AltChecker
by Cats1337
AltChecker is a server-side Fabric management tool for 1.21.1 that tracks player logins and flags accounts that may be alternate accounts of one person.
What it does
AltChecker records the IP address each player logs in from and keeps that history over time. Administrators can view grouped IP links for a player, which shows accounts that have shared an address and can point to possible alts. Each player also gets a risk score that blends VPN detection with behavioral signals, so staff can prioritize which accounts to check first.
Session tracking records join and leave times along with online or offline status. Access to commands is controlled by permissions, and IP addresses are masked in chat, with an optional hover or click reveal for staff holding the altcheck.view.ip permission. Admin utilities cover hiding, purging, cleanup and statistics, and the mod is open source under the GNU General Public License v2.0 only.
The mod is installed on the server alone, and players join without installing anything. Files target 1.21.1 only, and the latest Fabric file, 1.0, was published on 2026-04-13.
Key features
Login IP history
Every login IP is stored per player, so changes in address can be tracked over time.
Shared IP detection
Accounts that share IP addresses are linked, which helps staff identify possible alts.
Risk scoring
A risk score combines VPN detection with behavioral signals to rank accounts for staff attention.
Session tracking
Join and leave times and online or offline status are recorded for each player session.
Masked IP display
IP addresses are hidden in chat and revealed on hover or click only for players with the altcheck.view.ip permission.
Admin utilities
Commands to hide, purge and clean up records, along with statistics, support server administration.
Compatibility
Latest build for each game version and loader.
Only full game releases are shown. This mod publishes no snapshot builds.
Use cases
Who it’s for: Server administrators and moderators who manage player accounts and want to check login patterns without exposing player IP addresses in public chat.
How it compares
IP Security Suite is server-side only on Fabric for 1.21 – 1.21.3 and is open source under the GNU General Public License v3.0 only. AccountSwitcher is a client-side Fabric mod for 1.21.4 under All rights reserved, so it does not manage server accounts. EEssentials is a server-side Fabric mod for 1.20.1 – 1.21.1 under the GNU General Public License v3.0 only, covering general server commands rather than IP tracking.
| Mod | Loaders | Game versions | Environment | Updated |
|---|---|---|---|---|
| AltChecker | Fabric | 1.21.1 | Server-side only | 2026-04-13 |
| IP Security Suite | Fabric | 1.21 – 1.21.3 | Server-side only | 2024-11-17 |
| AccountSwitcher | Fabric | 1.21.4 | Client-side only | 2026-06-29 |
| EEssentials | Fabric | 1.20.1 – 1.21.1 | Server-side only | 2026-07-22 |
Questions
Does AltChecker need to be installed on the client?
Which loader does AltChecker support?
Can staff see player IP addresses in chat?
In the catalog
AltChecker is one of 8,016 Management mods on ModsFinder, and one of 5,930 Management mods for Fabric. 32,366 mods support Minecraft 1.21.1.
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What the author says
AltCheck is a mod focused on privacy and security. It provides tools to manage and verify accounts, with a comprehensive set of features to ease server administration while keeping sensitive data private.
Official listing
- Modrinth page
- modrinth.com/mod/altchecker
- Owner
- Cats1337
- Published
- 2026-04-13
- Updated on Modrinth
- 2026-04-13
- Facts verified
- 2026-10-15