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By: Shafna K

How to Configure NGINX as a Mail Proxy Server

Technical

By setting up  Nginx, you can enhance your email address as a proxy for IMAP, POP3, and SMTP protocols. Utilize NGINX to proxy IMAP, POP3, and SMTP protocols, consolidating them into a single endpoint for email clients. This approach offers numerous advantages, including:

1. Effortless Scalability: Easily expand the number of mail servers to accommodate growing demands.

2. Smart Routing: Select the most suitable mail server based on various criteria, such as the client’s IP address or proximity, to ensure optimal performance.

3. Load balancing: Efficiently distribute the email traffic laid across multiple mail servers for improved reliability and performance.

NGINX Open source is the freely available version of NGINX, and it may not include all modules by default. To enable email proxy functionality in NGINX Open Source, you would need to compile the required Mail modules using the --with-mail parameter during the NGINX compilation process.

* --with-mail Parameter: When compiling NGINX open source, adding the - -with-mail parameter tells the build process to include the necessary mail modules. These modules are responsible for handling email-related protocols such as IMAP, POP3, and SMTP.

$ ./configure --with-mail

* --with-mail_ssl_module Parameter: To support the SSL/TLS encryption for email traffic, you should also include the with--mail_ssl_module parameter during compilation. This module provides the required SSl/TLS support for securing email communication. It can be configured by typing the -nf-v command, and then looking for the with --mail_ssl_module line in the output:

$ nginx -V

configure arguments: ... with--mail_ssl_module

To fortify your mail proxy server, ensure you possess server certificates and a private key, obtainable from a reputable Certificate Authority (CA) or generated through an SSL library like OpenSSL.

You can activate the SSL/TLS globally for all mail proxy servers within the mail context using the ssl directive. Alternatively, enable the STLS and STARTTLS individually using the starttls directive:

ssl on;

Or

starttls on;

Incorporate SSL certificate by specifying the paths to the PEM-formatted certificates using the ssl_certificate directive. Likewise, denote the path to the private key with the ssl_certificate_key directive:

mail {
 #...
 ssl_certificate /etc/ssl/certs/server.crt;
 ssl_certificate_key /etc/ssl/certs/server.key;
}

For enhanced security, exclusively utilize robust  SSL/TLS versions and ciphers. Specify supported protocols with the ssl_protocols directive and acceptable ciphers with the ssl_ciphers directive. Example:

mail {
 #...
 ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
 ssl_ciphers HIGH:!aNULL:!MD5;
}

This configuration guarantees a resilient and encrypted communication channel for your mail proxy server, reinforcing the confidentiality and integrity of transmitted data.

Here’s a consolidated step-by-step process:

* Download the NGINX open-source code from the NGINX website.

* During the compilation process, specify the --with-mail parameter to enable the Mail modules.

* If you need SSL/TLS support for securing the email traffic, specify the  --with-mail_ssl_module parameter as well.

* Compile NGINX with these parameters.

* After successful compilation and installation, you can configure NGINX to act as an email proxy server and handle IMAP, POP3, and SMTP traffic.

If your email address is example@mycompany.com, and you want to set up NGINX as mail reverse proxy for your company’s mail server, you can configure NGINX as follows:

* Server Block Configuration: In your NGINX configuration file (usually found in /etc/nginx/nginx.conf or /etc/nginx/sites-available/), create a new server block to handle mail traffic.

We can specify settings for each server in your NGINX mail proxy configuration:

* Specify the protocol-specific port numbers using the listen directive for each server block in the NGINX mail proxy configuration.

* Define the protocol explicitly using the protocol directive in each server block of the NGINX mail proxy configuration. In the absence of a specified protocol, the system will automatically identify the protocol based on the port specified in the listen directive. Specify allowed authentication methods for each server by utilizing the imap_auth, pop3_auth, and smtp_auth directives in the NGINX mail proxy configuration.

server {
    listen    25;
    protocol  smtp;
    smtp_auth login plain cram-md5;

server {
    listen    110;
    protocol  pop3;
    pop3_auth plain apop cram-md5;
}
server {
    listen   143;
    protocol imap;
}

Next, Enhance the optimization of SSL/TLS for Mail Proxy by implementing the following guidelines:

Ensure the alignment of worker processes with processors by utilizing the worker_processes directive, placing it at the same level as the mail context in the NGINX configuration.

worker_processes auto;
mail {
    #...
}

* Enable the shared session cache and disable the default session cache by incorporating the ssl_session_cache directive into the NGINX configuration.

worker_processes auto;

mail {
    #...
    ssl_session_cache shared:SSL:10m;
    #...
}

If desired, you have the option to prolong the default session lifetime, which is initially set at 5 minutes, by making use of the ssl_session_timeout directive in the NGINX configuration.

worker_processes auto;

mail {
    #...
    ssl_session_cache   shared:SSL:10m;
    ssl_session_timeout 10m;
    #...
}

Now, we can see the complete example for optimizing SSL/TLs for mail proxy. In the following illustration, three email proxy servers—SMTP, POP3, and IMAP—are configured with both SSL and STARTTLS support. SSL session parameters are set to be cached.

The proxy server relies on an external HTTP authentication server, the configuration of which is outside the scope of this article. Clients will receive any error messages returned by the server.

worker_processes auto;

mail {
    server_name mail.example.com;
    auth_http   localhost:9000/cgi-bin/nginxauth.cgi;
    proxy_pass_error_message on;
    ssl                 on;
    ssl_certificate     /etc/ssl/certs/server.crt;
    ssl_certificate_key /etc/ssl/certs/server.key;
    ssl_protocols       TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers         HIGH:!aNULL:!MD5;
    ssl_session_cache   shared:SSL:10m;
    ssl_session_timeout 10m;
    server {
        listen     25;
        protocol   smtp;
        smtp_auth  login plain cram-md5;
    }
    server {
        listen    110;
        protocol  pop3;
        pop3_auth plain apop cram-md5;
}
     server {
        listen   143;
        protocol imap;
    }
}

To conclude, configuring NGINX as a mail proxy server involves configuring settings for specific protocols like SMTP, POP3, and IMAP, enhancing security with SSL/TLS support, and potentially utilizing an external HTTP authentication server. This setup optimizes performance, ensures secure communication, and provides flexibility in handling various email protocols, contributing to a robust and efficient mail infrastructure.


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