Thanks, unfortunately that doesn't get round the problem. Vsftpd (for example) is configured for anonymous access out of the box on Ubuntu, but the mainframe doesn't want to access anonymously, it insists on sending a username! What the people who designed the system were thinking I don't know.
I have managed to find a way round the problem, which is not optimal in a number of ways, but I thought it might be of interest to some so I'll explain what I did. The key was using a Java based ftp server called CrossFTP server
. It has a pretty straightforward GUI configuration system, that allows virtual users with silly passwords like "PASS". The complication with it is that to get it to bind to port 21 it has to be run as root, but it's designed to be run through Java web start, which should be run as a regular user.
I really, really
don't suggest anyone does this for a site on an untrusted network or for a proper FTP site, but I launched it as root using the command
user@local:~$ sudo javaws http://www.crossftp.com/crossftpserver.jnlp
Once it was launched it was simple to add in the FTP users the mainframe wanted to use, complete with weak passwords, and give them write access to a directory created just for receiving this data.
This doesn't answer the original question, but it enabled me to get things working. Of course, it requires root access on the local machine, and requires Java to be installed (I used the Ubuntu standard release of Sun Java 6, haven't tested any others), but I can live with that. Hope it helps other people!