Magento 2 – Varnish Cache Cleaning Problem (Purge: Method Not Allowed)

I have a magento 2.1 with a apache and varnish 4 installed on a Centos 7. (apache running in port number 8080 and varnish in port number 80 in the same server. The server's external IP address is: 79.175.173.210)

The problem is that 'Flushing Magento Cache' in admin panel won't actually flush full-page-cache.

I have used sudo varnishlog -g request -q 'ReqMethod eq "PURGE"' to view the varnish purge log. It will generate a purge request toward the varnish, but it would get a Method not allowed error message.
After "Flushing" magento cache, it would produce the following output:

*   << Request  >> 98310
-   Begin          req 98309 rxreq
-   Timestamp      Start: 1499706749.687418 0.000000 0.000000
-   Timestamp      Req: 1499706749.687418 0.000000 0.000000
-   ReqStart       79.175.173.210 39958
-   ReqMethod      PURGE
-   ReqURL         /
-   ReqProtocol    HTTP/1.1
-   ReqHeader      X-Magento-Tags-Pattern: .*
-   ReqHeader      X-Forwarded-For: 79.175.173.210
-   VCL_call       RECV
-   VCL_acl        NO_MATCH purge
-   Debug          "VCL_error(405, Method not allowed)"
-   VCL_return     synth
-   VCL_call       HASH
-   VCL_return     lookup
-   Timestamp      Process: 1499706749.687465 0.000047 0.000047
-   RespHeader     Date: Mon, 10 Jul 2017 17:12:29 GMT
-   RespHeader     Server: Varnish
-   RespHeader     X-Varnish: 98310
-   RespProtocol   HTTP/1.1
-   RespStatus     405
-   RespReason     Method Not Allowed
-   RespReason     Method not allowed
-   VCL_call       SYNTH
-   RespHeader     Content-Type: text/html; charset=utf-8
-   RespHeader     Retry-After: 5
-   VCL_return     deliver
-   RespHeader     Content-Length: 276
-   Debug          "RES_MODE 2"
-   RespHeader     Connection: keep-alive
-   Timestamp      Resp: 1499706749.687575 0.000158 0.000110
-   ReqAcct        48 0 48 208 276 484
-   End

The varnish config file is followed:

/etc/varnish/varnish.vcl

vcl 4.0;

import std;
# The minimal Varnish version is 4.0
# For SSL offloading, pass the following header in your proxy server or load balancer: 'X-Forwarded-Proto: https'

backend default {
    .host = "127.0.0.1";
    .port = "8080";
}

acl purge {
    "127.0.0.1";
    "localhost";
    "79.175.173.210";
}

sub vcl_recv {
    if (req.method == "PURGE") {
        if (client.ip !~ purge) {
            return (synth(405, "Method not allowed"));
        }
        if (!req.http.X-Magento-Tags-Pattern) {
            return (synth(400, "X-Magento-Tags-Pattern header required"));
        }
        ban("obj.http.X-Magento-Tags ~ " + req.http.X-Magento-Tags-Pattern);
        return (synth(200, "Purged"));
    }

    if (req.method != "GET" &&
        req.method != "HEAD" &&
        req.method != "PUT" &&
        req.method != "POST" &&
        req.method != "TRACE" &&
        req.method != "OPTIONS" &&
        req.method != "DELETE") {
          /* Non-RFC2616 or CONNECT which is weird. */
          return (pipe);
    }

    # We only deal with GET and HEAD by default
    if (req.method != "GET" && req.method != "HEAD") {
        return (pass);
    }

    # Bypass shopping cart, checkout and search requests
    if (req.url ~ "/checkout" || req.url ~ "/catalogsearch") {
        return (pass);
    }

    # normalize url in case of leading HTTP scheme and domain
    set req.url = regsub(req.url, "^http[s]?://", "");

    # collect all cookies
    std.collect(req.http.Cookie);

    # Compression filter. See https://www.varnish-cache.org/trac/wiki/FAQ/Compression
    if (req.http.Accept-Encoding) {
        if (req.url ~ "\.(jpg|jpeg|png|gif|gz|tgz|bz2|tbz|mp3|ogg|swf|flv)$") {
            # No point in compressing these
            unset req.http.Accept-Encoding;
        } elsif (req.http.Accept-Encoding ~ "gzip") {
            set req.http.Accept-Encoding = "gzip";
        } elsif (req.http.Accept-Encoding ~ "deflate" && req.http.user-agent !~ "MSIE") {
            set req.http.Accept-Encoding = "deflate";
        } else {
            # unkown algorithm
            unset req.http.Accept-Encoding;
        }
    }

    # Remove Google gclid parameters to minimize the cache objects
    set req.url = regsuball(req.url,"\?gclid=[^&]+$",""); # strips when QS = "?gclid=AAA"
    set req.url = regsuball(req.url,"\?gclid=[^&]+&","?"); # strips when QS = "?gclid=AAA&foo=bar"
    set req.url = regsuball(req.url,"&gclid=[^&]+",""); # strips when QS = "?foo=bar&gclid=AAA" or QS = "?foo=bar&gclid=AAA&bar=baz"

    # static files are always cacheable. remove SSL flag and cookie
        if (req.url ~ "^/(pub/)?(media|static)/.*\.(ico|css|js|jpg|jpeg|png|gif|tiff|bmp|mp3|ogg|svg|swf|woff|woff2|eot|ttf|otf)$") {
        unset req.http.Https;
        unset req.http.X-Forwarded-Proto;
        unset req.http.Cookie;
    }

    return (hash);
}

sub vcl_hash {
    if (req.http.cookie ~ "X-Magento-Vary=") {
        hash_data(regsub(req.http.cookie, "^.*?X-Magento-Vary=([^;]+);*.*$", "\1"));
    }

    # For multi site configurations to not cache each other's content
    if (req.http.host) {
        hash_data(req.http.host);
    } else {
        hash_data(server.ip);
    }

    # To make sure http users don't see ssl warning
    if (req.http.X-Forwarded-Proto) {
        hash_data(req.http.X-Forwarded-Proto);
    }

}

sub vcl_backend_response {
    if (beresp.http.content-type ~ "text") {
        set beresp.do_esi = true;
    }

    if (bereq.url ~ "\.js$" || beresp.http.content-type ~ "text") {
        set beresp.do_gzip = true;
    }

    # cache only successfully responses and 404s
    if (beresp.status != 200 && beresp.status != 404) {
        set beresp.ttl = 0s;
        set beresp.uncacheable = true;
        return (deliver);
    } elsif (beresp.http.Cache-Control ~ "private") {
        set beresp.uncacheable = true;
        set beresp.ttl = 86400s;
        return (deliver);
    }

    if (beresp.http.X-Magento-Debug) {
        set beresp.http.X-Magento-Cache-Control = beresp.http.Cache-Control;
    }

    # validate if we need to cache it and prevent from setting cookie
    # images, css and js are cacheable by default so we have to remove cookie also
    if (beresp.ttl > 0s && (bereq.method == "GET" || bereq.method == "HEAD")) {
        unset beresp.http.set-cookie;
        if (bereq.url !~ "\.(ico|css|js|jpg|jpeg|png|gif|tiff|bmp|gz|tgz|bz2|tbz|mp3|ogg|svg|swf|woff|woff2|eot|ttf|otf)(\?|$)") {
            set beresp.http.Pragma = "no-cache";
            set beresp.http.Expires = "-1";
            set beresp.http.Cache-Control = "no-store, no-cache, must-revalidate, max-age=0";
            set beresp.grace = 1m;
        }
    }

   # If page is not cacheable then bypass varnish for 2 minutes as Hit-For-Pass
   if (beresp.ttl <= 0s ||
        beresp.http.Surrogate-control ~ "no-store" ||
        (!beresp.http.Surrogate-Control && beresp.http.Vary == "*")) {
        # Mark as Hit-For-Pass for the next 2 minutes
        set beresp.ttl = 120s;
        set beresp.uncacheable = true;
    }
    return (deliver);
}

sub vcl_deliver {
    if (resp.http.X-Magento-Debug) {
        if (resp.http.x-varnish ~ " ") {
            set resp.http.X-Magento-Cache-Debug = "HIT";
        } else {
            set resp.http.X-Magento-Cache-Debug = "MISS";
        }
    } else {
        unset resp.http.Age;
    }

    unset resp.http.X-Magento-Debug;
    unset resp.http.X-Magento-Tags;
    unset resp.http.X-Powered-By;
    unset resp.http.Server;
    unset resp.http.X-Varnish;
    unset resp.http.Via;
    unset resp.http.Link;
}