On 10/19/2011 03:02 PM, David Stevens wrote:
Stefan,
Can't you achieve the same thing by reserving an
early block
of priorities (and a late one, for system stuff that should be
done late)?
The problem is that at the moment rules (in the 'root' table) can
have priorities [0, 1000]. So nothing prevents one to write a rule
with priority 0. However, due to how nwfilters works right now the
jumps into the protocol-specific tables will always be created
*before* those rules. I am trying to address this now with assigning
negative numbers to the chains to achieve the same sorting and
maintain backwards compatibility.
If you use negative numbers, then you lose the
capability of
ever extending priorities to interpret the negative number as
"from the end"
as done in ebtables/iptables line numbers. I think that is
more useful, and
having to do that outside of priorities would mean extra
parsing and encoding
to get that effect.
There is no relation between priorities and the ordering parameter
to the ebtables / iptables commands. The priorities were introduced
so that more complex filters can be built by composing them of
individual filters and yet have their filtering rules be created in
the 'proper' order that goes beyond of how they are reference
through filter references inside the filters and their appearance in
the XML.
I don't see how this could be changed, but I'd be curious to see
'how'.
I also think that nwfilters ought to reflect
the underlying filter
mechanisms as much as possible. Really, I'd prefer they were
simply
parameterized shell scripts of ebtables/iptables commands run
at significant
events (start-up, shutdown, migrate) instead of XML-encoded
things. Then
the full feature sets of ebtables/iptables would be available
"for free", instead
of requiring libvirt patches to, e.g., add "return/continue"
or multiple chains.
If you want to design another filtering subsytem for libvirt, please
go ahead. mwfilters currently works with XML and I don't see we can
change that so easily.
Barring that, at least I think what nwfilters
provides should be a close
map to ebtables/iptables capabilities. Mapping line numbers
into a wide range
It was not intended to be a 1:1 mapping but allow portability to
other system. Of course, the lack of similar functionality on other
system may be quite a bit of work to overcome first.
of priorities is straightforward, but if you use negative
numbers in an ordinary
sort, you can no longer use the sign as ebtables/iptables
does. Because
you've limited the range, you could do something hacky with
offsets (anything
below "-1000" is "from the end" or some such), but that's
arcane.
Right, 'arcane'...
Using priorities in multiple places is very
like programming in basic
and what both ebtables/iptables and nwfilters could use better
I think would
be the capability to label rules by name and reference the
label to identify the
rule location. Then you might, e.g., add a rule at "myrules +
5" and don't care
what particular priority/line number "myrules" is.
You have patches for that?
+-DLS
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