rsync - Jun 2018 - [Bug 13463] New: Please consider using the IP_FREEBIND socket option

https://bugzilla.samba.org/show_bug.cgi?id=13463

            Bug ID: 13463
           Summary: Please consider using the IP_FREEBIND socket option
           Product: rsync
           Version: 3.1.3
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: core
          Assignee: wayned at samba.org
          Reporter: andreas at canonical.com
        QA Contact: rsync-qa at samba.org

Ubuntu got a bug report
(https://bugs.launchpad.net/ubuntu/+source/rsync/+bug/1774788) showing that
rsync can fail to start during boot if a listen address is specified in the
rsyncd.conf config file.

Since the systemd service file does not have a dependency on
network-online.target, rsync tries to bind to the specific IP address from the
"address" option and fails if it's not available.

A workaround is to add "After=network-online.target" to the systemd
service
file, but that is unnecessary for the common case where there is no specific
address configuration. It's also a bit frowned upon in upstream systemd (see
https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/).

For such cases, it is suggested to use the IP_FREEBIND socket option in
linux.
>From the linux ip(7) manpage:
IP_FREEBIND (since Linux 2.4)
If enabled, this boolean option allows binding to an IP
address that is nonlocal or does not (yet) exist. This per‐
mits listening on a socket, without requiring the underlying
network interface or the specified dynamic IP address to be up
at the time that the application is trying to bind to it.
This option is the per-socket equivalent of the ip_nonlo‐
cal_bind /proc interface described below.

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--- Comment #1 from Carson Gaspar <carson at taltos.org> ---
If this is done, please make it optional. I want my daemon to break when given
an invalid config, e.g. a typo'd IP address. The fact that systemd folks are
crazy and people don't want to have proper startup dependencies isn't a
good
reason to make the rest of us suffer unconditionally.

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--- Comment #2 from Kevin Korb <rsync at sanitarium.net> ---
I agree with Carson.  If rsync is told to do the impossible it should fail with
an appropriate error and exit code.

Unfortunately I would also have to argue that the current behaviour is wrong
because it does not exit with an error code and because I think the error
message should also go to stderr instead of just syslog.  A non-0 exit code
would allow the launching script to simply keep trying until it works.

I think even the systemd people would prefer an explicit failure over an exit 0
with no output.  A program that fails to do what it was told to do shouldn't
be
effectively the same as /bin/true.

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--- Comment #3 from Andreas Hasenack <andreas at canonical.com> ---
Thanks for all the opinions. I have one remaining issue, and that is with
"systemctl start rsync.service" not detecting the failure right away.

The systemd unit file calls rsync like this:

[Service]
ExecStart=/usr/bin/rsync --daemon --no-detach

This is correct, specially the --no-detach option. systemd should be able to
tell immediately if the service started or not, but according to the
systemd.service manpage, it will signal success in any case for type=simple
services. It's not much better for type=exec.

If I run rsync with an invalid config, it exits non-zero right away:

root at j1-rsyncd:~# rsync --daemon --no-detach ; echo $?
10


But via systemctl, it exits 0:
root at j1-rsyncd:~# systemctl start rsync; echo $?
0

root at j1-rsyncd:~# systemctl status rsync
? rsync.service - fast remote file copy program daemon
     Loaded: loaded (/lib/systemd/system/rsync.service; disabled; vendor
preset: enabled)
     Active: failed (Result: exit-code) since Wed 2022-03-30 19:10:03 UTC; 3s
ago
       Docs: man:rsync(1)
             man:rsyncd.conf(5)
    Process: 4305 ExecStart=/usr/bin/rsync --daemon --no-detach (code=exited,
status=10)
   Main PID: 4305 (code=exited, status=10)
        CPU: 3ms

Mar 30 19:10:03 j1-rsyncd rsyncd[4305]: bind() failed: Cannot assign requested
address (address-family 2)
Mar 30 19:10:03 j1-rsyncd systemd[1]: Started fast remote file copy program
daemon.
Mar 30 19:10:03 j1-rsyncd rsyncd[4305]: unable to bind any inbound sockets on
port 873
Mar 30 19:10:03 j1-rsyncd systemd[1]: rsync.service: Main process exited,
code=exited, status=10/n/a
Mar 30 19:10:03 j1-rsyncd rsyncd[4305]: rsync error: error in socket IO (code
10) at socket.c(545) [Receiver=3.2.3]
Mar 30 19:10:03 j1-rsyncd systemd[1]: rsync.service: Failed with result
'exit-code'.


This seems to be the norm for this type of systemd service (type=simple,
type=exec), and looks like the most reliable way to have systemctl start detect
immediately if the service failed or not would be to implement systemd's
notify[1] mechanism in rsync.

Type=forking might be an alternative, but this timeout would have to be tuned:

root at j1-rsyncd:~# time systemctl start rsync
Job for rsync.service failed because a timeout was exceeded.
See "systemctl status rsync.service" and "journalctl -xeu
rsync.service" for
details.

real    1m30.246s


With TimeoutStartSec=5 in the unit file:

root at j1-rsyncd:~# time systemctl start rsync
Job for rsync.service failed because a timeout was exceeded.
See "systemctl status rsync.service" and "journalctl -xeu
rsync.service" for
details.

real    0m5.287s




1. https://www.freedesktop.org/software/systemd/man/sd_notify.html

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--- Comment #4 from Simon Deziel <simon at sdeziel.info> ---
Since rsyncd exits with error code 10 ("Error in socket I/O") there
are two
possible ways to improve the systemd unit:

[Service]
...
RestartForceExitStatus=10

Or:

[Service]
...
Restart=on-failure


Both should help with late showing IPv6 address due to DAD taking time.

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--- Comment #5 from Simon Deziel <simon at sdeziel.info> ---
The `Restart=on-failure` option was added in
https://github.com/WayneD/rsync/commit/d41bb98c09bf0b999c4eee4e2125c7e5d0747ec4

This should paper over the problem of late showing IPv6 addresses due to DAD
taking time.

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