inet_aton() returning success for invalid ip addresses?

Question

I am looking for some function to verify that if given string is a valid ipv4 address, but inet_aton() seems to be happy with strings like "11" and "1.1"
what is best way to validate an ipv4 string.

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

int
main(int argc, char *argv[])
{
    struct in_addr addr;

   if (argc != 2) {
        fprintf(stderr, "%s <dotted-address>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

   if (inet_aton(argv[1], &addr) == 0) {
        fprintf(stderr, "Invalid address\n");
        exit(EXIT_FAILURE);
    }

   printf("%s\n", inet_ntoa(addr));
    exit(EXIT_SUCCESS);
}

the ouput for some invalid strings are

[root@ ~]# ./a.out 1.1
1.0.0.1
[root@ ~]# ./a.out "1 some junk"
0.0.0.1
[root@ ~]# ./a.out "10 some junk"
0.0.0.10

I want a routine to reject any string not in dotted decimal notation x.x.x.x, x from 0 to 255

You can check this post to validate an ipv4 string. http://stackoverflow.com/questions/791982/determine-if-a-string-is-a-valid-ip-address-in-c — Ozan Deniz, May 01 '13 at 21:58
inet_aton is behaving as described in its manpage http://linux.die.net/man/3/inet_aton — jman, May 01 '13 at 22:04

score 3 · Accepted Answer · answered May 01 '13 at 22:06

This is the specified/documented behavior for inet_aton.

If you want to accept only dotted-quad decimal notation, use:

unsigned char *a = (void *)&addr, dummy;
if (sscanf(src, "%3hhd.%3hhd.%3hhd.%3hhd%c", a, a+1, a+2, a+3, &dummy)!=4) {
    /* error */
}

Alternatively, you might use the inet_pton function, which is more restrictive in the formats it accepts.

inet_aton() returning success for invalid ip addresses?

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