Does recvfrom() on an IPv6 socket always return an IPv6 address in 'from' parameter?-CodePudding

Does recvfrom() on an IPv6 socket always return an IPv6 address in the from parameter? My concern is if an IPv6 socket communicates with an IPv4 target by IPv4 mapped IPv6 address, is it possible that recvfrom() returns an IPv4 address directly?

CodePudding user response：

An AF_INET (IPv4) socket only works with IPv4 addresses.

An AF_INET6 (IPv6) socket only works with IPv6 addresses.

A dual-stack socket is an AF_INET6 socket with the IPV6_V6ONLY option turned off so it can work with both IPv4 and IPv6 peers. Since it is an IPv6 socket, IPv4 addresses will always be represented as IPv4-mapped IPv6 addresses. This is clearly stated in the documentation:

IP Addresses with a Dual-Stack Socket

Dual-stack sockets always require IPv6 addresses. The ability to interact with an IPv4 address requires the use of the IPv4-mapped IPv6 address format. Any IPv4 addresses must be represented in the IPv4-mapped IPv6 address format which enables an IPv6 only application to communicate with an IPv4 node. The IPv4-mapped IPv6 address format allows the IPv4 address of an IPv4 node to be represented as an IPv6 address. The IPv4 address is encoded into the low-order 32 bits of the IPv6 address, and the high-order 96 bits hold the fixed prefix 0:0:0:0:0:FFFF. The IPv4-mapped IPv6 address format is specified in RFC 4291. For more information, see www.ietf.org/rfc/rfc4291.txt. The IN6ADDR_SETV4MAPPED macro in Mstcpip.h can be used to convert an IPv4 address to the required IPv4-mapped IPv6 address format.

If the underlying protocol is actually IPv4, then the IPv4 address is mapped into an IPv4-mapped IPv6 address format. That is the, family field in the SOCKADDR structure indicates AF_INET6, but an IPv4-mapped IPv6 address is encoded in the IPv6 address structure. For a dual-stack socket in listening mode, this means that any accepted IPv4 connections will return an IPv4-mapped IPv6 address. For a dual-stack socket that is connecting to an IPv4 destination, the SOCKADDR structure passed to connect must be an IPv4-mapped IPv6 address. Applications must take care to handle these IPv4-mapped IPv6 addresses appropriately and only use them with dual stack sockets. If an IP address is to be passed to a regular IPv4 socket, the address must be a regular IPv4 address not a IPv4-mapped IPv6 address.