Starting with Fedora 17, SELinux prevents sshd to initiate connections to remote HTTP ports (80 and 443). In SELinux terms: sshd_t is not allowed to name_connect to http_port_t. This broke YubiKey authentication on a system with SELinux in enforcing mode, unless a custom SELinux policy was written and enabled.
Based on a bugreport in Red Hat Bugzilla, a boolean was added to the SELinux policy for Fedora 18 and up, that can be toggled to allow sshd (and some other SELinux types) to connect to remote HTTP ports.
To make a long story short, if you want to use a YubiKey on a system running Fedora 18 or higher (and probably RHEL7, eventually), you’ll need to toggle the authlogin_yubikey SELinux boolean, like so:
setsebool -P authlogin_yubikey 1
If you are using your own server via
url in the pam conf file and using a non-standard http port, you will need to add that port to the
http_port_t port list. For example, port
semanage port -a -t http_port_t -p tcp 12345