How to get real Client IP in .NET Behind Proxies

When your web server is hosted behind reverse proxies or load balancers, retrieving the real client IP address becomes a bit more complex. The default method of accessing HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"] in ASP.NET only returns the IP address of the proxy or load balancer, not the true source of the request — the client. This presents a significant challenge, particularly when you need to log client IP addresses for security audits, enforce access control, or prevent abuse.

The issue arises because, while proxies and load balancers forward requests to your server, they do so in a way that masks the real client’s IP. Without properly extracting the correct header information, your logs and security checks will show the IP of the proxy server, not the end user, which can complicate threat detection and incident response.

Why Your Current Code May Be Insecure

By default, many proxies and load balancers insert HTTP headers such as HTTP_X_CLIENTIP or X-Forwarded-For that can contain the real client IP address. However, these headers can easily be manipulated or spoofed by attackers if the server is not configured to only trust specific proxies. This is a security vulnerability, as attackers could potentially fake their IP address by sending custom headers, leading to false information in your logs and incorrect access control decisions.

If your application blindly trusts whatever IP is in these headers, it could allow malicious users to bypass security measures. For instance, an attacker could impersonate a trusted client or mask their identity, making it harder to detect malicious behavior like brute force attacks or IP-based restrictions.

To mitigate these risks, it’s crucial that your application is configured to trust only known proxies, and any incoming request headers should be validated before they are trusted. You must also ensure that proper configurations are in place to protect against header spoofing, and that only trusted proxy IPs are allowed to set these headers. Additionally, you may want to implement a layered approach by combining these headers with additional security checks (e.g., SSL certificates, IP filtering, etc.).

 

Safer Client IP Detection Strategy

  • Always validate proxy headers only from trusted networks

  • Prefer standard headers like X-Forwarded-For

  • Use built-in middleware in ASP.NET Core when possible

  • In classic ASP.NET, sanitize and extract IP cautiously

Additional Headers You Can Check (in order of reliability) to get client IP:

  1. HTTP_X_FORWARDED_FOR
  2. HTTP_X_CLIENTIP
  3. HTTP_CLIENT_IP
  4. HTTP_FORWARDED
  5. REMOTE_ADDR

Note: Only use these headers if you’re confident they’re set by trusted infrastructure.

Sample C# implementation:

public static string GetClientIPAddress()
{
    string ip = HttpContext.Current?.Request?.ServerVariables["HTTP_X_FORWARDED_FOR"];

    if (!string.IsNullOrEmpty(ip))
    {
        // X-Forwarded-For may contain multiple IPs, get the first
        string[] ipList = ip.Split(',');
        ip = ipList[0].Trim();
    }
    else
    {
        ip = HttpContext.Current?.Request?.ServerVariables["REMOTE_ADDR"];
    }

    return ip;
}

Conclusion

Don’t rely solely on REMOTE_ADDR or unverified headers in production. Always validate the source of these headers and use framework-level support where possible. This ensures you safely retrieve the client’s real IP address — which is essential for security, logging, and compliance.

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