Spoofing attacks based on these similarities are known as homograph spoofing attacks. Phishing and business email compromise scams generally involve an element of email spoofing. Packet filtering is one defense against IP spoofing attacks. Systems that do not use cryptographic hashes are vulnerable to IP address spoofing attacks. SYN cookie is a technique used to resist IP Spoofing attacks. This behavior is the vulnerability that allows ARP spoofing to occur. This spoofing technique was a known vulnerability. Frequently, caller ID spoofing is used for prank calls. Some upper layer protocols provide their own defense against IP spoofing attacks. However, it is very hard to track hackers that are utilizing MAC spoofing. Others engage in spoofing of email addresses (much easier than IP address spoofing). Sending no referrer information is not technically spoofing, though sometimes also described as such. Thus, DNSCrypt helps to prevent DNS spoofing. MAC address spoofing is limited to the local broadcast domain. This kind of spoofing attack is also known as script spoofing.