How to log what is drop

How to log what is drop

[IZEM Farid]

Hi all,

Just a Simple question.

How do i log what is being rejected or drop by my Firewall.

I configure my syslog.conf like this: 
	kern.*                                                  /var/log/kernel

And I had the following rule: 
	Iptables -A FORWARD -j LOG 

It seems that all connections is logged so it is very difficult to read the log.

Thanks,

Best regards,

Farid IZEM
Ingénieur Système Unix
Société ABX Logistics France
48-50, route principale du port
92232 Gennevilliers
Tél. : 01-41-47-61-78
Email : farid.izem@abxlogistics.fr

Re: How to log what is drop

[Gavin Hamill]

On Thursday 08 July 2004 08:30, IZEM Farid wrote:
> Hi all,
>
> Just a Simple question.
>
> How do i log what is being rejected or drop by my Firewall.
>
> I configure my syslog.conf like this:
> 	kern.*                                                  /var/log/kernel
>
> And I had the following rule:
> 	Iptables -A FORWARD -j LOG
>
> It seems that all connections is logged so it is very difficult to read the
> log.

Firstly, do you actually have DROP statements in your ruleset? If so, the LOG 
statement should appear JUST BEFORE the block of DROP statements. Since 
ACCEPT or DROP are one-way-streets for packets, you want to LOG just before 
they get DROPped, but not log any of the ACCEPTed packets.

Cheers,
Gavin.

Re: How to log what is drop

[Nils Juergens]

On Thu, 08.07.04, Gavin Hamill <gdh@acentral.co.uk> wrote:

> > And I had the following rule:
> > 	Iptables -A FORWARD -j LOG
> >
> > It seems that all connections is logged so it is very difficult to read the
> > log.
> 
> Firstly, do you actually have DROP statements in your ruleset? If so, the LOG 
> statement should appear JUST BEFORE the block of DROP statements. Since 
> ACCEPT or DROP are one-way-streets for packets, you want to LOG just before 
> they get DROPped, but not log any of the ACCEPTed packets.

A very good way to do this is via a special chain "logdrop"

iptables -N logdrop
iptables -A logdrop -j LOG --log-prefix ' log drop '
iptables -A logdrop -j DROP

And then you replace every -j DROP (that you want logged) with -j logdrop.

I think this is mentioned in a part of the netfilter docs.

hth,
Nils