Teams that cannot send messages through someone else's cloud run Mattermost - the open-core, self-hosted alternative to Slack. It provides public and private channels, threaded discussions, unlimited search history, file sharing with previews, one-to-one audio calls, and screen sharing, with desktop clients for Windows, macOS, and Linux plus iOS and Android apps. Messages support full Markdown, which suits engineering conversations with code blocks and logs. Playbooks turn repeatable processes such as incident response and release management into checklist-driven workflows with automated triggers and retrospectives. Integration is a core strength: prebuilt connectors for GitHub, GitLab, Jira, ServiceNow, and PagerDuty, plus webhooks, slash commands, bots, a REST API, and a plugin marketplace with 700+ entries - together making it a working surface for ChatOps rather than just a chat room. Playbooks add keyword and event triggers, task assignment, status broadcasting, and post-incident retrospectives, so operational knowledge is not trapped in individuals' heads. The server is a single Go binary backed by PostgreSQL, with React clients, released monthly under MIT license and deployable fully air-gapped - which is why governments and defense organizations run it inside closed networks, and why the same control applies to any team with confidentiality requirements. The compiled Team Edition is free for unlimited users with no message history cutoff, so costs stay flat as the team grows.

Openfire is a self-hosted XMPP server that gives your organization its own instant messaging network, with one-to-one chat, persistent group rooms, and federation to other XMPP servers, all administered from a web console. Team members connect with any XMPP client on desktop, mobile, or the web and get presence, rosters, offline message delivery, and file sharing inside conversations. Multi-user chat rooms stay available around the clock with configurable history, moderation roles, and access controls, so a support channel or project room keeps its context instead of dissolving into scattered direct messages. The plugin catalog installs from inside the admin console and adds capabilities like message archiving and compliance monitoring, a browser-based chat client, HTTP file upload, and push notifications for mobile clients. An LDAP directory or an existing database can supply users and groups, so accounts stay in sync with the directory your organization already maintains. Server-to-server federation connects your users with contacts on other XMPP domains over encrypted connections, and load balancing spreads busy deployments across multiple instances. TLS certificates, encrypted client ports, and IP-based access lists for the admin console keep the deployment under your control from the first setup wizard onward. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. Apache-2.0 licensed.