I haven't read all the answers, and the chosen response is basically correct, but when I have had this problem, the cause is a very simple mistake on my part. I am alternating between ssh and scp commands, and when I execute the scp command I simply copy the remote hostname, which some shells display, however the remote hostname displayed is shortened from the fully qualified domain name to just the name of the server. That is, the server is called "remoteservername", and its fully qualified domain name is "remoteservername.domainname.com" or something like that. If you try to execute an scp command with "remoteservername", the name cannot be resolved. The scp command has to be issued with "remoteservername.domainname.com".