Cybersecurity & Information Security

Zeek Logs: How to Investigate conn.log, dns.log, and http.log

7 min readPublished: August 5, 2026

Professional visual illustration on Zeek Logs in Network Security Monitoring

Quick answer

Zeek Logs investigation involves mapping Flow, timings, protocols, DNS/TLS/HTTP, and context to the asset. A single packet or connection is partial evidence, so a sequence is built and validated against additional sources.

Network traffic provides a perspective that is not solely dependent on the endpoint. It allows identification of who talked to whom, using what protocol, in what order, and at what volume, but requires an understanding of the limits of visibility and encryption. This article focuses on Zeek Logs and is intended for SOC analysts and Network Hunters. The goal is to provide a working method that can be applied in practice, in a professional interview, and in a work environment, without settling for a dictionary definition.

The main challenge is that the data is almost always partial. Query name, response code, TTL can indicate a direction, but their meaning depends on time, asset, user, and expected activity. Therefore, we will build the investigation around an inquiry question, required evidence, and clear completion criteria.

The practical scenario in the article is: investigating one session through four Logs. All examples are lab data or process descriptions. When it comes to Penetration Testing, Web, or Cloud, work only with explicit authorization, defined Scope, and the ability to stop the test.

Logs Structure and UID

The important fields are not necessarily those displayed at the top of the screen. In Zeek Logs, stable identifiers, time, source, destination, result, and context must be identified. Useful examples include query name, response code, TTL, subdomain entropy, NXDOMAIN ratio, resolver context. The goal is to enable Correlation between records and not just reading a single Event.

It is recommended to create a small Data dictionary: field name, meaning, format, source, expected Null values, and whether it is reliable for linking. This helps distinguish between a display field and an investigative identifier, and to identify when a Connector or version has changed the Schema.

conn.log

The topic 'conn.log' is a central part of working with Zeek Logs. It is recommended to break it down into three questions: what is the input, what decision do you want to make, and what evidence is sufficient to justify it. These questions prevent automatic tool use without understanding the goal.

In practice, record the query name, response code, TTL, subdomain entropy, NXDOMAIN ratio, compare to expected behavior, and define at least one Pivot. The result should be verifiable by another analyst, including limitations and next steps.

dns.log

The topic 'dns.log' is a central part of working with Zeek Logs. It is recommended to break it down into three questions: what is the input, what decision do you want to make, and what evidence is sufficient to justify it. These questions prevent automatic tool use without understanding the goal.

http.log and tls.log

The topic 'http.log and tls.log' is a central part of working with Zeek Logs. It is recommended to break it down into three questions: what is the input, what decision do you want to make, and what evidence is sufficient to justify it. These questions prevent automatic tool use without understanding the goal.

Correlation and Timeline

Timeline is the backbone of Zeek Logs. Normalize times to UTC or explicitly state the time zone, save both Event time and Ingestion time, and connect events by stable identifiers. The line should include time, source, entity, action, result, and reliability.

A gap or contradiction is not a document error but a finding. Clock drift, ingestion delay, NAT, reuse of PID, or a continuous session can change the order. Therefore, uncertainty ranges are noted, and a link back to the raw evidence is maintained.

When to return to PCAP

The topic 'When to return to PCAP' is a central part of working with Zeek Logs. It is recommended to break it down into three questions: what is the input, what decision do you want to make, and what evidence is sufficient to justify it. These questions prevent automatic tool use without understanding the goal.

Unique Examination Focus Points

In this topic, it is recommended to build a focused evidence map in advance. The main examination points are: query name, response code, TTL, subdomain entropy, NXDOMAIN ratio, resolver context, uid, conn.log, dns.log, http.log. The list is not an automatic Checklist; each item is chosen because it can link an entity, action, and time or explain legitimate behavior.

query name: Define the expected value, what would be considered abnormal, and what additional source would validate the finding.
response code: Define the expected value, what would be considered abnormal, and what additional source would validate the finding.
TTL: Define the expected value, what would be considered abnormal, and what additional source would validate the finding.
subdomain entropy: Define the expected value, what would be considered abnormal, and what additional source would validate the finding.
NXDOMAIN ratio: Define the expected value, what would be considered abnormal, and what additional source would validate the finding.
resolver context: Define the expected value, what would be considered abnormal, and what additional source would validate the finding.

When one of the focus points is unavailable, document the gap and choose an alternative. For example, if a Process identifier is not stable, you can use time, Host, User, and Parent; if Payload is encrypted, use Metadata, volume, frequency, and TLS/DNS context.

Recommended Workflow

Define the Scope and one working question regarding Zeek Logs.
List the data sources and required evidence: query name, response code, TTL, subdomain entropy.
Create a brief Baseline of normal behavior or expected outcome.
Perform the minimal test in a lab environment and save time, input, and output.
Build a Timeline or comparison table and separate facts from interpretation.
Pivot to an additional source to confirm or refute the initial explanation.
Summarize the decision, limitations, recommended action, and Retest criteria.

Practical Scenario

The chosen scenario is investigating one session through four Logs. The purpose of the exercise is not to prove attack capability, but to practice collecting, comparing, and documenting in a secure manner. Before starting, define simulated data, a time window, and an expected outcome.

At the end of the exercise, a product should be submitted that another analyst or tester can review: a screenshot or Export of the evidence, a short Timeline, an initial assumption, confirming evidence, a limitation, and a recommendation. When there is insufficient evidence, the correct conclusion is that the scenario was not proven.

Stage	What to perform	Output
Preparation	Define Scope, time, and target. List which fields or evidence from query name, response code, TTL are expected to appear.	Short test plan
Data Creation	Perform a safe, simulated action related to Zeek Logs, without real information or impact on a production system.	Controlled event/Request/Flow
Collection	Collect raw evidence and context from an additional source. Verify Time zone, identifiers, and integrity.	Two linked pieces of evidence
Analysis	Write what each piece of evidence proves, what it doesn't prove, and what is the possible legitimate explanation.	Interim conclusion
Completion	Choose closure, escalation, Finding, or Tuning; add a recommendation and Retest.	Documented output

Practical Checklist

Check and document: the five components of the Flow.
Check and document: start time, duration, and volume.
Check and document: DNS name and TLS metadata.
Check and document: HTTP method, host, and URI when visible.
Check and document: TCP flags and stream.
Check and document: connection to Host and Process.
Note Time zone, tool version, and collection time.
Save raw data before filtering or modification.
Write what the finding proves and what is still unknown.
Define owner and next action with a deadline.

Common Mistakes

Confusing Capture Filter with Display Filter.
Inferring content when traffic is encrypted.
Analyzing IP without DNS/TLS context.
Ignoring NAT or Proxy.
Focusing on a single Packet.
Not saving original Capture.

Summary and CTA

Zeek Logs: How to Investigate conn.log, dns.log, and http.log is a topic that combines technical knowledge with work discipline. Start with a question, collect only relevant evidence, preserve context and time, and choose an action that can be justified and re-tested.

In HPI's Cybersecurity & AI track, these principles are practiced using systems, logs, and labs. A natural next step is to go through the linked articles, perform the lab exercise, and save the output as part of a professional portfolio.

FAQ

Does Zeek Logs alone prove an attack or vulnerability?

No. It provides a signal or a finding that requires context, validation, and an additional source. A professional conclusion relies on a sequence of evidence and alignment with expected behavior.

What do you do when some data is missing?

Document the missing information, check for an alternative source, and reduce the level of certainty. Do not fill in fields by conjecture or present Unknown as normal.

How long should evidence be retained?

The retention time depends on policy, regulation, cost, and event type. It is important to pre-define Retention, Legal hold, and the ability to export evidence in a verifiable format.

How do you practice without risking a real system?

Use virtual machines, simulated data, CTF, or a dedicated lab. In authorized tests, define Scope, Stop conditions, and backup before starting work.