nginx installation configuration

To solve the above requirements, we need the following set of nginx modules:

• ngx_http_image_filter_module — for image resizing;
• ngx_http_proxy_module - for caching;
• ngx_http_secure_link_module — to protect against spam;

Modules ngx_http_image_filter_module и ngx_http_secure_link_module are not set by default, so they must be specified at the installation configuration stage :

phoinix@phoinix-work:~/src/nginx-0.8.29
$ ./configure --with-http_secure_link_module --with-http_image_filter_module


nginx configuration

Add a new one to our host configuration location and general cache options:

...
    proxy_cache_path /www/myprojects/cache levels=1:2 keys_zone=image-preview:10m;
...
    server {
...
        location ~ ^/preview/([cir])/(.+) {
        # Тип операции
            set                         $oper $1;
        # Параметры изображения и путь к файлу
            set                         $remn $2;
        # Проксируем на отдельный хост
            proxy_pass                  http://myproject.ru:81/$oper/$remn;
            proxy_intercept_errors      on;
            error_page                  404 = /preview/404;
        # Кеширование
            proxy_cache                 image-preview;
            proxy_cache_key             "$host$document_uri";
        # 200 ответы кешируем на 1 день
            proxy_cache_valid           200 1d;
        # остальные ответы кешируем на 1 минуту
            proxy_cache_valid           any 1m;
        }
        
        # Возвращаем ошибку
        location = /preview/404 {
            internal;
            default_type                image/gif;
            alias                       /www/myprojects/image/noimage.gif;
        }
...
    }
...


We also add a new host to the config:

server {
    server_name                     myproject.ru;
    listen                          81;

    access_log                      /www/myproject.ru/logs/nginx.preview.access_log;
    error_log                       /www/myproject.ru/logs/nginx.preview.error_log info;

    # Указываем секретное слово для md5
    secure_link_secret              secret;

    # Ошибки отправляем она отдельный location
    error_page                      403 404 415 500 502 503 504 = @404;

    # location Для фильтра size
    location ~ ^/i/[^/]+/(.+) {
        
        # грязный хак от Игоря Сысоева *
        alias                       /www/myproject.ru/images/$1;
        try_files                   "" @404;
    
        # Проверяем правильность ссылки и md5
        if ($secure_link = "") { return 404; }
        
        # Используем соответсвующий фильтр
        image_filter                size;
    }

    # По аналогии остальные location для других фильтров
    location ~ ^/c/[^/]+/(d+|-)x(d+|-)/(.+) {
        set                         $width  $1;
        set                         $height $2;
        
        alias                       /www/myproject.ru/images/$3;
        try_files                   "" @404;
    
        if ($secure_link = "") { return 404; }
    
        image_filter                crop  $width  $height;
    }
    
    location ~ ^/r/[^/]+/(d+|-)x(d+|-)/(.+) {
        set                         $width  $1;
        set                         $height $2;

        alias                       /www/myproject.ru/images/$3;
        try_files                   "" @404;

        if ($secure_link = "") { return 404; }

        image_filter                resize  $width  $height;
    }

    location @404 { return 404; }
}


As a result, additional images can be taken from the links:

• myproject.ru/preview/i[md5]/[path_to_image]
• myproject.ru/preview/c[md5]/[size]/[path_to_image]
• myproject.ru/preview/r[md5]/[size]/[path_to_image]

* try_files - sensitive to spaces and Russian characters, so I had to make a crutch with alias.

Usage in a web application

At the web application level, the following procedure can be done (Perl):

sub proxy_image {
    use Digest::MD5     qw /md5_hex/;
    my %params = @_;
    my $filter = {
                    size    => 'i',
                    resize  => 'r',
                    crop    => 'c'            
                  }->{$params{filter}} || 'r';
    my $path = ($filter ne 'i' ?
                    ( $params{height} || '_' ) . 'x' . ( $params{width} || '_' ) . '/' :
                    ()
               ) . $params{source};
    my $md5 = md5_hex( $path . 'secret' );
    $path = '/preview/' . $filter . '/' . $md5 . '/' . $path;
    return $path;
}

my $preview_path = &proxy_image(
                    source  => 'image1.jpg',
                    height  => 100,
                    width   => 100,
                    filter  => 'resize'
                );


Although I would still recommend calculating the dimensions preview.

Rake

When deleting the original image, the previews, of course, will not be deleted from the cache until the cache is invalidated, and in our case, the previews can exist for a day after deletion, but this is the maximum in time.

original

Source: habr.com