Network Incident Response — Network Forensics

Network-specific evidence collection and analysis during security incidents. This skill covers network artifacts only: packet captures, flow records (NetFlow/sFlow/IPFIX), ARP/MAC/CAM tables, routing table state, and device syslog events. It does not cover general incident response lifecycle (NIST 800-61), endpoint forensics, malware analysis, or organizational communication plans.

The procedure follows an event-driven lifecycle shaped around forensic evidence: preserve volatile data → triage scope → detect lateral movement → verify containment → reconstruct timeline → document findings. All commands are read-only. Containment verification confirms that previously applied controls are effective — it does not execute containment actions.

Commands use [Cisco], [JunOS], or [EOS] vendor labels where syntax diverges. See references/cli-reference.md for the full command reference and references/forensics-workflow.md for evidence methodology, chain-of-custody templates, and timeline reconstruction guidance.

When to Use

• Active security incident requiring network-level evidence collection (packet captures, flow analysis, device logs)
• Post-incident network forensics — reconstructing what happened on the network after a confirmed security event
• Lateral movement investigation — tracing attacker movement between internal hosts using flow records, ARP/MAC table changes, and routing state analysis
• Unauthorized access investigation — identifying how an external or internal actor reached target systems via network path analysis
• Data exfiltration analysis — quantifying outbound data transfers via flow record byte counts and packet capture content analysis
• Containment verification — confirming (read-only) that ACLs, null routes, or VLAN isolation applied by responders are blocking attacker traffic effectively

Prerequisites

• Device CLI access — read-only access to network devices in the incident scope is sufficient for all evidence collection commands. No enable/configure privilege is required.
• Flow collection infrastructure — NetFlow, sFlow, or IPFIX collectors must be receiving exports from network devices. Verify with flow export commands in references/cli-reference.md. Without flow data, lateral movement analysis (Step 3) is limited to ARP/MAC/syslog correlation.
• Centralized logging — device syslog events must be forwarded to a SIEM or syslog server. Local device log buffers are small and rotate quickly. Missing centralized logs create timeline gaps.
• NTP synchronization — all devices must be time-synchronized. Verify with [Cisco] show ntp status, [JunOS] show system ntp, [EOS] show ntp status. Skewed clocks corrupt timeline correlation.
• Known-good baseline — saved copies of routing tables, ARP tables, and device configurations from before the incident for comparison. Without baselines, anomaly detection relies on general heuristics rather than delta analysis.

Procedure

Follow these six steps in order. Earlier steps capture volatile evidence before it ages out; later steps analyze and document. Each step references specific commands from references/cli-reference.md and methodology from references/forensics-workflow.md.

Step 1: Evidence Preservation

Capture volatile network evidence before it ages out or is overwritten. Follow the volatility ordering from references/forensics-workflow.md — most volatile first.

1a. ARP / MAC / CAM tables (highest volatility — minutes to hours):

Collect the current ARP and MAC address tables from every device in the incident scope. These tables map IP addresses to MAC addresses and MAC addresses to physical switch ports — essential for identifying which hosts were connected where.

• [Cisco] — show arp and show mac address-table
• [JunOS] — show arp no-resolve and show ethernet-switching table
• [EOS] — show arp and show mac address-table

Save output to files with timestamps. ARP entries typically age out in 4 hours; CAM entries in 5 minutes. Delay here means losing L2 mapping.

1b. Active packet captures (real-time — exists only while traffic flows):

If the incident is active and the investigation requires payload-level evidence, initiate packet captures on relevant interfaces immediately.

• [Cisco] — monitor capture CAP1 interface <intf> both then monitor capture CAP1 start — export with monitor capture CAP1 export flash:evidence.pcap
• [JunOS] — monitor traffic interface <intf> write-file /var/tmp/capture.pcap
• [EOS] — bash tcpdump -i <intf> -w /mnt/flash/evidence.pcap -c 10000

Performance note: On-device packet capture consumes CPU. Monitor device health during capture and set packet count or duration limits.

1c. Routing table snapshots (hours — convergence overwrites state):

• [Cisco] — show ip route and show ip route summary
• [JunOS] — show route and show route summary
• [EOS] — show ip route and show ip route summary

Also capture routing protocol adjacency state (show ip ospf neighbor, show ip bgp summary or vendor equivalents) to document peering status at the time of collection.

1d. Flow export verification (hours to days — collector retention):

Confirm that flow data from the incident time window is available in the flow collector. Verify export is active and records exist:

• [Cisco] — show flow monitor and show flow exporter <name> statistics
• [JunOS] — show services flow-monitoring version-ipfix template and show services accounting status
• [EOS] — show flow tracking and show flow tracking counters

1e. Device configuration and comprehensive state:

Save the running configuration and full technical support output for each device in scope:

• [Cisco] — show tech-support | redirect flash:tech-<hostname>-<date>.txt
• [JunOS] — request support information | save /var/tmp/tech-<hostname>-<date>.txt
• [EOS] — show tech-support | redirect flash:tech-<hostname>-<date>.txt

Compute SHA-256 hashes immediately after saving evidence files (see references/forensics-workflow.md for hash verification commands).

Step 2: Initial Triage

Determine the scope of the incident — affected devices, time window, involved IP addresses — using log and flow data collected in Step 1.

Identify the time window: Find the earliest indicator (first alert, first anomalous event) and the latest known malicious activity. Add a buffer of ±2 hours to account for undetected precursor activity.

Identify involved IPs: Extract unique source and destination IP addresses from alerts, SIEM events, and flow records within the time window. Classify each as internal, external, or infrastructure.

Identify affected devices: Determine which network devices handled traffic to/from involved IPs. Use routing tables to trace the forwarding path and identify all transit devices.

Scope assessment output: A list of (1) affected time window, (2) involved IP addresses with classification, (3) affected network devices, and (4) evidence types available for each device. This scoping drives the depth of Steps 3–5.

Step 3: Lateral Movement Detection

Trace internal-to-internal connections that indicate attacker movement between hosts. Lateral movement leaves evidence in flow records (new internal connections), ARP/MAC tables (new L2 entries), and syslog (authentication events, new sessions).

Flow record analysis: Query the flow collector for internal-to-internal connections involving known compromised IPs during the incident time window. Look for:

• Connections to ports commonly used for lateral movement (SMB/445, RDP/3389, SSH/22, WinRM/5985, WMI/135)
• Connections from a compromised host to hosts it has never contacted before (new destination analysis)
• High byte-count transfers between internal hosts (staging or exfil prep)
• Sequential connections from one host to many hosts in a short time window (scanning behavior)

ARP/MAC table analysis: Compare current ARP/MAC tables against baseline captures. Look for:

• New MAC addresses on access ports (rogue devices)
• MAC address appearing on a different port than baseline (device moved or MAC spoofing)
• Multiple IP addresses mapped to a single MAC (IP aliasing, potential MITM)

Syslog correlation: Review authentication events on network devices during the incident window. Attacker lateral movement often involves:

• Failed authentication attempts from internal IPs against network device management interfaces
• Successful logins from unexpected source IPs
• Configuration view commands from unusual user accounts

Step 4: Containment Verification (Read-Only)

Verify that containment measures applied by the incident response team are functioning as intended. This step is strictly read-only — it confirms effectiveness, it does not apply containment.

ACL hit count verification: Confirm that blocking ACLs are matching the attacker's traffic. Rising hit counters on deny rules confirm the ACL is intercepting traffic.

• [Cisco] — show access-lists <containment-acl-name> — check hit counters on deny entries
• [JunOS] — show firewall filter <containment-filter> — check term counters for deny actions
• [EOS] — show access-lists <containment-acl-name> — check per-entry match counts

Routing containment verification: If null routes or route modifications were applied for containment, verify they are present and effective:

• Confirm the null route exists in the routing table (show ip route <attacker-prefix> should show Null0/discard)
• Verify no more-specific routes bypass the null route
• Check routing protocol advertisements to confirm containment routes are not being overridden by dynamic protocols

Network isolation verification: If VLAN isolation was applied, verify that the isolated segment has no unintended paths:

• Check the routing table for routes to/from the isolated VLAN
• Verify trunk port allowed VLAN lists exclude the isolated VLAN on uplinks
• Confirm no layer-3 interfaces provide alternative paths

Step 5: Timeline Reconstruction

Build a unified chronological sequence of network events from all evidence sources. This timeline is the primary deliverable of network forensics investigation.

Source integration: Merge events from syslog, flow records, routing changes, and ARP/MAC transitions into a single timeline sorted by UTC timestamp. Follow the full timeline reconstruction methodology in references/forensics-workflow.md.

Key timeline elements:

1. Anchor events — high-confidence events that serve as fixed points (first alert, interface state changes, BGP/OSPF adjacency changes)
2. Correlated events — events linked by shared IP addresses, timestamps, or session identifiers across multiple devices
3. Gaps — time periods with missing evidence from devices that should have been active (document explicitly as uncertainty)
4. Phase transitions — points where activity shifts from reconnaissance to access, access to lateral movement, lateral movement to objective or exfiltration

Timeline validation: Cross-reference the reconstructed timeline against multiple evidence sources. Events confirmed by two or more independent sources (e.g., firewall deny in syslog + flow record for same session) are high confidence. Single-source events are medium confidence.

Step 6: Post-Incident Documentation

Compile investigation findings into a structured evidence package. This documentation supports organizational incident response and any subsequent legal or compliance review.

Required documentation artifacts:

• Evidence inventory with chain-of-custody records (use the template in references/forensics-workflow.md)
• Reconstructed timeline of network events (from Step 5)
• Lateral movement map showing affected hosts and connection paths (from Step 3, if lateral movement was detected)
• Containment verification results (from Step 4)
• List of affected network devices with evidence types collected
• Identified gaps in evidence and their impact on conclusions

Threshold Tables

Evidence Priority Classification

Containment Verification Criteria

Decision Trees

Evidence Collection Priority

Incident reported
├── Is the incident currently active?
│   ├── Yes — Active threat
│   │   ├── Is payload-level evidence needed?
│   │   │   ├── Yes → Start packet capture immediately (Step 1b)
│   │   │   └── No → Proceed to ARP/MAC collection (Step 1a)
│   │   └── Simultaneously: collect ARP/MAC tables (Step 1a)
│   │       └── Then: routing snapshots (Step 1c) → flow verification (Step 1d)
│   │
│   └── No — Post-incident investigation
│       ├── How long ago did the incident occur?
│       │   ├── < 4 hours → ARP/MAC tables may still have entries (Step 1a)
│       │   ├── 4–24 hours → ARP tables likely aged out; start with flow data
│       │   └── > 24 hours → Rely on syslog and flow collector retention
│       └── Verify flow data and syslog coverage for incident window (Steps 1d, 1e)
│
├── Has containment been applied?
│   ├── Yes → Add containment verification (Step 4) after triage
│   │   └── Check ACL counters, null routes, VLAN isolation
│   └── No → Skip Step 4, proceed through Steps 1–3, 5–6
│
└── Proceed to initial triage (Step 2)

Report Template

NETWORK FORENSICS EVIDENCE SUMMARY
=====================================
Incident Reference:   [ticket/tracking number]
Investigation Period: [start] — [end] (UTC)
Network Scope:        [number] devices across [number] sites
Analyst:              [name/identifier]
Collection Date:      [date evidence collection began]

EVIDENCE INVENTORY:
| # | Device | Evidence Type | File | SHA-256 | Collected At |
|---|--------|--------------|------|---------|-------------|
| 1 | [host] | [type] | [file] | [hash] | [time UTC] |

INCIDENT TIMELINE:
| # | Time (UTC) | Device | Event | Details | Confidence |
|---|-----------|--------|-------|---------|------------|
| 1 | [time] | [host] | [event] | [details] | [H/M/L] |

LATERAL MOVEMENT MAP (if detected):
- Source host → destination host : port (first seen, last seen, byte count)
- [list all observed internal-to-internal attacker paths]

CONTAINMENT VERIFICATION:
| Control | Device | Status | Evidence |
|---------|--------|--------|----------|
| [ACL/route/VLAN] | [host] | [Effective/Bypassed] | [counter values] |

EVIDENCE GAPS:
- [device/time period with missing evidence and impact on conclusions]

RECOMMENDATIONS:
1. [network-level remediation or monitoring improvement]

Troubleshooting

Insufficient Flow Data Coverage

Symptom: Flow records do not exist for devices or time windows critical to the investigation.

Diagnosis: Verify flow export configuration on each device using commands in references/cli-reference.md. Check collector storage — retention may have expired for the incident time window.

Workaround: Substitute with syslog events (lower fidelity but covers event timestamps) and ARP/MAC table correlation. Document the flow gap and its impact on lateral movement analysis completeness.

Time Synchronization Gaps

Symptom: Events from different devices appear out of order or correlation produces implausible sequences.

Diagnosis: Check NTP status on each device. Compare timestamps of events that should be near-simultaneous (e.g., both ends of a link-down event logged by adjacent devices).

Workaround: Calculate clock offset per device and apply correction to the timeline. Note the correction in evidence documentation. Reduce correlation confidence for events involving desynchronized devices.

Evidence Overwritten by Log Rotation

Symptom: Syslog events from the incident time window no longer exist on the device or in the SIEM.

Diagnosis: Check device log buffer size (show logging to see buffer capacity and oldest retained message). Check SIEM retention policy for the relevant index.

Workaround: Use flow records or SNMP trap history as alternative event sources. Note the syslog gap in the timeline with an explicit confidence reduction for that time period.

Packet Capture Performance Impact

Symptom: Device CPU spikes or forwarding performance degrades during on-device packet capture.

Diagnosis: Monitor CPU utilization during capture. On-device capture processes packets in software, bypassing hardware forwarding.

Workaround: Limit captures with ACL filters (capture only relevant traffic), set packet count limits (-c flag), use span/mirror sessions to an external capture appliance instead of on-device capture, or reduce capture duration. If performance impact is unacceptable, stop capture and rely on flow records for metadata-level analysis.

Incomplete ARP/MAC Table Recovery

Symptom: ARP or MAC address tables are mostly empty — entries have already aged out by the time evidence collection begins.

Diagnosis: Default ARP aging is 4 hours; default CAM aging is 5 minutes. If more than 4 hours have elapsed since the incident, ARP entries for inactive hosts will be gone.

Workaround: Cross-reference DHCP lease logs for IP-to-MAC mappings during the incident window. Use flow records to identify involved IP addresses without L2 mapping. Check if any NMS polled ARP/MAC tables via SNMP during the incident window.