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PSAD (Port Scan Attack Detector) monitors iptables log messages, identifies port scans and suspicious packet signatures, and reports the source, ports, scan type, packet counts and danger level. On a Debian or Ubuntu VPS, reliable detection requires three things in order: install PSAD, make the firewall log traffic that reaches its deny path, and tune /etc/psad/psad.conf for your workload.
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Keep automatic blocking disabled while you validate alerts. PSAD can only analyze packets your firewall logs, and spoofed traffic can make an innocent address look malicious.
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PSAD is a set of lightweight daemons that consumes firewall log messages rather than inspecting every packet independently. iptables records packets that match logging rules; PSAD parses those messages, correlates activity from each source and classifies likely scans or attack signatures.
Scan types and signatures
TCP flag analysis helps PSAD identify SYN, FIN and XMAS scans. Its TCP, UDP and ICMP signatures are derived from Snort rules and are intended to recognize probes associated with backdoors, denial-of-service tools, operating-system fingerprinting and related traffic. An alert can include the source address, packet count, scanned-port range, danger level, reverse-DNS data and WHOIS information.
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The firewall-log boundary
PSAD is not a substitute for a firewall. It analyzes packets that have already reached an iptables logging rule, normally after the firewall has decided that the traffic is not allowed. A scan detected this way was unsuccessful at that firewall boundary; detection does not mean the service was compromised.
Prerequisites on a VPS
- A Debian or Ubuntu image with root or
sudoaccess. - iptables rules that log unwanted traffic before dropping or rejecting it.
- A known location for kernel firewall messages. PSAD uses
/var/log/messagesby default, but rsyslog or another syslog arrangement may write those messages elsewhere. - An email or syslog destination if you want notifications beyond the local PSAD views.
- An authorized system from which you can perform a controlled scan for verification.
Before changing rules, save your current firewall configuration and make sure you have console or out-of-band access. A logging or blocking mistake can lock you out of SSH.
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Install PSAD on Debian or Ubuntu
- Refresh the package index as normal repository maintenance:
sudo apt-get update- Install the distribution package:
sudo apt-get install psadThe documented package command is apt-get install psad; the update step is routine repository hygiene, not a PSAD-specific requirement. The package installs the PSAD daemons and service integration. If the package is unavailable for your image, the project README documents a bundled install.pl script that can install or upgrade PSAD and its Perl dependencies. Treat that script as an alternative installation path and review what it changes before running it on a production host.
Make iptables generate useful evidence
PSAD cannot detect traffic that the firewall never logs. The project documentation puts the dependency plainly: PSAD is only as good as the logging rules in the iptables ruleset.
Order rules so legitimate traffic is not reported
Place established-connection and explicitly permitted-service rules before the logging rule. Put the log-and-drop path after those accepts and before the end of the chain. The exact ports and interfaces depend on your VPS policy.
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# Illustrative policy; adapt the allowed services to this VPS
sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT
sudo iptables -A INPUT -j LOG --log-prefix 'IPTABLES-DROP: '
sudo iptables -A INPUT -j DROPThe broad documentation examples are:
iptables -A INPUT -j LOG
iptables -A FORWARD -j LOGDo not paste those broad rules blindly into a production firewall. Logging every packet can create noise, and a VPS that does not route traffic may not need a FORWARD rule. Add a distinctive prefix when you want PSAD to identify a dedicated logging path; the prefix must match the value searched in FW_MSG_SEARCH when broad searching is disabled.
Confirm where firewall messages are written
The PSAD man page names /var/log/messages as the default input. Distribution logging differs, so generate or inspect a known firewall event and find the corresponding kernel message in the actual syslog or rsyslog file. If the message is in another file, use that path for historical analysis and adjust PSAD’s message-file setting as appropriate for your installation.
Start the daemons and inspect their output
After installation, start the PSAD service through the package’s service integration and verify that its daemons are running with the init or systemd tooling provided by your VPS distribution. There is no single service command that is guaranteed across all Debian- and Ubuntu-derived images, so use the command documented by the image’s service manager.
Use the normal PSAD status and alert views supplied by the package. When no alert appears, inspect the firewall log first: the decisive question is whether the packet reached a LOG rule before it was dropped. Also verify that PSAD is reading the same file in which the kernel message was written.
Configure /etc/psad/psad.conf
The packaged configuration is normally /etc/psad/psad.conf. The -c option lets you select a different configuration file, which is useful for testing changes away from the production configuration.
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| Setting | Documented default or value | Purpose and tuning note |
|---|---|---|
FW_SEARCH_ALL |
Y |
Parse all iptables messages. Set to N when you want PSAD to consider only prefixes listed in FW_MSG_SEARCH. |
FW_MSG_SEARCH |
DROP |
Search string used when matching is restricted. Add the exact log prefix used by your firewall if it does not contain the default text. |
| Danger thresholds | 5, 15, 150, 1,500 and 10,000 packets for levels 1 through 5 | Packet-count boundaries for danger levels. They are packaged defaults, not universal values for every VPS. |
CHECK_INTERVAL |
5 seconds |
Polling interval for checking new log data. |
PORT_RANGE_SCAN_THRESHOLD |
1 |
Minimum scanned-port range used when deciding whether to raise a port-range alert. |
ENABLE_PERSISTENCE |
Y |
Retains long-running scan data instead of allowing it to time out under the default behavior. |
ALERTING_METHODS |
ALL |
Enables the documented syslog and email reporting methods. |
IGNORE_PORTS, IGNORE_INTERFACES, IGNORE_LOG_PREFIXES |
Use as needed | Suppress known benign sources such as port-knocking sequences, trusted interfaces or dedicated audit prefixes. |
ENABLE_AUTO_IDS |
N |
Automatic firewall blocking is disabled by default. |
Choose a log-search strategy
With FW_SEARCH_ALL Y, PSAD parses all iptables messages and can see rules that do not share a dedicated prefix. This is simple but may include unrelated firewall activity. Setting it to N narrows matching to the strings in FW_MSG_SEARCH; use the exact prefix from your logging rule and test it by examining a real log line.
Set thresholds from a baseline
The five packet-count defaults provide a starting point, not a promise that level 3 or level 5 means the same thing on every service. A public web server, a VPN endpoint and a private database host have different normal traffic. Observe alerts during ordinary operation, identify expected health checks and monitoring probes, and change one threshold or exclusion at a time. Keep a record of the previous value so a noisy change can be reversed.
Verify detection with an authorized test
Only scan a system you own or are explicitly authorized to test. A controlled test should prove the complete path: the probe reaches the VPS, iptables logs it, PSAD reads the message and an alert contains useful classification data.
- Confirm the PSAD daemons are running.
- Confirm an unwanted packet reaches the final logging-and-deny path rather than an earlier accept rule.
- From an authorized test host, generate a limited scan appropriate to your change window. Do not use a third-party target.
- Locate the corresponding iptables log entry and note its timestamp, source address, destination port and log prefix.
- Open the PSAD alert view or notification and check for the source, scanned ports, packet count, scan type or signature and danger level.
- If the alert is absent, compare the firewall message with the configured search string and verify that PSAD is reading the correct log file.
- Review the result for false positives before changing thresholds or enabling any response action.
A successful test demonstrates detection, not prevention. The firewall rule performed the blocking; PSAD classified and reported the event.
Analyze scans recorded before installation
PSAD can process an existing iptables log and exit instead of waiting for new events. This is useful when the daemon was installed after a scan or when you want to evaluate a configuration change against historical data.
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sudo psad -A --messages-file /path/to/iptables-logUse a copy of the log when experimenting with thresholds. Replace the path with the file that actually contains the kernel firewall messages; on some systems that may be /var/log/messages, while others use a different syslog file. Historical analysis does not alter the firewall rules, but it can produce alerts based on old sources that are no longer active.
Control false positives without hiding attacks
Benign traffic can resemble reconnaissance. Common examples include uptime checks that touch several ports, internal vulnerability scanners, port-knocking sequences and monitoring interfaces that generate distinctive log prefixes.
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- Prefer the narrowest documented exclusion: an individual port, interface or log prefix rather than disabling all searches.
- Document why the exception exists and who owns the generating system.
- Recheck exclusions after network changes; a trusted internal address can be reassigned.
- Use historical analysis to compare a proposed exclusion with earlier events before applying it to live monitoring.
Do not raise every danger threshold merely to silence one noisy service. A broad threshold change can delay recognition of a real scan elsewhere on the host.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Should you enable automatic blocking?
Usually not at first. ENABLE_AUTO_IDS is N in the documented configuration, and that conservative default is appropriate while you learn the VPS’s normal traffic.
Why automatic blocking is risky
The PSAD man page warns that spoofed packets can make an innocent, well-known address appear to be the scanner. If automatic blocking reacts to that alert, it can deny legitimate access to a customer, monitoring service or administrator. Because PSAD evaluates traffic already dropped by the firewall, automatic blocking adds a response action to an event that was already unsuccessful at the firewall boundary; it does not improve the initial drop.
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- Run in alert-only mode long enough to establish normal sources and verify classification quality.
- Maintain an allowlist for trusted management networks before enabling any automated response.
- Review the generated firewall rules after a controlled test and record how to list, add and remove PSAD-managed blocks using the commands documented in the PSAD man page.
- Define a rollback path that works from the VPS console if SSH access is denied.
- Set an owner and review schedule for temporary blocks; do not allow an automated list to become permanent without oversight.
Enable automatic blocking only when the operational safeguards are stronger than the convenience it provides.
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Troubleshoot missing or noisy alerts
| Symptom | Likely cause | Action |
|---|---|---|
| No PSAD alert after an authorized scan | The packet matched an earlier accept rule, or no log rule ran before the drop. | Trace the packet through the chain and place logging after legitimate accepts but before the final deny. |
| A firewall log exists but PSAD ignores it | PSAD is reading a different file, or the search string does not match the line. | Confirm the active syslog path; with FW_SEARCH_ALL N, add the exact prefix to FW_MSG_SEARCH. |
| Alerts contain unrelated firewall traffic | FW_SEARCH_ALL Y is parsing messages outside the intended chain. |
Use a dedicated prefix and restrict matching, or refine the firewall logging policy. |
| Repeated alerts for expected checks | Monitoring, health checks or port knocking resembles a scan. | Verify the source and then use the narrowest suitable ignore setting. |
| Alerts arrive late | The polling interval, syslog delivery or daemon state is delaying processing. | Check that the daemons are running, confirm log timestamps and review CHECK_INTERVAL before changing it. |
| Administrators are locked out after a response rule | Automatic blocking treated a trusted or spoofed source as hostile. | Use console access to remove the PSAD-managed block, restore the allowlist and keep ENABLE_AUTO_IDS N while investigating. |
IPv6, nftables and deployment boundaries
The documented PSAD design is centered on iptables log messages. The cited documentation does not establish current native nftables support, so verify compatibility with the exact VPS image, firewall frontend and IPv4/IPv6 arrangement before relying on PSAD in that environment.
If IPv6 traffic is enabled, test it separately rather than assuming an IPv4 logging rule covers it. Confirm that the firewall family, logging format and syslog path used by the image are all supported by the PSAD package version you install.
Where PSAD fits in a VPS security plan
PSAD is a focused detector for firewall-visible reconnaissance and suspicious signatures. It is useful when you already operate an iptables policy and want classification, alerting and historical analysis without deploying a full network sensor. It does not inspect traffic that bypasses iptables logging, replace service hardening, or prove that an accepted connection is safe.
Evaluate it against these deployment questions:
- Does the host firewall produce consistent logs for denied traffic?
- Are the scan and signature types relevant to the services exposed by this VPS?
- Do syslog and email alerts reach an operator who can act on them?
- Can you replay historical logs while tuning?
- Can you maintain exclusions for known scanners without hiding unexpected sources?
- Does the target distribution’s IPv4, IPv6 and nftables setup match PSAD’s iptables-centered design?
- Is alert-only response sufficient, or can you safely operate a reviewed firewall-blocking workflow?
No performance benchmark is established here. Measure CPU, memory and log volume on your own VPS during normal traffic and during a controlled test, especially if the host already runs a busy firewall or writes large kernel logs.
Quick Recap
A practical operating routine
- After installation, verify daemon status and identify the real firewall-log file.
- Review the iptables chain order whenever an exposed service or allow rule changes.
- Check alert output on a schedule and investigate new sources, signatures and danger levels.
- Use
psad -Aagainst a copied historical log before deploying a threshold or exclusion change. - Audit
IGNORE_PORTS,IGNORE_INTERFACESandIGNORE_LOG_PREFIXESso exceptions still match current ownership and network layout. - Re-test from an authorized host after firewall, syslog or PSAD package upgrades.
- Keep automatic blocking disabled unless allowlists, console recovery and rule-review procedures are tested.
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