i've try the command line : s6-tcpserver -v -4 0.0.0.0 2121
stupid-ftpd has no inetd mode. It binds and listens to its socket
in other words, it's not suitable to run under s6-tcpserver.
s6-tcpserver only works with servers that do not listen to the network
themselves. For FTP, you have a choice - wuftpd, vsftpd, busybox ftpd,
name a few, all have an inetd mode and can work under s6-tcpserver.
But stupid-ftpd is not one of them.
Sorry for my english i'am french.
C'est pas une excuse. ;)