How to get a string from a bytes array?

Question

I'm creating my own DNS server and host blocker, I want to get host from DNS request message byte[]

dns message hex dump:

e07901000001000000000000057961686f6f03636f6d0000010001
.y...........yahoo.com.....

code:

using System;
using System.Text;

public class Program
{
    public static void Main()
    {
        string b64 = "4HkBAAABAAAAAAAABXlhaG9vA2NvbQAAAQAB";
        int pad = b64.Length % 4;
        if (pad > 0 )
        {
            b64 += new string('=', 4 - pad);
        }
        byte[] decoded = Convert.FromBase64String(b64);
        int start = 13;
        int end = start;
        while(decoded[end] != 0){
            end++;
        }
        
        int hostLength = end-start;
        byte[] byteHost = new byte[hostLength];
        Array.Copy(decoded, start, byteHost, 0, hostLength);
        string host = Encoding.Default.GetString(byteHost);
        Console.WriteLine(host); // yahoo♥com
    }
}

The questions:

is my method above to get host name right/efficient/fastest ?
why I get weird character replacing the dot yahoo♥com ?

change to Encoding.ASCII or Encoding.UTF8 has no effect

For reference: it can be interesting to write a DNS server as an academic exercise, but I wouldn't do this for "real things" - just get a pi-hole; it has custom host blocking inbuilt, etc — Marc Gravell
– Marc Gravell, Commented Jan 28, 2021 at 9:11
Does using this matches your goal and provides the excpected result: new string(decoded.Select(b => (char)b).Where(char.IsLetterOrDigit).ToArray()) ? You can replace IsLetterOrDigit by any logic. — user12031933
– user12031933, Commented Jan 28, 2021 at 11:26
@uingtea Your code outputs yahoo♥com and the snippet àyyahoocom. What do you expect in fact ? yahoo.com ? — user12031933
– user12031933, Commented Jan 28, 2021 at 11:58

Marc Gravell · Accepted Answer · 2021-01-28 09:14:47Z

5

There's no need for the second array; Encoding.GetString allows you to pass in an offset and count, so: GetString(decoded, start, hostLength)
Never use Encoding.Default; that is badly named - it should be called Encoding.Wrong :) Find out what encoding the data is in (probably UTF-8 or ASCII), and use that
You should be able to use IndexOf to find the terminating '\0'; also consider what your code should do if it doesn't find one

As for the unusual character: the data contains an 03 byte where you would expect the .; check the DNS protocol specification to see if this is expected. 03 is ETX (end of text). Beyond that: I don't know.

edited Jan 28, 2021 at 9:14

answered Jan 28, 2021 at 9:07

Marc Gravell

1.1m273 gold badges2.6k silver badges3k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

uingtea Over a year ago

thanks, about IndexOf i try Array.IndexOf(decoded, (byte)0); but it return 3 not 18

Marc Gravell Over a year ago

@uingtea you can provide the start offset too

uingtea · Accepted Answer · 2021-01-29 21:13:47Z

Found the answer, 03 is not ETX but length of the next string, let see the the example

00 05 79 61 68 6F 6F 03 63 6F 6D
.  .  y  a  h  o  o  .  c  o  m

05 mean is the length of yahoo and 03 is for com

Valid host or domain name contains only ASCII range from 44-127 or [a-z0-9-\.], domain like bücher.nu will be converted into xn--bcher-kva.nu, so I replace byte like 03,0C,09 or under 44 with the dot .

and thanks to @Marc Gravell for GetString(decoded, start, hostLength)

/*
I0sBAAABAAAAAAAABmMtcmluZwZtc2VkZ2UDbmV0AAABAAE
ldgBAAABAAAAAAAABWZwLXZwCWF6dXJlZWRnZQNuZXQAAAEAAQ
4HkBAAABAAAAAAAABXlhaG9vA2NvbQAAAQAB
*/
string b64 = "4VoBAAABAAAAAAAAIGYyNWIzNjgyMGUyNDljNGQxY2I0YzQzNGUxNjc5YTljA2Nsbwxmb290cHJpbnRkbnMDY29tAAABAAE";
int pad = b64.Length % 4;
if (pad > 0)
{
    b64 += new string ('=', 4 - pad);
}

byte[] decoded = Convert.FromBase64String(b64);
int start = 13;
int end = start;
while (decoded[end] != 0)
{
    if(decoded[end] < 44)
        decoded[end] = 0x2e;
    end++;
}

int hostLength = end - start;
string host = Encoding.ASCII.GetString(decoded, start, hostLength);
Console.WriteLine(host);

edit: micro optimization, benchmark with 1e9 or 1 billion loop

Convert.ToChar() finished in 00:00:04

for(int i =0; i<1e9; i++){
    while (decoded[end] != 0)
    {
        if(decoded[end] < 44)
            decoded[end] = 0x2e;
        host += Convert.ToChar(decoded[end]);
        end++;
    }
}

VS Encoding.ASCII.GetString() finished in 00:03:20 (200 seconds)

for(int i =0; i<1e9; i++){
    while (decoded[end] != 0)
    {
        if(decoded[end] < 44)
            decoded[end] = 0x2e;
        end++;
    }
    int hostLength = end - start;
    string host = Encoding.ASCII.GetString(decoded, start, hostLength);

Collectives™ on Stack Overflow

How to get a string from a bytes array?

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related