If your scan engine already holds credentials for a host, it can ask that host which ports are open instead of probing for them.
Every scan begins with the same question: which ports on this host are open? Everything after it, from identifying services to checking for vulnerabilities to evaluating policy, depends on the answer being right. The traditional answer comes from the outside: the scan engine sends traffic to a range of ports and infers each port's state from how the host responds. That approach is the industry standard, and it works well when a clear network path exists between the engine and the host. Hardened hosts can stay silent rather than replying, which forces the engine to wait out timeouts. Rate limiting and intrusion prevention can throttle a burst of probes, and genuinely open ports go missing when they do. Large port ranges take time to cover thoroughly, and that time comes out of your scan window.
There is a more direct route on any host where the scan engine already holds valid credentials: ask the host itself. This is credentialed discovery, so a credential that matches the host is the precondition for everything that follows. The engine connects to the port that credential uses, authenticates with a credential you already manage, and the host's operating system returns an authoritative list of the ports it is listening on. That list covers both TCP and UDP ports. There is no probing, no inference, and nothing to wait out.
Three things to know before enabling pre-port discovery
Pre-port discovery can report ports that a firewall or other network control stops your scan engine from reaching, and on those hosts you get fewer results and a longer scan.
SSH and the Scan Assistant are tried on their standard ports, TCP 22 and TCP 21047, unless you set a different port on the credential's restriction.
Credential coverage decides which hosts benefit, and a host with no matching credential falls back to a network port scan.
Each of these is covered in full in the configuration and troubleshooting documentation.
Why probing from the outside can hit a wall
A network port scan works by inference. The engine sends traffic to each port in a configured range and reads the host's response, or its silence, as evidence about that port's state. Inference is the whole method, and its accuracy depends on the path between the engine and the host behaving predictably.
Several common conditions break that assumption. A hardened host that drops unsolicited traffic instead of refusing it gives the engine nothing to work with, so the engine waits for a timeout and then records an ambiguous result. Rate limiting and intrusion prevention are built to react to exactly the traffic pattern a port scan produces, and a throttled probe looks the same to the engine as a closed port. Wide port ranges make both problems worse, because every additional port is another probe, another possible timeout, and more scan time.
The outcome is a picture that can be partial on one scan and different on the next, on the hosts where an accurate picture matters most.
If you already have credentials, ask the host
Credentialed pre-port discovery replaces that inference with a question, and it is available from version 8.58. The engine connects to the port a credential uses, authenticates, and reads the list of listening ports from the host. For any host where that succeeds, the engine skips the network port scan and moves straight to examining the ports the host reported. That is what pre-port discovery means: discovering ports before, and in place of, the network port scan.
There is nothing new to deploy, because pre-port discovery reuses the credentials you already configure for authenticated scanning. You do not have to choose a method: when more than one credential fits a host, the engine prefers the Scan Assistant, then SSH, then a direct Windows connection, and it uses the first one that authenticates.
A host with no matching credential, or one where authentication does not succeed, falls back to a network port scan automatically, and that fallback is not reported as an error.
The option is a per-template checkbox under Asset Discovery, and it is off by default. Everything after port discovery is unchanged: fingerprinting, vulnerability checks, and policy evaluation run as they do today. The configuration guide has the console and REST API steps.
Pre-port discovery's trade-off, stated plainly
A host reports what it is listening on, and it has no way of knowing what sits between it and your scan engine. A network port scan never ran into that, because it only ever reported a port it could actually reach. Pre-port discovery trades that outside-in view for the host's authoritative inside-out view, and the trade has a cost worth understanding first.
After discovery, the engine still connects to each reported port to identify the service on it. A port the engine cannot reach has to time out before the engine moves on. Three things follow on that host: no service is identified on the unreachable port, the scan takes longer, and the engine can read repeated connection failures as a sign that the host has stopped responding. In that case it stops examining the host early and reports a finding saying the host scan was terminated because of excessive connection errors.
The finding describes the engine's experience of the host, not the health of the host. The port is genuinely open, and the host answered every question pre-port discovery asked it. The troubleshooting documentation covers what can block the path and how to test reachability from the engine.
Who should turn on pre-port discovery?
Pre-port discovery is a good fit where the scan engine has broad network reachability to the hosts it scans, and where you already use SSH, Scan Assistant, or Windows credentials for authenticated scanning. It pays off most on hardened or rate-limited hosts and on large port ranges, which are the cases where a network port scan has been slow or inconsistent. A responsive host on a fast network may show little difference.
Approach it with more care where the engine is deliberately segmented from the hosts it scans and allowed through on only specific ports, or where the firewall rules between the engine and those hosts are restrictive or not fully known. In those environments, confirm reachability first, or keep using the network port scan.
Because this is a per-template setting, both approaches can coexist: a pre-port discovery template for the estate your engine can reach broadly, and a standard template for the hosts that segmentation deliberately keeps at a distance.
Try it on one template
One template at a time is the easiest way to judge the difference. Enable pre-port discovery on a single template, scan a representative group of hosts with it, and compare the results against what those same hosts returned before. If the port lists and the scan times look the way you expect, widen it from there. The configuration and troubleshooting guide has the details.
Further reading
Credentialed pre-port discovery: How to enable it in the console and over the REST API, how it picks a credential and a port, what your template's port settings still control, and what to check when a host does not behave the way you expect.