Linux Applications: The Daemons and Tools That Run on Your Server

The programs you install, the daemons that keep running, and the names you'll meet in every server tour.

How to Read This Index

Commands are what you type; applications are the actual programs you install and run — the web server fronting your site, the database behind it, the cache between them, the mail daemon sending your password resets, the container runtime that holds it all together. We've grouped them by family below in roughly the order a real server grows into them: serve traffic, store data, cache it, schedule it, ship logs, talk to other boxes. Every entry links to its own page; if you already know what you're looking for, jump to the Full Alphabetical List at the bottom.

1. Web Servers, Proxies, and Caches

The front door — what answers when someone types your domain. Most stacks pair an HTTP server with a reverse proxy or cache; sometimes one program does both.

  • Nginx — high-performance web server, reverse proxy, load balancer, and HTTP cache; the default choice today
  • Apache — the original mass-deployed web server; still everywhere, still solid
  • HAProxy — the load balancer of choice when you need raw TCP/HTTP routing at scale
  • Varnish — caching HTTP accelerator that sits in front of slow backends
  • Squid — forward and reverse web proxy with a long pedigree
  • Apache Tomcat — Java servlet container; what runs your .war files

2. Databases and Data Stores

Where your application's state lives. Pick relational for ACID guarantees, document for flexible schemas, columnar for time series, key-value for raw speed.

  • MySQL — the most-deployed relational database on the planet
  • MariaDB — the community-driven MySQL fork, drop-in compatible
  • PostgreSQL — the relational database experienced engineers reach for
  • MongoDB — document store; JSON in, JSON out
  • Cassandra — wide-column store built for write-heavy, multi-datacenter workloads
  • InfluxDB — time series database for metrics and sensor data
  • Elasticsearch — distributed search and analytics engine
  • Solr — the other open-source search platform, built on Lucene

3. Caches, Queues, and Streaming

The middle tier — speed up reads, decouple producers from consumers, smooth out spikes.

  • Redis — in-memory key-value store; cache, queue, and pub/sub in one process
  • Memcached — the original in-memory cache, simpler and narrower than Redis
  • RabbitMQ — message broker built around the AMQP protocol
  • Kafka — durable, partitioned event log; the spine of modern data pipelines

4. Container Runtimes and Virtualization

How you isolate one workload from another on the same kernel — or run a whole different kernel.

  • Docker — the container runtime that made containers mainstream
  • Kubernetes — container orchestration at scale; the cluster-level scheduler
  • KVM — full hardware virtualization built into the Linux kernel

5. Mail Servers

The most under-appreciated workload on any server — until you're trying to figure out why your password resets aren't arriving.

  • Postfix — the default modern MTA on most Linux distros
  • Exim — the other major MTA; Debian's historical default
  • Dovecot — IMAP and POP3 server; what your client talks to
  • fetchmail — pulls mail from remote servers into a local inbox
  • SpamAssassin — the rule-based spam filter every mail server ends up bolting on

6. DNS, Time, and Directory

The quiet infrastructure: names, clocks, and identity. Get any of these wrong and nothing works.

  • Bind — the reference DNS server; runs a huge fraction of the internet's authoritative zones
  • NTP — Network Time Protocol daemon; without it, certificates expire early and logs lie
  • OpenLDAP — the open-source directory server for users and groups

7. Security, VPN, and Certificates

The tools that decide who gets in, what's encrypted, and which IP just got banned for trying ten thousand passwords.

  • Fail2Ban — watches logs, bans IPs that misbehave; the lightweight intrusion blocker
  • firewalld — dynamic firewall manager; the friendly face on nftables/iptables
  • WireGuard — modern VPN; tiny config, fast, built into the kernel
  • OpenVPN — the older, full-featured VPN with TLS at its core
  • Let's Encrypt — the certificate authority that made HTTPS free
  • Cryptsetup — disk encryption via LUKS; what protects data at rest

8. Logging, Monitoring, and Metrics

The tools that tell you what happened, what's happening, and what's about to break.

  • Rsyslog — the syslog daemon most distros ship today
  • Graylog — log aggregation and search platform
  • Logstash — the L in ELK; ingests, parses, and forwards logs
  • Kibana — the visualization frontend for Elasticsearch
  • Grafana — dashboarding for time series data, from any source
  • Nagios — the granddaddy of open-source server monitoring
  • Icinga — the modernized Nagios fork
  • Zabbix — all-in-one monitoring with its own agent and database
  • Cacti — RRDtool-based graphing for SNMP devices and servers

9. Storage, Filesystems, and RAID

When one disk isn't enough — pool them, mirror them, share them, cache them.

  • LVM — Logical Volume Manager; resize and snapshot volumes without unmounting
  • mdadm — Linux software RAID; turn N disks into one fault-tolerant array
  • Bcache — use an SSD as a cache in front of a slower HDD
  • NFS — Network File System; the classic Unix way to share a directory across the network
  • Samba — SMB/CIFS server; how Linux serves files to Windows clients
  • AutoFS — mounts filesystems on demand, unmounts them when idle
  • Ceph — scalable distributed storage; object, block, and file in one cluster
  • GlusterFS — distributed filesystem; pool storage across many servers
  • Hadoop — HDFS plus the batch-processing ecosystem around it

10. File Transfer and Sync

When the file has to get from this machine to that one — securely or not, scheduled or not.

  • Proftpd — full-featured FTP server with extensive configuration
  • vsftpd — very secure FTP daemon; smaller, safer, fewer features
  • Nextcloud — self-hosted file sync and collaboration; the open Dropbox
  • OwnCloud — Nextcloud's predecessor; the original

11. Configuration Management and Infrastructure as Code

The tools that let you describe a server in a file, then create a hundred of them.

  • Ansible — agentless configuration management; SSH plus YAML
  • Puppet — declarative config management with its own DSL and agent
  • Terraform — provision infrastructure across clouds from one config
  • Consul — service discovery and distributed KV; the substrate underneath a lot of microservice stacks
  • Jenkins — the CI/CD workhorse; the build farm in a box
  • Git — the distributed version control system; what everything else builds on
  • SVN — Subversion; the centralized predecessor still alive in many enterprises

12. Scheduling and Languages

Things you'll find running on almost any server — even when nobody installed them on purpose.

  • Cron — the scheduler that has run periodic jobs on Unix since 1975
  • Java — the JVM and its toolchain; the runtime behind a startling amount of infrastructure

13. CMS, Wiki, and Web Apps

The user-facing applications you actually deploy on top of the stack above.

  • WordPress — the CMS that runs roughly 40% of the web
  • Drupal — the enterprise-leaning CMS
  • Joomla — the other long-running PHP CMS
  • MediaWiki — the software behind Wikipedia; what to use when you need a wiki

14. Media and IoT

The odd-shaped tools — useful enough to belong, distinct enough not to fit a bigger family.

  • Mencoder — command-line video encoder from the MPlayer project
  • Node-RED — flow-based programming for wiring together IoT devices and APIs

Full Alphabetical List

Every application in the knowledge base, A to Z. If you know the name, jump straight to it.

  • Ansible — agentless configuration management over SSH
  • Apache — the original mass-deployed web server
  • Apache Tomcat — Java servlet container for .war applications
  • AutoFS — on-demand filesystem automounter
  • Bcache — block-layer cache that puts an SSD in front of an HDD
  • Bind — the reference DNS server
  • Cacti — RRDtool-based graphing for SNMP and servers
  • Cassandra — wide-column NoSQL store for write-heavy workloads
  • Ceph — distributed object, block, and file storage
  • Consul — service discovery and distributed key-value store
  • Cron — schedule commands and scripts at fixed intervals
  • Cryptsetup — LUKS disk encryption tool
  • Docker — the container runtime that made containers mainstream
  • Dovecot — IMAP and POP3 server
  • Drupal — enterprise-leaning PHP content management system
  • Elasticsearch — distributed search and analytics engine
  • Exim — flexible mail transfer agent; Debian's historical default
  • Fail2Ban — bans IPs that fail logins too often
  • fetchmail — pulls mail from remote servers into a local inbox
  • firewalld — dynamic firewall manager
  • Git — the distributed version control system
  • GlusterFS — distributed filesystem that pools storage across servers
  • Grafana — dashboards and visualization for time series data
  • Graylog — log aggregation, search, and alerting
  • Hadoop — HDFS and batch data processing
  • HAProxy — high-availability TCP/HTTP load balancer
  • Icinga — modernized Nagios fork
  • InfluxDB — time series database
  • Java — the JVM and its toolchain
  • Jenkins — continuous integration and delivery server
  • Joomla — long-running PHP CMS
  • Kafka — distributed event-streaming platform
  • Kibana — visualization frontend for Elasticsearch
  • Kubernetes — container orchestration platform
  • KVM — kernel-based virtual machine; full virtualization in Linux
  • Let's Encrypt — free, automated SSL/TLS certificates
  • Logstash — log ingestion and parsing pipeline
  • LVM — Logical Volume Manager
  • MariaDB — community-driven MySQL fork
  • mdadm — Linux software RAID management
  • MediaWiki — the wiki engine behind Wikipedia
  • Memcached — simple in-memory cache
  • Mencoder — command-line video encoder from MPlayer
  • MongoDB — document-oriented NoSQL database
  • MySQL — the most-deployed relational database
  • Nagios — the original open-source monitoring system
  • Nextcloud — self-hosted file sync and collaboration
  • NFS — Network File System; the classic Unix file share
  • Nginx — high-performance web server and reverse proxy
  • Node-RED — flow-based IoT and integration programming
  • NTP — Network Time Protocol daemon
  • OpenLDAP — open-source LDAP directory server
  • OpenVPN — TLS-based VPN
  • OwnCloud — self-hosted file sharing and collaboration; Nextcloud's predecessor
  • Postfix — the default modern MTA on most Linux distros
  • PostgreSQL — the relational database experienced engineers reach for
  • Proftpd — full-featured FTP server
  • Puppet — declarative configuration management with agents
  • RabbitMQ — AMQP message broker
  • Redis — in-memory key-value store; cache, queue, and pub/sub
  • Rsyslog — the syslog daemon most distros ship
  • Samba — SMB/CIFS file and print server
  • Solr — Lucene-based open-source search platform
  • SpamAssassin — rule-based spam filter
  • Spark — distributed data processing engine
  • Squid — forward and reverse web proxy
  • SVN — Subversion; centralized version control
  • Terraform — multi-cloud infrastructure as code
  • Varnish — caching HTTP accelerator
  • vsftpd — very secure FTP daemon
  • WireGuard — modern, fast, kernel-based VPN
  • WordPress — the CMS behind roughly 40% of the web
  • Zabbix — all-in-one monitoring with agent and database

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