The Tool Desk
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Start by defining the connection that fails
Before changing configuration, write down the failing flow so that each check tests the same connection:
- Source and destination hostnames and IP addresses.
- Protocol and destination port.
- When the failure occurred and the symptom: timeout, refusal, reset, intermittent loss, slow response, or low throughput.
- Whether a comparable flow to the same service or from the same host works.
Separate a reachability failure from a response-time or throughput problem. IBM z/OS Communications Server supports both TCP/IP and SNA; the checks below apply to TCP/IP flows. If the application uses SNA, use the relevant SNA diagnostics instead of treating TCP/IP results as conclusive.
Check the z/OS stack and server process first
Verify basic TCP/IP operation
IBM’s “Steps for diagnosing problems connecting to a server” procedure starts at the z/OS host. Ping the loopback address and a configured home address to check basic local TCP/IP operation. A successful ping is not proof that the remote application is healthy or listening on the required port, so continue through the application and network checks.
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Confirm that the application is operational
Check that the server application is running and able to accept the relevant work. Inspect the z/OS system log and the output for the relevant started task or job. IBM identifies the system log as a primary place to look for TCP/IP and IP-application messages; TCP/IP and standard Communications Server applications commonly issue messages beginning with EZ. Record the message text and timestamps so they can be compared with the failure time and any later trace.
Inspect local addresses, interfaces, and routes
Use NETSTAT output to compare the active stack state with the intended configuration. Do not assume that a profile or definition you expect to be active is the one currently in effect.
NETSTAT HOMEshows configured home addresses.NETSTAT DEVorNETSTAT DEVLINKSshows device and interface information.NETSTAT ROUTEshows route information.NETSTAT CONNorNETSTAT SOCKETSlets you inspect connections or sockets.
From z/OS, run TRACERTE destination to examine the observed packet path toward the destination. IBM also documents packet-size options for investigating path behavior. A missing hop is not, by itself, proof that traffic stopped there: the available guidance describes what traceroute is for, but does not establish that every router will answer its probes.
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Check OSA-Express state when it is in the path
For an OSA-Express interface or link, DISPLAY TCPIP,,OSAINFO retrieves information from the feature. Compare relevant values with NETSTAT DEVLINKS output to see whether Communications Server and OSA-Express report consistent information.
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Check network access and IP security
If the stack and route appear correct, inspect the controls that can prevent a server from sending or receiving socket data. IBM’s server-connectivity procedure includes the network-access display command and a check of IP security rules:
DISPLAY TCPIP,,NETSTAT,ACCESS,NETWORK— inspect network access configuration and whether it affects the server.- Review the applicable IP security rules to determine whether they prevent this flow.
These checks establish whether z/OS controls could block the connection; they do not replace any policy or firewall checks required elsewhere in the particular network architecture.
Test hostname resolution from both sides
If the application connects by hostname, test the name in the environment where the client runs and on z/OS. This can expose a difference between what each side resolves, even when a connection by IP address behaves differently.
- On a distributed host, run
nslookup hostname. - On z/OS, use
TSO NSLOOKUP hostname. - If a lookup fails, investigate DNS reachability and resolver configuration for the environment that failed. In IBM’s ClearCase TSO Client guidance, adding the distributed name to the local host table is also described as a possible configuration approach in that product context.
The ClearCase instructions provide a concrete two-sided check, not a complete DNS procedure for every middleware product or z/OS installation.
Use a packet trace when command output is not enough
If logs and command output do not show where the connection stalls, collect a TCP/IP packet trace using component SYSTCPDA. Correlate the trace’s flow endpoints and timestamps with the failure time. Seeing whether requests and replies reach the z/OS host can help narrow the boundary; IBM notes that timestamps can help distinguish delay at the z/OS end from delay elsewhere on the network.
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Follow site procedures for collecting, retaining, and protecting traces. Packet traces can contain sensitive traffic metadata.
Use console NETSTAT syntax where appropriate
For console use, NETSTAT commands take the form DISPLAY TCPIP,<proc>,NETSTAT,.... For example, IBM’s diagnostic-command guidance gives DISPLAY TCPIP,tcpproc,NETSTAT,ROUTE. Replace the procedure name with the one used by the local TCP/IP stack, and confirm command syntax for the installed release.
Keep release and configuration scope in mind
The IBM server-connection procedure described here is for z/OS 2.5. IBM’s z/OS 3.2 IP Diagnosis Guide is identified as supporting IPv4 and IPv6 unless otherwise noted. Check documentation for the installed release before relying on exact syntax, behavior, or security configuration.
IBM’s z/OS Development and Test Environment networking troubleshooting guidance has a narrower scope: for a “Cannot connect” symptom in that environment, it recommends checking startup messages and consistency among device-map, VTAM, and TCP/IP definitions. Apply those checks when troubleshooting that ZD&T configuration, not as a universal procedure for every IBM Z deployment.
The checks above do not provide a universal firewall command sequence, TLS diagnosis, connection-pool analysis, or distributed-platform packet-capture procedure. Those are follow-on branches to select according to the application and network architecture once the z/OS-side evidence has narrowed the problem.
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