Drop a packet while keeping the conntrack alive

Drop a packet while keeping the conntrack alive

[Nicolas Maître]

Hi,

I've developed a connection tracking module which I'd like to use for
detecting packets that may be dropped with an iptables rule.
That is, the connection tracker can detect that a packet is not  valid
in the context of the tracked connection. So, I'd like the
administrator to be able to add a rule that would allow to drop/reject
that packet.
A possible use case is to block packets from an attacker (assumed as
such as the packet is not conform considering the current state of the
connection).

As far as I understand it, the connection tracker usually handle this
by returning a negative/null value (-NF_ACCEPT, NF_DROP, ...) which
marks the packet invalid, so that it is possible to use the
xt_conntrack extension to match it.

I've looked at the TCP connection tracker. If I understand well, the
current behavior is that if we've got an INVALID packet, it kills the
conntrack, then create it again if we see an ACK later on. Am I right?

That is, it seems that by any way we invalidate a packet, it always
destroy the conntrack so that we haven't the connection state at our
disposal anymore. I would like to avoid that for the protocol I track.
Is there a way to do so? Am I missing something?



Thank you.

--
nimai

Re: Drop a packet while keeping the conntrack alive

[Pablo Neira Ayuso]

Hi Nicolas,

On Thu, Mar 21, 2013 at 01:57:33AM +0100, Nicolas Maître wrote:
> Hi,
> 
> I've developed a connection tracking module which I'd like to use for
> detecting packets that may be dropped with an iptables rule.
> That is, the connection tracker can detect that a packet is not  valid
> in the context of the tracked connection. So, I'd like the
> administrator to be able to add a rule that would allow to drop/reject
> that packet.
> A possible use case is to block packets from an attacker (assumed as
> such as the packet is not conform considering the current state of the
> connection).
> 
> As far as I understand it, the connection tracker usually handle this
> by returning a negative/null value (-NF_ACCEPT, NF_DROP, ...) which
> marks the packet invalid, so that it is possible to use the
> xt_conntrack extension to match it.
> 
> I've looked at the TCP connection tracker. If I understand well, the
> current behavior is that if we've got an INVALID packet, it kills the
> conntrack, then create it again if we see an ACK later on. Am I right?

Packets that are invalid do not destroy the conntrack entry. The TCP
tracker explicitly destroys the entry by calling nf_ct_kill function
in some situations (RST seen / tracking is out-of-sync).
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Re: Drop a packet while keeping the conntrack alive

[Nicolas Maître]

On Thu, Mar 21, 2013 at 2:53 AM, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> Packets that are invalid do not destroy the conntrack entry. The TCP
> tracker explicitly destroys the entry by calling nf_ct_kill function
> in some situations (RST seen / tracking is out-of-sync).
>

Well, it means that nf_conntrack_put doesn't kill the conntrack as I
thought. I still can't understand how the nfct from skb works. What is
that "use" value that makes nf_conntrack_put() call
nf_conntrack_destroy() when it equals zero?
Well, I suppose it's not that important for me. At least I have my
answer, thank you :)

--
nimai

On Thu, Mar 21, 2013 at 2:53 AM, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> Hi Nicolas,
>
> On Thu, Mar 21, 2013 at 01:57:33AM +0100, Nicolas Maître wrote:
>> Hi,
>>
>> I've developed a connection tracking module which I'd like to use for
>> detecting packets that may be dropped with an iptables rule.
>> That is, the connection tracker can detect that a packet is not  valid
>> in the context of the tracked connection. So, I'd like the
>> administrator to be able to add a rule that would allow to drop/reject
>> that packet.
>> A possible use case is to block packets from an attacker (assumed as
>> such as the packet is not conform considering the current state of the
>> connection).
>>
>> As far as I understand it, the connection tracker usually handle this
>> by returning a negative/null value (-NF_ACCEPT, NF_DROP, ...) which
>> marks the packet invalid, so that it is possible to use the
>> xt_conntrack extension to match it.
>>
>> I've looked at the TCP connection tracker. If I understand well, the
>> current behavior is that if we've got an INVALID packet, it kills the
>> conntrack, then create it again if we see an ACK later on. Am I right?
>
> Packets that are invalid do not destroy the conntrack entry. The TCP
> tracker explicitly destroys the entry by calling nf_ct_kill function
> in some situations (RST seen / tracking is out-of-sync).
>



-- 
Nicolas
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Re: Drop a packet while keeping the conntrack alive

[Pablo Neira Ayuso]

On Thu, Mar 21, 2013 at 04:17:47PM +0100, Nicolas Maître wrote:
> On Thu, Mar 21, 2013 at 2:53 AM, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > Packets that are invalid do not destroy the conntrack entry. The TCP
> > tracker explicitly destroys the entry by calling nf_ct_kill function
> > in some situations (RST seen / tracking is out-of-sync).
> 
> Well, it means that nf_conntrack_put doesn't kill the conntrack as I
> thought. I still can't understand how the nfct from skb works. What is
> that "use" value that makes nf_conntrack_put() call
> nf_conntrack_destroy() when it equals zero?

That "use" field in the nf_conn object is a refcounter. In SMP, you
may have several packets traveling through the stack referencing to
the same conntrack object. The refcount is initially set to 1 because
there is a timer that releases the conntrack if it expires.

You may also have a conntrack entries that are related to another
master conntrack (eg. FTP helper scenario: the control flow is the
master conntrack, and all data flows are related to it). In that case,
the refcount is also incremented depending on the amount of references
to it.

nf_conntrack_find_get increments the refcount, nf_ct_put decrements it
and it checks if it has become 0, in that case the nf_conn object is
released.

This article may help you to understand better how conntrack works:

https://www.usenix.org/publications/login/june-2006-volume-31-number-3/netfilters-connection-tracking-system

Regards.
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Re: Drop a packet while keeping the conntrack alive

[Nicolas Maître]

On Thu, Mar 21, 2013 at 5:07 PM, Pablo Neira Ayuso <pablo@netfilter.org> wrote:

> That "use" field in the nf_conn object is a refcounter. In SMP, you
> may have several packets traveling through the stack referencing to
> the same conntrack object. The refcount is initially set to 1 because
> there is a timer that releases the conntrack if it expires.
>
> You may also have a conntrack entries that are related to another
> master conntrack (eg. FTP helper scenario: the control flow is the
> master conntrack, and all data flows are related to it). In that case,
> the refcount is also incremented depending on the amount of references
> to it.
>
> nf_conntrack_find_get increments the refcount, nf_ct_put decrements it
> and it checks if it has become 0, in that case the nf_conn object is
> released.

Now that's clearer. Thank you very much!


--
Nicolas Maître
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Re: Drop a packet while keeping the conntrack alive

[Jorge Dávila]

Hi list,

An example about ...

With the current user space tools
there are ways for tracking INVALID connections.

INVALID connections, IMHO, should be "active" if you create a firewall
with stateless packet filtering and of course, setting the "needed"
values for the kernel parameters.

Jorge Isaac

2013/3/20, Nicolas Maître <nimai@skynet.be>:
> Hi,
>
> I've developed a connection tracking module which I'd like to use for
> detecting packets that may be dropped with an iptables rule.
> That is, the connection tracker can detect that a packet is not  valid
> in the context of the tracked connection. So, I'd like the
> administrator to be able to add a rule that would allow to drop/reject
> that packet.
> A possible use case is to block packets from an attacker (assumed as
> such as the packet is not conform considering the current state of the
> connection).
>
> As far as I understand it, the connection tracker usually handle this
> by returning a negative/null value (-NF_ACCEPT, NF_DROP, ...) which
> marks the packet invalid, so that it is possible to use the
> xt_conntrack extension to match it.
>
> I've looked at the TCP connection tracker. If I understand well, the
> current behavior is that if we've got an INVALID packet, it kills the
> conntrack, then create it again if we see an ACK later on. Am I right?
>
> That is, it seems that by any way we invalidate a packet, it always
> destroy the conntrack so that we haven't the connection state at our
> disposal anymore. I would like to avoid that for the protocol I track.
> Is there a way to do so? Am I missing something?
>
>
>
> Thank you.
>
> --
> nimai
> --
> To unsubscribe from this list: send the line "unsubscribe netfilter-devel"
> in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>


-- 
Jorge Isaac Dávila López
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