On Tue, 25 Feb 2020 21:37:45 +0100 (CET) Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx> wrote: > On Tue, 25 Feb 2020, Stefano Brivio wrote: > > > > The logic could be changed in the user rules from > > > > > > iptables -I INPUT -m set --match-set c src --bytes-gt 800 -j DROP > > > > > > to > > > > > > iptables -I INPUT -m set --match-set c src --bytes-lt 800 -j ACCEPT > > > [ otherwise DROP ] > > > > > > but of course it might be not so simple, depending on how the rules are > > > built up. > > > > Yes, it would work, unless the user actually wants to check with the > > same counter how many bytes are sent "in excess". > > You mean the counters are still updated whenever the element is matched in > the set and then one could check how many bytes were sent over the > threshold just by listing the set elements. Yes, exactly -- note that it was possible (and, I think, used) before. > > Now, I see the conceptual problem about matching: if the rule isn't > > matching, and counters count matched packets, counters shouldn't > > increase. But still, I think there are a number of facts to be > > considered: > > > > - the man page says (and has said for a number of years): > > > > If the packet is matched an element in the set, match only if > > the byte counter of the element is greater than the given value > > as well. > > > > which actually makes the problem undecidable: matching depends on > > matching itself. Trying some "common sense" interpretation, I would > > read this as: > > > > If the packet matches an *element* in the set, this *rule* will > > match only if the byte counter of the element is greater than > > the given value. > > > > that is, by separating the meaning of "element matching" from "rule > > matching", this starts making sense. > > Yes, you are right. Sometimes I think I'm far from the best at writing > documentation... So I'm going to update the manpage with your sentence. Wait, though: that's only the case if we update the counters for matching *elements* and not necessarily matching *rules*, which was the case before 4750005a85f7, or with this patch. Otherwise, the sentence I wrote is not accurate. I can try to come up with another one to describe the current behaviour, but I'll need some calm minutes with pencil and paper tomorrow. > > - I spent the past two hours trying to think of an actual case that was > > affected by 4750005a85f7, *other than the "main" bug it fixes*, that > > is, "! --update-counters" was ignored altogether, and I couldn't. > > > > Even if we had a --bytes-lt option, it would be counter-intuitive, > > because the counter would be updated until bytes are less than the > > threshold, and then the rule would stop matching, meaning that the > > user most probably thinks: > > > > "Drop matching packets as long as less than 800 bytes are sent" > > > > and what happens is: > > > > "Count and drop matching packets until 800 bytes are sent, then > > stop dropping and counting them" > > Again, yes, that's what would happen. > > > The only "functional" case I can think of is something like > > --bytes-lt 800 -j ACCEPT. User probably thinks: > > > > "Don't let more than 800 bytes go through" > > > > and what happens is: > > > > "Let up to 800 bytes, or 799 bytes plus one packet, go through, > > counting the bytes in packets that were let through" > > > > which isn't much different from the expectation. > > > > - and then, > > > > > > Other than this, I'm a bit confused. How could --packets-gt and > > > > --bytes-gt be used, if counters don't increase as long as the rule > > > > doesn't match? > > > > > > I almost added to my previous mail that the 'ge' and 'gt' matches are not > > > really useful at the moment... > > > > ...yes, I can't think of any other use for those either. > > Those could really be useful if the counters could be decremented. > Otherwise I think the counter matching in the sets is not as useful as it > seems to be. Still, if counters are updated with just matching element, but not necessarily matching rule, they should be as useful as in the hypothesis of introducing a "decrementing" feature -- one just needs to adjust the rule logic to that. > > > > > What's really missing is a decrement-counters flag: that way one could > > > > > store different "quotas" for the elements in a set. > > > > > > > > I see, that would work as well. > > > > > > The other possibility is to force counter update. I.e. instead of > > > > > > iptables -I INPUT -m set --match-set c src --bytes-gt 800 -j DROP > > > > > > something like > > > > > > iptables -I INPUT -m set --match-set c src --update-counters \ > > > --bytes-gt 800 -j DROP > > > > > > but that also requires some internal changes to store a new flag, because > > > at the moment only "! --update-counters" is supported. So there'd be then > > > a fine-grained control over how the counters are updated: > > > > > > - no --update-counters flag: update counters only if the whole rule > > > matches, including the counter matches > > > - --update-counters flag: update counters if counter matching is false > > > > ...this should probably be "in any case", also if it's true. > > Yes, but now I don't really like the name itself: --force-update-counters > or something like that would be more clear. > > > > - ! --update-counters flag: don't update counters > > > > I think that would fix the issue as well, I'm just struggling to find a > > sensible use case for the "no --update-counters" case -- especially one > > where there would be a substantial issue with the change I proposed. > > The no update counter flag was introduced to handle when one needs to > match in the same set multiple times, i.e. there are multiple rules with > the same set. Like you need to match in the raw/mangle/filter tables as > well. Unfortunately I can't recall the usercase. Okay, but what you're describing is the "! --update-counters" option. That works, didn't work before 4750005a85f7, but would still work with this change. What I meant is really the case where "--update-counters" (or "--force-update-counters") and "! --update-counters" are both absent: I don't see any particular advantage in the current behaviour for that case. -- Stefano
