[Bug 13463] New: Please consider using the IP_FREEBIND socket option

[samba-bugs at samba.org]

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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