sql*plus 11.2 client connection to oracle timing out on Linux

One of our DBAs is seeing connection issues with his sql*plus client. Whenever he connects to the Oracle v10g server, the connection seems to time out for no apparent reason.

We tried checking iptables but we have no state-based rules.

We also have older servers on the same switch so we know it isn't a switch issue. My hypothesis: It's a client issue.

This is kind of a shot in the dark but can anyone suggest some troubleshooting steps to help out? I'll update the question with whatever details you may need to make a recommendation. Just ask in comments.

Update

Per comments:

• I tested telneting to the Oracle DB server to port 1521. That worked.

• Also, the initial test query works. However, after an hour, when I try the query again I get ORA-03135: connection lost contact. Full error below.

ERROR at line 1:
ORA-03135: connection lost contact
Process ID: 14848
Session ID: 1219 Serial number: 1551

I had a similar problem after a firewall upgrade. Users could connect to the database just fine, but if their session was idle for some time, the connection would be terminated by firewall. Our solution was to add

SQLNET.EXPIRE_TIME = 10

to sqlnet.ora file. This keeps connections alive by probing them in 10 minute intervals.

StackExchange Database Administrators Q#28574, answer score: 2