Syncthing — Open Source Peer-to-Peer 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

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

Syncthing vs Alternatives

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 — 81.6K+ ⭐ | MPL-2.0
• Website: syncthing.net