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Chapter 1 tweaks

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Emil Hernvall 2018-03-15 14:16:22 +01:00
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commit cf153a7847
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chapter1.md
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@ -92,12 +92,12 @@ a lookup using the `dig` tool:
;; ANSWER SECTION:
google.com. 204 IN A 172.217.18.142
```
;; Query time: 0 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Wed Jul 06 13:24:19 CEST 2016
;; MSG SIZE rcvd: 44
```
We're using the `+noedns` flag to make sure we stick to the original format.
There are a few things of note in the output above:
@ -328,19 +328,18 @@ pub struct BytePacketBuffer {
impl BytePacketBuffer {
// This gives us a fresh buffer for holding the packet contents, and a field for
// keeping track of where we are.
pub fn new() -> BytePacketBuffer {
BytePacketBuffer {
buf: [0; 512],
pos: 0
}
}
```
This gives us a fresh buffer for holding the packet contents, and a field for
keeping track of where we are. When handling the reading of domain names later
on, we'll need a way of reading and manipulating our buffer position:
// When handling the reading of domain names, we'll need a way of
// reading and manipulating our buffer position.
```rust
fn pos(&self) -> usize {
self.pos
}
@ -356,11 +355,8 @@ on, we'll need a way of reading and manipulating our buffer position:
Ok(())
}
```
Next up is a method for reading a single byte, and moving one step forward:
```rust
// A method for reading a single byte, and moving one step forward
fn read(&mut self) -> Result<u8> {
if self.pos >= 512 {
return Err(Error::new(ErrorKind::InvalidInput, "End of buffer"));
@ -370,34 +366,27 @@ Next up is a method for reading a single byte, and moving one step forward:
Ok(res)
}
```
We might also want to retrieve a byte at our current position without moving
forward:
// Methods for fetching data at a specified position, without modifying
// the internal position
```rust
fn get(&mut self, pos: usize) -> Result<u8> {
if pos >= 512 {
return Err(Error::new(ErrorKind::InvalidInput, "End of buffer"));
}
Ok(self.buf[pos])
}
```
Sometimes we want to get a full range of the buffer:
```rust
fn get_range(&mut self, start: usize, len: usize) -> Result<&[u8]> {
if start + len >= 512 {
return Err(Error::new(ErrorKind::InvalidInput, "End of buffer"));
}
Ok(&self.buf[start..start+len as usize])
}
```
In many cases we'll want to read a two-byte integer or a four-byte integer:
// Methods for reading a u16 and u32 from the buffer, while stepping
// forward 2 or 4 bytes
```rust
fn read_u16(&mut self) -> Result<u16>
{
let res = ((try!(self.read()) as u16) << 8) |
@ -415,135 +404,86 @@ In many cases we'll want to read a two-byte integer or a four-byte integer:
Ok(res)
}
```
And then we get to the tricky part of correctly reading domain names, handling
any jump we might encounter:
```rust
// The tricky part: Reading domain names, taking labels into consideration.
// Will take something like [3]www[6]google[3]com[0] and append
// www.google.com to outstr.
fn read_qname(&mut self, outstr: &mut String) -> Result<()>
{
```
Since we might encounter jumps, we'll keep track of our position
locally as opposed to using the position within the struct.
```rust
// Since we might encounter jumps, we'll keep track of our position
// locally as opposed to using the position within the struct. This
// allows us to move the shared position to a point past our current
// qname, while keeping track of our progress on the current qname
// using this variable.
let mut pos = self.pos();
```
We'll keep track of whether or not we've jumped for use later on
```rust
// track whether or not we've jumped
let mut jumped = false;
```
Our delimeter which we append for each label. Since we don't want a dot at the
beginning of the domain name we'll leave it empty for now and set it to "." at
the end of the first iteration.
```rust
// Our delimeter which we append for each label. Since we don't want a dot at the
// beginning of the domain name we'll leave it empty for now and set it to "." at
// the end of the first iteration.
let mut delim = "";
loop {
```
We're at the beginning of a label, so our current position is the length byte.
```rust
// At this point, we're always at the beginning of a label. Recall
// that labels start with a length byte.
let len = try!(self.get(pos));
```
`len* is a two byte sequence, where the two highest bits of the first byte is
set, represents a offset relative to the start of the buffer. We handle this
by jumping to the offset, setting a flag to indicate that we shouldn't update
the shared buffer position once done.
```rust
// If len has the two most significant bit are set, it represents a jump to
// some other offset in the packet:
if (len & 0xC0) > 0 {
```
When a jump is performed, we only modify the shared buffer position once, and avoid
making the change later on.
```rust
// 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));
}
```
Read another byte, and calculate the jump offset:
```rust
// Read another byte, calculate offset and perform the jump by
// updating our local position variable
let b2 = try!(self.get(pos+1)) as u16;
let offset = (((len as u16) ^ 0xC0) << 8) | b2;
pos = offset as usize;
```
Indicate that a jump was performed.
```rust
// Indicate that a jump was performed.
jumped = true;
```
Restart the loop and retry at the new position.
```rust
continue;
}
```
Move a single byte forward to move past the length byte.
```rust
// The base scenario, where we're reading a single label and
// appending it to the output:
else {
// Move a single byte forward to move past the length byte.
pos += 1;
```
Domain names are terminated by an empty label of length 0, so if the length is zero
we're done.
```rust
// Domain names are terminated by an empty label of length 0, so if the length is zero
// we're done.
if len == 0 {
break;
}
```
Append the delimiter to our output buffer first.
```rust
// Append the delimiter to our output buffer first.
outstr.push_str(delim);
```
Extract the actual ASCII bytes for this label and append them to the output buffer.
// Extract the actual ASCII bytes for this label and append them to the output buffer.
```rust
let str_buffer = try!(self.get_range(pos, len as usize));
outstr.push_str(&String::from_utf8_lossy(str_buffer).to_lowercase());
delim = ".";
```
Move forward the full length of the label.
```rust
// Move forward the full length of the label.
pos += len as usize;
}
```
}
If a jump has been performed, we've already modified the buffer position state and
shouldn't do so again.
```rust
// If a jump has been performed, we've already modified the buffer position state and
// shouldn't do so again.
if !jumped {
try!(self.seek(pos));
}
Ok(())
} // End of read_qname
```
Oh, and we're done:
```rust
} // End of BytePacketBuffer
```