How do I get sub string from string

So the text contains multiple lines and you want the whole line?

string result = allLines.Split(new[] { Environment.NewLine }, StringSplitOptions.None)
  .FirstOrDefault(l=> l.TrimStart().StartsWith("Connection-specific DNS Suffix . :"));

it's null if it was not found.

if(result != null) result = result.Trim(); 

If it doesn't contain multiple lines and you want to find it in the middle of the string:

string result = null;
string textToFind = "Connection-specific DNS Suffix  . :";
int startIndex = text.IndexOf(textToFind);
if (startIndex >= 0)
{ 
    string behind = text.Substring(startIndex + textToFind.Length).TrimStart();
    int endIndex = behind.IndexOf(" ");
    if (endIndex >= 0)
        behind = behind.Substring(0, endIndex).TrimEnd();
    result = string.Format("{0} {1}", textToFind, behind);
}

Note that string comparisons are case sensitive in .NET by default. If you want to ignore the case (so that Connection and connection are treated the same) you can use the overload of String.IndexOf that takes a StringComparison, for example:

int startIndex = text.IndexOf(textToFind, StringComparison.OrdinalIgnoreCase);

int startIndex = s.IndexOf("Connection-specific DNS Suffix", StringComparison.CurrentCultureIgnoreCase);
string substring  = s.Substring(startIndex, s.IndexOf(Environment.NewLine, startIndex) - startIndex);

it will work in all the situations

 string your_string = "tomany_words_Connection-specific DNS Suffix . :abcd.comLinkbfdbfdb";
        string check_string="Connection-specific DNS Suffix . :";
        int index;
        int last_index;
        string final;
        if (true == your_string.Contains(check_string))
        {
            index=your_string.IndexOf(check_string);
            last_index = your_string.IndexOf("Link");

            final = your_string.Substring(index + check_string.Length, last_index - (index + check_string.Length));              
            Console.WriteLine(final);
        }