when you want to fuzz a network service and you can not/do not want to modify the source (or just have a binary), then this LD_PRELOAD library will allow for sending input to stdin which the target binary will think is coming from a network socket.

This is desock_dup.c from the amazing preeny project https://github.com/zardus/preeny

It is packaged in AFL++ to have it at hand if needed