[Planetlab-users] sendto: operation not permitted

[Neil Spring]

Patrick,

PlanetLab's vnet works on the assumption that you can send tcp  
packets so long as the source port is one your slice "owns" via  
opening a socket and calling bind, and that you can only receive tcp  
packets to a destination port that you "own" in the same way.

We send gobs of tcp packets, without EPERM on sendto.   If it's  
letting a packet through after 100,000 retries, that sounds like a bug.

Planetlab rate limiting has been found to delay packets by 30 seconds  
(I'm as surprised as anyone, but I heard this from a reputable source  
on sunday).  I'd guess that if you were really blowing the queue,  
you'd get ENOBUFF rather than EPERM.

I'm pretty sure Mark wrote a vnet faq, or at least the documentation  
should get you pointed in the right direction.

-neil

On Nov 9, 2006, at 2:58 AM, Patrick Verkaik wrote:

>
> Hi,
>
> I have a question about the following sendto() behaviour that I  
> don't quite understand.
>
> We're sending raw TCP packets using sendto() but getting  
> intermittent EPERM errors. We've found that repeatedly retrying the  
> sendto() (with exactly the same packet) until it no longer gives  
> EPERM eventually gets the packet through. During a short connection  
> (perhaps lasting 10 seconds and sending across less than 100 bytes  
> of data) we sometimes see about 100,000 failed sendto() attempts  
> (with a usleep(1) separating the attempts roughly 20,000 attempts).
>
> Another curious fact is that we only see this behaviour when we're  
> tunneling the reverse TCP traffic into the sending host. The host  
> therefore doesn't see e.g. SYN/ACK packets coming back in response  
> to outgoing SYNs.
>
> Can anyone explain this? Is this how Planetlab implements rate  
> limiting or prevents SYN-flooding attacks being launched from  
> Planetlab?
>
> (Btw: we're running as root and using the node's IP address as  
> source IP address.)
>
> 	Thanks,
>
> 	Patrick Verkaik
> 	Barath Raghavan
>
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