Hah, so this is another corner case I found in bridge testing. In https://bugzilla.gnome.org/show_bug.cgi?id=733641 I wrote:

"Say you have a bridge device 'br0' and a slaved interface 'eth0'. If you try
and bring up the slaved interface first:

# ifup eth0

you'll get:

Error: Connection activation failed: Master connection not found or invalid

so obviously you have to bring up the bridge device first."

But that wasn't actually entirely true. In fact, NetworkManager usually does a good thing in this case: 'ifup slave' brings up both slave and bridge, and everything works, and everyone's happy.

The "Master connection not found or invalid" failure is actually a corner case I just happened to hit on the path I was testing. I'm pretty sure I've isolated how it happens. Here's how to reproduce:

1. Delete /etc/sysconfig/network-scripts/ifcfg-* and delete all NetworkManager profiles listed in 'nmcli con show', so you have a completely blank slate

2. Hand-edit /etc/sysconfig/network-scripts/ifcfg-eth0 and /etc/sysconfig/network-scripts/ifcfg-br0 defining a simple, correct bridged connection

3. Run 'nmcli con show', see that still no connections are listed

4. Run 'ifup eth0', observe the error occur

5. Run 'nmcli con show', see that a connection has been created for eth0 *but not for br0*

The problem is that NetworkManager isn't quite clever enough to handle this case. It *does* try and handle it. If you just create a simple single connection behind its back - say, create /etc/sysconfig/network-scripts/ifcfg-eth0 , defining a simple DHCP connection, and immediately run 'ifcfg-eth0' - some smarts somewhere in NetworkManager realize it should check if an ifcfg-eth0 file has appeared and create a new NetworkManager connection based on it if it exists. That simple single connection case works.

But the smarts aren't smart enough to handle a bridge. At step 5 above, you can see that NM is smart enough to create a connection for the slave, but it's not smart enough to go the extra mile and check if an ifcfg-br0 (or whatever) has appeared, and if so, create a connection for that too. So src/nm-manager.c 's find_master() function gets called still without, so far as NetworkManager is concerned, the 'br0' connection really existing at all, and that function (correctly, in a sense) bails with the "Master connection not found or invalid" error.

Suggestion: whatever bit of NM does the 'check if an ifcfg file matching the connection on which ifup was called has appeared, and create a connection if so' should be extended to check if the new connection is a bridge slave, and if so, also check for an ifcfg file matching the name of the bridge master and create a connection for that too, before actually trying to bring up the slave.

I listed hand-editing as a handy way to reproduce above, but the way I hit this issue at first was with virt-manager. virt-manager sets up bridged connections by simply creating the config files in /etc/sysconfig/network-scripts and then immediately starting to do 'ifup' calls, so it runs into this bug in some cases (whether it hits this bug or another one depends on exactly what connections you start with before running virt-manager, and what state they're in when you try to create and bring up the bridge in virt-manager).