Messages in this thread | | | Subject | Re: [RFC PATCH] network: return errors if we know tcp_connect failed | From | Eric Dumazet <> | Date | Thu, 11 Nov 2010 22:14:38 +0100 |
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Le jeudi 11 novembre 2010 à 16:03 -0500, Eric Paris a écrit : > THIS PATCH IS VERY POSSIBLY WRONG! But if it is I want some feedback. > > Basically what I found was that if I added an iptables rule like so: > > iptables -A OUTPUT -p tcp --dport 80 -j DROP > > And then ran a web browser like links it would just hang on 'establishing > connection.' I expected that the application would immediately, or at least > very quickly, get notified that the connect failed. This waiting for timeout > would be expected if something else dropped the SYN or if we were dropping the > SYN/ACK packet coming back, but I figured if we knew we threw away the SYN we knew > right away that the connection was denied and we should be able to indicate > that to the application. 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. > > What I discovered was that we actually had 2 problems in making it possible. > For userspace to quickly realize the connect failed. The first was a problem > in the netfilter code which wasn't passing errors back up the stack correctly, > due to what I believe to be a mistake in precedence rules. > > http://marc.info/?l=netfilter-devel&m=128950262021804&w=2 > > And the second was that tcp_connect() was just ignoring the return value from > tcp_transmit_skb(). Maybe this was intentional but I really wish we could > find out that connect failed long before the minutes long timeout. Once I > fixed both of those issues I find that links gets denied (with EPERM) > immediately when it calls connect(). Is this wrong? Is this bad to tell > userspace more quickly what happened? Does passing this error code back up > the stack here break something else? Why do some functions seem to pay > attention to tcp_transmit_skb() return codes and some functions just ignore > it? What do others think? >
I think its an interesting idea, but a temporary memory shortage would abort the connect().
We could imagine some special handling of the first packet of a flow being DROPED for whatever reason (flow control...)
So it needs some refinement I think.
SYN packets should be allowed to be re-transmitted before saying a TCP connect() cannot succeed.
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