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Beej's Guide to Network Programming

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Saved by 9 people (-2 private), first by anonymouse user on 2008-01-24


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You'll see that the ai_addr field in the struct addrinfo is a pointer to a struct sockaddr. This is where we start getting into the nitty-gritty details of what's inside an IP address structure.

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oftentimes, a call to getaddrinfo() to fill out your struct addrinfo for you is all you'll need. You will, however, have to peer inside these structs to get the values out, so I'm presenting them here.

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So if you have declared ina to be of type struct sockaddr_in, then ina.sin_addr.s_addr references the 4-byte IP address (in Network Byte Order).

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destination address

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And this is the important bit: a pointer to a struct sockaddr_in can be cast to a pointer to a struct sockaddr and vice-versa. So even though connect() wants a struct sockaddr*, you can still use a struct sockaddr_in and cast it at the last minute!

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sin_port must be in Network Byte Order (by using htons()

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struct sockaddr_storage that is designed to be large enough to hold both IPv4 and IPv6 structures.

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What's important is that you can see the address family in the ss_family field—check this to see if it's AF_INET or AF_INET6 (for IPv4 or IPv6). Then you can cast it to a struct sockaddr_in or struct sockaddr_in6 if you wanna.

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bunch of functions that allow you to manipulate IP addresses.

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inet_pton(), converts an IP address in numbers-and-dots notation into either a struct in_addr or a struct in6_addr

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inet_ntop()

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try to use getaddrinfo() to get all the struct sockaddr info, instead of packing the structures by hand.

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The place most people get stuck around here is what order to call these things in.

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getaddrinfo()

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It helps set up the structs you need later on.

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You give this function three input parameters, and it gives you a pointer to a linked-list, res, of results.

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The node parameter is the host name to connect to, or an IP address.

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Next is the parameter service, which can be a port number, like "80", or the name of a particular service

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Finally, the hints parameter points to a struct addrinfo that you've already filled out with relevant information.

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getaddrinfo(NULL, "3490", &hints, &servinfo)

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servinfo now points to a linked list of 1 or more struct addrinfos

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Also, you'll see the AI_PASSIVE flag in there; this tells getaddrinfo() to assign the address of my local host to the socket structures. This is nice because then you don't have to hardcode it.

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getaddrinfo(argv[1], NULL, &hints, &res

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What you really want to do is use the values from the results of the call to getaddrinfo(), and feed them into socket() directly like this

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(This is commonly done if you're going to listen() for incoming connections on a specific port

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Once you have a socket, you might have to associate that socket with a port on your local machine

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The port number is used by the kernel to match an incoming packet to a certain process's socket descriptor.

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If you're going to only be doing a connect() (because you're the client, not the server), this is probably be unnecessary.

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bind(sockfd, res->ai_addr, res->ai_addrlen);

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