rsync daemon behaving eratically - linux

Question

I'm running an rsync daemon (providing a mirror for the SaneSecurity signatures).
rsync is started like this (from runit):
/usr/bin/rsync -v --daemon --no-detach
And the config contains:
use chroot = no
munge symlinks = no
max connections = 200
timeout = 30
syslog facility = local5
transfer logging = no
log file = /var/log/rsync.log
reverse lookup = no
[sanesecurity]
comment = SaneSecurity ClamAV Mirror
path = /srv/mirror/sanesecurity
read only = yes
list = no
uid = nobody
gid = nogroup
But what I'm seeing is a lot of "lingering" rsync processes:
# ps auxwww|grep rsync
root 423 0.0 0.0 4244 1140 ? Ss Oct30 0:00 runsv rsync
root 2529 0.0 0.0 11156 2196 ? S 15:00 0:00 /usr/bin/rsync -v --daemon --no-detach
nobody 4788 0.0 0.0 20536 2860 ? S 15:10 0:00 /usr/bin/rsync -v --daemon --no-detach
nobody 5094 0.0 0.0 19604 2448 ? S 15:13 0:00 /usr/bin/rsync -v --daemon --no-detach
root 5304 0.0 0.0 11156 180 ? S 15:15 0:00 /usr/bin/rsync -v --daemon --no-detach
root 5435 0.0 0.0 11156 180 ? S 15:16 0:00 /usr/bin/rsync -v --daemon --no-detach
root 5797 0.0 0.0 11156 180 ? S 15:19 0:00 /usr/bin/rsync -v --daemon --no-detach
nobody 5913 0.0 0.0 20536 2860 ? S 15:20 0:00 /usr/bin/rsync -v --daemon --no-detach
nobody 6032 0.0 0.0 20536 2860 ? S 15:21 0:00 /usr/bin/rsync -v --daemon --no-detach
root 6207 0.0 0.0 11156 180 ? S 15:22 0:00 /usr/bin/rsync -v --daemon --no-detach
nobody 6292 0.0 0.0 20544 2744 ? S 15:23 0:00 /usr/bin/rsync -v --daemon --no-detach
root 6467 0.0 0.0 11156 180 ? S 15:25 0:00 /usr/bin/rsync -v --daemon --no-detach
root 6905 0.0 0.0 11156 180 ? S 15:29 0:00 /usr/bin/rsync -v --daemon --no-detach
(it's currently 15:30)
So there's processes (not even having dropped privileges!) hanging around since 15:10, 15:13 and the like.
And what are they doing?
Let's check:
# strace -p 5304
strace: Process 5304 attached
select(4, [3], NULL, [3], {25, 19185}^C
strace: Process 5304 detached
<detached ...>
# strace -p 5797
strace: Process 5797 attached
select(4, [3], NULL, [3], {48, 634487}^C
strace: Process 5797 detached
<detached ...>
This happended with both rsync from Ubuntu Xenial as well as installed from PPA (currently using rsync 3.1.2-1~ubuntu16.04.1york0 )

Answer 1

One process is created for each connection. Before a client selects the module the process does not know if it should drop privileges.
You can easily create such a process.
nc $host 873
You will notice that the connection will not be closed after 30s because the timeout is just a disk i/o timeout. The rsync client have a --contimeout option, but it seems that a server side option is missing.

Answer 2

In the end, I resorted to invoking rsync from (x)inetd instead of running it standalone.
service rsync
{
disable = no
socket_type = stream
wait = no
user = root
server = /usr/bin/timeout
server_args = -k 60s 60s /usr/bin/rsync --daemon
log_on_failure += USERID
flags = IPv6
}
As an additional twist, I wrapped the rsync invocation with timeout, adding another safeguard against long-running processes.