Ubuntu Manpage: syncthing-faq - Frequently Asked Questions

Provided by: syncthing_1.27.2~ds4-1ubuntu0.24.04.3_amd64

NAME

       syncthing-faq - Frequently Asked Questions

       • General

         • What is Syncthing?

         • Is it “syncthing”, “Syncthing” or “SyncThing”?

         • What things are synced?

         • Is synchronization fast?

         • How does Syncthing differ from BitTorrent/Resilio Sync?

         • Is there an iOS client?

         • Should I keep my device IDs secret?

       • Troubleshooting

         • Where are the Syncthing logs?

         • Why is the sync so slow?

         • Why does it use so much CPU?

         • Why is the setup more complicated than BitTorrent/Resilio Sync?

         • Why do I get “Host check error” in the GUI/API?

         • My Syncthing database is corrupt

         • Why do I see Syncthing twice in task manager?

         • How can I view the history of changes?

         • Does the audit log contain every change?

         • Why does Syncthing connect to this unknown/suspicious address?

         • I am seeing the error message “folder marker missing”. What do I do?

       • Usage

         • What if there is a conflict?

         • How do I serve a folder from a read only filesystem?

         • I really hate the .stfolder directory, can I remove it?

         • Am I able to nest shared folders in Syncthing?

         • How do I rename/move a synced folder?

         • How do I configure multiple users on a single machine?

         • Does Syncthing support syncing between folders on the same system?

         • When  I do have two distinct Syncthing-managed folders on two hosts, how does Syncthing handle moving
           files between them?

         • Can I help initial sync by copying files manually?

         • Is Syncthing my ideal backup application?

         • How can I exclude files with brackets ([]) in the name?

         • How do I access the web GUI from another computer?

         • I don’t like the GUI or the theme. Can it be changed?

         • How do I upgrade Syncthing?

         • Where do I find the latest release?

         • How do I run Syncthing as a daemon process on Linux?

         • How do I increase the inotify limit to get my filesystem watcher to work?

         • How do I reset the GUI password?

GENERAL

What is Syncthing?
Syncthing is an application that lets you synchronize your files across multiple devices. This means the
creation, modification or deletion of files on one machine will automatically be replicated to your other
devices. We believe your data is your data alone and you deserve to choose where it is stored. Therefore
Syncthing does not upload your data to the cloud but exchanges your data across your machines as soon as
they are online at the same time.

Is it “syncthing”, “Syncthing” or “SyncThing”?
It’s Syncthing, although the command and source repository is spelled syncthing so it may be referred to
in that way as well. It’s definitely not SyncThing, even though the abbreviation st is used in some
circumstances and file names.

What things are synced?
The following things are always synchronized:

• File contents

• File modification times

The following may be synchronized or not, depending:

• File permissions (when supported by file system; on Windows only the read only bit is synchronized)

• Symbolic links (synced, except on Windows, but never followed)

• File or directory owners and groups (when enabled)

• Extended attributes (when enabled)

• POSIX or NFS ACLs (as part of extended attributes)

The following are not synchronized;

• Directory modification times (not preserved)

• Hard links (followed, not preserved)

• Windows junctions (synced as ordinary directories; require enabling in the configuration on a
per-folder basis)

• Resource forks (not preserved)

• Windows ACLs (not preserved)

• Devices, FIFOs, and other specials (ignored)

• Sparse file sparseness (will become sparse, when supported by the OS & filesystem)

• Syncthing internal files and folders (e.g. .stfolder, .stignore, .stversions, temporary files, etc.)

Is synchronization fast?
Syncthing segments files into pieces, called blocks, to transfer data from one device to another.
Therefore, multiple devices can share the synchronization load, in a similar way to the torrent protocol.
The more devices you have online, the faster an additional device will receive the data because small
blocks will be fetched from all devices in parallel.

Syncthing handles renaming files and updating their metadata in an efficient manner. This means that
renaming a file will not cause a retransmission of that file. Additionally, appending data to existing
files should be handled efficiently as well.

Temporary files are used to store partial data downloaded from other devices. They are automatically
removed whenever a file transfer has been completed or after the configured amount of time which is set
in the configuration file (24 hours by default).

How does Syncthing differ from BitTorrent/Resilio Sync?
The two are different and not related. Syncthing and BitTorrent/Resilio Sync accomplish some of the same
things, namely syncing files between two or more computers.

BitTorrent Sync, now called Resilio Sync, is a proprietary peer-to-peer file synchronization tool
available for Windows, Mac, Linux, Android, iOS, Windows Phone, Amazon Kindle Fire and BSD. [1] Syncthing
is an open source file synchronization tool.

Syncthing uses an open and documented protocol, and likewise the security mechanisms in use are well
defined and visible in the source code. Resilio Sync uses an undocumented, closed protocol with unknown
security properties.

[1] https://en.wikipedia.org/wiki/Resilio_Sync

Is there an iOS client?
There are no plans by the current Syncthing team to officially support iOS in the foreseeable future.

iOS has significant restrictions on background processing that make it very hard to run Syncthing
reliably and integrate it into the system.

However, there is a commercial packaging of Syncthing for iOS that attempts to work within these
limitations. [2]

[2] https://www.mobiussync.com

Should I keep my device IDs secret?
No. The IDs are not sensitive. Given a device ID it’s possible to find the IP address for that device, if
global discovery is enabled on it. Knowing the device ID doesn’t help you actually establish a connection
to that device or get a list of files, etc.

For a connection to be established, both devices need to know about the other’s device ID. It’s not
possible (in practice) to forge a device ID. (To forge a device ID you need to create a TLS certificate
with that specific SHA-256 hash. If you can do that, you can spoof any TLS certificate. The world is
your oyster!)

TROUBLESHOOTING

Where are the Syncthing logs?
Syncthing logs to stdout by default. On Windows Syncthing by default also creates syncthing.log in
Syncthing’s home directory (run syncthing --paths to see where that is). The command line option
--logfile can be used to specify a user-defined logfile. If you only have access to a running instance’s
GUI, check under the Actions - About menu item to see the used paths.

If you’re running a process manager like systemd, check there. If you’re using a GUI wrapper integration,
it may keep the logs for you.

Why is the sync so slow?
When troubleshooting a slow sync, there are a number of things to check.

First of all, verify that you are not connected via a relay. In the “Remote Devices” list on the right
side of the GUI, double check that you see “Address: <some address>” and not “Relay: <some address>”.
[image]

If you are connected via a relay, this is because a direct connection could not be established. Double
check and follow the suggestions in Firewall Setup to enable direct connections.

Second, if one of the devices is a very low powered machine (a Raspberry Pi, or a phone, or a NAS, or
similar) you are likely constrained by the CPU on that device. See the next question for reasons
Syncthing likes a faster CPU.

Third, verify that the network connection is OK. Tools such as iperf or just an Internet speed test can
be used to verify the performance here.

Why does it use so much CPU?
1. When new or changed files are detected, or Syncthing starts for the first time, your files are hashed
using SHA-256.

2. Data that is sent over the network is compressed (optionally) and encrypted (always). When receiving
data it must be decrypted and then (if compressed) decompressed.

3. There is a certain amount of housekeeping that must be done to track the current and available
versions of each file in the index database.

4. By default Syncthing uses periodic scanning every hour when watching for changes or every minute if
that’s disabled to detect file changes. This means checking every file’s modification time and
comparing it to the database. This can cause spikes of CPU usage for large folders.

Hashing, compression and encryption cost CPU time. Also, using the GUI causes a certain amount of extra
CPU usage to calculate the summary data it presents. Note however that once things are in sync CPU usage
should be negligible.

To minimize the impact of this, Syncthing attempts to lower the process priority when starting up.

To further limit the amount of CPU used when syncing and scanning, set the environment variable
GOMAXPROCS to the maximum number of CPU cores Syncthing should use at any given moment. For example,
GOMAXPROCS=2 on a machine with four cores will limit Syncthing to no more than half the system’s CPU
power.

Why is the setup more complicated than BitTorrent/Resilio Sync?
Security over convenience. In Syncthing you have to setup both sides to connect two devices. An attacker
can’t do much with a stolen device ID, because you have to add the device on the other side too. You have
better control where your files are transferred.

This is an area that we are working to improve in the long term.

Why do I get “Host check error” in the GUI/API?
Since version 0.14.6 Syncthing does an extra security check when the GUI/API is bound to localhost -
namely that the browser is talking to localhost. This protects against most forms of DNS rebinding
attack <https://en.wikipedia.org/wiki/DNS_rebinding> against the GUI.

To pass this test, ensure that you are accessing the GUI using an URL that begins with http://localhost,
http://127.0.0.1 or http://[::1]. HTTPS is fine too, of course.

If you are using a proxy in front of Syncthing you may need to disable this check, after ensuring that
the proxy provides sufficient authentication to protect against unauthorized access. Either:

• Make sure the proxy sets a Host header containing localhost, or

• Set gui.insecureSkipHostcheck in the advanced settings, or

• Bind the GUI/API to a non-localhost listen port.

In all cases, username/password authentication and HTTPS should be used.

My Syncthing database is corrupt
This is almost always a result of bad RAM, storage device or other hardware. When the index database is
found to be corrupt Syncthing cannot operate and will note this in the logs and exit. To overcome this
delete the database folder inside Syncthing’s data directory and re-start Syncthing. It will then need to
perform a full re-hashing of all shared folders. You should check your system in case the underlying
cause is indeed faulty hardware which may put the system at risk of further data loss.

Why do I see Syncthing twice in task manager?
One process manages the other, to capture logs and manage restarts. This makes it easier to handle
upgrades from within Syncthing itself, and also ensures that we get a nice log file to help us narrow
down the cause for crashes and other bugs.

How can I view the history of changes?
The web GUI contains a Recent Changes button under the device list which displays changes since the last
(re)start of Syncthing. With the --audit option you can enable a persistent, detailed log of changes and
most activities, which contains a JSON-formatted sequence of events in the
~/.local/state/syncthing/audit-_date_-_time_.log file.

Does the audit log contain every change?
The audit log (and the Recent Changes window) sees the changes that your Syncthing sees. When Syncthing
is continuously connected it usually sees every change happening immediately and thus knows which node
initiated the change. When topology gets complex or when your node reconnects after some time offline,
Syncthing synchronises with its neighbours: It gets the latest synchronised state from the neighbour,
which is the result of all the changes between the last known state (before disconnect or network delay)
and the current state at the neighbour, and if there were updates, deletes, creates, conflicts, which
were overlapping we only see the latest change for a given file or directory (and the node where that
latest change occurred). When we connect to multiple neighbours Syncthing decides which neighbour has the
latest state, or if the states conflict it initiates the conflict resolution procedure, which in the end
results in a consistent up-to-date state with all the neighbours.

Why does Syncthing connect to this unknown/suspicious address?
If you see outgoing connections to odd and unexpected addresses these are most likely connections to
relay servers. Relay servers are run by volunteers all over the world. They usually listen on ports 443
or 22067, though this is controlled by the user running it. You can compare the address you are concerned
about with the current list of active relays <https://relays.syncthing.net>. Relays do not and can not
see the data transmitted via them.

I am seeing the error message “folder marker missing”. What do I do?
Syncthing uses a specific marker usually called .stfolder to determine whether a folder is healthy. This
is a safety check to ensure that your folder is properly readable and present on disk. For example, if
you remove a USB drive from your computer or unmount a filesystem, then syncthing must know whether you
have really deleted all of your files. Therefore, syncthing always checks that the .stfolder is present.

When this error appears, syncthing assumes that the folder has encountered some type of error and will
stop syncing it until the .stfolder reappears. Once that happens, all changes made to the folder locally
will be synced (i.e. missing files will be considered deletions).

• If you get this error message, check the folder in question on your storage. If you have unmounted the
folder (or a parent of it), you must remount it for syncthing to resume syncing this folder.

• If you have moved the folder, you must either move it back to its original location, or remove the
folder from within the syncthing UI and re-add it at its new location.

• If the folder is present on disk, with all of its children files and directories, but the .stfolder is
still missing:

It is possible that a file cleaning software has removed the .stfolder. Some software removes empty
folders, and the .stfolder is often empty. This happens particularly often on Android. To remediate,
recreate the .stfolder and add a dummy file in it, or add an exception to your cleaning software.

If you are still unsure what has happened, you can remove the folder from within the syncthing UI and
re-add it at the same location. This causes syncthing to attempt an automatic re-creation of the
.stfolder. Next, it will also reset the database state of this folder. It will be considered a “new”
folder, meaning that its files will be merged with files from remote devices.

Also see the marker FAQ for more information about the folder marker.

USAGE

What if there is a conflict?
SEE ALSO:
Conflicting Changes

How do I serve a folder from a read only filesystem?
Syncthing requires a “folder marker” to indicate that the folder is present and healthy. By default this
is a directory called .stfolder that is created by Syncthing when the folder is added. If this folder
can’t be created (you are serving files from a CD or something) you can instead set the advanced config
Marker Name to the name of some file or folder that you know will always exist in the folder.

I really hate the .stfolder directory, can I remove it?
See the previous question.

Am I able to nest shared folders in Syncthing?
Sharing a folder that is within an already shared folder is possible, but it has its caveats. What you
must absolutely avoid are circular shares. This is just one example, there may be other undesired
effects. Nesting shared folders is not supported, recommended or coded for, but it can be done
successfully when you know what you’re doing - you have been warned.

How do I rename/move a synced folder?
Syncthing doesn’t have a direct way to do this, as it’s potentially dangerous to do so if you’re not
careful - it may result in data loss if something goes wrong during the move and is synchronized to your
other devices.

The easy way to rename or move a synced folder on the local system is to remove the folder in the
Syncthing UI, move it on disk, then re-add it using the new path.

It’s important to do this when the folder is already in sync between your devices, as it is otherwise
unpredictable which changes will “win” after the move. Changes made on other devices may be overwritten,
or changes made locally may be overwritten by those on other devices.

An alternative way is to shut down Syncthing, move the folder on disk (including the .stfolder marker),
edit the path directly in config.xml in the configuration folder (see Syncthing Configuration) and then
start Syncthing again.

How do I configure multiple users on a single machine?
Each user should run their own Syncthing instance. Be aware that you might need to configure listening
ports such that they do not overlap (see Syncthing Configuration).

Does Syncthing support syncing between folders on the same system?
No. Syncthing is not designed to sync locally and the overhead involved in doing so using Syncthing’s
method would be wasteful. There are better programs to achieve this such as rsync
<https://rsync.samba.org/> or Unison <https://www.cis.upenn.edu/~bcpierce/unison>.

When I do have two distinct Syncthing-managed folders on two hosts, how does Syncthing handle moving files
between them?
Syncthing does not specially handle this case, and most files will most likely get re-downloaded.

In detail, the behavior depends on the scan order. If you have folders A and B, and move files from A to
B, if A gets scanned first, it will announce the removal of the files to others who will then remove the
files. As you rescan B, B will announce the addition of new files, and other peers will have nowhere to
get them from apart from re-downloading them.

If B gets rescanned first, B will announce additions first, and remote peers will then reconstruct the
files (not rename, more like copying block by block) from A, and then as A gets rescanned, it will remove
the files from A.

A workaround would be to copy first from A to B, rescan B, wait for B to copy the files on the remote
side, and then delete from A.

Can I help initial sync by copying files manually?
If you have a large folder that you want to keep in sync over a not-so-fast network, and you have the
possibility to move all files to the remote device in a faster manner, here is a procedure to follow:

• Create the folder on the local device, but don’t share it with the remote device yet.

• Copy the files from the local device to the remote device using regular file copy. If this takes a long
time (perhaps requiring travelling there physically), it may be a good idea to make sure that the files
on the local device are not updated while you are doing this.

• Create the folder on the remote device, and copy the Folder ID from the folder on the local device, as
we want the folders to be considered the same. Then wait until scanning the folder is done.

• Now share the folder with the other device, on both sides. Syncthing will exchange file information,
updating the database, but existing files will not be transferred. This may still take a while
initially, be patient and wait until it settled.

Is Syncthing my ideal backup application?
No. Syncthing is not a great backup application because all changes to your files (modifications,
deletions, etc.) will be propagated to all your devices. You can enable versioning, but we encourage you
to use other tools to keep your data safe from your (or our) mistakes.

How can I exclude files with brackets ([]) in the name?
The patterns in .stignore are glob patterns, where brackets are used to denote character ranges. That is,
the pattern q[abc]x will match the files qax, qbx and qcx.

To match an actual file called q[abc]x the pattern needs to “escape” the brackets, like so: q\[abc\]x.

On Windows, escaping special characters is not supported as the \ character is used as a path separator.

How do I access the web GUI from another computer?
The default listening address is 127.0.0.1:8384, so you can only access the GUI from the same machine.
This is for security reasons. To access it from another computer, change the GUI listen address option
in the web GUI from 127.0.0.1:8384 to 0.0.0.0:8384, or change the config.xml:

Then the GUI is accessible from everywhere. There is no filtering based on e.g. source address (use a
firewall for that). You should set a password and enable HTTPS with this configuration. You can do this
from inside the GUI.

If both your computers are Unix-like (Linux, Mac, etc.) you can also leave the GUI settings at default
and use an SSH port forward to access it. For example,

$ ssh -L 9090:127.0.0.1:8384 user@othercomputer.example.com

will log you into othercomputer.example.com, and present the remote Syncthing GUI on
http://localhost:9090 on your local computer.

If you only want to access the remote GUI and don’t want the terminal session, use this example:

$ ssh -N -L 9090:127.0.0.1:8384 user@othercomputer.example.com

If only your remote computer is Unix-like, you can still access it with SSH from Windows. Under Windows
10 or later you can use the same ssh command if you install the OpenSSH Client
<https://learn.microsoft.com/windows-server/administration/openssh/openssh_install_firstuse>.

I don’t like the GUI or the theme. Can it be changed?
You can change the theme in the settings. Syncthing ships with other themes than the default.

If you want a custom theme or a completely different GUI, you can add your own. By default, Syncthing
will look for a directory gui inside the Syncthing home folder. To change the directory to look for
themes, you need to set the STGUIASSETS environment variable. To get the concrete directory, run
syncthing with the --paths parameter. It will print all the relevant paths, including the “GUI override
directory”.

To add e.g. a red theme, you can create the file red/assets/css/theme.css inside the GUI override
directory to override the default CSS styles.

To create a whole new GUI, you should checkout the files at
https://github.com/syncthing/syncthing/tree/main/gui/default to get an idea how to do that.

How do I upgrade Syncthing?
If you use a package manager such as Debian’s apt-get, you should upgrade using the package manager. If
you use the binary packages linked from Syncthing.net, you can use Syncthing’s built-in automatic upgrade
functionality.

• If automatic upgrades is enabled (which is the default), Syncthing will upgrade itself automatically
within 24 hours of a new release.

• The upgrade button appears in the web GUI when a new version has been released. Pressing it will
perform an upgrade.

• To force an upgrade from the command line, run syncthing --upgrade.

Note that your system should have CA certificates installed which allows a secure connection to GitHub
(e.g. FreeBSD requires sudo pkg install ca_root_nss). If curl or wget works with normal HTTPS sites, then
so should Syncthing.

Where do I find the latest release?
We release new versions through GitHub. The latest release is always found on the release page
<https://github.com/syncthing/syncthing/releases/latest>. Unfortunately GitHub does not provide a single
URL to automatically download the latest version. We suggest to use the GitHub API
<https://api.github.com/repos/syncthing/syncthing/releases/latest> and parsing the JSON response.

How do I run Syncthing as a daemon process on Linux?
If you’re using systemd, runit, or upstart, we ship example configurations
<https://github.com/syncthing/syncthing/tree/main/etc>.

If however you’re not using one of these tools, you have a couple of options. If your system has a tool
called start-stop-daemon installed (that’s the name of the command, not the package), look into the local
documentation for that, it will almost certainly cover 100% of what you want to do. If you don’t have
start-stop-daemon, there are a bunch of other software packages you could use to do this. The most well
known is called daemontools, and can be found in the standard package repositories for almost every
modern Linux distribution. Other popular tools with similar functionality include S6 and the
aforementioned runit.

How do I increase the inotify limit to get my filesystem watcher to work?
You are probably reading this because you encountered the following error with the filesystem watcher on
linux:
Failed to start filesystem watcher for folder yourLabel (yourID): failed to setup inotify handler.
Please increase inotify limits, see https://docs.syncthing.net/users/faq.html#inotify-limits

Linux typically restricts the amount of watches per user (usually 8192). When you have more directories
you need to adjust that number.

On many Linux distributions you can run the following to fix it:

echo "fs.inotify.max_user_watches=204800" | sudo tee -a /etc/sysctl.conf

On Arch Linux and potentially others it is preferred to write this line into a separate file, i.e. you
should run:

echo "fs.inotify.max_user_watches=204800" | sudo tee -a /etc/sysctl.d/90-override.conf

This only takes effect after a reboot. To adjust the limit immediately, run:

echo 204800 | sudo tee /proc/sys/fs/inotify/max_user_watches

How do I reset the GUI password?
If you’ve forgotten / lost the GUI password, you can reset it using the --gui-password (and possibly
--gui-user) options to the syncthing generate subcommand. This should be done while Syncthing is not
running.

1. Stop Syncthing: syncthing cli operations shutdown

2. syncthing generate --gui-password=myNewPassword --gui-user=newUserName

3. Restart Syncthing as usual.

Alternatively, in step 2, you can manually delete the <user> and <password> XML tags from the <gui> block
in file config.xml. The location of the file depends on the OS and is described in the configuration
documentation.

For example, the two emphasized lines below would be removed from the file.

<gui enabled="true" tls="false" debugging="false">
<address>127.0.0.1:8384</address>
<user>syncguy</user>
<password>$2a$10$s9wWHOQe...Cq7GPye69</password>
<apikey>9RCKohqCAyrj5RjpyZdR2wXmQ9PyQFeN</apikey>
<theme>default</theme>
</gui>

AUTHOR

       The Syncthing Authors

COPYRIGHT

       2014-2019, The Syncthing Authors

v1.27.0                                           Dec 21, 2023                                  SYNCTHING-FAQ(7)