Connection default can be configured in NetworkManager.conf (see `man NetworkManager.conf`).

By specifying a "match-device", this is actually really powerful. For example,
one can configure that WiFi has worse default-route metric then WWAN:

  [connection.route-metric-wlan0]
  device-match=interface-name:wlan0
  ipv4.route-metric=1000

  [connection.route-metric-wwan]
  device-match=type:gsm,type:cdma
  ipv4.route-metric=599


It would be however great to apply default value based on other connection properties. Like:

  [connection.default-stable-id]
  connection-match=is:wifi.cloned-mac-address=stable,unset:connection.stable-id
  connection.stable-id=value

that is, depending on other connection properties, specify a default value.


One difficulty is that libnm doesn't have a convenient API to access connection properties by their name (see bug 732292).




Also, what would be actually more useful is with bug 776276. If the user could tag connections with arbitrary user-data:

For example, I want for all my Wi-Fis to use "wifi.cloned-mac-addres=random" by default. But certain Wi-Fis, I want to tag them (by setting user.my.public-wifi-stable=1) and select the "stable" cloned-mac-address.

  [connection.public-wifi-stable]
  device-match=type:wifi
  connection-match= is:user.my.public-wifi-stable=1 , \ 
                    unset:wifi.cloned-mac-address , \
                    unset:connection.stable-id
  connection.stable-id=$boot-id
  wifi.cloned-mac-address=stable

  [connection.public-wifi]
  device-match=type:wifi
  wifi.cloned-mac-address=random

Note that currently connection.stable-id cannot be configured as global default, but let's assume it could, and setting it to $boot-id would use the value from /proc/sys/kernel/random/boot_id.