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Splunk Cheatsheet

Splunk is a powerful platform for searching, monitoring, and analyzing machine-generated data via a web-style interface. It's widely used for security information and event management (SIEM), IT operations, and business analytics.

Installation and Setup

Download and Install Splunk Enterprise

# Download Splunk Enterprise (Linux)
wget -O splunk-9.1.2-b6b9c8185839-linux-2.6-amd64.deb \
  'https://download.splunk.com/products/splunk/releases/9.1.2/linux/splunk-9.1.2-b6b9c8185839-linux-2.6-amd64.deb'

# Install on Ubuntu/Debian
sudo dpkg -i splunk-9.1.2-b6b9c8185839-linux-2.6-amd64.deb

# Install on CentOS/RHEL
sudo rpm -i splunk-9.1.2-b6b9c8185839.x86_64.rpm

# Start Splunk
sudo /opt/splunk/bin/splunk start --accept-license

# Enable boot start
sudo /opt/splunk/bin/splunk enable boot-start

Splunk Universal Forwarder

# Download Universal Forwarder
wget -O splunkforwarder-9.1.2-b6b9c8185839-linux-2.6-amd64.deb \
  'https://download.splunk.com/products/universalforwarder/releases/9.1.2/linux/splunkforwarder-9.1.2-b6b9c8185839-linux-2.6-amd64.deb'

# Install Universal Forwarder
sudo dpkg -i splunkforwarder-9.1.2-b6b9c8185839-linux-2.6-amd64.deb

# Start forwarder
sudo /opt/splunkforwarder/bin/splunk start --accept-license

Docker Installation

# Run Splunk Enterprise in Docker
docker run -d -p 8000:8000 -p 9997:9997 \
  -e SPLUNK_START_ARGS='--accept-license' \
  -e SPLUNK_PASSWORD='changeme123' \
  --name splunk splunk/splunk:latest

# Run Universal Forwarder in Docker
docker run -d -p 9997:9997 \
  -e SPLUNK_START_ARGS='--accept-license' \
  -e SPLUNK_PASSWORD='changeme123' \
  --name splunk-forwarder splunk/universalforwarder:latest

Basic Search Commands

Search Fundamentals

# Basic search
index=main error

# Search with time range
index=main error earliest=-24h latest=now

# Search multiple indexes
(index=main OR index=security) error

# Search with wildcards
index=main source="*access*" status=404

# Case-insensitive search
index=main Error OR error OR ERROR

Search Operators

# AND operator (implicit)
index=main error failed

# OR operator
index=main (error OR warning)

# NOT operator
index=main error NOT warning

# Field searches
index=main host=webserver01 sourcetype=access_combined

# Quoted strings
index=main "connection refused"

# Field existence
index=main user=*

# Field non-existence
index=main NOT user=*

Time Modifiers

# Relative time
earliest=-1h latest=now
earliest=-7d@d latest=@d
earliest=-1mon@mon latest=@mon

# Absolute time
earliest="01/01/2024:00:00:00" latest="01/31/2024:23:59:59"

# Snap to time
earliest=-1d@d latest=@d  # Yesterday from midnight to midnight
earliest=@w0 latest=@w6   # This week from Sunday to Saturday

Data Processing Commands

Field Extraction

# Extract fields with regex
index=main|rex field=_raw "(?<username>\w+)@(?<domain>\w+\.\w+)"

# Extract multiple fields
index=main|rex "user=(?<user>\w+).*ip=(?<ip>\d+\.\d+\.\d+\.\d+)"

# Extract with named groups
index=main|rex field=message "Error: (?<error_code>\d+) - (?<error_msg>.*)"

# Extract fields from specific field
index=main|rex field=url "\/(?<category>\w+)\/(?<item>\w+)"

Field Operations

# Create new fields
index=main|eval new_field=field1+field2

# String operations
index=main|eval upper_user=upper(user)
index=main|eval user_domain=user."@".domain

# Conditional fields
index=main|eval status_desc=case(
  status>=200 AND status<300, "Success",
  status>=400 AND status<500, "Client Error",
  status>=500, "Server Error",
  1=1, "Unknown"
)

# Mathematical operations
index=main|eval response_time_ms=response_time*1000
index=main|eval percentage=round((part/total)*100, 2)

Data Transformation

# Remove fields
index=main|fields - _raw, _time

# Keep only specific fields
index=main|fields user, ip, action

# Rename fields
index=main|rename src_ip as source_ip, dst_ip as dest_ip

# Convert field types
index=main|eval response_time=tonumber(response_time)
index=main|eval timestamp=strftime(_time, "%Y-%m-%d %H:%M:%S")

Statistical Commands

Basic Statistics

# Count events
index=main|stats count

# Count by field
index=main|stats count by user

# Multiple statistics
index=main|stats count, avg(response_time), max(bytes) by host

# Distinct count
index=main|stats dc(user) as unique_users

# List unique values
index=main|stats values(user) as users by host

Advanced Statistics

# Percentiles
index=main|stats perc50(response_time), perc95(response_time), perc99(response_time)

# Standard deviation
index=main|stats avg(response_time), stdev(response_time)

# Range and variance
index=main|stats min(response_time), max(response_time), range(response_time), var(response_time)

# First and last values
index=main|stats first(user), last(user) by session_id

Time-based Statistics

# Statistics over time
index=main|timechart span=1h count by status

# Average over time
index=main|timechart span=5m avg(response_time)

# Multiple metrics over time
index=main|timechart span=1h count, avg(response_time), max(bytes)

# Fill null values
index=main|timechart span=1h count|fillnull value=0

Filtering and Sorting

Where Command

# Filter results
index=main|stats count by user|where count > 100

# String comparisons
index=main|where like(user, "admin%")
index=main|where match(email, ".*@company\.com")

# Numeric comparisons
index=main|where response_time > 5.0
index=main|where bytes > 1024*1024  # Greater than 1MB

Search and Where

# Search command for filtering
index=main|search user=admin OR user=root

# Complex search conditions
index=main|search (status>=400 AND status<500) OR (response_time>10)

# Search with wildcards
index=main|search user="admin*" OR user="*admin"

Sorting

# Sort ascending
index=main|stats count by user|sort user

# Sort descending
index=main|stats count by user|sort -count

# Multiple sort fields
index=main|stats count, avg(response_time) by user|sort -count, user

# Sort with limit
index=main|stats count by user|sort -count|head 10

Advanced Search Techniques

Subsearches

# Basic subsearch
index=main user=[search index=security action=login|return user]

# Subsearch with formatting
index=main [search index=security failed_login|stats count by user|where count>5|format]

# Subsearch with specific return
index=main ip=[search index=blacklist|return ip]

Joins

# Inner join
index=main|join user [search index=user_info|fields user, department]

# Left join
index=main|join type=left user [search index=user_info|fields user, department]

# Join with multiple fields
index=main|join user, host [search index=inventory|fields user, host, asset_tag]

Lookups

# CSV lookup
index=main|lookup user_lookup.csv user OUTPUT department, manager

# Automatic lookup (configured in transforms.conf)
index=main|lookup geoip clientip

# External lookup
index=main|lookup dnslookup ip OUTPUT hostname

Transactions

# Group events into transactions
index=main|transaction user startswith="login" endswith="logout"

# Transaction with time constraints
index=main|transaction user maxspan=1h maxpause=10m

# Transaction with event count
index=main|transaction session_id maxevents=100

# Transaction statistics
index=main|transaction user|stats avg(duration), count by user

Visualization and Reporting

Chart Commands

# Simple chart
index=main|chart count by status

# Chart over time
index=main|chart count over _time by status

# Chart with functions
index=main|chart avg(response_time), max(response_time) over host by status

# Chart with bins
index=main|chart count over response_time bins=10

Top and Rare

# Top values
index=main|top user

# Top with limit
index=main|top limit=20 user

# Top by another field
index=main|top user by host

# Rare values
index=main|rare user

# Top with percentage
index=main|top user showperc=true

Geostats

# Geographic statistics
index=main|iplocation clientip|geostats count by Country

# Geostats with latfield and longfield
index=main|geostats latfield=latitude longfield=longitude count by region

# Geostats with globallimit
index=main|iplocation clientip|geostats globallimit=10 count by City

Security and SIEM Use Cases

Failed Login Detection

# Failed login attempts
index=security sourcetype=linux_secure "Failed password"
|rex field=_raw "Failed password for (?<user>\w+) from (?<src_ip>\d+\.\d+\.\d+\.\d+)"
|stats count by user, src_ip
|where count > 5
|sort -count

Brute Force Detection

# Brute force attack detection
index=security action=login result=failure
|bucket _time span=5m
|stats dc(user) as unique_users, count as attempts by src_ip, _time
|where attempts > 20 OR unique_users > 10
|sort -attempts

Privilege Escalation

# Sudo usage monitoring
index=security sourcetype=linux_secure "sudo"
|rex field=_raw "sudo:\s+(?<user>\w+)\s+:\s+(?<command>.*)"
|stats count, values(command) as commands by user
|sort -count

Network Traffic Analysis

# Large data transfers
index=network
|stats sum(bytes_out) as total_bytes by src_ip, dest_ip
|where total_bytes > 1073741824  # 1GB
|eval total_gb=round(total_bytes/1073741824, 2)
|sort -total_gb

Malware Detection

# Suspicious process execution
index=endpoint process_name=*
|search (process_name="*.tmp" OR process_name="*.exe" OR process_name="powershell.exe")
|stats count, values(command_line) as commands by host, process_name
|where count > 10

Performance Optimization

Search Optimization

# Use specific indexes
index=main sourcetype=access_combined

# Filter early in search
index=main error earliest=-1h|stats count by host

# Use fast commands first
index=main|where status=404|stats count by uri

# Avoid wildcards at the beginning
index=main uri="/api/*" NOT uri="*debug*"

Field Extraction Optimization

# Extract only needed fields
index=main|rex field=_raw "user=(?<user>\w+)"|fields user, _time

# Use field extraction in search
index=main user=admin|stats count

# Limit field extraction scope
index=main|head 1000|rex field=_raw "pattern"

Memory and CPU Optimization

# Use summary indexing for frequent searches
|collect index=summary_index source="daily_stats"

# Use report acceleration
|sistats count by user

# Limit search scope
index=main earliest=-1h latest=now|head 10000

Configuration and Administration

Index Management

# Create new index
/opt/splunk/bin/splunk add index myindex -maxDataSize 1000 -maxHotBuckets 10

# List indexes
/opt/splunk/bin/splunk list index

# Clean index
/opt/splunk/bin/splunk clean eventdata -index myindex

User Management

# Add user
/opt/splunk/bin/splunk add user username -password password -role user

# List users
/opt/splunk/bin/splunk list user

# Change user password
/opt/splunk/bin/splunk edit user username -password newpassword

Data Input Configuration

# Monitor file
/opt/splunk/bin/splunk add monitor /var/log/messages -index main

# Monitor directory
/opt/splunk/bin/splunk add monitor /var/log/ -index main

# Network input
/opt/splunk/bin/splunk add tcp 9999 -sourcetype syslog

Forwarder Configuration

# Add forward server
/opt/splunkforwarder/bin/splunk add forward-server splunk-server:9997

# Add monitor to forwarder
/opt/splunkforwarder/bin/splunk add monitor /var/log/apache2/access.log -index web

# List forward servers
/opt/splunkforwarder/bin/splunk list forward-server

REST API Usage

Authentication

# Get session key
curl -k -u admin:password https://localhost:8089/services/auth/login \
  -d username=admin -d password=password

# Use session key
curl -k -H "Authorization: Splunk <session_key>" \
  https://localhost:8089/services/search/jobs

Search API

# Create search job
curl -k -u admin:password https://localhost:8089/services/search/jobs \
  -d search="search index=main|head 10"

# Get search results
curl -k -u admin:password \
  https://localhost:8089/services/search/jobs/<sid>/results \
  --get -d output_mode=json

Data Input API

# List data inputs
curl -k -u admin:password https://localhost:8089/services/data/inputs/monitor

# Add monitor input
curl -k -u admin:password https://localhost:8089/services/data/inputs/monitor \
  -d name=/var/log/myapp.log -d index=main

Troubleshooting

Common Issues

# Check Splunk status
/opt/splunk/bin/splunk status

# Check license usage
/opt/splunk/bin/splunk list licenser-localslave

# Restart Splunk
/opt/splunk/bin/splunk restart

# Check configuration
/opt/splunk/bin/splunk btool inputs list
/opt/splunk/bin/splunk btool outputs list

Log Analysis

# Check Splunk internal logs
tail -f /opt/splunk/var/log/splunk/splunkd.log

# Check metrics
tail -f /opt/splunk/var/log/splunk/metrics.log

# Check audit logs
tail -f /opt/splunk/var/log/splunk/audit.log

Performance Monitoring

# Internal Splunk metrics
index=_internal source=*metrics.log group=per_index_thruput
|stats sum(kb) as total_kb by series
|sort -total_kb

# Search performance
index=_audit action=search
|stats avg(total_run_time) as avg_runtime by user
|sort -avg_runtime

# License usage
index=_internal source=*license_usage.log type=Usage
|stats sum(b) as bytes by idx
|eval GB=round(bytes/1024/1024/1024,2)
|sort -GB

Best Practices

Search Best Practices

  1. Use specific time ranges - Avoid searching "All time"
  2. Filter early - Use index, sourcetype, and host filters first
  3. Use fast commands - stats, chart, timechart are faster than transaction
  4. Avoid wildcards - Especially at the beginning of search terms
  5. Use summary indexing - For frequently run searches

Data Onboarding Best Practices

  1. Plan index strategy - Separate indexes by data type and retention
  2. Configure sourcetypes - Proper field extraction and parsing
  3. Set up proper time extraction - Ensure accurate timestamps
  4. Use Universal Forwarders - For distributed data collection
  5. Monitor license usage - Stay within license limits

Security Best Practices

  1. Use role-based access - Limit user permissions appropriately
  2. Enable SSL - For web interface and forwarder communication
  3. Regular backups - Backup configuration and critical data
  4. Monitor admin activities - Track configuration changes
  5. Keep Splunk updated - Apply security patches regularly

Resources