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How to replace HTTP response data in nginx using lua

This is an example of how to use body_filter_by_lua_block to replace parts of the response by a string. Note that since it’s a Lua feature, you’ll be able to use arbitrary code when modifying the body, including network requests, parsing etc. Note: This will replace in the response send to the client, not in the request sent to the the reverse proxy. See How to replace string in reverse proxy HTTP request using Lua & nginx for a Lua-based approach on how to replace a string in the request.

header_filter_by_lua_block { ngx.header.content_length = nil }
body_filter_by_lua_block {
        local body = ngx.arg[1]
        if body then
                body = ngx.re.gsub(body, "string_to_replace", "string_by_which_to_replace")
        end
        ngx.arg[1] = body
}

This is typically used in a location {} block to only replace strings for specific sets of URLs:

location /config {
   proxy_pass http://127.0.0.1:12345/;
    proxy_set_header X-Forwarded-For $remote_addr;
    proxy_set_header Host meet.techoverflow.net;

    header_filter_by_lua_block { ngx.header.content_length = nil }
    body_filter_by_lua_block {
            local body = ngx.arg[1]
            if body then
                    body = ngx.re.gsub(body, "myvar", "myothervar")
            end
            ngx.arg[1] = body
    }
}