I originally composed this for the *nix forum, but seems better suited here.
I've spent all day tracking down a problem that has plagued my network intermittently for a while, slow to establish telnet sessions, and FTP failing at times with !Error received: Blocking call cancelled
There's lot's of info on the net about this specific FTP error, but nothing helped. Most suggested setting FTP to passive mode or vise versa, others suggested lengthening the timeout value (which does work but is not a solution, waiting minutes for a session to be established).
Since none of the documented solutions helped I started looking closer at FTP, which eventually turned on the lightbulb that led me to DNS. When I set up BIND I didn't bother to set up the in-addr.arpa section for reverse resolution, thinking I didn't really need it based on previous experience with DNS.
Now after setting up the reverse resolution zone and the PTR records, FTP session establishment is lightning fast, and so is telnet session establishment.
Anyway thought I'd post it here since I saw many complaints when researching the problem from others that believed it to be slow servers at their ISP, or problems with their FTP client, or routing problems (spent hours looking at routes). Reverse resolution not being set up was never mentioned as a possible cause. Because of how FTP establishes a session I am now convinced that this is *the* problem when this error crops up, and I know there are ISP's out there running DNS without reverse lookup set up, or you may be unlucky enough to get a host/IP without a PTR record if they do have reverse lookup set up.
Anyway hope it helps someone else out there.