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Re: processing order for configuration files in /etc/network/interfaces.d


On Mon, Aug 27, 2018 at 12:01:23PM -0400, Greg Wooledge wrote:
> On Mon, Aug 27, 2018 at 06:41:25PM +0300, Reco wrote:
> > Last sentence says to me that wordexp output should be always sorted.
> This only tells us that it *reads* the config files in glob-sorted order.
> And peeking into the actual source code of ifupdown, yes, it appears to
> do this.  (File config.c starting at line 436, in the stretch source.)
> It even uses wordexp() as advertised.
> What's less clear to me at this moment is what happens *after* the
> interface configuration file(s) have been read into memory.
> Moving over to main.c, it looks like it reads the interfaces (line 639),
> and that function was called from line 1190.  After line 1190, it loops
> over the "target_iface" array, and processes them in the order they appear
> in that array.
> So... next question is how they get packed into that array.
> Returning to the select_interfaces function, it looks like the ordering
> is created by a call to find_allowup.
> And... this is where I stopped reading, because it got confusing.  Maybe
> someone else can take a stab at deciphering that part.

select_interfaces() is called on "ifup -a" invocation.
read_interfaces() is called by it.
read_interfaces() calls read_interfaces_defn().
read_interfaces_defn() parses "iface" and "source" directives, and calls
read_interfaces_defn() once again on each file specified by "source" in
order defined by wordexp(3).

Therefore defn[] array should be filled in this order (assuming that one
specifies "source" only at /e/n/i):

1) any interface definition at /e/n/i in order encountered, until
"source" directive.
2) any interface definition provided by "source" directive in order
defined by wordexp(3).
3) any interface definition at /e/n/i in order encountered, after the
"source" directive.

I'd use ltrace(1) to check it, but building test environment is
something that I lack the time to do.


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