The built in NFS server allows you to create and modify fileshares to NFS clients (mainly Un*x like systems). Network File System (NFS) is a distributed file system protocol originally developed by Sun Microsystems which allows users on a client computer to access files over a computer network, similar to the way local storage is accessed.
It's essentially the equivalent of samba in a Linux environment.
Enable legacy (NFS V2) support - NFS currently has 3 versions - NFS 2/3/4. FreshTomato defaults to v3, but it can also be set to “speak” v2 also.
IP Address/Subnet - Limits connectivity to the IP address/subnet you specify in this field.
Access - Defines the share permissions (how the share can be accessed)
Sync/Async - when set to Sync (Synchronous), the FreshTomato will only acknowledge data after it's been written out. The Async(hronous) setting configures for the opposite behaviour. The NFS server will acknowledge data before it's committed to disk, which can lead to data corruption if the server crashes.
Subtree_check - This option disables subtree checking, which has mild security implications, but can improve reliability in some circumstances. In NFS, if a subdirectory of a filesystem is exported, but the whole filesystem isn't, then whenever a NFS request arrives, the server must check not only that the accessed file is in the appropriate filesystem (which is easy) but also that it is in the exported tree (which is harder). This check is called the subtree_check.
In order to perform this check, the server must include some information about the location of the file in the “filehandle” that is given to the client. This can cause problems with accessing files that are renamed while a client has them open (though in many simple cases it will still work).
Other_options - Here you can specify advanced options. Please refer to the official NFS documentation for additional details.
NFS man(ual) page