How to run multiple networked applications concurrently on DOS?

Question

For science, I am working on a Vagrant box for DOS. The virtual machine needs to support a remote command execution service, such as sshd or telnetd, so that administrators can run COMMAND.COM scripts and one liners in an automated manner. And the virtual machine should run an FTP server as well, so that administrators can copy files to and from the host.

It's not easy, but it turns out that FreeDOS has a community of retro programmers, who have kindly supplied a kind of telnet server (rmenu) as well as an FTP server (FTPSRV from mTCP). Other common tools like an rsync, wget, and curl have varying degrees of support for DOS.

Both rmenu and FTPSRV appear to work, but only if one of these is run directly from the main COMMAND.COM context. When FTPSRV, or other network applications like wget are run via rmenu, then they are not able to make successful network connections. Is this a particular limitation with rmenu, or is it difficult for DOS to bind multiple applications to ports at once?

Currently using FDNET to connect the guest to the VirtualBox network, in case that matters.

Answer 1

How to run multiple networked applications concurrently on DOS?

You cannot.
Wikipedia explains it simply:

DOS is a single-user, single-tasking operating system with basic kernel functions that are non-reentrant: only one program at a time can use them and DOS itself has no functionality to allow more than one program to execute at a time.

However

DOS is not a multitasking operating system. DOS did however provide a Terminate and Stay Resident (TSR) function which allowed programs to remain resident in memory.

So if the "networked application" was written as a TSR, then you could achieve some level of concurrency.