libssh2 is hanging during a file transfert

[Brian Candler]

My first thought is it could be an MTU problem.

Try running tcpdump at both ends - you should see the same packets (*).  
If you find one side sends a maximum-sized packet, but the other side 
receives nothing, then that's very likely the issue.

If you don't have access to the far side, then reduce the TCP MSS on 
your side and try again.  (Or at worst, just reduce the MTU on your 
interface).  At least you'll know if that's the problem or not.

Regards,

Brian.

(*) Maybe multiple packets will appear as one packet, if your NIC 
reassembles TCP segments.  Turning off TCP offload on your NIC can 
sometimes be worth trying too.