SAP Note 25917 - Changes to /etc/hosts are not accepted.

Symptom:

Changes to the operating system tables hosts or services are only effective after restarting R/3.

Cause and prerequisites

For performance reasons, the SAP network interface (NI) buffers the host and service names. The system only calls the host and service names once to translate host names to IP addresses or service names to port numbers (and vice versa).
The NI level does have calls to flush these buffers, but this function is not yet used in all R/3 components.

Solution

prior to R/3 Release 3.1G:
The corresponding application server must be restarted.
To avoid such situations you should make sure when adding new hosts that you do not access the new name before it is known by the operating system.

as of R/3 Release 3.1G:
As of R/3 Release 3. 1G you have the option of resetting these buffers when the system is in operation. Use Transaction SM51: "Goto ==> Host name buffer ==> Reset".

Additional key words

host name, service name, hosts table, services table, buffer