After switching one of my self-hosted WordPress blogs to all-HTTPS, I ran into an odd problem: Jetpack Related comments stopped working after a while.
After going back and forth with Jetpack support and my web host, it turned out the problem was with the SSL configuration on my site. Jetpack has to download a copy of your posts in order to calculate recommendations, and it uses libcurl to do that. Curl has stopped supporting the RC4 cipher in SSL connections because weaknesses have been found in it…and that’s what my server was using! (Ack!) I assume it was an old compatibility setting that never got updated.
Jetpack needed to reindex the site, but couldn’t retrieve anything, so it got stuck on “Indexing request queued and waiting…” Disconnecting and reconnecting didn’t work. Jetpack thought it was connected, so it didn’t report an error. (I assume it uses a different library for some things.) Pages were loading the script and the placeholder, but didn’t have suggestions to put there. And of course it wasn’t done indexing, so it didn’t offer a reindex button on the debug page.
What to do:
SSL ciphers are a server configuration setting, not a problem with your SSL certificate, so you don’t need to revoke and reissue the cert. If your hosting provider manages your server, you can ask them to disable RC4. If you run your own server, you’ll need to look up how to disable RC4 on IIS, Apache, NginX, etc. You can verify your site’s settings at Qualys’ SSL Server Test: Look for RC4 in the results and see if it’s labeled Yes or No.
If Jetpack doesn’t start indexing after you change your config, try turning off the Related Posts module and turning it back on. It only took a few minutes before recommendations started appearing on the site again.
There is one downside, which is that some older browsers (specifically Internet Explorer on Windows XP) may not be able to connect. As always, it’s a trade-off.