On 11/24/20 7:01 AM, Daniel Henrique Barboza wrote:
On 11/24/20 12:30 AM, Laine Stump wrote:
When libvirt added support for firewalld, we were unable to use
firewalld's higher level rules, because they weren't detailed enough
and could not be applied to the iptables FORWARD or OUTPUT chains
(only to the INPUT chain). Instead we changed our code so that rather
than running the iptables/ip6tables/ebtables binaries ourselves, we
would send these commands to firewalld as "passthrough commands", and
firewalld would run the appropriate program on our behalf.
This was done under the assumption that firewalld was somehow tracking
all these rules, and that this tracking was benefitting proper
operation of firewalld and the system in general.
Several years later this came up in a discussion on IRC, and we
learned from the firewalld developers that, in fact, adding iptables
and ebtables rules with firewalld's passthrough commands actually has
*no* advantage; firewalld doesn't keep track of these rules in any
way, and doesn't use them to tailor the construction of its own rules.
That's odd. Why bother providing the mechanism if you're not building
anything upon it?
Sometimes ideas don't work out and/or plans change, and there are
multiple levels of stuff based on those earlier ideas that are already
in place.
Might as well drop it entirely and tell users to just
use the command line.
Actually, when firewalld added in their nftables backend, they did not
add support for adding nftables rules via their passthrough command API.
But dropping something "entirely" from the API first requires that all
users of that part of the API stop using it. This entire dance can take
years.
At least it'll not cause confusion.
I was going to ask you to mention the man page/doc where it is stated
that
the passthrough commands aren't tracked, but I have a hunch that this
might
not be officially documented (I just searched in 'man
firewalld.direct' and
'man firewalld', haven't found any info about this behavior). So yeah, it
seems like "a guy on IRC told us that ..." is as far as we'll have to
document this, ATM.
Well, not just "a guy on IRC", but Eric Garver, the main upstream
maintainer/developer of firewalld.
Meanwhile, users have been complaining for some time that whenever
firewalld is restarted on a system with libvirt virtual networks
and/or nwfilter rules active, the system logs would be flooded with
warning messages whining that [lots of different rules] could not be
deleted because they didn't exist. For example:
firewalld[3536040]: WARNING: COMMAND_FAILED:
'/usr/sbin/iptables -w10 -w --table filter --delete LIBVIRT_OUT
--out-interface virbr4 --protocol udp --destination-port 68
--jump ACCEPT' failed: iptables: Bad rule
(does a matching rule exist in that chain?).
(See https://bugzilla.redhat.com/1790837 for many more examples and a
discussion)
Note that these messages are created by iptables, but are logged by
firewalld - when an iptables/ebtables command fails, firewalld grabs
whatever is in stderr of the program, and spits it out to the system
log as a warning. We've requested that firewalld not do this (and
instead leave it up to the calling application to do the appropriate
logging), but this request has been respectfully denied.
But combining the two problems above ( 1) firewalld doesn't do
anything useful when you use it as a proxy to add/remove iptables
rules, 2) firewalld often insists on logging lots of
annoying/misleading/useless "error" messages when you use it as a
proxy to remove iptables rules that don't already exist), leads to a
solution - simply stop using firewalld to add and remove iptables
rules. Instead, exec iptables/ip6tables/ebtables directly in the same
way we do when firewalld isn't active.
We still need to keep track of whether or not firewalld is active, as
there are some things that must be done, e.g. we need to add some
actual firewalld rules in the firewalld "libvirt" zone, and we need to
take notice when firewalld restarts, so that we can reload all our
rules.
This patch doesn't remove the infrastructure that allows having
different firewall backends that perform their functions in different
ways, as that will very possibly come in handy in the future when we
want to have an nftables direct backend, and possibly a "pure"
firewalld backend (now that firewalld supports more complex rules, and
can add those rules to the FORWARD and OUTPUT chains). Instead, it
just changes the action when the selected backend is "firewalld" so
that it adds rules directly rather than through firewalld, while
leaving as much of the existing code intact as possible.
After reading all patches I can't help but think that, in the end,
you're having to dance around firewalld ankwardness just to get regular
use case to work properly.
Well, firewalld would like it if it was the only arbiter of access to a
host's packet filtering engine(s). That's closer to being a possibility
(e.g. firewalld can now add rules to the FORWARD and OUTPUT chains) but
still not reality, and until it is a reality, other software will need
to synchronize itself with firewalld. In the meantime, it would have
been nice if firewalld provided a "startup completed" event for people
like libvirt to use for synchronization, but it doesn't. We *could*
insist that firewalld add such an event, and remain broken until it was
provided, but this clever hack gives us the same effects, can be done
immediately, and will work with any version of firewalld, so it's a
win-win-win.
Queing 'iptables -V' to not get Libvirt rules
overwritten by firewalld purge protocol in patch 7 represents my 2020
experience: every day getting flabbergasted by random stuff that
shouldn't happen.
Yes, "random stuff that shouldn't happen" would be a nice synopsis for
this year.
Anyway, good story!
... as would this :-)
