When any Network File System (NFS) client host/server/machine wants to access a remote file/directory then its operating system sends a request to the Network File System (NFS) server. The request basically specifies, among others, a file/directory identifier, the operation (read, write, change permission, etc.), and the identity of the user on whose behalf the operation/process is to be done.
By default, the user identity is detect with the UNIX numeric user and group policy. With this scheme, also called AUTH_UNIX, the server basically believes anything that the client sends it.
