getaddrinfo Reading IP Address value from res->ai_addr

[Marc Majka]

The value is a struct sockaddr, which is commonly used by IP networking code to hold an address.  The definition of a "struct sockaddr" (see <sys/sockaddr.h>) is mostly a storage container.  It specifies what kind of address is in the container (the "address family"), and then just has a buffer that holds the value of the address.  

In many cases, there's no need to interpret what's stored in a sockaddr at all.  Most networking APIs use sockaddr structures.  So, for example, you can call getaddrinfo() to obtain one or more addresses (as sockaddrs) for some service that you want to contact, then just try making a socket connection with the connect() call.  You get your socket, but you don't really need to know what kind of address (the address family) was used.   This means that the same code will work with any kind of networking that's supported by your operating system.  Your code really doesn't need to know if the system used IPv4, IPv6, or perhaps some other networking protocol to make the connection, since all you really want is a socket.

If you need to dig inside the sockaddr, your code will need to first look at the sa_family variable.  Then it will need to handle each type of address family that you expect (or need to support).  Typically, you can simply cast the "struct sockaddr" type to another structure that breaks down the raw data buffer part of the sockaddr into meaningful components.  For example, if the address family is IPv4 (AF_INET - see <sys/socket.h>) then you can cast the struct sockaddr to a "struct sockaddr_in" (see <netinet/in.h>).  That structure breaks down the sockaddr address buffer into a port (TCP or UDP port number) and an IPv4 address.  If the family is AF_INET6, then you can cast to a "struct sockaddr_in6" (see <netinet6/in6.h>) where the sockaddr address is broken down into a port number, IPv6 flow information, an IPv6 address, and a scope ID.


--
Marc Majka

On 13 Jan, 2012, at 08:59, Martin McCormick wrote:

> I am experimenting with getaddrinfo and getnameinfo and have
> gotten a little confused as to the best way to extract the
> IP address recovered after the function runs. The element in the
> structure 
> res->ai_addr is a socket address which, if I am reading the
> documentation correctly, will give me the binary form of the
> internet address of the host I just looked up.
> 
> 	Is that correct? Also, what is the easiest way to
> extract that value in to an unsigned long?
> 
> 	Wikipedia has an excellent sample program that uses
> getaddrinfo to resolve a name to an IP address and then
> getnameinfo to reverse the lookup back to a name, but
> getnameinfo just takes res->ai_addr as an argument and works
> great, but one doesn't see how it extracted the IP address.
> 
> 	Thanks for all good suggestions.
> 
> Martin McCormick WB5AGZ  Stillwater, OK 
> Systems Engineer
> OSU Information Technology Department Telecommunications Services Group
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