What causes "failed to get hostname: %w" in kubernetes/kops?

Test SSH connectivity manually: ssh @ hostname

failed to get hostname: %w

Error message

failed to get hostname: %w

What it means

getHostname runs `hostname` over SSH and wraps any failure from runCommand. Since the hostname becomes the node name for registration, kOps aborts bootstrap data generation when it cannot be determined. All SSH-layer failures (session open, auth, non-zero exit) surface wrapped here.

Source

Thrown at pkg/commands/toolbox_enroll.go:419

		// We send both to stderr, so we don't "corrupt" stdout
		session.Stdout = io.MultiWriter(os.Stderr, session.Stdout)
		session.Stderr = io.MultiWriter(os.Stderr, session.Stderr)
	}
	if s.sudo {
		command = "sudo " + command
	}
	if err := session.Run(command); err != nil {
		return output, fmt.Errorf("error running command %q: %w", command, err)
	}
	return output, nil
}

// getHostname gets the hostname of the SSH target.
// This is used as the node name when registering the node.
func (s *SSHHost) getHostname(ctx context.Context) (string, error) {
	output, err := s.runCommand(ctx, "hostname", ExecOptions{Echo: true})
	if err != nil {
		return "", fmt.Errorf("failed to get hostname: %w", err)
	}

	hostname := output.Stdout.String()
	hostname = strings.TrimSpace(hostname)
	if len(hostname) == 0 {
		return "", fmt.Errorf("hostname was empty")
	}
	return hostname, nil
}

type BootstrapData struct {
	// NodeupScript is a script that can be used to bootstrap the node.
	NodeupScript []byte
	// NodeupConfig is structured configuration, provided by kops-controller (for example).
	NodeupConfig *nodeup.Config
	// NodeupScriptAdditionalFiles are additional files that are needed by the nodeup script.
	NodeupScriptAdditionalFiles map[string][]byte
}

View on GitHub (pinned to 4c8573c808)

Solutions

Test SSH connectivity manually: ssh <user>@<host> hostname
Check SSH user/key and sudo configuration used by kops toolbox enroll
Confirm the node's sshd is up and the security group/firewall allows port 22
Inspect the wrapped cause with errors.Unwrap / %v for the root SSH error

Example fix

// before
hostname, err := host.getHostname(ctx) // opaque failure
// after
hostname, err := host.getHostname(ctx)
if err != nil {
    return fmt.Errorf("enroll: check SSH access to %s: %w", host.addr, err)
}

Defensive patterns

Strategy: try-catch

Validate before calling

out, err := exec.Command("ssh", "-o", "BatchMode=yes", user+"@"+host, "hostname").Output()
if err != nil || len(strings.TrimSpace(string(out))) == 0 {
    return fmt.Errorf("cannot determine hostname of %s", host)
}

Type guard

func isHostnameLookupFailure(err error) bool {
    return err != nil && strings.Contains(err.Error(), "failed to get hostname")
}

Try / catch

hostname, err := host.getHostname(ctx)
if err != nil {
    return fmt.Errorf("verify SSH access/credentials for node enrollment: %w", err)
}

Prevention

Test SSH access with the exact user/key before enrolling
Confirm sshd is running and port 22 is open
Avoid custom shells/profiles that interfere with non-interactive commands

When it happens

Trigger: buildHostData calls getHostname and the SSH `hostname` command fails: connection dropped, authentication rejected, command exited non-zero, or sudo is misconfigured.

Common situations: Wrong SSH credentials/user in enroll flags; host unreachable or firewalled; sshd not running on a freshly provisioned node; sudo not permitted for the enroll user.

Related errors

AI-assisted analysis of kubernetes/kops@4c8573c808 (2026-09-05). Data as JSON: /api/errors/c12f3d835537e1a5. Report an issue: GitHub.