I think you need to use the "Target" knockd config param, otherwise it uses the interface's primary ip addr. See 'man knockd' and scroll to the bottom, in the "KNOCK/EVENT DIRECTIVES" section.
** Changed in: knockd (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1873186 Title: knockd does not work on additional IP address of a NIC To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/knockd/+bug/1873186/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs