n, sa, err := c.fd.readFrom(b)
	switch sa := sa.(type) {
	case *syscall.SockaddrInet4:
		addr = &IPAddr{IP: sa.Addr[0:]}
		n = stripIPv4Header(n, b)
	case *syscall.SockaddrInet6:
		addr = &IPAddr{IP: sa.Addr[0:], Zone: zoneCache.name(int(sa.ZoneId))}
	}
	return n, addr, err
}

func stripIPv4Header(n int, b []byte) int {
	if len(b) < 20 {
		return n
	}
	l := int(b[0]&0x0f) << 2
	if 20 > l || l > len(b) {
		return n
	}