Subject: Re: [RFC PATCH] network: return errors if we know tcp_connect failed

From: Eric Paris <[email-masked]>
Date: Mon, 15 Nov 2010 10:47:46 -0500

On Mon, 2010-11-15 at 11:32 +0100, Patrick McHardy wrote:
> On 13.11.2010 00:14, Hua Zhong wrote:
> >> On 11.11.2010 22:58, Hua Zhong wrote:
> >>>> Yes, I realize this is little different than if the
> >>>> SYN was dropped in the first network device, but it is different
> >>>> because we know what happened!  We know that connect() call failed
> >>>> and that there isn't anything coming back.
> >>>
> >>> I would argue that -j DROP should behave exactly as the packet is
> >> dropped in the network, while -j REJECT should signal the failure to
> >> the application as soon as possible (which it doesn't seem to do).
> >>
> >> It sends an ICMP error or TCP reset. Interpretation is up to TCP.
> > 
> > Huh? It's the OUTPUT chain we are talking about. There is no ICMP error or
> > TCP reset.
> 
> Of course there is.
> 
> ICMP (default):
> 
> iptables -A OUTPUT -p tcp -j REJECT
> 
> TCP reset:
> 
> iptables -A OUTPUT -p tcp -j REJECT --reject-with tcp-reset
> 
> The second one will cause a hard error for the connection.

Well I'm (I guess?) surprised that the --reject-with icmp doesn't do
anything with a local outgoing connection but --reject-with tcp-reset
does something like what I'm looking for.

I notice the heavy lifting for this is done in 
net/ipv4/netfilter/ipt_REJECT.c::send_rest()
(and something very similar for IPv6)

I really don't want to duplicate that code into SELinux (for obvious
reasons) and I'm wondering if anyone has objections to me making it
available outside of netlink and/or suggestions on how to make that code
available outside of netfilter (aka what header to expose it, and does
it still make logical sense in ipt_REJECT.c or somewhere else?)

-Eric