Sequence of commands to the error:
set service dns forwarding listen-on bond027 (a non existent bonding interface)
commit
ERROR!
[ service dns forwarding listen-on bond027 ]
interface bond027 does not exist on system
but if you try "discard" nothing happens and if you "save" you obtain this line in the configuration file
"listen-on bond027" under dns section, which should not exist...
As @UnicronNL says, lines about nonexistent interfaces have no effect on dnsmasq functionality.
But what's worse, is that making it a commit fail will break the configs of those people who carelessly left a nonexistent interface in their DNS forwarding config, it will fail to load at boot time after upgrade.
As much as I hate generating configs that make no sense, leaving those people with potentially inaccessible systems after they upgrade (DNS loads before SSH AFAIR) is not an acceptable cost of somewhat tidier generated configs.
Sorry but I don't understand.
If this problem in configuration script could break the boot config why do you set the state "Wontfix" at the ticket? Ok, dnsmasq is able to work also with non existent interface but I think it is important to block the configurator script to insert in the configuration file a wrong line, right?
As it is now it can not break the config, that is why "wontfix".
If we block it then configs that have non existent interfaces in them (due to breakage or removed and forgot to remove from dns forwarding) will fail at boot.
Ok, but why don't deny the possibility to the user to insert an inexistent interface with "set service dns...." ?
I think that, if in future dnsmasq checks for inexistent interfaces and we permit to the user to insert it in the configuration, we will have some problems.
Curiously, the rewrite introduced exactly the problem @UnicronNL warned against. Entering an invalid interface at set time is only one part of the story — the worst case no set-time or commit-time check can protect against is when a once valid interface is removed, e.g. by pulling a physical or virtual NIC out of the router. Then that validation becomes a time bomb because the config will stop loading.
I've made the script display a prominent warning at commit time when a user tries to enter an invalid interface. Hope this solution satifsies everyone because this is the only one we've got.