slackhq/nebula · error

unexpected PacketConn: %T %#v

Error message

unexpected PacketConn: %T %#v

What it means

Type-guard failure at the end of the Darwin NewListener: ListenPacket returned something that is not a *net.UDPConn (its concrete type and value are printed). Practically unreachable with the "udp" network string; if hit, it means an unexpected netpoll wrapper was returned.

Source

Thrown at udp/udp_darwin.go:61

		}

		err = rc.Control(func(fd uintptr) {
			c.sysFd = fd
		})
		if err != nil {
			return nil, fmt.Errorf("failed to get udp fd: %w", err)
		}

		la, err := c.LocalAddr()
		if err != nil {
			return nil, err
		}
		c.isV4 = la.Addr().Is4()

		return c, nil
	}

	return nil, fmt.Errorf("unexpected PacketConn: %T %#v", pc, pc)
}

func NewListenConfig(multi bool) net.ListenConfig {
	return net.ListenConfig{
		Control: func(network, address string, c syscall.RawConn) error {
			if multi {
				var controlErr error
				err := c.Control(func(fd uintptr) {
					if err := syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
						controlErr = fmt.Errorf("SO_REUSEPORT failed: %v", err)
						return
					}
				})
				if err != nil {
					return err
				}
				if controlErr != nil {
					return controlErr

View on GitHub (pinned to dd8f660c0a)

Solutions

1. This is a defensive assertion: on macOS the listener first tries to obtain a udp_sock via Control-based fd extraction from a *net.UDPConn and returns early; reaching this error means the ListenPacket-based fallback returned a PacketConn that is neither a udp_sock nor a *net.UDPConn
2. Check which nebula UDP path is active on this Darwin build; a non-standard build tag or stubbed network stack can produce an unexpected PacketConn concrete type
3. Update Go and nebula: changes in net package behavior across Go versions can alter the concrete type returned by ListenPacket
4. If reproducible, file a nebula issue with the %T/%#v detail printed in the error — it names the unexpected concrete type

When it happens

Trigger: Thrown at udp/udp_darwin.go:61 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of slackhq/nebula@dd8f660c0a (2026-09-03). Data as JSON: /api/errors/a0c64f8f21782079. Report an issue: GitHub.