Ian Smith wrote:
On Tue, 20 Feb 2007, Julian Elischer wrote:
> admin wrote:
> > Wrong: the implied "check-state" done by the "limit" lets the connection
> > through (i.e. performs the action) iff there's state recorded for it
> > (src-addr+src-port+dst-addr+dst-port). If however it's a SYN packet
> > incoming and the number of current states is trying to cross the limit,
> > the SYN packet is implicitly dropped and the search terminates.
> > This is not to say that I completely understand the things going on when
> > the connections start building up (different timeouts?) but the above
> > conclusion is based on what simulation has shown. The whole ruleset fits
> > on one screen, there's an "allow ip from any to any" in the end, so I'm
> > pretty sure I'm not crazy :-)
> One thing to keep in mind is that a 'check-state' rule works by effectively
> jumping to the rule that did the 'keep-state' and re-executing it..
> (and incrementing its stats).
What if the action of the rule that does the 'keep-state' (here a limit
src-addr) is a skipto, rather than an allow / fwd / divert etc rule that
would terminate the search? Does 're-executing' here imply anything
about whether the skipto's conditional branch is or is not taken?
if the keep-state rule is a skipto then the check-state will skip to..
I use that in some rules.
here's a snippet that I have when acting as a filtering transparent bridge,
with DNS replies (from the outside) being diverted to a snooper.
The bridge is between em2 (inside) and em3 (outside).
01200 17588453 1257728932 skipto 1210 udp from any to any dst-port 53 recv em2
01201 16192860 2721432229 skipto 1220 udp from any 53 to any recv em3
01202 43658738 21537916245 skipto 1250 ip from any to any
01210 34023270 3994649771 skipto 1240 ip from any to any keep-state
01220 0 0 check-state
01229 2364 441701 deny ip from any to any
01240 16190496 2720990528 divert 5001 udp from any 53 to any recv em3
01242 17832774 1273659243 deny ip from any to any
(note this requires patches to allow divert from a bridge.)
It's convoluted I know but it only (well, mostly) diverts packets from
the outside that are part of a conversation that was started already
(probably by a dns request from inside) DNS packets from the outside
that are not a response to something we already asked for are discarded
(a DOS protection)
note that all the packets that went to the check-state, (1220) ended up going
back to 1210
where they did the skipto 1240 and were filtered again.
I bought into this because admin said that more connections were being
allowed than the limit src-addr clause should allow, and I assumed that
the skipto branch was not being taken on over-limit packets, and that
the following fwd rule (allowing any type of packets including SYN) was
being executed, which would account for what he'd said was happening.
admin above asserts that my assumption was wrong, and that in a match
beyond the limit number of connections for that src/dest address, the
setup packet is 'implicitly dropped and the search terminates', and
while I can't find that stated as such in ipfw(8), he may be right.
Which still doesn't explain why connections from a particular IP beyond
his specified limit are allowed to be established, as originally stated.
[shrug] Over to the ipfw gurus.
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