Quite a few of my rsync jobs failed after I upgraded from FreeNAS-11.3-U3 to FreeNAS-11.3-U5. I tracked down the problem to differing rsync versions. FreeNAS rsync is 3.1.3 and I'm able to rsync to systems that have 3.1. I'm unable to rsync to systems that have rsync 3.0 installed. It seems pretty unusual that a minor version upgrade would break rsync.
The log shows
opening tcp connection to moby port 873
Connected to moby (129.59.XX.XX)
sending daemon args: --server --sender -vvvlogDtpre.iLsfxC . etc/ (5 args)
@Error: access denied to etc from hadal (10.2.XX.XX)
rsync error: error starting client-server protocol (code 5) at main.c(1671) [Receiver=3.1.3]
[Receiver] _exit_cleanup(code=5, file=main.c, line=1671): about to call exit(5)
The log shows
opening tcp connection to moby port 873
Connected to moby (129.59.XX.XX)
sending daemon args: --server --sender -vvvlogDtpre.iLsfxC . etc/ (5 args)
@Error: access denied to etc from hadal (10.2.XX.XX)
rsync error: error starting client-server protocol (code 5) at main.c(1671) [Receiver=3.1.3]
[Receiver] _exit_cleanup(code=5, file=main.c, line=1671): about to call exit(5)