Networking, Part 2: Using getaddrinfo - angrave/SystemProgramming GitHub Wiki

The function getaddrinfo can convert a human readable domain name (e.g. www.illinois.edu) into an IPv4 and IPv6 address. In fact it will return a linked-list of addrinfo structs:

struct addrinfo {
    int              ai_flags;
    int              ai_family;
    int              ai_socktype;
    int              ai_protocol;
    socklen_t        ai_addrlen;
    struct sockaddr *ai_addr;
    char            *ai_canonname;
    struct addrinfo *ai_next;
};

It's very easy to use. For example, suppose you wanted to find out the numeric IPv4 address of a webserver at www.bbc.com. We do this in two stages. First use getaddrinfo to build a linked-list of possible connections. Secondly use getnameinfo to convert the binary address into a readable form.

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

struct addrinfo hints, *infoptr; // So no need to use memset global variables

int main() {
    hints.ai_family = AF_INET; // AF_INET means IPv4 only addresses

    int result = getaddrinfo("www.bbc.com", NULL, &hints, &infoptr);
    if (result) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(result));
        exit(1);
    }

    struct addrinfo *p;
    char host[256];

    for (p = infoptr; p != NULL; p = p->ai_next) {

        getnameinfo(p->ai_addr, p->ai_addrlen, host, sizeof (host), NULL, 0, NI_NUMERICHOST);
        puts(host);
    }

    freeaddrinfo(infoptr);
    return 0;
}

Typical output:

212.58.244.70
212.58.244.71

Magic! No seriously, a system called "DNS" (Domain Name Service) is used. If a machine does not hold the answer locally then it sends a UDP packet to a local DNS server. This server in turn may query other upstream DNS servers.

DNS by itself is fast but not secure. DNS requests are not encrypted and susceptible to 'man-in-the-middle' attacks. For example, a coffee shop internet connection could easily subvert your DNS requests and send back different IP addresses for a particular domain

There are three basic system calls you need to connect to a remote machine:

getaddrinfo -- Determine the remote addresses of a remote host
socket      -- Create a socket
connect     -- Connect to the remote host using the socket and address information

The getaddrinfo call if successful, creates a linked-list of addrinfo structs and sets the given pointer to point to the first one.

The socket call creates an outgoing socket and returns a descriptor (sometimes called a 'file descriptor') that can be used with read and write etc. In this sense it is the network analog of open that opens a file stream - except that we haven't connected the socket to anything yet!

Finally the connect call attempts the connection to the remote machine. We pass the original socket descriptor and also the socket address information which is stored inside the addrinfo structure. There are different kinds of socket address structures (e.g. IPv4 vs IPv6) which can require more memory. So in addition to passing the pointer, the size of the structure is also passed:

// Pull out the socket address info from the addrinfo struct:
connect(sockfd, p->ai_addr, p->ai_addrlen);

As part of the clean up code call freeaddrinfo on the top-most addrinfo struct:

void freeaddrinfo(struct addrinfo *ai);

No. Error handling with getaddrinfo is a little different:

• The return value is the error code (i.e. don't use errno)
• Use gai_strerror to get the equivalent short English error text:

int result = getaddrinfo(...);
if (result) { 
    const char *mesg = gai_strerror(result); 
    ...
}

Yes! Use the addrinfo structure that is passed into getaddrinfo to define the kind of connection you'd like.

For example, to specify stream-based protocols over IPv6:

struct addrinfo hints;
memset(&hints, 0, sizeof (hints));

hints.ai_family = AF_INET6; // Only want IPv6 (use AF_INET for IPv4)
hints.ai_socktype = SOCK_STREAM; // Only want stream-based connection

The old function gethostbyname is deprecated; it's the old way convert a host name into an IP address. The port address still needs to be manually set using htons function. It's much easier to write code to support IPv4 AND IPv6 using the newer getaddrinfo

Yes and no. It's easy to create a simple TCP client - however network communications offers many different levels of abstraction and several attributes and options that can be set at each level of abstraction (for example we haven't talked about setsockopt which can manipulate options for the socket).

For more information see Beej's guide.