Thread: Extracting and comparing DNS servers from a .txt file

Code:

#include <stdio.h>
#include <string.h>
#include <inttypes.h>
 
#define FRM_SCN "%" SCNu32 ".%" SCNu32 ".%" SCNu32 ".%" SCNu32
#define FRM_PRI "%" PRIu32 ".%" PRIu32 ".%" PRIu32 ".%" PRIu32
 
void print_ip(uint32_t ip)
{
    printf(FRM_PRI "\n", ip >> 24 & 0xff,
                         ip >> 16 & 0xff,
                         ip >>  8 & 0xff,
                         ip       & 0xff);
}
 
int main()
{
    FILE *in = fopen("input.txt", "r");
 
    char line[1024];
    while (fgets(line, sizeof line, in))
    {
        uint32_t a, b, c, d;
 
        // get index to first digit in the line, or to '\0' if none
        unsigned digit = strcspn(line, "0123456789");
 
        if (line[digit]
            && sscanf(line + digit, FRM_SCN, &a, &b, &c, &d) == 4
            && a < 256 && b < 256 && c < 256 && d < 256)
            {
                // 32-bit ip's can be stored in uint32_t
                uint32_t ip = a << 24 | b << 16 | c << 8 | d;
                print_ip(ip);
 
                // Or you can store it in a string
                char ip_str[20];
                sprintf(ip_str, FRM_PRI, a, b, c, d);
                printf("%s\n\n", ip_str);
            }
    }
 
    fclose(in);
 
    return 0;
}

Hey John, thanks a lot once again for your response.
Many concepts escaping my understanding there, so it's a bit of copying/pasting blindly this time. I guess I just need to modify that "input.txt" to "dns.txt" (the file where I store the redirected info) and just choose one of the blocks, either storing in string or uint32_t, right?

What would ip_str store, though? Just the DNS string?

Anyway, FRM_PRI (on the "void print_ip" function) expects a parenthesis or it won't let me compile. I don't know if I missed something while declaring it...

https://cboard.cprogramming.com/imag...AASUVORK5CYII=

It may have been overly-complicated (showing two different ways and using inttypes).
How about this.

Code:

#include <stdio.h>
#include <string.h>
 
int main()
{
    FILE *in = fopen("input.txt", "r");
 
    char line[1024];
    while (fgets(line, sizeof line, in))
    {
        unsigned a, b, c, d;
 
        // get index to first digit in the line, or to '\0' if none
        unsigned digit = strcspn(line, "0123456789");
 
        if (line[digit] // if digit is not index of '\0'
            // and if we're able to read 4 unsigned ints with dots between them
            && sscanf(line + digit, "%u.%u.%u.%u, &a, &b, &c, &d) == 4
            // and if all the values are in range
            && a < 256 && b < 256 && c < 256 && d < 256)
            {
                // then reconstruct the string from the unsigned ints
                char ip_str[20];
                sprintf(ip_str, "%u.%u.%u.%u, a, b, c, d);
 
                // Now ip_str has the ip as a string.
                // Do whatever you want with it.
                printf("%s\n", ip_str);
            }
    }
 
    fclose(in);
 
    return 0;
}

I didn't test it and forgot some closing double-quotes.
But you put them in the wrong place.
Note that you should probably use _pclose to close a FILE* opened with _popen.

Code:

FILE* dns = _popen(str, "r");
while (fgets(line, sizeof line, dns))
{
    unsigned a = 0, b = 0, c = 0, d = 0;
 
    // get index to first digit in the line, or to '\0' if none
    unsigned digit = strcspn(line, "0123456789");
 
    if (line[digit] // if digit is not index of '\0'
        // and if we're able to read 4 unsigned ints with dots between them
        && sscanf(line + digit, "%u.%u.%u.%u", &a, &b, &c, &d) == 4
        // and if all the values are in range
        && a < 256 && b < 256 && c < 256 && d < 256)
    {
        // then reconstruct the string from the unsigned ints
        char ip_str[20];
        sprintf(ip_str, "%u.%u.%u.%u", a, b, c, d);
 
        // Now ip_str has the ip as a string.
        // Do whatever you want with it.
        printf("%s\n", ip_str);
    }
}
 
_pclose(dns);

As usual, I don't understand what you are trying to do. The code doesn't make sense to me and you've never really been able to explain what in the world you are trying to do overall. I'm lost. It's frustrating.

Is this one program or more than one?
What is it's purpose?
What exactly does the input (from all sources if more than one) look like?
What exactly do you want the output to be?

Yes, sorry. I've tried to simplify working in different modules now.

First one should export to dns_responden.txt those DNS servers. Working fine so far:

Code:

void extraerDNS() {

    FILE* responsesdns = fopen("dns_responden.txt", "w");
    FILE* dns = _popen(str, "r");
    while (fgets(line, sizeof line, dns))
    {
    unsigned a = 0, b = 0, c = 0, d = 0;
    unsigned digit = strcspn(line, "0123456789");

    if (line[digit]
        && sscanf(line + digit, "%u.%u.%u.%u", &a, &b, &c, &d) == 4
        && a < 256 && b < 256 && c < 256 && d < 256)
    {
        char ip_str[20];
        sprintf(ip_str, "%u.%u.%u.%u", a, b, c, d);
        fprintf(responsesdns, "%s\n", ip_str);
    }
}

_pclose(dns);
}

dns_responden.txt looks like this, format-wise (fictional DNS servers):

Code:

80.51.41.651
80.08.21.229

Now, back to my main, the backbone of my program, where I had implemented previous help to ping through different IP's (also from a redirected .txt file, namely ips_responden.txt):

Code:

        FILE* responses = fopen("ips_responden.txt", "w");
        while (fgets(ch, sizeof(ch), fp))
        {
            printf("\n    Lanzando ping a %s", ch);
            sprintf(ping_cmd, "ping %s", ch);
            FILE* f = _popen(ping_cmd, "r");
            while (fgets(line, sizeof line, f)) {
                if (strstr(line, term)) {
                    fprintf(responses, "%s", ch);
                }
            }
            fclose(f);
        }
        fclose(responses);
        fclose(fp);

        int best_response = 0;
        char best_ip[sizeof(ipresp)] = "";
        int best_valid = 0;
        responses = fopen("ips_responden.txt", "r");
        if (responses != NULL)
        {
            while (fgets(ipresp, sizeof(ipresp), responses))
            {
                int response1 = velocidad_media(ipresp);
                if (!best_valid || response1 < best_response) {
                    best_response = response1;
                    strcpy(best_ip, ipresp);
                    best_valid = 1;
                }
            }
            printf("\n\n    Velocidad de respuesta media: %d ms.\n", best_response);
            printf("\n    IP seleccionada: %s", best_ip);
        }
        extraerDNS();

This came in hefty to compare among IP's, keeping the lower value as best_response and its corresponding IP as best_ip. The latter phase of my program involves almost the same, but considerably shorter since I don't need to ping before in order to select among those which respond.
As you can see, this final line invoked this new module, i.e. redirects the DNS servers to the .txt file.

I'm implementing the following block immediately afterwards just to be able to compare between them, too, and be able to print the "winner" as well. First off, I'll open the redirected .txt to be able to do the rest:

Code:

        FILE* responsesdns = fopen("dns_responden.txt", "r");
        int best_response_dns = 0;
        char best_dns[sizeof(ip_str)] = "";
        int best_valid_dns = 0;
        if (responsesdns != NULL)
        {
            while (fgets(ip_str, sizeof(ip_str), responsesdns))
            {
                int dns1 = velocidad_media(ip_str);
                if (!best_valid_dns || dns1 < best_response_dns) {
                    best_response_dns = dns1;
                    strcpy(best_dns, ip_str);
                    best_valid_dns = 1;
                }
            }
            printf("\n\n    Velocidad de respuesta media: %d ms.\n", best_response_dns);
            printf("\n    DNS seleccionada: %s", best_dns);
        }

... but it doesn't print a thing. In fact, I'm getting just a "0" for best_response_dns and a null value for best_dns. I don't know how to fix it.

Does this make more sense now?
Please forgive my sloppy approach. As you can see, I'm just learning the basics and it's too frustrating as well. And your help is much appreciated, mind you.

Are you sure velocidad_media is working correctly with the input from fgets? Recall that fgets stores the newline from reading the line at the end of the string if there is space for it.

best_response_dns sounds like a poor name because it sounds like the same kind of thing as best_dns. I'd rename it to best_response_time.

best_valid_dns sounds badly named too. It has nothing to do with the best valid dns... rather, it is a flag as to whether the first IP address in the list has been processed. This means that you need to check this flag before you blindly start printing anything.

Update: I've tried out, through a different file and folder (where I've copied the same dns_responden.txt file) the following code, in order to follow laserlight's signature hint. Compiles and works perfect, the way I want it to do:

Code:

#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int velocidad_media(const char* ip)
{
    const char* const Delim = "Media = ";
    printf("\nExtrayendo velocidad media de %s", ip);

    char cmd[64];
    sprintf(cmd, "ping %s", ip);
    FILE* p = _popen(cmd, "r");
    char line[256];
    while (fgets(line, sizeof(line), p))
    {
        char* s = NULL;
        if ((s = strstr(line, Delim)) != NULL)
        {
            _pclose(p);
            return atoi(s + strlen(Delim));
        }
    }
    _pclose(p);

    return 0;
}

void main()
{
    FILE* responsesdns = fopen("dns_responden.txt", "r");
    int best_response_time = 0;
    char ip_str[100], best_dns[sizeof(ip_str)] = "";
    int best_valid_dns = 0;
    if (responsesdns != NULL)
    {
        while (fgets(ip_str, sizeof(ip_str), responsesdns))
        {
            int dns1 = velocidad_media(ip_str);
            if (!best_valid_dns || dns1 < best_response_time) {
                best_response_time = dns1;
                strcpy(best_dns, ip_str);
                best_valid_dns = 1;
            }
        }
        printf("\n\n    Velocidad de respuesta media: %d ms.\n", best_response_time);
        printf("\n    DNS seleccionada: %s", best_dns);
    }
}

Now, implementing it to my program, I've created the mediaDNS() module, just copied/pasted from the previous main:

Code:

void mediaDNS()
{
    FILE* responsesdns = fopen("dns_responden.txt", "r");
    int best_response_time = 0;
    char ip_str[100], best_dns[sizeof(ip_str)] = "";
    int best_valid_dns = 0;
    if (responsesdns != NULL)
    {
        while (fgets(ip_str, sizeof(ip_str), responsesdns))
        {
            int dns1 = velocidad_media(ip_str);
            if (!best_valid_dns || dns1 < best_response_time) {
                best_response_time = dns1;
                strcpy(best_dns, ip_str);
                best_valid_dns = 1;
            }
        }
        printf("\n\n    Velocidad de respuesta media: %d ms.\n", best_response_time);
        printf("\n    DNS seleccionada: %s", best_dns);
    }
}

I'm invoking it just inside the program's main, just after extracting the DNS servers to the redirected file (extraerDNS module), both inside and outside its loop...:

Code:

        int best_response = 0;
        char best_ip[sizeof(ipresp)] = "";
        int best_valid = 0;
        responses = fopen("ips_responden.txt", "r");
        if (responses != NULL)
        {
            while (fgets(ipresp, sizeof(ipresp), responses))
            {
                int response1 = velocidad_media(ipresp);
                if (!best_valid || response1 < best_response) {
                    best_response = response1;
                    strcpy(best_ip, ipresp);
                    best_valid = 1;
                }
            }
            printf("\n\n    Velocidad de respuesta media: %d ms.\n", best_response);
            printf("\n    IP seleccionada: %s", best_ip);
            fclose(responses);
            extraerDNS();
        }
        mediaDNS();

And nothing. Well, it gives me a 0 for best_response and null value for best_ip. I can't understand what's happening between both approaches.

Originally Posted by JSteel

I just wanted to transpose the previous block with slightly different nomenclature in order to avoid mixing variables.

When you start getting worried about "mixing variables", it means that it is time to break your function into smaller functions that each does one thing and does it well.

Originally Posted by JSteel

As for the velocidad_media function, sure, it gets the average time of IP's in the previous block (the one involving variable ipresp instead of ip_str), so I don't know how would it be affecting this time. Once again, maybe you're right and I'm not getting it straight at all. But I think I've kept the same syntax and structure and it's so weird it's not working this time. I'm really lost on it.

Then don't assume. Test! Write unit tests that throw various input at the function and see if it returns what you expect it to return, whether a valid value or an error code. In more advanced unit testing, you would mock the DNS servers so that for a known mock IP address, the mocked DNS server waits for some time that you know in advance and hence can check if the time returned is in range of that known time.