Imported from kubeshark/kubeshark (skills/kfl/SKILL.md). Install upstream with npx skills add kubeshark/kubeshark --skill kfl. Copyright stays with the author.

KFL2 — Kubeshark Filter Language

You are a KFL2 expert. KFL2 is built on Google's CEL (Common Expression Language) and is the query language for all Kubeshark traffic analysis. It operates as a display filter — it doesn't affect what's captured, only what you see.

Think of KFL the way you think of SQL for databases or Google search syntax for the web. Kubeshark captures and indexes all cluster traffic; KFL is how you search it.

For the complete variable and field reference, see references/kfl2-reference.md.

Core Syntax

KFL expressions are boolean CEL expressions. An empty filter matches everything.

Operators

String Functions

str.contains(substring)      // Substring search
str.startsWith(prefix)       // Prefix match
str.endsWith(suffix)         // Suffix match
str.matches(regex)           // Regex match
size(str)                    // String length

Collection Functions

size(collection)             // List/map/string length
key in map                   // Key existence
map[key]                     // Value access
map_get(map, key, default)   // Safe access with default
value in list                // List membership

Time Functions

timestamp("2026-03-14T22:00:00Z")   // Parse ISO timestamp
duration("5m")                        // Parse duration
now()                                 // Current time (snapshot at filter creation)

Negation

!http                                // Everything that is NOT HTTP
http && status_code != 200           // HTTP responses that aren't 200
http && !path.contains("/health")    // Exclude health checks
!(src.pod.namespace == "kube-system")  // Exclude system namespace

Protocol Detection

Boolean flags that indicate which protocol was detected. Use these as the first filter term — they're fast and narrow the search space immediately.

Kubernetes Context

The most common starting point. Filter by where traffic originates or terminates.

Pod and Service Fields

src.pod.name == "orders-594487879c-7ddxf"
dst.pod.namespace == "production"
src.service.name == "api-gateway"
dst.service.namespace == "payments"

Pod fields fall back to service data when pod info is unavailable, so dst.pod.namespace works even for service-level entries.

Summary Name and Namespace

Convenience variables that pick the best available identity for a peer:

src.name == "api-gateway"                    // pod > service > dns > process
dst.name.contains("payment")                 // works across identity types
src.namespace == "production"                 // pod namespace, falls back to service
dst.namespace != "kube-system"               // exclude system namespace

Aggregate Collections

Match against any direction (src or dst):

"production" in namespaces           // Any namespace match
"orders" in pods                     // Any pod name match
"api-gateway" in services            // Any service name match

Labels and Annotations

map_get(local_labels, "app", "") == "checkout"   // Safe access with default
map_get(remote_labels, "version", "") == "canary"
"tier" in local_labels                            // Label existence check

Always use map_get() for labels and annotations — direct access like local_labels["app"] errors if the key doesn't exist.

Node and Process

node_name == "ip-10-0-25-170.ec2.internal"
local_process_name == "nginx"
remote_process_name.contains("postgres")

DNS Resolution

src.dns == "api.example.com"
dst.dns.contains("redis")

HTTP Filtering

HTTP is the most common protocol for API-level investigation.

Fields

Common Patterns

// Error investigation
http && status_code >= 500                           // Server errors
http && status_code == 429                           // Rate limiting
http && status_code >= 400 && status_code < 500      // Client errors

// Endpoint targeting
http && method == "POST" && path.contains("/orders")
http && url.matches(".*/api/v[0-9]+/users.*")

// Performance
http && elapsed_time > 5000000                       // > 5 seconds
http && response_body_size > 1000000                 // > 1MB responses

// Header inspection
http && "authorization" in request.headers
http && request.headers["content-type"] == "application/json"

// GraphQL (subset of HTTP)
gql && method == "POST" && status_code >= 400

// Only eBPF-intercepted TLS traffic (decrypted HTTPS)
tls && http && status_code >= 500

Note on tls: The tls flag is an alias for capture_source == "ebpf_tls". It indicates traffic captured via eBPF TLS interception, not TLS protocol dissection.

DNS Filtering

DNS issues are often the hidden root cause of outages.

dns && "api.external-service.com" in dns_questions
dns && dns_response && status_code != 0              // Failed lookups
dns && "A" in dns_question_types                     // A record queries
dns && size(dns_questions) > 1                       // Multi-question

Database and Messaging Protocols

Redis

redis && redis_type == "GET"                         // Command type
redis && redis_key.startsWith("session:")            // Key pattern
redis && redis_command.contains("DEL")               // Command search
redis && redis_total_size > 10000                    // Large operations

Kafka

kafka && kafka_api_key_name == "PRODUCE"             // Produce operations
kafka && kafka_client_id == "payment-processor"      // Client filtering
kafka && kafka_request_summary.contains("orders")    // Topic filtering
kafka && kafka_size > 10000                          // Large messages

MongoDB

mongodb && mongodb_command == "find"                  // Find operations
mongodb && mongodb_collection == "users"              // Collection filtering
mongodb && mongodb_database == "mydb"                 // Database filtering
mongodb && !mongodb_success                           // Failed operations
mongodb && mongodb_error_code != 0                    // Error code filtering
mongodb && mongodb_total_size > 10000                 // Large operations

MySQL

mysql && mysql_command == "COM_QUERY"                 // SQL queries
mysql && mysql_query.contains("SELECT")               // SELECT statements
mysql && mysql_database == "orders_db"                // Database filtering
mysql && !mysql_success                               // Failed queries
mysql && mysql_error_code != 0                        // Error code filtering
mysql && mysql_total_size > 10000                     // Large queries

PostgreSQL

postgresql && postgresql_command == "COM_QUERY"              // Query commands
postgresql && postgresql_query.contains("SELECT")            // SELECT statements
postgresql && postgresql_database == "orders_db"             // Database filtering
postgresql && postgresql_user == "admin"                     // User filtering
postgresql && !postgresql_success                            // Failed queries
postgresql && postgresql_error_code != ""                    // Error code filtering (SQLSTATE string)
postgresql && postgresql_total_size > 10000                  // Large queries

Note: postgresql_error_code is a string (SQLSTATE code like "23505"), not an int. This differs from MySQL's mysql_error_code which is an int.

gRPC

gRPC is a sub-protocol of HTTP/2. All HTTP variables are also available on gRPC entries.

grpc && grpc_method == "SayHello"                     // Method filtering
grpc && grpc_status != 0                              // Non-OK status codes
grpc && grpc_status == 14                             // UNAVAILABLE
grpc && grpc_method.contains("Create")                // Method pattern
grpc && elapsed_time > 1000000                        // Slow gRPC calls (>1s)

AMQP, LDAP, RADIUS, Diameter

amqp && amqp_method == "basic.publish"               // AMQP publish
ldap && ldap_type == "bind"                          // LDAP bind requests
radius && radius_code_name == "Access-Request"       // RADIUS auth
diameter && diameter_method.contains("Credit")       // Diameter credit control

For the full variable list for these protocols, see references/kfl2-reference.md.

Transport Layer (L4)

TCP/UDP Fields

tcp && tcp_error_type != ""                          // TCP errors
udp && udp_length > 1000                             // Large UDP packets

Connection Tracking

conn && conn_state == "open"                         // Active connections
conn && conn_local_bytes > 1000000                   // High-volume
conn && "HTTP" in conn_l7_detected                   // L7 protocol detection
tcp_conn && conn_state == "closed"                   // Closed TCP connections

Flow Tracking (with Rate Metrics)

flow && flow_local_pps > 1000                        // High packet rate
flow && flow_local_bps > 1000000                     // High bandwidth
flow && flow_state == "closed" && "TLS" in flow_l7_detected
tcp_flow && flow_local_bps > 5000000                 // High-throughput TCP

Network Layer

src.ip == "10.0.53.101"
dst.ip.startsWith("192.168.")
src.port == 8080
dst.port >= 8000 && dst.port <= 9000

Time-Based Filtering

timestamp > timestamp("2026-03-14T22:00:00Z")
timestamp >= timestamp("2026-03-14T22:00:00Z") && timestamp <= timestamp("2026-03-14T23:00:00Z")
timestamp > now() - duration("5m")                   // Last 5 minutes
elapsed_time > 2000000                               // Latency > 2 seconds

Building Filters: Progressive Narrowing

The most effective investigation technique — start broad, add constraints:

// Step 1: Protocol + namespace
http && dst.pod.namespace == "production"

// Step 2: Add error condition
http && dst.pod.namespace == "production" && status_code >= 500

// Step 3: Narrow to service
http && dst.pod.namespace == "production" && status_code >= 500 && dst.service.name == "payment-service"

// Step 4: Narrow to endpoint
http && dst.pod.namespace == "production" && status_code >= 500 && dst.service.name == "payment-service" && path.contains("/charge")

// Step 5: Add timing
http && dst.pod.namespace == "production" && status_code >= 500 && dst.service.name == "payment-service" && path.contains("/charge") && elapsed_time > 2000000

Performance Tips

1. Protocol flags first — http && ... is faster than ... && http
2. startsWith/endsWith over contains — prefix/suffix checks are faster
3. Specific ports before string ops — dst.port == 80 is cheaper than url.contains(...)
4. Use map_get for labels — avoids errors on missing keys
5. Keep filters simple — CEL short-circuits on &&, so put cheap checks first

Type Safety

KFL2 is statically typed. Common gotchas:

• status_code is int, not string — use status_code == 200, not "200"
• elapsed_time is in microseconds — 5 seconds = 5000000
• timestamp requires timestamp() function — not a raw string
• Map access on missing keys errors — use key in map or map_get() first
• List membership uses value in list — not list.contains(value)