The PIX became the first commercially available firewall product to introduce protocol specific filtering with the introduction of the "fixup" command. The PIX "fixup" capability allows the firewall to apply additional security policies to connections identified as using specific protocols. Protocols for which specific fixup behaviors were developed include DNS and SMTP. The DNS fixup originally implemented a very simple but effective security policy; it allowed just one DNS response from a DNS server on the Internet (known as "outside" interface) for each DNS request from a client on the protected (known as "inside") interface. "Inspect" has superseded "fixup" in later versions of PIX OS.