analyzing-network-traffic-for-incidents

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Summary

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SKILL.md

Analyzing Network Traffic for Incidents

When to Use

SIEM alerts on anomalous network traffic patterns requiring deeper investigation
C2 beaconing is suspected and needs confirmation through packet-level analysis
Data exfiltration volume or destination must be quantified from network evidence
Lateral movement between systems needs to be traced through network connections
An IDS/IPS alert requires packet-level validation to confirm or dismiss

Do not use for host-based forensic analysis (process execution, file system artifacts); use endpoint forensics tools instead.

Prerequisites

Full packet capture (PCAP) infrastructure or on-demand capture capability (network tap, SPAN port)
Wireshark installed on the analysis workstation with appropriate display filters knowledge
Zeek (formerly Bro) deployed for network metadata generation (conn.log, dns.log, http.log, ssl.log)
NetFlow/IPFIX collection from network devices for traffic flow analysis
Network architecture diagram showing VLAN layout, firewall placement, and monitoring points
Threat intelligence feeds for correlating observed network indicators

Workflow

Step 1: Capture or Acquire Network Traffic

Obtain the relevant traffic data for the investigation:

Live Capture (if incident is active):

# Capture on specific interface filtering by host
tcpdump -i eth0 -w capture.pcap host 10.1.5.42

# Capture C2 traffic to specific external IP
tcpdump -i eth0 -w c2_traffic.pcap host 185.220.101.42

# Capture with rotation (1GB files, keep 10)
tcpdump -i eth0 -w capture_%Y%m%d%H%M.pcap -C 1000 -W 10

From Existing Infrastructure:

Export PCAP from full packet capture appliance (Arkime/Moloch, ExtraHop, Corelight)
Pull Zeek logs from the Zeek cluster for the investigation timeframe
Export NetFlow data from network devices for high-level traffic analysis

# Extract connections to external IPs with regular intervals
cat conn.log | zeek-cut ts id.orig_h id.resp_h id.resp_p duration orig_bytes resp_bytes \
  | awk '$4 ~ /^185\.220/' | sort -t. -k1,1n -k2,2n

# Filter for traffic to suspected C2 IP
ip.addr == 185.220.101.42

# Filter HTTPS traffic to non-standard ports
tcp.port != 443 && ssl

# Filter DNS queries for suspicious domains
dns.qry.name contains "evil" or dns.qry.name matches "^[a-z0-9]{32}\."

# Filter HTTP POST (common C2 check-in method)
http.request.method == "POST" && ip.dst == 185.220.101.42

Key protocols for lateral movement detection:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
SMB (TCP 445):     PsExec, file share access, ransomware propagation
RDP (TCP 3389):    Remote desktop sessions
WinRM (TCP 5985):  PowerShell remoting
WMI (TCP 135):     Remote command execution
SSH (TCP 22):      Linux lateral movement
DCE/RPC (TCP 135): DCOM-based lateral movement

# SMB lateral movement
smb2 && ip.src == 10.1.5.42 && ip.dst != 10.1.5.42

# RDP connections from compromised host
tcp.dstport == 3389 && ip.src == 10.1.5.42

# Kerberos ticket requests (potential pass-the-ticket)
kerberos.msg_type == 12 && ip.src == 10.1.5.42

# NTLM authentication (potential pass-the-hash)
ntlmssp.auth.username && ip.src == 10.1.5.42

# Identify large outbound transfers in Zeek conn.log
cat conn.log | zeek-cut ts id.orig_h id.resp_h id.resp_p orig_bytes \
  | awk '$5 > 100000000' | sort -t$'\t' -k5 -rn

# DNS tunneling detection (high volume of TXT queries)
cat dns.log | zeek-cut query qtype | grep TXT | cut -f1 \
  | rev | cut -d. -f1,2 | rev | sort | uniq -c | sort -rn | head

# Unusual protocol usage (ICMP tunneling, DNS over HTTPS)
cat conn.log | zeek-cut proto id.resp_p orig_bytes | awk '$1 == "icmp" && $3 > 1000'

# Large HTTP POST uploads
http.request.method == "POST" && tcp.len > 10000

# FTP data transfers
ftp-data && ip.src == 10.0.0.0/8

# DNS with large TXT responses (tunneling)
dns.resp.type == 16 && dns.resp.len > 200

Term	Definition
PCAP (Packet Capture)	File format storing raw network packets captured from a network interface for offline analysis
Beaconing	Regular, periodic network connections from a compromised host to a C2 server, identifiable by consistent timing intervals
JA3/JA3S	TLS client and server fingerprinting method based on the ClientHello and ServerHello parameters; unique per application
NetFlow/IPFIX	Network traffic metadata (source, destination, ports, bytes, duration) collected by routers and switches without full packet capture
DNS Tunneling	Technique encoding data in DNS queries and responses to exfiltrate data or maintain C2 through DNS protocol
Network Tap	Hardware device that creates an exact copy of network traffic for monitoring without impacting network performance
Zeek Logs	Structured metadata logs generated by the Zeek network analysis framework covering connections, DNS, HTTP, SSL, and more

NETWORK TRAFFIC ANALYSIS REPORT
=================================
Incident:         INC-2025-1547
Analyst:          [Name]
Capture Source:   Arkime full packet capture
Analysis Period:  2025-11-15 14:00 UTC - 2025-11-15 18:00 UTC
Total PCAP Size:  4.7 GB

C2 COMMUNICATIONS
Source:           10.1.5.42 (WKSTN-042)
Destination:      185.220.101.42:443 (HTTPS)
Beacon Interval:  60 seconds ± 12% jitter
Sessions:         237 connections over 4 hours
JA3 Hash:         a0e9f5d64349fb13191bc781f81f42e1
TLS Certificate:  CN=update.evil[.]com (self-signed)
Total Data Sent:  147 MB (outbound)
Total Data Recv:  2.3 MB (inbound - commands)

LATERAL MOVEMENT
10.1.5.42 → 10.1.10.15 (SMB, TCP 445) - 14:35 UTC
10.1.5.42 → 10.1.10.20 (RDP, TCP 3389) - 14:42 UTC
10.1.5.42 → 10.1.1.5  (LDAP, TCP 389) - 15:10 UTC

EXFILTRATION SUMMARY
Protocol:         HTTPS to C2 server
Volume:           147 MB outbound
Duration:         14:23 UTC - 18:00 UTC
Files Extracted:  [list if recoverable from unencrypted channels]

DNS ANALYSIS
Suspicious Queries: 0 DNS tunneling indicators
DGA Detection:      0 algorithmically generated domains

EVIDENCE REFERENCES
PCAP File:        INC-2025-1547_capture.pcap (SHA-256: ...)
Zeek Logs:        /logs/zeek/2025-11-15/ (conn.log, ssl.log, dns.log)

analyzing-network-traffic-for-incidents

Summary

SKILL.md

Analyzing Network Traffic for Incidents

When to Use

Prerequisites

Workflow

Step 1: Capture or Acquire Network Traffic

Step 2: Identify C2 Communications

Step 3: Analyze Lateral Movement Traffic

Step 4: Detect Data Exfiltration

Step 5: Extract and Correlate IOCs

Step 6: Document Network Forensic Findings

Key Concepts

Tools & Systems

Common Scenarios

Scenario: Confirming C2 Beaconing and Quantifying Exfiltration

Output Format

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