Flow Filtering

Flow filtering allows you to include or exclude network flows based on various criteria before they are exported, reducing data volume and focusing on relevant traffic.

Overview

Mermin supports filtering flows by:

• Source/destination IP addresses and ports

• Network protocols and interface names

• TCP flags, ICMP types

• Connection states

Filters use glob patterns aligned with OpenTelemetry Binary Instrumentation (OBI) conventions.

Configuration

filter "source" {
  address = {
    match = "10.0.0.0/8"
    not_match = "10.1.0.0/16"
  }
  port = {
    match = "80,443,8080-8090"
    not_match = ""
  }
}

filter "destination" {
  address = {
    match = ""
    not_match = "169.254.0.0/16"  # Exclude link-local
  }
  port = {
    match = ""
    not_match = ""
  }
}

filter "network" {
  transport = {
    match = "tcp,udp"
    not_match = ""
  }
  type = {
    match = "ipv4"
    not_match = ""
  }
  interface_name = {
    match = "eth*"
    not_match = ""
  }
}

filter "flow" {
  connection_state = {
    match = "established"
    not_match = ""
  }
  tcp_flags = {
    match = "SYN,ACK"
    not_match = "RST"
  }
}

Filter Syntax

Match Patterns

• match: Include flows matching this pattern (empty = match all)

• not_match: Exclude flows matching this pattern (takes precedence)

Glob Patterns

IP addresses and CIDRs:

• 10.0.0.0/8: CIDR notation

• 192.168.1.*: Glob wildcard

• 10.0.0.1,10.0.0.2: Comma-separated list

Ports:

• 80: Single port

• 80,443,8080: Multiple ports

• 8000-8999: Port range

Protocols:

• tcp,udp: Protocol names

• established,close_wait: Connection states

Source and Destination Filters

address

Filter by IP address.

Examples:

filter "source" {
  address = {
    match = "10.0.0.0/8"          # Only RFC1918
    not_match = "10.0.0.0/24"      # Exclude subnet
  }
}

port

Filter by port number.

Examples:

filter "source" {
  port = {
    match = "1024-65535"  # Only ephemeral ports
    not_match = ""
  }
}

Network Filters

transport

Filter by transport protocol.

Values: tcp, udp, icmp, icmpv6

type

Filter by IP version.

Values: ipv4, ipv6

interface_name

Filter by network interface.

Example:

filter "network" {
  interface_name = {
    match = "eth*"
    not_match = "docker*"
  }
}

Flow Filters

connection_state

Filter by TCP connection state.

Values: established, syn_sent, syn_received, fin_wait, close_wait, closing, last_ack, time_wait, closed

tcp_flags

Filter by TCP flags.

Values: SYN, ACK, FIN, RST, PSH, URG

Other Flow Attributes

• ip_dscp_name: DSCP value

• ip_ecn_name: ECN value

• ip_ttl: TTL value

• icmp_type_name: ICMP type

• icmp_code_name: ICMP code

Common Filtering Scenarios

HTTP/HTTPS Only

filter "destination" {
  port = {
    match = "80,443"
    not_match = ""
  }
}

Exclude Internal Traffic

filter "source" {
  address = {
    match = ""
    not_match = "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
  }
}

TCP Only, Established Connections

filter "network" {
  transport = {
    match = "tcp"
    not_match = ""
  }
}

filter "flow" {
  connection_state = {
    match = "established"
    not_match = ""
  }
}

Complete Example

# Source filters
filter "source" {
  address = { match = "", not_match = "" }
  port = { match = "", not_match = "" }
}

# Destination filters
filter "destination" {
  address = { match = "", not_match = "" }
  port = { match = "80,443", not_match = "" }  # HTTP/HTTPS only
}

# Network filters
filter "network" {
  transport = { match = "tcp,udp", not_match = "icmp" }
  type = { match = "ipv4", not_match = "" }
  interface_name = { match = "", not_match = "" }
}

# Flow filters
filter "flow" {
  connection_state = { match = "", not_match = "" }
  tcp_flags = { match = "", not_match = "" }
}

Best Practices

1. Start permissive: Begin with no filters, add as needed

2. Monitor impact: Check flow reduction with metrics

3. Test incrementally: Add one filter at a time

4. Document rationale: Comment why filters are applied

5. Use not_match carefully: Exclusions can hide important traffic

Next Steps

• Configuration Examples: See complete filter configurations

• Flow Span Options: Configure flow generation

• OTLP Export: Configure export options