Is there a way to DROP on OUTPUT without returning EPERM?

Is there a way to DROP on OUTPUT without returning EPERM?

[Mike Kasick]

I'm trying to emulate network packet loss by randomly dropping packets with
netfilter and the statistic match module.  Everything works fine with the
following firewall rule:

iptables -A INPUT -i eth1 -m statistic --mode random --probability 0.5 -j DROP

However, since packets are dropped by the receiving machine, they're still
making it out on the wire.  I also want to test dropping packets on the
sending machine such that they never make it to the wire, unfortunately the
analogous rule:

iptables -A OUTPUT -o eth1 -m statistic --mode random --probability 0.5 -j DROP

has the side effect of returning EPERM for write/send syscalls for which
packets are dropped.  While this behavior makes sense in the usual case,
I'd really like to drop packets silently without the user application being
notified.

Is there a way I can achieve a truly silent DROP on the OUTPUT chain?  Is
there an extension target that does this that I've missed?

Thanks!

Re: Is there a way to DROP on OUTPUT without returning EPERM?

[G.W. Haywood]

Hi there,

On Thu, 10 Jan 2008, Mike Kasick wrote:

> iptables -A OUTPUT -o eth1 -m statistic --mode random --probability 0.5 -j DROP
>
> has the side effect of returning EPERM ... I'd really like to drop
> packets silently without the user application being notified.
>
> Is there a way I can achieve a truly silent DROP on the OUTPUT chain?  Is
> there an extension target that does this that I've missed?

Instead of DROPping it, why not just send it somewhere else?

--

73,
Ged.

Re: Is there a way to DROP on OUTPUT without returning EPERM?

[Benny Amorsen]

Mike Kasick <mkasick-nf@club.cc.cmu.edu> writes:

> iptables -A OUTPUT -o eth1 -m statistic --mode random --probability 0.5 -j DROP
>
> has the side effect of returning EPERM for write/send syscalls for which
> packets are dropped.

It would make more sense if DROP dropped silently and REJECT returned
EPERM. Hard to fix now, of course.


/Benny