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Re: [libvirt] [PATCH V2 00/10] Make inner workings of nwfilters more flexible + extensions



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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