I have this configuration for static html pages, where redirects are done from flat files as well:

old-location.html

<!-- {new-location.html} -->
<!DOCTYPE html>
<html>
<head>
<title>Redirecting to new location</title>
<link rel="canonical" href="new-location.html" />
<meta name="robots" content="noindex, follow">
<noscript><meta http-equiv="refresh" content="0;URL='new-location.html'" /></noscript>
<script>window.location = "new-location.html"</script>
</head>
<body>
<a href="new-location.html">[old-location.html moved here]</a>
</body>

The html is supposed to only serve as a graceful chain of fallbacks, while the comment on the first line is a configuration hook for proper http redirects that, since there's no runtime environment, need to be done by the webserver.

Currently I do this in Openresty, with Lua's matching patterns, finding the new-location and then setting it as a 301 redirect.

header_filter_by_lua_block { 
local address = ngx.var.document_root .. ngx.var.document_uri
local file = io.open(address, "rb")
local content = file:read(100)
file:close()
local location = string.match(content, "{(%g+)}")
ngx.header['location'] = location
ngx.status = 301
}
-- ideally I should intercept the response body and spare the extra file read, but it seems that even with Lua this is not possible

However, not everybody accepts to switch to a new webserver. So I wonder if there's a way to do it with off-the-shelf Nginx or, even better, with a generic method that's supported by most/all webservers.

Since the server side includes (is there a way to retrieve a ssi variable from the file and set it as header?) and the sub_module (replace anything except the needed part with some regex, maybe?) both do parse the entire body already, I thought there might be a way, but I don't quite know where to start looking.

Questions about server-side-includes

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