This is a nice feature if you want to use QEMU to debug a NuttX board and learn more about its initialization:
$ qemu-system-arm -net nic,model=stellaris -net user,hostfwd=tcp:127.0.0.1:10023-10.0.2.15:23,hostfwd=tcp:127.0.0.1:10021-10.0.2.15:21 -M lm3s6965evb -kernel nuttx -nographic -s -S
$ arm-none-eabi-gdb-py -i=mi -ix=./tools/nuttx-gdbinit nuttx
(gdb) target extended-remote:1234
Note: 10.0.2.15 is the internal IP of the emulated lm3s6965evb ethernet on QEMU. The hostfwd command just forward the access to the localhost port 10023 to 23 (telnet) of the emulated board. Same to port 10021 and 21 (ftp).