Name and Address Conversion: DNS Resolution & getaddrinfo()

Explanation

Hardcoding IP addresses in client-server programs compromises architectural flexibility and breaks IPv4/IPv6 portability. getaddrinfo() accepts a hostname, a service name (e.g., 'http' or '80'), and an optional hints structure specifying criteria such as socket type (SOCK_STREAM). It queries DNS and /etc/hosts, returning a linked list of addrinfo structures containing valid sockaddr pointers ready for direct invocation with socket() and connect(). This protocol-independent design enables seamless migration to IPv6.

Key points

• Legacy functions gethostbyname() and gethostbyaddr() are obsolete and non-reentrant.
• getaddrinfo() provides modern protocol-independent, thread-safe name-to-address resolution supporting both IPv4 and IPv6.
• freeaddrinfo() frees the linked list of struct addrinfo dynamically allocated by getaddrinfo().
• gai_strerror() converts getaddrinfo error return codes into descriptive error strings.

Example

Code examples

struct addrinfo hints, *res, *p;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC; // IPv4 or IPv6
hints.ai_socktype = SOCK_STREAM;

int status = getaddrinfo("www.purbanchaluniversity.edu.np", "80", &hints, &res);
if (status != 0) {
    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
    exit(1);
}
// Iterate linked list and connect
for (p = res; p != NULL; p = p->ai_next) {
    int sfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
    if (connect(sfd, p->ai_addr, p->ai_addrlen) == 0) break; // Connected!
    close(sfd);
}
freeaddrinfo(res);

Exam questions

• [8 Marks] Explain getaddrinfo(), freeaddrinfo(), and gai_strerror(). Why are they preferred over gethostbyname()?
• [5 Marks] Write a C program to resolve a given domain name to its corresponding IP address using getaddrinfo().