[GH-ISSUE #2] Jump query issue #5

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opened 2026-03-07 21:26:19 +03:00 by kerem · 0 comments

kerem commented

2026-03-07 21:26:19 +03:00

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Originally created by @SUMUKHA-PK on GitHub (Feb 18, 2019).
Original GitHub issue: https://github.com/EmilHernvall/dnsguide/issues/2

if (len & 0xC0) == 0xC0 {
                // Update the buffer position to a point past the current
                // label. We don't need to touch it any further.
                if !jumped {
                    try!(self.seek(pos+2));
                }

and

"" I mentioned earlier that each label is preceeded by a single byte length. The additional thing we need to consider is that if the two Most Significant Bits of the length is set, we can instead expect the length byte to be followed by a second byte. These two bytes taken together, and removing the two MSB's, indicate the jump position. In the example above, we've got 0xC00C. The bit pattern of the the two high bits expressed as hex is 0xC000 (in binary 11000000 00000000), so we can find the jump position by xoring our two bytes with this mask to unset them: 0xC00C ^ 0xC000 = 12. Thus we should jump to byte 12 of the packet and read from there. Recalling that the length the DNS header happens to be 12 bytes, we realize that it's instructing us to start reading from where the question part of the packet begins, which makes sense since the question starts with the query domain which in this case is "google.com". ""

Shoudnt we jump by the number indicated after the set flag? Why are we jumping from pos to pos+2 ?

Originally created by @SUMUKHA-PK on GitHub (Feb 18, 2019). Original GitHub issue: https://github.com/EmilHernvall/dnsguide/issues/2 ``` if (len & 0xC0) == 0xC0 { // Update the buffer position to a point past the current // label. We don't need to touch it any further. if !jumped { try!(self.seek(pos+2)); } ``` and "" I mentioned earlier that each label is preceeded by a single byte length. The additional thing we need to consider is that if the two Most Significant Bits of the length is set, we can instead expect the length byte to be followed by a second byte. These two bytes taken together, and removing the two MSB's, indicate the jump position. In the example above, we've got 0xC00C. The bit pattern of the the two high bits expressed as hex is 0xC000 (in binary 11000000 00000000), so we can find the jump position by xoring our two bytes with this mask to unset them: 0xC00C ^ 0xC000 = 12. Thus we should jump to byte 12 of the packet and read from there. Recalling that the length the DNS header happens to be 12 bytes, we realize that it's instructing us to start reading from where the question part of the packet begins, which makes sense since the question starts with the query domain which in this case is "google.com". "" Shoudnt we jump by the number indicated after the set flag? Why are we jumping from ```pos``` to ```pos+2``` ?