# Syncthing — Open Source Peer-to-Peer File Synchronization

> Syncthing is a continuous file synchronization tool that syncs files between devices directly — no cloud, no servers, no accounts. Encrypted, private, and decentralized.

## Install

Save the content below to `.claude/skills/` or append to your `CLAUDE.md`:

## Quick Use

```bash
# Linux/macOS
curl -s https://api.github.com/repos/syncthing/syncthing/releases/latest | 
  grep browser_download_url | grep linux-amd64 | head -1 | cut -d'"' -f4 | xargs curl -L -o syncthing.tar.gz
tar xzf syncthing.tar.gz && cd syncthing-*/
./syncthing

# Or Docker
docker run -d --name syncthing -p 8384:8384 -p 22000:22000/tcp -p 22000:22000/udp 
  -v syncthing-config:/var/syncthing/config -v /path/to/sync:/var/syncthing/data 
  syncthing/syncthing:latest
```

Open `http://localhost:8384` — add devices and select folders to sync.

## Intro

**Syncthing** is an open-source, peer-to-peer continuous file synchronization tool. Unlike Dropbox or Google Drive, Syncthing syncs files directly between your devices without routing through any cloud server — your data never leaves your network, and no third party has access to your files.

With 81.6K+ GitHub stars and MPL-2.0 license, Syncthing is one of the most popular open-source tools on GitHub, trusted by millions of users for private, secure file synchronization.

## What Syncthing Does

- **Direct Sync**: Files sync directly between devices — no cloud server intermediary
- **End-to-End Encryption**: All communication encrypted with TLS 1.3
- **No Account Required**: No signup, no login, no cloud subscription
- **Cross-Platform**: Windows, macOS, Linux, Android, FreeBSD, Solaris
- **Selective Sync**: Choose which folders to sync on each device
- **Versioning**: File versioning with configurable retention (trash can, simple, staggered)
- **Ignore Patterns**: .stignore files for excluding files/folders (like .gitignore)
- **Conflict Resolution**: Automatic conflict detection with both versions preserved
- **NAT Traversal**: Works through firewalls with relay fallback
- **Web UI**: Beautiful management interface at localhost:8384

## Architecture

```
┌──────────────┐           ┌──────────────┐
│  Device A    │◄─────────▶│  Device B    │
│  (Desktop)   │  Direct   │  (Laptop)    │
│  Syncthing   │  P2P      │  Syncthing   │
└──────┬───────┘           └──────────────┘
       │
       │ Direct P2P
       │
┌──────┴───────┐
│  Device C    │
│  (Phone)     │
│  Syncthing   │
└──────────────┘

No central server. All devices connect directly.
If direct connection fails → encrypted relay.
```

## Self-Hosting

### Docker Compose

```yaml
services:
  syncthing:
    image: syncthing/syncthing:latest
    hostname: my-syncthing
    ports:
      - "8384:8384"    # Web UI
      - "22000:22000/tcp"  # Sync protocol
      - "22000:22000/udp"  # QUIC sync
      - "21027:21027/udp"  # Discovery
    volumes:
      - syncthing-config:/var/syncthing/config
      - /path/to/documents:/var/syncthing/Documents
      - /path/to/photos:/var/syncthing/Photos
    environment:
      PUID: 1000
      PGID: 1000
    restart: unless-stopped

volumes:
  syncthing-config:
```

## Key Features

### Folder Configuration

```
Folder Types:
├── Send & Receive — Full two-way sync (default)
├── Send Only — This device is the "master" copy
└── Receive Only — This device only receives updates

Versioning:
├── Trash Can — Deleted/replaced files moved to .stversions/
├── Simple — Keep N old versions
├── Staggered — Keep versions at increasing intervals
└── External — Run custom script on file changes
```

### .stignore (Exclusion Patterns)

```
// .stignore file
(?d).DS_Store
(?d)Thumbs.db
(?d)desktop.ini
node_modules
.git
*.tmp
*.log
~*
```

### Multi-Device Setup

```
Step 1: Install Syncthing on Device A
Step 2: Install Syncthing on Device B
Step 3: On Device A, click "Add Remote Device"
Step 4: Enter Device B's ID (shown in Device B's web UI)
Step 5: Accept connection on Device B
Step 6: Share folders between devices
```

Device IDs are automatically generated cryptographic identifiers — no passwords to share.

### Use Cases

| Scenario | Setup |
|----------|-------|
| Laptop ↔ Desktop sync | Send & Receive on both |
| Phone photo backup | Send Only on phone, Receive Only on server |
| Team shared folder | Send & Receive on all team devices |
| NAS backup | Send Only from source, Receive Only on NAS |
| Dotfiles sync | Sync ~/.config across Linux machines |

## Syncthing vs Alternatives

| Feature | Syncthing | Dropbox | Resilio Sync | Nextcloud |
|---------|----------|---------|-------------|-----------|
| Open Source | Yes (MPL-2.0) | No | No | Yes |
| P2P (no server) | Yes | No | Yes | No (server) |
| Cloud storage | No | Yes (cloud) | No | Self-hosted |
| Encryption | E2E (TLS) | At rest | E2E | At rest |
| Account needed | No | Yes | Yes | Yes |
| File versioning | Yes | 30-day | Yes | Yes |
| Mobile | Android | iOS+Android | iOS+Android | iOS+Android |
| iOS app | Community (Möbius) | Official | Official | Official |

## FAQ

**Q: Without a central server, how do devices find each other?**
A: Syncthing uses global discovery servers (only to exchange IP addresses, no file data) plus local network discovery (mDNS). Once peers find each other, they establish a direct P2P connection. If a direct link fails, traffic flows through an encrypted relay server.

**Q: Is there iOS support?**
A: There's no official iOS app (Apple heavily restricts background sync). The community project Möbius Sync provides an iOS client. If iOS sync is a hard requirement, consider Nextcloud or Resilio.

**Q: How large or how many files can it sync?**
A: There's no limit. Users report successfully syncing hundreds of GB and even TB-scale datasets. Syncing millions of files works fine, though the initial scan may take a while.

## Sources & Credits

- GitHub: [syncthing/syncthing](https://github.com/syncthing/syncthing) — 81.6K+ ⭐ | MPL-2.0
- Website: [syncthing.net](https://syncthing.net)

---
Source: https://tokrepo.com/en/workflows/syncthing-open-source-peer-peer-file-synchronization-a1ace164
Author: Script Depot