We are configuring a Linux backup server at work that all of our other servers (also mostly Linux) will send backups to. The backups are hundreds of gigabytes in size, and have thousands of files, 99% of which don't change from day to day.
Normally, this is the perfect use case for rsync. Unfortunately, one of our requirements is that the backup server be write-only, so that servers can upload backups to it, but nothing can have access to those backups remotely, in case any of those servers get compromised. We currently have vsftpd setup to work in write-only mode, which works great, except for the fact that hundreds of gigabytes of data needs to go through the network every night.
As far as I know, because rsync does a comparison of the remote files with local files, it needs both read and write access on the remote server.
My question is this: is there any mode that rsync can run in where the backup server can reveal the checksums, and filenames of the files on it, but not the actual data? That means it would still be a write-only server, but we could do differential backups instead of full backups.