Using AF_INET6 Socket to Send IPv4 UDP Communications

You can use the AF_INET6 socket for both IPv6 and IPv4 communications; IPv6 uses the POSIX function call getaddrinfo() rather than the IPv4 gethostbyname() function call. For IPv4 communications, create an AF_INET6 socket and pass it a sockaddr_in6 structure that contains an IPv4-mapped IPv6 address (for example, ::FFFF:1.2.3.4). The figure below shows the sequence of events for an application that uses an AF_INET6 socket to send IPv4 packets.

Figure 5-2 Title not available (Using AF_INET6 Socket to Send IPv4 UDP Communications)

Application calls getaddrinfo() and passes:
- the host name (host2).
- the AF_INET6 address family hint, which asks the Name Service for an IPv6 address corresponding to the host name.
- The AI_V4MAPPED flag hint, which tells the function that if the Name Service finds no IPv6 address but finds an IPv4 address for host2, return the IPv4 address within an IPv4-mapped IPv6 address. See getaddrinfo(3N) later in this document for a description of hints and flags values.
The search finds the IPv4 address 1.2.3.4 for host2 in the Name Service database.
Because getaddrinfo() had the AI_V4MAPPED flag set, the function returns the IPv4 -mapped address ::FFFF:1.2.3.4.
The application calls the socket() function to open an IPv6 AF_INET6 socket.
The application calls the sendto() function toward the ::FFFF:1.2.3.4 address.
The socket layer passes the sendto request, socket information and IPv4-mapped IPv6 address to the UDP/IP module.
The UDP/IP module:
1. identifies the IPv4-mapped IPv6 address.
2. puts the 1.2.3.4 address into an IPv4 packet header.
3. passes the packet to the IPv4 module for transmission.

Using AF_INET6 Socket to Send IPv4 UDP Communications

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