Synology’s “secret” telnet password….

If you ever had to recover a Synology nas box in recovery-mode, pre DiskStation installation or after a failed DiskStation install….. Then you would need the “secret” telnet password for admin (or in some cases root)

In case you don’t want to contact Synology here is how it’s generated

• 1st character = month in hexadecimal, lower case (1=Jan, … , a=Oct, b=Nov, c=Dec)
• 2-3 = month in decimal, zero padded and starting in 1 (01, 02, 03, …, 11, 12)
• 4 = dash
• 5-6 = day of the month in hex (01, 02 .., 0A, .., 1F)
• 7-8 = greatest common divisor between month and day, zero padded. This is always a number between 01 and 12.

So, let’s say today is October 15, the password would be: a10-0f05 (a = month in hex, 10 = month in dec, 0f = day in hex, 05 = greatest divisor between 10 and 15).

In some cases the clock is also set to factory default… then try the password: 101-0101

Based on the original correct_password.c source,here is a short snippet to generate the daily password.

#include <stdlib.h> 
#include <time.h> 
#include <stdio.h> 

void main()
{
    struct timeval tvTime;
    struct tm tmOutput;

    gettimeofday(&tvTime, 0);
    localtime_r(&(tvTime.tv_sec), &tmOutput);

    tmOutput.tm_mon += 1;
    printf("password for today is: %x%02d-%02x%02d\n\n",
        tmOutput.tm_mon, tmOutput.tm_mon, tmOutput.tm_mday,
        gcd(tmOutput.tm_mon, tmOutput.tm_mday));
}

int gcd(int a, int b)
{
    return (b?gcd(b,a%b):a);
}