Why Expires header set to past date leading Varnish to skip caching?

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Thank you for your clarification.

Do WP Fastest Cache has any feature to control server-cache expiration time? Like an option to enable, and it adds X-Cdn-Expires header or something like that?

are you using Varnish Cache?

Yes, I’m using mainly Varnish cache, but I’m asking this question for all server-side page caching like Varnish, also Nginx proxy cache, Cloudflare’s page caching, and any other HTTP caching.

yes but I added it because we had lots of problem about the browser caching.

I recommend you to add Varnish Cache integration and set a cache timeout rule.

Enabling Varnish Cache integration only enables cleaning mechanism. It doesn’t make really sense to enable the Varnish Cache integration to clean cache if there’s nothing to clean.

For now, I tagged all .html pages cached by WP Fastest Cache with an header on my VirtualHost:

<Directory ~ "wp\-content\/cache\/all(\/.*)*\/index\.html$">
    Header set X-Varnish-Platform "wpfastestcache"
</Directory>

Then, I bypass the default built-in VCL in Varnish:

sub vcl_backend_response {
    if (beresp.ttl < 86400s && beresp.http.Content-Type ~ "text/html" && beresp.http.X-Varnish-Platform ~ "wpfastestcache") {
        set beresp.ttl = 86400s;
        set beresp.grace = 86400s;
        return(deliver);
    }
}

So it is forcing Varnish to cache page for 1 day (86400 sec) if response Content-Type is text/html and if it is tagged with the header X-Varnish-Platform (so, it is not caching anything that is not cached by WP Fastest Cache, making it to follow all exclusion rules implemented in WP Fastest Cache settings).

However, a proper implementation would be very welcome. The cache timeout settings itself could be served as custom header response on cached HTML pages so any server-side caching system can catch and use it.