What Syncthing actually is
Syncthing is a small background client that you manage through a local web interface. Each install generates a Device ID and a cryptographic keypair, and devices use those to verify each other. Devices that connect and approve each other form a cluster and can share chosen folders.
There is no hosted file library behind it. Your files live on your own machines, so capacity is limited by your disks rather than a plan tier. That is the "no storage limits" appeal Sezov points to. Syncthing's own FAQ describes the design this way: your data is exchanged across your machines as soon as they are online at the same time. The practical rule is that a device that is off can't hand its files to anyone.
The setup flow, as Sezov describes it
On a desktop, the interface is at [url]http://localhost:8384[/url]. The default listen address binds it to the local machine, so nothing else can reach it.
- Find your Device ID. The Identification link in the top-right box shows it. Send it to whoever you want to share with.
- Add the other device. Use Add Remote Device and paste in the other person's Device ID.
- Create a folder. Use Add Folder and set a label, an optional group, the local path, and optionally a readable Folder ID. The Folder ID must be unique within your cluster.
- Choose recipients. The Sharing tab lets you pick which connected devices get the folder.
- Accept on the other side. The recipient approves the share and chooses where to store it.
Sezov's own organizing tip is to keep every shared folder under a single parent directory, ~/Share in his case. On Windows you could use something like a Share folder in your user profile. That is his habit, not a Syncthing requirement. It does help you see at a glance what is and isn't being shared.
The Syncthing FAQ also says you must set up both sides of a connection. That is deliberate: an attacker can't do much with a stolen Device ID, because the other end has to approve the device too.
Windows gotcha: Public network profile and relays
Windows users should know about one common surprise. Syncthing's FAQ says Windows sets new networks to "Public" by default, which commonly prevents Syncthing from making direct connections and forces it through a relay.
To check whether you're on a relay, open the GUI and look at the Remote Devices list. The Connection Type should not start with "Relay."
Relays are not a failure or a security hole:
- Relaying is on by default and is only used when two devices can't communicate directly.
- The connection stays end-to-end encrypted, and the relay only retransmits encrypted data.
- There is a privacy cost. The relay learns your IP and device ID and can see how much traffic flows between devices.
- Relays are slower. Sezov calls them a last resort, and Syncthing periodically retries a direct connection and drops the relay if one succeeds.
Syncthing's FAQ suggests switching the network profile to "Private" if you want direct connections. Do that only on a network you actually trust, such as your home network. Don't flip a coffee-shop Wi-Fi to Private just to speed up a sync. A Syncthing forum thread adds a caution about the alternative of allowing Public access through the firewall: that is unsafe in principle, because someone could try to open your Syncthing web GUI on public Wi-Fi. A password mitigates that risk but doesn't remove it.
Sync is not backup
This is the most important caveat. Sezov doesn't dwell on it, but the Syncthing FAQ is blunt. It says Syncthing is not a great backup application because all changes to your files, including deletions, propagate to all your devices.
So if you delete or corrupt a file on one machine, that change spreads. The same is true of ransomware-encrypted files. Syncthing has optional file versioning, but it is configured per folder and is off by default. It is not a substitute for a real backup with history that lives somewhere else.
Möbius Sync's FAQ says the same about its own app: it is not a backup tool and users should back up their data.
Phones: where the "it just works" story gets complicated
Android
Sezov recommends Syncthing-Fork. Its history has shifted. A GitHub repository for the Android wrapper says the earlier maintainer, Catfriend1, no longer seems to be actively maintaining the project. The new maintainer says they will continue development there. The README also says the project is mid-migration and recommends the Play Store channel as the most stable install route. That repository is itself marked archived as of December 27, 2025. Check the current project and download channel yourself before you install anything.
iOS
Möbius Sync is a third-party app. Its FAQ confirms what Sezov found: no iOS app can run continuously in the background, so syncing happens while the app is open, shortly after, and in brief iOS-scheduled windows. That scheduling is adaptive and unpredictable. Its FAQ suggests you may see roughly one to two hours of sync activity per day once things settle. An iPhone will not behave like an always-connected desktop peer.
Möbius also has limits and costs. Its FAQ says free use is capped at 20 MB of total file storage. After that, a one-time in-app purchase unlocks unlimited sync. For photos, its FAQ says that only the Send Only folder type is supported, and that deletions on the iPhone will also delete on other devices unless you account for that. If you're counting savings, factor in that purchase.
Filenames: the colon problem
Sezov's example is a music file with a colon in its name that Android refuses to store. Syncthing can only create files the target system can represent, so one bad filename can block a single file without breaking everything else. Syncthing's documentation describes a related issue with case sensitivity. Windows, macOS and, to an extent, Android treat file.txt and FILE.txt as the same file, while Syncthing treats them as different. Syncthing will report a case conflict when that happens, and you have to settle on a consistent name by hand.
If you share a folder between Linux and Windows, expect this class of problem. It's another reason to test with a small folder first.
The always-on server idea
Sezov's best tip is a small always-on peer, such as a Raspberry Pi or a Libre Computer board. The server stays online, so your other devices can sync with it even when your laptop is closed.
His hardware advice, in his words a personal recommendation:
- Use an external USB disk for the shares. He uses a 5TB spinning disk.
- Use a powered USB hub. He tried running the disk from the Pi's power supply and says it went badly.
- Keep all the shares on the external disk. If the Pi dies, the disk can be plugged into a computer.
A single server with a single disk is still a single point of failure. It improves availability and it is not a backup.
Locking down the GUI
To reach the Pi's console from another machine, Sezov says to set the GUI Listen Address to 0.0.0.0:8384 and set a GUI username and password under Actions → Settings → GUI.
Syncthing's FAQ explains what that setting does. The default address is 127.0.0.1:8384, which is local-only for security. Changing it to 0.0.0.0:8384 makes the interface reachable from anywhere on the network. The docs add that there is no source-address filtering, so you should use a firewall for that. They say you should set a password and enable HTTPS.
The password is not optional. If you only need occasional remote access and both machines are Unix-like, the FAQ also describes an SSH port forward as a way to leave the GUI at its local-only default.
What it can't do
Syncthing can't create a public link. There is no Dropbox-style URL you can email to someone outside your cluster. Everyone who needs the files has to run Syncthing and be approved. For a household or a small team that may be fine. For sending a file to a client, it won't work.
Does it really replace cloud sync?
Sezov's claim is modest: it replaced cloud sync for him, with family files, photos, PDFs, design documents and a music library. That is plausible. Here is a balanced read of the trade-offs.
| Consideration | Hosted cloud sync | Syncthing |
|---|---|---|
| Storage limit | Plan tier | Your own disks |
| Monthly fee | Usually yes | No (iOS app may need a one-time purchase) |
| Public share links | Yes | No |
| Devices must be online | No | Yes, at least one peer |
| Built-in version history | Often | Optional, off by default |
| Who maintains hardware and uptime | Provider | You |
Your costs don't disappear. You pay in disks, power, setup time and maintenance. If you already own the hardware, the math is friendly. If you'd buy a Pi, a drive and a hub just to cancel a few dollars a month, the savings may take a while to show up.
Bottom line
Syncthing is a good fit if you want to keep files on hardware you control and share them with people willing to install a client. Start with one test folder and confirm it transfers between two machines. Check the Connection Type for relays. Keep a separate backup. If you expose the GUI beyond localhost, set a password first. Whether it can replace your whole cloud subscription depends on how much of that subscription is really just syncing.