How to Check if a String is a Valid IPv6 Address in C

Check if a String is a Valid IPv6 Address in C

To check if a string is a valid IPv6 address in C, you can use functions like inet_pton or getaddrinfo to convert the address from its text form to a binary format. If the conversion is successful, the address is valid. In this tutorial, we cover multiple approaches to validate an IPv6 address using different functions.

---

Example 1: Using inet_pton to Validate IPv6 Address

In this example, we will use the inet_pton function to convert an IPv6 address string into its binary form. A successful conversion indicates that the address is valid.

main.c

#include <stdio.h>
#include <stdlib.h>
#include <arpa/inet.h>

int main() {
    const char *ipv6 = "2001:0db8:85a3:0000:0000:8a2e:0370:7334";
    struct in6_addr addr;

    // Convert IPv6 address from text to binary form
    int result = inet_pton(AF_INET6, ipv6, &addr);

    if (result == 1) {
        printf("'%s' is a valid IPv6 address.\n", ipv6);
    } else if (result == 0) {
        printf("'%s' is not a valid IPv6 address.\n", ipv6);
    } else {
        perror("inet_pton");
    }
    return 0;
}

Explanation:

1. The variable ipv6 is a constant character pointer that holds the IPv6 address string to be validated.
2. struct in6_addr addr is declared to store the binary representation of the IPv6 address.
3. The function inet_pton (which stands for “presentation to network”) converts the IPv6 address from its text format to a binary format. It returns 1 if the conversion is successful, 0 if the input is not a valid IPv6 address, and -1 if an error occurs.
4. Based on the return value of inet_pton, the program prints whether the IPv6 address is valid or not.

Output:

'2001:0db8:85a3:0000:0000:8a2e:0370:7334' is a valid IPv6 address.

Example 2: Using getaddrinfo to Validate IPv6 Address

In this example, we will use the getaddrinfo function to validate an IPv6 address. By setting the hints to use IPv6 (AF_INET6), we can check whether the provided address is valid. If getaddrinfo returns 0, the address is valid.

main.c

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

int main() {
    const char *ipv6 = "2001:db8::1";
    struct addrinfo hints, *res;
    int status;

    // Set the hints for IPv6
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_INET6;

    // Validate the IPv6 address using getaddrinfo
    status = getaddrinfo(ipv6, NULL, &hints, &res);
    if (status == 0) {
        printf("'%s' is a valid IPv6 address.\n", ipv6);
        freeaddrinfo(res);
    } else {
        printf("'%s' is not a valid IPv6 address: %s\n", ipv6, gai_strerror(status));
    }
    return 0;
}

Explanation:

1. The variable ipv6 stores the IPv6 address string to be validated.
2. A struct addrinfo hints is declared and initialized to zero using memset to ensure all fields are set to default values.
3. The member hints.ai_family is set to AF_INET6 to indicate that we are interested in IPv6 addresses only.
4. The getaddrinfo function is called with the IPv6 address, NULL for the service, and the hints structure. If it returns 0, the address is valid; otherwise, an error message is provided.
5. If the address is valid, the allocated memory for the results is freed using freeaddrinfo.

Output:

'2001:db8::1' is a valid IPv6 address.

Conclusion

In this tutorial, we explored two methods to check if a string is a valid IPv6 address in C. Using inet_pton and getaddrinfo, you can easily validate IPv6 addresses. Both approaches offer reliable ways to ensure that your IPv6 address strings are correctly formatted and valid.