"ENOENT has many meanings, and iptables prints just one of them,
which is potentially misleading."
Thanks, this will be corrected I take it?
The reason you have to specify -p tcp is so that CT can take a
reference to the particular helper instance (which is identified
by name, L3 family, _and_ L4 family) and/or load a module that
does provide such instances, respectively.
Is there a need for the L4 inclusion (seems that it causes the initial
error message)?
Also, there are helpers (the sip one comes to mind) that work both with
tcp and udp protocols and are potentially great candidates to be used
with the two-dimensional ipsets (hash:ip,port). As it stands though, I
have to create two iptables statements *and* use one-dimensional ipset
to load and use that particular helper. Wouldn't it be easier if the L4
restriction is dropped/amended (if it is indeed the source of that
error) so that protocols could be omitted when using two-dimensional sets?
While we do know whata L4 protocol a packet is once it is received
from the network, there is no chance to load the required module
on the fly due to interrupt context constraint, so hypothetically
allowing "(-p all) -j CT" would be unreliable, packets would not
"get helped".
As such, specify the L4 beforehand is a requirement.
I thought the helper module is loaded once the iptables statement is
created, isn't that the case? Also, if the helper itself allows for "-p
all" (or "-p tcp" or "-p udp" at the very least) what then? The "L4
protocol" restriction won't be needed, would it?
To me it seems rather unnecessary to have to specify a protocol where I
could have ipset take care of that, otherwise I am left with the
alternative of using one-dimensional sets only (I haven't yet tested the
iface-type sets, so I strongly suspect I may have a bumpy ride there as
well).
Precisely. "-p all" just is not the same as "-p tcp". I guess
one /could/ add a PF_UNSPEC instance to nf_conntrack_ftp.c,
thereby allowing to use -j CT without any -p (or nonsensical values
for -p) and ignore all packets that are not TCP.
But that would cause some practical overhead as a packet's verdict
would not be known until the helper code is executed. (Currently
the check can be done outside the helper because the L4 is
known in advance.)
What I still can't get my head around yet is why the hesitation for not
using a helper module, which was already specified (and I suspect loaded
in memory) in the iptables statement - is there a concern that more than
one module may be used for a particular packet? If that is so, then I
could easily prepare a clash with using an ftp helper module selecting
the same ip:protocol:port triplet for, say, ftp helper module as well as
sip. What happens then?
On a side note, looking at Eric's "helper-recommandation.pdf" (I guess,
I should have spell-checked the name of that file as well), as far as I
understand it, there are two different methods of
loading/creating/setting up helper modules:
1. Using the "FORWARD" chain in filter as well as specifying "RELATED"
and helper name matches;
2. Using the PREROUTING/OUTPUT chain in raw by specifying the CT target
with a helper name match (this is what I used in my examples up to now).
Are the two methods above interchangeable (i.e. use one or the other,
but not both) or should I configure both (in which case I haven't tried
my ipset trick using the first scenario!)? In that file there is no
indication whether I should use one or the other (or both!).
If I have to use the first method above as well, why is the "FORWARD"
chain used (at least in the examples given in that article)? If I want
to set up a helper on outgoing packets then using the OUTPUT chain in
filter seems to be the best solution.
I also take it either of the above methods is the preferred option
instead of using modprobe (and force the loading of that helper module).
Am I correct in thinking that?
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