Question & Answer
Question
FTP sessions to servers generally work well. But on occasion, a transfer attempt will fail when trying to open the data connection (after the RETR or STOR command) and no data is transferred.
One (or more) of the following messages will be seen:
EZA2589E Connection to server interrupted or timed out. Waiting for data connect
425 Can't open data connection
425 Unable to open data connection
Answer
The most likely cause is that the ephemeral port number selected for the data connection is being blocked by some device on the network. This will typically be a firewall, but routers and even some switches can be configured to block specific port numbers.
You should work with the network administrators to identify all of the port numbers that are being blocked. Then add port reservation statements (PORT and PORTRANGE with RESERVED as the job name field) for the list of ports so that they will not be selected as an ephemeral port. For example: the ports associated with the Internet Relay Chat (IRC) protocol are commonly blocked, so the following statements would be be added to avoid use of those ports:
PORT 194 TCP RESERVED
PORT 194 UDP RESERVED
PORTRANGE 6665 5 TCP RESERVED
PORTRANGE 6665 5 UDP RESERVED
If further diagnostics are needed, collect a packet trace of the failure, preferably one at the client system and one at the server simultaneously. See TechNote 1292013 for directions for collecting this on a z/OS system. And filter each trace using the IP address of the other system.
Product Synonym
ZOSCS COMMSERVER
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Document Information
Modified date:
13 June 2018
UID
dwa1453249