## ref:
## - https://www.digitalocean.com/community/tools/nginx?domains.0.reverseProxy.reverseProxy=true
map $remote_addr
    $proxy_forwarded_elem
{
    ## IPv4 addresses can be sent as-is
    ~^[0-9.]+$         "for=$remote_addr";
    ## IPv6 addresses need to be bracketed and quoted
    ~^[0-9A-Fa-f:.]+$  "for=\"[$remote_addr]\"";
    ## Unix domain socket names cannot be represented in RFC 7239 syntax
    default            "for=unknown";
}

## ref:
## - https://www.digitalocean.com/community/tools/nginx?domains.0.reverseProxy.reverseProxy=true
## - https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Forwarded
map $http_forwarded
    $proxy_add_forwarded
{
    volatile;

    ## if the incoming Forwarded header is syntactically valid, append to it
    "~^(,[ \\t]*)*([!#$%&'*+.^_`|~0-9A-Za-z-]+=([!#$%&'*+.^_`|~0-9A-Za-z-]+|\"([\\t \\x21\\x23-\\x5B\\x5D-\\x7E\\x80-\\xFF]|\\\\[\\t \\x21-\\x7E\\x80-\\xFF])*\"))?(;([!#$%&'*+.^_`|~0-9A-Za-z-]+=([!#$%&'*+.^_`|~0-9A-Za-z-]+|\"([\\t \\x21\\x23-\\x5B\\x5D-\\x7E\\x80-\\xFF]|\\\\[\\t \\x21-\\x7E\\x80-\\xFF])*\"))?)*([ \\t]*,([ \\t]*([!#$%&'*+.^_`|~0-9A-Za-z-]+=([!#$%&'*+.^_`|~0-9A-Za-z-]+|\"([\\t \\x21\\x23-\\x5B\\x5D-\\x7E\\x80-\\xFF]|\\\\[\\t \\x21-\\x7E\\x80-\\xFF])*\"))?(;([!#$%&'*+.^_`|~0-9A-Za-z-]+=([!#$%&'*+.^_`|~0-9A-Za-z-]+|\"([\\t \\x21\\x23-\\x5B\\x5D-\\x7E\\x80-\\xFF]|\\\\[\\t \\x21-\\x7E\\x80-\\xFF])*\"))?)*)?)*$" "$http_forwarded, $proxy_forwarded_elem";

    ## otherwise, replace it
    default  "$proxy_forwarded_elem";
}