PHP $_SERVER['HTTP_HOST'] vs. $_SERVER['SERVER_NAME'], am I understanding the man pages correctly?

user_142233 0
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I did a lot of searching and also read the PHP $_SERVER docs. Do I have this right regarding which to use for my PHP scripts for simple link definitions used throughout my site?

$_SERVER['SERVER_NAME'] is based on your web server's config file (Apache2 in my case), and varies depending on a few directives: (1) VirtualHost, (2) ServerName, (3) UseCanonicalName, etc.

$_SERVER['HTTP_HOST'] is based on the request from the client.

Therefore, it would seem to me that the proper one to use in order to make my scripts as compatible as possible would be $_SERVER['HTTP_HOST']. Is this assumption correct?

Followup comments:

I guess I got a little paranoid after reading this article and noting that some folks said "they wouldn't trust any of the $_SERVER vars":

Apparently the discussion is mainly about $_SERVER['PHP_SELF'] and why you shouldn't use it in the form action attribute without proper escaping to prevent XSS attacks.

My conclusion about my original question above is that it is "safe" to use $_SERVER['HTTP_HOST'] for all links on a site without having to worry about XSS attacks, even when used in forms.

Please correct me if I'm wrong.

owasp security apache php
9 Answers

I am not sure and not really trust $_SERVER['HTTP_HOST'] because it depend on header from client. In another way, if a domain requested by client is not mine one, they will not getting into my site because DNS and TCP/IP protocol point it to the correct destination. However I don't know if possible to hijack the DNS, network or even Apache server. To be safe, I define host name in environment and compare it with $_SERVER['HTTP_HOST'].

Add SetEnv MyHost domain.com in .htaccess file on root and add ths code in Common.php

if (getenv('MyHost')!=$_SERVER['HTTP_HOST']) {
  header($_SERVER['SERVER_PROTOCOL'].' 400 Bad Request');
  exit();
}

I include this Common.php file in every php page. This page doing anything required for each request like session_start(), modify session cookie and reject if post method come from different domain.

user_1863340

First I want to thank you for all the good answers and explanations. This is the method I created based upon all your answer to get the base url. I only use it in very rare situations. So there is NOT a big focus on security issues, like XSS attacks. Maybe someone needs it.

// Get base url
function getBaseUrl($array=false) {
    $protocol = "";
    $host = "";
    $port = "";
    $dir = "";  

    // Get protocol
    if(array_key_exists("HTTPS", $_SERVER) && $_SERVER["HTTPS"] != "") {
        if($_SERVER["HTTPS"] == "on") { $protocol = "https"; }
        else { $protocol = "http"; }
    } elseif(array_key_exists("REQUEST_SCHEME", $_SERVER) && $_SERVER["REQUEST_SCHEME"] != "") { $protocol = $_SERVER["REQUEST_SCHEME"]; }

    // Get host
    if(array_key_exists("HTTP_X_FORWARDED_HOST", $_SERVER) && $_SERVER["HTTP_X_FORWARDED_HOST"] != "") { $host = trim(end(explode(',', $_SERVER["HTTP_X_FORWARDED_HOST"]))); }
    elseif(array_key_exists("SERVER_NAME", $_SERVER) && $_SERVER["SERVER_NAME"] != "") { $host = $_SERVER["SERVER_NAME"]; }
    elseif(array_key_exists("HTTP_HOST", $_SERVER) && $_SERVER["HTTP_HOST"] != "") { $host = $_SERVER["HTTP_HOST"]; }
    elseif(array_key_exists("SERVER_ADDR", $_SERVER) && $_SERVER["SERVER_ADDR"] != "") { $host = $_SERVER["SERVER_ADDR"]; }
    //elseif(array_key_exists("SSL_TLS_SNI", $_SERVER) && $_SERVER["SSL_TLS_SNI"] != "") { $host = $_SERVER["SSL_TLS_SNI"]; }

    // Get port
    if(array_key_exists("SERVER_PORT", $_SERVER) && $_SERVER["SERVER_PORT"] != "") { $port = $_SERVER["SERVER_PORT"]; }
    elseif(stripos($host, ":") !== false) { $port = substr($host, (stripos($host, ":")+1)); }
    // Remove port from host
    $host = preg_replace("/:\d+$/", "", $host);

    // Get dir
    if(array_key_exists("SCRIPT_NAME", $_SERVER) && $_SERVER["SCRIPT_NAME"] != "") { $dir = $_SERVER["SCRIPT_NAME"]; }
    elseif(array_key_exists("PHP_SELF", $_SERVER) && $_SERVER["PHP_SELF"] != "") { $dir = $_SERVER["PHP_SELF"]; }
    elseif(array_key_exists("REQUEST_URI", $_SERVER) && $_SERVER["REQUEST_URI"] != "") { $dir = $_SERVER["REQUEST_URI"]; }
    // Shorten to main dir
    if(stripos($dir, "/") !== false) { $dir = substr($dir, 0, (strripos($dir, "/")+1)); }

    // Create return value
    if(!$array) {
        if($port == "80" || $port == "443" || $port == "") { $port = ""; }
        else { $port = ":".$port; } 
        return htmlspecialchars($protocol."://".$host.$port.$dir, ENT_QUOTES); 
    } else { return ["protocol" => $protocol, "host" => $host, "port" => $port, "dir" => $dir]; }
}
user_29422110
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