How come
apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 0xcbcb082a1bb943db
would fail but
apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:80 0xcbcb082a1bb943db
would not?
How do I check what is being blocked?
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muru
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The OpenPGP HTTP Key Protocol operates, by default, on port 11371.
The Ubuntu Keyserver is run on port 80 (by default, used by www-http) to reduce problems that can be caused by firewalls. So you need the port and protocol to make sure that GPG (which is running the request behind-the-scenes) knows exactly where to go to get the key.
Your first example is equivalent to:
apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:11371 0xcbcb082a1bb943db
Since you need to specify :80 when using apt-key adv, your network administrator or ISP has blocked port 11371.
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4Ubuntu's keyserver listens on port 80 so that you don't have any problem if your ISP/proxy/firewall blocks some ports. – Andrea Corbellini Jul 16 '15 at 15:51
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2@AndreaCorbellini (and Cliff) But it also works/listens on 11371. – Thomas Ward Jul 18 '15 at 23:32