CALLBACK Option

You can use the CALLBACK option in LOGNAME entries to specify that no transaction takes place until the calling system is called back. There are two reasons to set up CALLBACK: For security purposes, if you call back a machine, you can be sure it is the right machine. For accounting purposes, if you are doing long data transmissions, you can choose the machine that is billed for the longer call.

The string CALLBACK=yes specifies that your computer must call the remote computer back before any file transfers can take place.

The default for the CALLBACK option is CALLBACK=no. If you set CALLBACK to yes, then the permissions that affect the rest of the conversation must be specified in the MACHINE entry corresponding to the caller. Do not specify these permissions in the LOGNAME, or in the LOGNAME entry that the remote machine might have set for your host.

If two sites have the CALLBACK option set for each other, a conversation never gets started.