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CGI client variables
Last update:
May 18, 2026
The following table describes common CGI environment variables the browser creates and passes in the request header:
CGI client variable | Description |
HTTP_REFERER | The referring document that linked to or submitted form data. |
HTTP_USER_AGENT | The browser that the client is currently using to send the request. Format: software/version library/version. |
HTTP_IF_MODIFIED_SINCE | The last time the page was modified. The browser determines whether to set this variable, usually in response to the server having sent the LAST_MODIFIED HTTP header. It can be used to take advantage of browser-side caching. |
| HTTP_URL | The URL path in an encoded format. |
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