I also submitted this as a Fedora bug question, here.
I'm not sure if this is a bug, but here is my sudden issue.
The Linux/LXC single-box cluster setup
I use Fedora x86_64 (currently Fedora-25) as the LXC/HOST O/S. I use CentOS-6 x86_64 (currently CentOS-6.9 Final) for the six (qty. 6) LXC/GUEST O/S'.
This was working for a long time (a few years), but suddenly does not after a 'sudo dnf -y update' (HOST) and 'sudo yum -y update' (GUESTS). It has been a few months since I booted this HOST/GUESTS LXC "cluster" and, as usual, O/S updates are the first thing that I perform. This may provide a hint if some underlying system-level component(s)/behavior(s) changed during that time.
The Fedora HOST and CentOS-6 GUESTS are on the same subnet, and share the same default router: 192.168.0.0/24; 192.168.0.1 (all standard stuff).
The Fedora Host does not have any firewall/firewalld RPM packages installed, and therefore doesn't not run a firewall. I removed this long ago to simplify things.
After performing the above O/S updates to the HOST and GUESTS, from within any GUEST, I can no longer (a) successfully ping/ssh guest-to-guest or (b) ping the default router.
I can, however, ping/ssh HOST-to-GUEST and GUEST-to-HOST with no issue.
From any computer outside this setup -- which, by the way, are also on the same subnet and share the same default router as above -- I can ping/ssh to the HOST but cannot to any of the GUESTS.
Other than performing the aforementioned O/S updates, I didn't alter anything.
Here is output from the HOST: HOST.txt
Here is output from a GUEST: ONE_GUEST.txt
Note that the GUESTS are named vps00, vps01, vps02, vps03, vps04 and vps10, and have identical configurations except MAC and IP addresses (so I only provided output for one of them). While the HOST is named lxc-host. Throughout the attachments, you'll see some in-line notes that I annotated them with.
Any ideas? Thank you in advance. :)