Confused over name of eth connection (and more...)

[Colin Helliwell <colin helliwell ln-systems com>]

Sorry this is long....
I'm back getting to grips with NM, after time out on other aspects. I'm using the git master (1.9.1), on a 
SysV system, no graphical desktop, no netconfig or resolvconf.
Looks like quite a few updates since I was last engrossed in it, so it may be that my configs and - more 
likely - understanding is a bit out of date.

The system has eth0 and a gsm connection (via ModemManager). 
I have in /etc/network/interfaces:
 auto lo
 iface lo inet loopback
 auto eth0
 iface eth0 inet dhcp

NetworkManager.conf is:
 [main]
 plugins=ifupdown,keyfile
 dns=default
 rc-manager=file

 [connectivity]

 [ifupdown]
 managed=true

First off, up/down: from reboot
root@wg:~# nmcli -c no conn show
NAME             UUID                                  TYPE      DEVICE
eth0             129206d8-d28f-47c7-9f9a-f9ddb1cb5218  ethernet  eth0
Ifupdown (eth0)  681b428f-beaf-8932-dce4-687ed5bae28e  ethernet  --
O2               42bf3c07-9dd7-45f6-ba08-37dfc8a7a537  gsm       --
Vodafone         e0f97177-2f8e-4ed4-9942-7b4e499397cc  gsm       --
root@wg:~# nmcli -c no conn down eth0
Connection 'eth0' successfully deactivated (D-Bus active path: 
/org/freedesktop/NetworkManager/ActiveConnection/1)
root@wg:~# nmcli -c no conn show
NAME             UUID                                  TYPE      DEVICE
Ifupdown (eth0)  681b428f-beaf-8932-dce4-687ed5bae28e  ethernet  eth0
O2               42bf3c07-9dd7-45f6-ba08-37dfc8a7a537  gsm       --
Vodafone         e0f97177-2f8e-4ed4-9942-7b4e499397cc  gsm       --
root@wgs:~# nmcli -c no conn up eth0
Error: unknown connection 'eth0'.
root@wg:~# nmcli -c no conn up 681b428f-beaf-8932-dce4-687ed5bae28e
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/3)
root@wg:~# nmcli -c no conn show
NAME             UUID                                  TYPE      DEVICE
Ifupdown (eth0)  681b428f-beaf-8932-dce4-687ed5bae28e  ethernet  eth0
O2               42bf3c07-9dd7-45f6-ba08-37dfc8a7a537  gsm       --
Vodafone         e0f97177-2f8e-4ed4-9942-7b4e499397cc  gsm       --
root@wg:~# nmcli -c no conn down eth0
Error: 'eth0' is not an active connection.
Error: no active connection provided.

Puzzling how eth0 is-then-isn't a valid connection name..?  Maybe it's to do with the ifupdown/managed 
setting - and, really, I'm not sure whether I want to be using this or not.
[The overall goal is to understand whether/how the system will behave, for example, if the eth connection 
dies for some reason. But it looks like I've got something fouled up that I need to better understand first]

This also causes me confusion if I try to take the eth down:

root@wg:~# nmcli -c no conn show
NAME             UUID                                  TYPE      DEVICE
eth0             e76aaae3-b320-4f64-9402-2a4ff8f28e01  ethernet  eth0
Ifupdown (eth0)  681b428f-beaf-8932-dce4-687ed5bae28e  ethernet  --
O2               42bf3c07-9dd7-45f6-ba08-37dfc8a7a537  gsm       --
Vodafone         e0f97177-2f8e-4ed4-9942-7b4e499397cc  gsm       --
root@wg:~# cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 192.168.10.254
root@wg:~# nmcli conn up O2
PPP generic driver version 2.4.2
PPP BSD Compression module registered
PPP Deflate Compression module registered
Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/2)
root@wg:~# cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 192.168.10.254
nameserver 82.132.254.2
nameserver 82.132.254.3
root@wg:~# nmcli conn down eth0
Connection 'eth0' successfully deactivated (D-Bus active path: 
/org/freedesktop/NetworkManager/ActiveConnection/1)
root@wg:~# cat /etc/resolv.conf
# Generated by NetworkManager
search lan
nameserver 192.168.10.254
nameserver 82.132.254.2
nameserver 82.132.254.3
root@wg:~# ping www.google.com
PING www.google.com (172.217.17.100): 56 data bytes
64 bytes from 172.217.17.100: seq=0 ttl=51 time=22.627 ms                <- Ethernet speed
^C
--- www.google.com ping statistics ---
2 packets transmitted, 1 packets received, 50% packet loss
round-trip min/avg/max = 22.627/22.627/22.627 ms
root@wg:~# nmcli -c no conn show
NAME             UUID                                  TYPE      DEVICE
Ifupdown (eth0)  681b428f-beaf-8932-dce4-687ed5bae28e  ethernet  eth0
O2               42bf3c07-9dd7-45f6-ba08-37dfc8a7a537  gsm       ttyMux1
Vodafone         e0f97177-2f8e-4ed4-9942-7b4e499397cc  gsm       --
root@wg:~# nmcli conn down 681b428f-beaf-8932-dce4-687ed5bae28e
Connection 'Ifupdown (eth0)' successfully deactivated (D-Bus active path: 
/org/freedesktop/NetworkManager/ActiveConnection/3)
root@wg:~# cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 82.132.254.2
nameserver 82.132.254.3
root@wg:~# ping www.google.com
PING www.google.com (173.194.122.242): 56 data bytes
64 bytes from 173.194.122.242: seq=0 ttl=41 time=592.168 ms                <- GSM speed
64 bytes from 173.194.122.242: seq=1 ttl=41 time=531.795 ms
64 bytes from 173.194.122.242: seq=2 ttl=41 time=511.506 ms
64 bytes from 173.194.122.242: seq=3 ttl=41 time=531.117 ms
64 bytes from 173.194.122.242: seq=4 ttl=41 time=510.785 ms
^C
--- www.google.com ping statistics ---
6 packets transmitted, 5 packets received, 16% packet loss
round-trip min/avg/max = 510.785/535.474/592.168 ms


Any advice please on things to check and a good way to work through and tidy this?

(Not at all keen on the new style 'standard input' presentation on a console/tty, tbh - the old style 
colouring used to work ok I think)