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ConfigServer Security & Firewall (CSF) is a Linux-server firewall and login-failure detection suite. It applies stateful packet-inspection rules through Ubuntu’s netfilter firewall layer, using iptables or nftables underneath, while its Login Failure Daemon (LFD) watches for suspicious authentication activity.
On Ubuntu, the dependable installation sequence is: install the required Perl and networking dependencies, download a stable CSF archive, run the supplied pre-install test, install CSF and LFD, then edit /etc/csf/csf.conf. Keep your existing SSH session open while applying the policy and prove that a second session works before disconnecting the first.
What CSF does on an Ubuntu server
CSF (ConfigServer Security & Firewall) is designed for servers rather than Ubuntu desktop machines. Its documentation describes it as a “Stateful Packet Inspection (SPI) firewall and Login/Intrusion Detection and Security application for Linux servers.” CSF provides a central configuration file, address allow and deny lists, blocklist controls, and IPSET-related options. LFD operates alongside the firewall to detect login failures and other intrusion indicators.
Ubuntu’s kernel uses the netfilter subsystem to decide what happens to traffic entering or passing through the host. Ubuntu’s default firewall configuration frontend is UFW. CSF is another management layer for the same underlying firewall capabilities, so the important operational decision is which tool owns the host policy. Running two independently managed policy sets can produce confusing or contradictory results.
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Before you install CSF
Confirm administrative access
- Use an account with
sudorights or a root shell. - Have console, out-of-band, or hosting-panel access available in case a firewall change interrupts remote access.
- Identify the SSH port actually used by the server. Do not assume it is the distribution default; inspect the SSH daemon configuration or the service’s listening sockets.
- Record the ports required by every service that must be reachable, including administration, web, mail, DNS, monitoring, backup, or application traffic.
Choose one firewall policy owner
Check the current UFW state before installing CSF:
sudo ufw status verboseIf UFW is already enforcing rules, document those rules and decide whether CSF will replace them. Do not make both tools independent owners of production policy. Plan the change during a maintenance window, keep a console path available, and make the ownership decision explicit in your operating runbook.
Protect the current SSH session
For a remote server, leave the current SSH connection open. Before applying CSF, make sure the configured inbound TCP list will allow the real SSH port and every other required service port. After the change, open a second SSH session from a separate terminal and authenticate successfully. Only then should you close the original session.
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The current project installation guidance lists these packages for Debian and Ubuntu:
sudo apt-get update
sudo apt-get install -y ipset libcrypt-ssleay-perl libio-socket-inet6-perl libio-socket-ssl-perl libnet-libidn-perl libsocket6-perl perl wgetThe list supplies the Perl runtime and networking modules used by CSF, the wget downloader, and IPSET support. Installing the packages before unpacking the archive avoids a failed test caused by missing modules.
A separate dependency variation is documented for Debian 13: that platform’s list also includes libwww-perl and postfix. Those additions are not a blanket requirement for every Ubuntu release. Add them only when the package guidance for your specific platform or the installer indicates they are needed.
Download and unpack a stable CSF release
Stable archives are available without a license. Use the archive format that fits the tools on your server:
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TGZ archive
sudo mkdir -p /usr/local/src/csf
cd /usr/local/src/csf
sudo wget -O csf.tgz https://download.configserver.dev/csf.tgz
sudo tar -xzf csf.tgz
cd csfZIP archive
If you use the ZIP distribution, download https://download.configserver.dev/csf.zip and extract it with a ZIP utility:
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sudo mkdir -p /usr/local/src/csf
cd /usr/local/src/csf
sudo wget -O csf.zip https://download.configserver.dev/csf.zip
sudo unzip csf.zip
cd csfThe TGZ download is available at https://download.configserver.dev/csf.tgz. If the archive service exposes a release tag, it can be used to request a specific release. The insiders channel is separate from stable releases and requires a valid sponsorship license; insiders builds arrive before public stable releases and may contain unresolved issues. Use stable for a normal production installation unless you have a deliberate reason and rollback plan for insiders builds.
Run the pre-install test
Run the test script from the extracted files before installing services. If you are in the directory above csf, use:
sudo perl csf/csftest.plIf your shell is already inside the extracted csf directory, the equivalent path is:
sudo perl ./csftest.plA successful test reports:
RESULT: csf should function on this serverDo not proceed as if the host is ready when the test reports missing modules, unsupported firewall capabilities, or another failure. Resolve the stated prerequisite first and run the test again. If the result does not indicate that CSF should function, retain your existing firewall policy and use the available console or distribution support path to investigate.
Install CSF and verify CSF/LFD
The extracted archive includes the installer supplied by the project. From the extracted directory, inspect the files so you run the installer that is actually present:
ls -laRun that installer with root privileges. Current CSF archives commonly provide an install.sh script, in which case the command is:
sudo sh ./install.shIf the archive presents a differently named installer, use that supplied filename rather than guessing. The quick-install flow is intended to leave both CSF and LFD installed with minimal configuration.
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Confirm that the services are present and running:
sudo systemctl status csf lfdLook for an active/running state for both units. If either unit failed, read its service output before changing firewall rules:
sudo systemctl status csf
sudo systemctl status lfdKeep the original SSH session open while you diagnose a service failure. A failed install is safer to repair before any restrictive policy is applied.
Configure /etc/csf/csf.conf
The main CSF configuration file is:
/etc/csf/csf.confOpen it with an editor that preserves the file’s formatting and comments:
sudo nano /etc/csf/csf.confRead the comments shipped in the file. They explain the accepted syntax and identify options that vary by release. Make a backup before editing:
sudo cp -p /etc/csf/csf.conf /etc/csf/csf.conf.backupSet inbound and outbound TCP policy
Configure the file’s TCP inbound and outbound settings for the services this host must use. The inbound list should include the actual SSH port and every service that must accept connections from outside. The outbound list should permit destinations required by the operating system and applications, such as package retrieval, DNS resolution, monitoring, updates, or external APIs, according to your security policy.
Do not copy a universal port list from another server. Inventory this machine’s requirements first. A server that only provides an internal application has a different exposure profile from one that publishes a web service, mail service, or public DNS. If SSH has been moved to a nonstandard port, that exact port must be present in the TCP inbound setting before you reload CSF.
Set inbound and outbound UDP policy
Use the corresponding UDP inbound and outbound settings for services that genuinely require UDP. Include only the ports needed by the host’s installed services and network design. TCP and UDP entries are independent; allowing a TCP port does not automatically allow the same numeric UDP port.
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Review logging
Review the logging controls in csf.conf before going live. Logs are essential when validating a new policy and diagnosing blocked traffic, but an overly verbose policy can make operational review difficult. Choose a level that lets you identify denied connections and LFD events without overwhelming your log collection system.
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Use CSF’s address-list mechanisms for hosts that must always be allowed or denied. Add trusted administration or monitoring addresses deliberately and keep the list current. A stale allow entry can bypass a later policy change, while an incorrect deny entry can block your own management network.
Review blocklists
Blocklist settings determine whether CSF consumes external or locally maintained lists of addresses considered hostile. Validate the origin and maintenance process for every list you enable. A blocklist is an input to policy, not proof that every listed address is malicious; monitor for false positives and remove a source that causes legitimate traffic to fail.
Review IPSET options
IPSET-related settings affect how large address collections are represented and applied. The required ipset package must be installed before using those options. Follow the comments in the shipped configuration for syntax and compatibility with the host’s firewall backend.
Apply the configuration safely
Check the edited file
- Confirm that the SSH port in the inbound TCP setting matches the daemon currently serving your session.
- Confirm that every required application port is present in the correct protocol and direction.
- Check for accidental whitespace, malformed commas, or edits to option names.
- Ensure your current source address is not unintentionally denied.
- Keep the original SSH window open while applying the change.
Reload CSF and LFD
After saving the file, apply configuration changes with the vendor-documented command:
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sudo systemctl status csf lfdTest from a second connection
From another terminal or a separate administration workstation, open a new SSH session to the server. Test the other required services from an appropriate client as well. Verify both expected allows and intentional denies. Only after the second SSH login and essential service checks succeed should you close the original session.
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Recover from an SSH lockout
If a reload blocks your current connection, do not repeatedly terminate sessions. Keep any surviving session open and use the provider’s console or out-of-band access if available. From that administrative path:
- Open
/etc/csf/csf.confand correct the TCP inbound entry for the actual SSH port. - Remove an incorrect deny or add the legitimate administration address to the appropriate allow list.
- Save the file and run
sudo csf -ra. - Test a new SSH session before ending the console session.
If the service itself is not running, inspect systemctl status csf lfd and correct the configuration or dependency error first. Keep a copy of the last known-good configuration so a rollback does not depend on memory during an incident.
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CSF and UFW: which approach fits?
Both tools manage the Linux firewall layer, but they present different operating models. The choice should follow the server’s existing administration practice, not the assumption that more features are always safer.
| Decision point | CSF | UFW |
|---|---|---|
| Primary role | Server firewall with Login/Intrusion Detection through LFD | Ubuntu’s default firewall configuration frontend |
| Rule-management model | Central configuration in /etc/csf/csf.conf, with address lists, blocklists, and IPSET-related controls |
Ubuntu’s standard frontend for defining firewall rules |
| Underlying layer | Front end to iptables or nftables | Uses Ubuntu’s netfilter firewall layer through its frontend |
| Login-failure monitoring | LFD is included with the CSF installation | UFW’s documented role is Ubuntu’s firewall frontend, not an equivalent LFD component |
| Operational fit | Useful when you want firewall policy and login-failure detection managed as one server-oriented suite | Useful when the host already follows Ubuntu’s default firewall workflow |
| Main risk | Applying a restrictive central policy without preserving SSH or required service ports | Maintaining UFW rules alongside CSF can create conflicting ownership and unexpected results |
Whichever tool you select, document it as the single policy owner. If you migrate from UFW to CSF, export or record the existing rules, implement their intended equivalents in CSF, and test from a second connection before retiring the old workflow.
Common installation and configuration problems
The test script reports missing Perl modules
Recheck the dependency command and install the package named by the failure. Pay attention to similarly named Perl networking modules; CSF requires the complete dependency set, not only the perl interpreter. Run the test again after installation.
The archive will not extract
Verify that the download completed and that the command matches the archive type. Use tar -xzf for the TGZ file and a ZIP extractor for the ZIP file. Download the stable archive again if the local file is incomplete.
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Inspect each unit with systemctl status, then read the reported configuration or dependency error. Restore the last known-good csf.conf backup if a syntax edit prevents startup, and rerun the service check before attempting another policy change.
A required service stopped responding after reload
Confirm that its port is included in the correct TCP or UDP direction, verify the service is listening on the expected address and port, and check whether an address-list or blocklist entry overrides the general rule. Apply the correction with sudo csf -ra and retest from the client side.
Rules behave differently after UFW changes
Stop treating both tools as independent policy owners. Review which service is currently loading rules, document the intended owner, and remove the unplanned second control path during a controlled maintenance window.
Quick Recap
Ongoing maintenance checklist
- Keep the stable CSF archive source bookmarked: https://download.configserver.dev/csf.tgz or https://download.configserver.dev/csf.zip.
- Track the installed release and compare it with the current stable channel before upgrading.
- Review
/etc/csf/csf.confafter upgrades because shipped options and comments can change. - Remove retired service ports and stale allow-list entries.
- Review LFD and firewall logs for repeated login failures, unexpected denied traffic, and false positives.
- Test SSH and essential application connections after every policy change.
- Keep console or out-of-band recovery access documented and functional.
- Use the insiders channel only when its sponsorship-license requirement and pre-release risk are acceptable for your environment.
Recommended first-day validation
- Record the server’s listening services and the SSH port.
- Confirm the dependency installation completed without errors.
- Run
csftest.pland verify the exact successful result. - Install CSF and confirm both
csfandlfdare running. - Back up
/etc/csf/csf.conf. - Set only the inbound and outbound TCP/UDP ports the host requires.
- Review logging, address lists, blocklists, and IPSET settings.
- Apply with
sudo csf -rawhile retaining the original SSH session. - Log in through a second SSH session and test each required service.
- Record the final policy owner and rollback procedure in the server runbook.
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