Determine if a string is a valid IPv4 address in C

Question

What would be a good way to determine if a string contains an IPv4 address? Should I use isdigit()?

Answer 1

Heres the start of a function I've been working on, although not complete, it may spark ideas or comments. The thought behind the function is;

1. Check the passed char pointer or string is an IPv4 address without port using its min/max size, how many dots in the string, and if the colon : character exists or not.
2. If the string is not IPv4 with or without a port then check if the string is IPv6, if not IPv6 then IP format not recognized because it isn't implemented yet.

I think it depends on how deep you want to go into the issue, how deep you want to go to understand what problems could occur.

#include 
#include 
#include 
#include 
#include 

int isIP(char *ip)
{
    char *data_ptr = ip;    // Create a pointer to passed data
    int orig_str_size = 0;  // Create an int to hold passed data size
    int str_index = 0;      // Create an int to iterate individual ip characters
    int dot_count = 0;      // Create an int to check for the number of dots

    // Count the number of characters in data_ptr
    while (*data_ptr++ != '\0'){ orig_str_size++; }

    if(orig_str_size <= 0) // If nothing
    {
        printf("Get a grip, ip is empty\n\n");
        exit(0);
    }
    else // If within IPv4 size range
    if(orig_str_size >= 7 && orig_str_size <= INET_ADDRSTRLEN)
    {
        char *data1_ptr = ip; // Create a pointer to passed data
        printf("Within IPv4 range, %i characters in length\n\n", orig_str_size);

        // Count the number of dots in the string, 3 for IPv4
        for(str_index; str_index < orig_str_size; str_index++)
        {
            if(data1_ptr[str_index] == '.'){ dot_count++; }
        }

        // If theres 3 dots, while ignoring dots, check each char is a digit 
        if(dot_count == 3)
        {

            printf("There's three dots in the string\n\n");
            data1_ptr = ip;
            str_index = 0;

            // Iterate the string char by char
            for(str_index; str_index < orig_str_size; str_index++)
            {
                // Ignoring dots
                if(data1_ptr[str_index] != '.')
                { 
                    // If isdigit() is happy its a digit and isalpha() happy not alphabetic
                    if(isdigit(data1_ptr[str_index]) && !isalpha(data1_ptr[str_index]))
                    {
                        printf("Digit found and is not alphabetic\n\n");
                        continue;
                    }
                    else
                    if(!isdigit(data1_ptr[str_index]) && isalpha(data1_ptr[str_index]))
                    {
                        printf("Not a recognised IPv4 address, character detected in string\n\n");
                        exit(0);
                    }
                }
            }

            return 0;
        }
    }
    else // If IPv6
    if(orig_str_size > 0 && orig_str_size > INET_ADDRSTRLEN && orig_str_size <= INET6_ADDRSTRLEN)
    {
        printf("Within IPv6 range %i\n\n", orig_str_size);
        return 0;
    }
    else
    {
        printf("Unknown target format, the format you provided as a target is not implemented\n\n");
        exit(0);
    }

}

TCP/IP Internetworking

RFC791 - Internet Protocol - https://tools.ietf.org/html/rfc791

The CISCO Internetworking handbook http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook

The Open Systems Interconnection Reference Model http://docwiki.cisco.com/wiki/Internetworking_Basics#Open_Systems_Interconnection_Reference_Model

CISCO Troubleshooting TCP/IP Networks https://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1907.pdf

What is the largest TCP/IP network port number allowable for IPv4?