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Mastodon

The leading ActivityPub server implementation. Self-hosted, federated microblogging.

Overview​

Mastodon is the flagship of the Fediverse — a decentralized social network where anyone can run their own server while remaining connected to the broader network via ActivityPub.

Key Features​

Federation​

┌─────────────────┐         ┌─────────────────┐
│  mastodon.social│         │ fosstodon.org   │
│    (Server A)   │◄───────►│   (Server B)    │
└────────┬────────┘   AP    └────────┬────────┘
         │                           │
         │ Your posts               │ Their posts
         │ reach their             │ appear in
         │ followers               │ your feed
         ▼                           ▼
┌─────────────────────────────────────────────┐
│           Unified Social Experience          │
└─────────────────────────────────────────────┘
  • Choose your server — Join any instance or run your own
  • Follow anyone — Across any ActivityPub server
  • No lock-in — Move servers, keep followers
  • Interoperable — Works with Pleroma, Pixelfed, PeerTube, etc.

Content Features​

  • 500 character posts (default, configurable)
  • Content warnings — Hide sensitive content
  • Image descriptions — Accessibility support
  • Polls — Multiple choice voting
  • Threads — Long-form via reply chains
  • Bookmarks — Save posts privately
  • Lists — Organize follows

Privacy Controls​

VisibilityWho sees it
PublicEveryone, appears in feeds
UnlistedAnyone with link, not in feeds
Followers-onlyOnly your followers
DirectOnly mentioned users

Moderation​

  • Server-level rules — Each instance sets policies
  • User controls — Mute, block, filter
  • Report system — Flag problematic content
  • Domain blocks — Defederate from bad actors
  • Secure mode — Require signed requests

Self-Hosting​

Why Self-Host?​

  • Complete control — Your rules, your moderation
  • Custom branding — Your domain, your identity
  • No algorithm — Chronological by default
  • Privacy — Your data on your server
  • Independence — Can't be deplatformed

Requirements​

  • Domain name — your-instance.social
  • Server — VPS, dedicated, or cloud
  • PostgreSQL — Database
  • Redis — Caching
  • Storage — For media (S3 optional)

Docker Deployment​

# docker-compose.yml
version: '3'
services:
  db:
    image: postgres:14-alpine
    volumes:
      - ./postgres:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: password

  redis:
    image: redis:7-alpine

  web:
    image: tootsuite/mastodon
    command: bash -c "rm -f /mastodon/tmp/pids/server.pid; bundle exec rails s -p 3000"
    depends_on:
      - db
      - redis
    ports:
      - "3000:3000"
    volumes:
      - ./public/system:/mastodon/public/system

Configuration​

# .env.production

# Federation
LOCAL_DOMAIN=your-instance.social
WEB_DOMAIN=your-instance.social

# Database
DB_HOST=db
DB_USER=mastodon
DB_PASS=password
DB_NAME=mastodon_production

# Redis
REDIS_HOST=redis

# Secrets (generate with rake secret)
SECRET_KEY_BASE=...
OTP_SECRET=...

# Storage (optional S3)
S3_ENABLED=true
S3_BUCKET=your-bucket

ActivityPub Implementation​

Mastodon fully implements:

  • Server-to-Server — Federation with other instances
  • Client-to-Server — API for clients
  • WebFinger — User discovery (@user@domain)
  • HTTP Signatures — Request authentication

Mastodon API​

Third-party apps use the REST API:

// Fetch home timeline
const response = await fetch('https://mastodon.social/api/v1/timelines/home', {
  headers: {
    'Authorization': `Bearer ${accessToken}`
  }
});

const posts = await response.json();

Many Nostr apps (like Ditto) implement the Mastodon API for compatibility.

Comparison with Nostr​

AspectMastodonNostr
IdentityServer-basedKey-based
Data locationServer storesRelays store
MigrationPossible, complexEasy (keys)
ModerationServer adminsClient-side
ProtocolActivityPubNostr

Bridging​

Connect Mastodon to Nostr via Ditto or Mostr:

Nostr user ──► Mostr bridge ──► Mastodon
                    │
                    ▼
            Appears as @user@mostr.pub
InstanceFocus
mastodon.socialGeneral (flagship)
fosstodon.orgOpen source
infosec.exchangeSecurity
hachyderm.ioTech
mas.toGeneral

Or run your own!

See Also​