(This is a guest post by Antoni Sawicki aka tenox)

TL;DR
RCPD plays nicely with NAT, re-implemented as a stand alone daemon, from scratch, without ined, r-daemons. Fully open, no security or authentication. Docker friendly.

I often find myself needing to copy a file or two, in or out of an emulated system with some weird networking setup like qemu net user, SLiRP, NAT, etc. Or from/to a physical machine, but having server running in a Docker container, sometimes on a Mac, which has even weirder network contraptions.

Most of the old file transfer protocols like FTP have tendency to open a reverse connection from the server back to the client, which doesn’t play very well with NAT. Even TFTP (who would have thought!) requires nf_conntrack, STUN, proxies, connection helpers, punching holes, UPnP and other nonsense. While they can be individually worked out, it’s rather not universal. Vintage OS SSH/SCP is either non existent or ciphers are too old to play well with modern SSH server. NFS is mostly OK but it has it’s own quirks and you can’t run it in a Docker container. I seen people doing shit like this. I’m also guilty of using Kermit to transfer files over network from time to time. Overall quite frustrating situation.

It turns out that the rcp protocol is very easy on NAT, as it uses just a single, outgoing TCP connection. Not only it worked in all my use cases, but also Berkeley r-commands are ubiquitous on pretty much all vintage OSes that I work with. From Unix, VMS to Windows NT. There are ports to OS/2, DOS and everything else. A recent practical example was how Neozeed and I copied a VBScript in to Windows 2000 64bit install.

The server side however is somewhat of a challenge. It has been obsolete for quite some time now. For starters, R-daemons require inetd, which is mostly not a thing on modern Unix. Then there are actual rshd/rexec/rlogin/rcpd daemons. I spent several hours trying to hack netkit-rsh and rsh-redone to play nicely in Docker, without good results. Even if it could be done, there still are /etc/hosts.equiv, ~/.rhosts and other bullshit “security” contraptions to deal with.

Annoyed with all this nonsense decided to write RCPD from scratch. Docker friendly, fully open, no security or authentication of any kind. I also didn’t want remote shell (rsh/rlogin/rexec), just the file copy part. The original rcpd was using rshd under the hood. This is now completely eliminated and the shell stuff is synthetically generated to satisfy the protocol.

Now if I want to copy a file in to a vintage system I can simply do this:

That’s it. It just works™ with net user and Docker on a Mac vmnet.

The best part that the whole thing has been written and debugged entirely by Claude!

Available from https://github.com/tenox7/rcpd