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问题:

I'm trying out the <link rel="dns-prefetch"> and <link rel="preconnect"> tags and I'm trying to see whether they help for my site. I can't find any online resources about how verify if these hints are working using browser dev tools, extensions, or other software. It seems like you just evaluate whether they may be useful to you based on some criteria and then drop them in and hope for the best.

In my case, I have a single page app that renders the entire contents of the <body> in the browser, so the browser can't really scan the initial HTML to lookahead for domains to resolve so it seemed like this might be useful for me.

回答1:

Run your page through webpagetest.org. Requests to the domains you specified in your dns-prefetch or preconnect tags should begin sooner because the initial connection will have been established.

This will show in the waterfall graph, for those requests - at the left of the bar the DNS, connect and SSL (if applicable) segments will detach from the response and move to the left in the waterfall, to reflect the fact that they occurred earlier.

回答2:

In order to just make sure that the features are working in a given browser (very synthetic test), you can do as follows

Test 1: test `dns-prefetch` (just DNS) with Chrome

serve the following HTML on localhost

<!doctype html><html><head>
    <link rel="dns-prefetch" href="//ajax.googleapis.com">
</head><body></html>

go to chrome://net-internals/#dns and clear host cache
open new tab in Chrome on http://localhost
refresh chrome://net-internals/#dns and observe it to have the DNS entry - this confirms that DNS resolution has been done

Test 2: test `preconnect` (DNS+TLS+TCP) with Chrome and Fiddler (Windows-only)

serve the following HTML on localhost

<!doctype html><html><head>
    <link rel="preconnect" href="https://ajax.googleapis.com">
</head><body></html>

go to chrome://net-internals/#dns and clear host cache
start Fiddler and make it listen to the traffic
open new tab in Chrome on http://localhost
observe Fiddler to have a "Tunnel to ajax.googleapis.com:443" session - this confirms that DNS resolution and TLS handshake were done (and you can probably trust the browser that it established a TCP connection too)