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