What causes "invalid engine %s" in probelabs/goreplay?

Use exactly one of: pcap, raw_socket, af_packet, vxlan

invalid engine %s

Error message

invalid engine %s

What it means

EngineType.Set parses a user-supplied engine string into the internal EngineType enum. Recognized values are pcap, raw_socket, af_packet and vxlan; any other value returns "invalid engine <value>". It is a config-parsing validation error, thrown before any capture begins.

Source

Thrown at internal/capture/capture.go:125

	EngineAFPacket
	EngineVXLAN
)

// Set is here so that EngineType can implement flag.Var
func (eng *EngineType) Set(v string) error {
	switch v {
	case "", "libpcap":
		*eng = EnginePcap
	case "pcap_file":
		*eng = EnginePcapFile
	case "raw_socket":
		*eng = EngineRawSocket
	case "af_packet":
		*eng = EngineAFPacket
	case "vxlan":
		*eng = EngineVXLAN
	default:
		return fmt.Errorf("invalid engine %s", v)
	}
	return nil
}

func (eng *EngineType) String() (e string) {
	switch *eng {
	case EnginePcapFile:
		e = "pcap_file"
	case EnginePcap:
		e = "libpcap"
	case EngineRawSocket:
		e = "raw_socket"
	case EngineAFPacket:
		e = "af_packet"
	case EngineVXLAN:
		e = "vxlan"
	default:
		e = ""

View on GitHub (pinned to 251e45abd2)

Solutions

Use exactly one of: pcap, raw_socket, af_packet, vxlan
Lowercase and trim the value before assigning (engine parsing is case-sensitive)
Log/print the invalid value to spot the typo (it is embedded in the error message)
Normalize old config names in a migration map if upgrading between versions

Example fix

// before
cfg.Engine.Set("afpacket") // error: invalid engine afpacket
// after
cfg.Engine.Set("af_packet")

Defensive patterns

Strategy: validation

Validate before calling

var validEngines = map[string]bool{"pcap": true, "raw_socket": true, "af_packet": true, "vxlan": true}
func validEngine(v string) bool { return validEngines[strings.ToLower(strings.TrimSpace(v))] }

Try / catch

if err := cfg.Engine.Set(v); err != nil {
    return fmt.Errorf("engine %q: %w (valid: pcap, raw_socket, af_packet, vxlan)", v, err)
}

Prevention

Use only the exact strings: pcap, raw_socket, af_packet, vxlan
Normalize config values with strings.ToLower before parsing
Validate engine names at config-load time, not at capture time

When it happens

Trigger: Setting the listener config's engine (e.g. config.Engine.Set("<value>") or a config file field) to any string other than "pcap", "raw_socket", "af_packet", or "vxlan".

Common situations: Typo in config file (e.g. "afpacket" without underscore, "rawsocket"); upper/lowercase mismatch ("PCAP"); renaming engines across versions; copy-pasting engine names from other tools.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of probelabs/goreplay@251e45abd2 (2026-09-02). Data as JSON: /api/errors/3f54c0b3484867b5. Report an issue: GitHub.