caddyserver/caddy · error

generating random confirmation bytes: %v

Error message

generating random confirmation bytes: %v

What it means

Returned by `caddy start` when crypto/rand.Read fails while generating the 32 random confirmation bytes for the parent-child handshake. Go documents rand.Read as never failing on supported platforms (it panics on unsupported ones), so encountering this error is exceptional — it indicates a broken system entropy source in a custom build or unusual platform.

Source

Thrown at cmd/commandfuncs.go:111

		cmd.Args = append(cmd.Args, "--watch")
	}
	if pidfileFlag != "" {
		cmd.Args = append(cmd.Args, "--pidfile", pidfileFlag)
	}
	stdinPipe, err := cmd.StdinPipe()
	if err != nil {
		return caddy.ExitCodeFailedStartup,
			fmt.Errorf("creating stdin pipe: %v", err)
	}
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr

	// generate the random bytes we'll send to the child process
	expect := make([]byte, 32)
	_, err = rand.Read(expect)
	if err != nil {
		return caddy.ExitCodeFailedStartup,
			fmt.Errorf("generating random confirmation bytes: %v", err)
	}

	// begin writing the confirmation bytes to the child's
	// stdin; use a goroutine since the child hasn't been
	// started yet, and writing synchronously would result
	// in a deadlock
	go func() {
		_, _ = stdinPipe.Write(expect)
		stdinPipe.Close()
	}()

	// start the process
	err = cmd.Start()
	if err != nil {
		return caddy.ExitCodeFailedStartup,
			fmt.Errorf("starting caddy process: %v", err)
	}

View on GitHub (pinned to 50e54ee279)

Solutions

1. Verify entropy availability: cat /proc/sys/kernel/random/entropy_stat on Linux
2. Upgrade the Go toolchain used to build Caddy (modern rand.Read cannot return an error)
3. Run on a supported platform/kernel for the Go version in use

When it happens

Trigger: Effectively unreachable with standard Go on Linux/macOS/Windows; conceivable on platforms where getrandom(2) is unavailable such as very old kernels or restricted sandboxes using Go versions predating the fallback chain.

Common situations: Near-zero incidence; would surface only on exotic kernels, gVisor-style sandboxes with blocked entropy syscalls, or heavily patched Go runtimes.

Related errors

• reading envfile flag: %v
• opening listener for success confirmation: %v
• creating stdin pipe: %v
• starting caddy process: %v
• caddy process exited with error: %v

AI-assisted analysis of caddyserver/caddy@50e54ee279 (2026-08-15). Data as JSON: /api/errors/c34f73e7f68c7c3c. Report an issue: GitHub.