mirror of
https://github.com/EmilHernvall/dnsguide.git
synced 2024-12-22 15:44:17 +07:00
312 lines
7.9 KiB
Markdown
312 lines
7.9 KiB
Markdown
2 - Building a stub resolver
|
|
============================
|
|
|
|
While it's slightly satisfying to know that we're able to successfully parse DNS
|
|
packets, it's not much use to just read them off disk. As our next step, we'll
|
|
use it to build a `stub resolver`, which is a DNS client that doesn't feature
|
|
any built-in support for recursive lookup and that will only work with a DNS
|
|
server that does. Later we'll implement an actual recursive resolver to lose
|
|
the need for a server.
|
|
|
|
### Extending BytePacketBuffer for writing
|
|
|
|
In order to be able to service a query, we need to be able to not just read
|
|
packets, but also write them. To do so, we'll need to extend `BytePacketBuffer`
|
|
with some additional methods:
|
|
|
|
```rust
|
|
impl BytePacketBuffer {
|
|
|
|
- snip -
|
|
|
|
fn write(&mut self, val: u8) -> Result<()> {
|
|
if self.pos >= 512 {
|
|
return Err("End of buffer".into());
|
|
}
|
|
self.buf[self.pos] = val;
|
|
self.pos += 1;
|
|
Ok(())
|
|
}
|
|
|
|
fn write_u8(&mut self, val: u8) -> Result<()> {
|
|
self.write(val)?;
|
|
|
|
Ok(())
|
|
}
|
|
|
|
fn write_u16(&mut self, val: u16) -> Result<()> {
|
|
self.write((val >> 8) as u8)?;
|
|
self.write((val & 0xFF) as u8)?;
|
|
|
|
Ok(())
|
|
}
|
|
|
|
fn write_u32(&mut self, val: u32) -> Result<()> {
|
|
self.write(((val >> 24) & 0xFF) as u8)?;
|
|
self.write(((val >> 16) & 0xFF) as u8)?;
|
|
self.write(((val >> 8) & 0xFF) as u8)?;
|
|
self.write(((val >> 0) & 0xFF) as u8)?;
|
|
|
|
Ok(())
|
|
}
|
|
```
|
|
|
|
We'll also need a function for writing query names in labeled form:
|
|
|
|
```rust
|
|
fn write_qname(&mut self, qname: &str) -> Result<()> {
|
|
for label in qname.split('.') {
|
|
let len = label.len();
|
|
if len > 0x3f {
|
|
return Err("Single label exceeds 63 characters of length".into());
|
|
}
|
|
|
|
self.write_u8(len as u8)?;
|
|
for b in label.as_bytes() {
|
|
self.write_u8(*b)?;
|
|
}
|
|
}
|
|
|
|
self.write_u8(0)?;
|
|
|
|
Ok(())
|
|
}
|
|
|
|
} // End of BytePacketBuffer
|
|
```
|
|
|
|
### Extending DnsHeader for writing
|
|
|
|
Building on our new functions we can extend our protocol representation
|
|
structs. Starting with `DnsHeader`:
|
|
|
|
```rust
|
|
impl DnsHeader {
|
|
|
|
- snip -
|
|
|
|
pub fn write(&self, buffer: &mut BytePacketBuffer) -> Result<()> {
|
|
buffer.write_u16(self.id)?;
|
|
|
|
buffer.write_u8(
|
|
(self.recursion_desired as u8)
|
|
| ((self.truncated_message as u8) << 1)
|
|
| ((self.authoritative_answer as u8) << 2)
|
|
| (self.opcode << 3)
|
|
| ((self.response as u8) << 7) as u8,
|
|
)?;
|
|
|
|
buffer.write_u8(
|
|
(self.rescode as u8)
|
|
| ((self.checking_disabled as u8) << 4)
|
|
| ((self.authed_data as u8) << 5)
|
|
| ((self.z as u8) << 6)
|
|
| ((self.recursion_available as u8) << 7),
|
|
)?;
|
|
|
|
buffer.write_u16(self.questions)?;
|
|
buffer.write_u16(self.answers)?;
|
|
buffer.write_u16(self.authoritative_entries)?;
|
|
buffer.write_u16(self.resource_entries)?;
|
|
|
|
Ok(())
|
|
}
|
|
|
|
}
|
|
```
|
|
|
|
### Extending DnsQuestion for writing
|
|
|
|
Moving on to `DnsQuestion`:
|
|
|
|
```rust
|
|
impl DnsQuestion {
|
|
|
|
- snip -
|
|
|
|
pub fn write(&self, buffer: &mut BytePacketBuffer) -> Result<()> {
|
|
buffer.write_qname(&self.name)?;
|
|
|
|
let typenum = self.qtype.to_num();
|
|
buffer.write_u16(typenum)?;
|
|
buffer.write_u16(1)?;
|
|
|
|
Ok(())
|
|
}
|
|
|
|
}
|
|
```
|
|
|
|
### Extending DnsRecord for writing
|
|
|
|
`DnsRecord` is for now quite compact as well, although we'll eventually add
|
|
quite a bit of code here to handle different record types:
|
|
|
|
```rust
|
|
impl DnsRecord {
|
|
|
|
- snip -
|
|
|
|
pub fn write(&self, buffer: &mut BytePacketBuffer) -> Result<usize> {
|
|
let start_pos = buffer.pos();
|
|
|
|
match *self {
|
|
DnsRecord::A {
|
|
ref domain,
|
|
ref addr,
|
|
ttl,
|
|
} => {
|
|
buffer.write_qname(domain)?;
|
|
buffer.write_u16(QueryType::A.to_num())?;
|
|
buffer.write_u16(1)?;
|
|
buffer.write_u32(ttl)?;
|
|
buffer.write_u16(4)?;
|
|
|
|
let octets = addr.octets();
|
|
buffer.write_u8(octets[0])?;
|
|
buffer.write_u8(octets[1])?;
|
|
buffer.write_u8(octets[2])?;
|
|
buffer.write_u8(octets[3])?;
|
|
}
|
|
DnsRecord::UNKNOWN { .. } => {
|
|
println!("Skipping record: {:?}", self);
|
|
}
|
|
}
|
|
|
|
Ok(buffer.pos() - start_pos)
|
|
}
|
|
|
|
}
|
|
```
|
|
|
|
### Extending DnsPacket for writing
|
|
|
|
Putting it all together in `DnsPacket`:
|
|
|
|
```rust
|
|
impl DnsPacket {
|
|
|
|
- snip -
|
|
|
|
pub fn write(&mut self, buffer: &mut BytePacketBuffer) -> Result<()> {
|
|
self.header.questions = self.questions.len() as u16;
|
|
self.header.answers = self.answers.len() as u16;
|
|
self.header.authoritative_entries = self.authorities.len() as u16;
|
|
self.header.resource_entries = self.resources.len() as u16;
|
|
|
|
self.header.write(buffer)?;
|
|
|
|
for question in &self.questions {
|
|
question.write(buffer)?;
|
|
}
|
|
for rec in &self.answers {
|
|
rec.write(buffer)?;
|
|
}
|
|
for rec in &self.authorities {
|
|
rec.write(buffer)?;
|
|
}
|
|
for rec in &self.resources {
|
|
rec.write(buffer)?;
|
|
}
|
|
|
|
Ok(())
|
|
}
|
|
|
|
}
|
|
```
|
|
|
|
### Implementing a stub resolver
|
|
|
|
We're ready to implement our stub resolver. Rust includes a convenient
|
|
`UDPSocket` which does most of the work.
|
|
|
|
```rust
|
|
fn main() -> Result<()> {
|
|
// Perform an A query for google.com
|
|
let qname = "google.com";
|
|
let qtype = QueryType::A;
|
|
|
|
// Using googles public DNS server
|
|
let server = ("8.8.8.8", 53);
|
|
|
|
// Bind a UDP socket to an arbitrary port
|
|
let socket = UdpSocket::bind(("0.0.0.0", 43210))?;
|
|
|
|
// Build our query packet. It's important that we remember to set the
|
|
// `recursion_desired` flag. As noted earlier, the packet id is arbitrary.
|
|
let mut packet = DnsPacket::new();
|
|
|
|
packet.header.id = 6666;
|
|
packet.header.questions = 1;
|
|
packet.header.recursion_desired = true;
|
|
packet
|
|
.questions
|
|
.push(DnsQuestion::new(qname.to_string(), qtype));
|
|
|
|
// Use our new write method to write the packet to a buffer...
|
|
let mut req_buffer = BytePacketBuffer::new();
|
|
packet.write(&mut req_buffer)?;
|
|
|
|
// ...and send it off to the server using our socket:
|
|
socket.send_to(&req_buffer.buf[0..req_buffer.pos], server)?;
|
|
|
|
// To prepare for receiving the response, we'll create a new `BytePacketBuffer`,
|
|
// and ask the socket to write the response directly into our buffer.
|
|
let mut res_buffer = BytePacketBuffer::new();
|
|
socket.recv_from(&mut res_buffer.buf)?;
|
|
|
|
// As per the previous section, `DnsPacket::from_buffer()` is then used to
|
|
// actually parse the packet after which we can print the response.
|
|
let res_packet = DnsPacket::from_buffer(&mut res_buffer)?;
|
|
println!("{:#?}", res_packet.header);
|
|
|
|
for q in res_packet.questions {
|
|
println!("{:#?}", q);
|
|
}
|
|
for rec in res_packet.answers {
|
|
println!("{:#?}", rec);
|
|
}
|
|
for rec in res_packet.authorities {
|
|
println!("{:#?}", rec);
|
|
}
|
|
for rec in res_packet.resources {
|
|
println!("{:#?}", rec);
|
|
}
|
|
|
|
Ok(())
|
|
}
|
|
```
|
|
|
|
Running it will print:
|
|
|
|
```text
|
|
DnsHeader {
|
|
id: 6666,
|
|
recursion_desired: true,
|
|
truncated_message: false,
|
|
authoritative_answer: false,
|
|
opcode: 0,
|
|
response: true,
|
|
rescode: NOERROR,
|
|
checking_disabled: false,
|
|
authed_data: false,
|
|
z: false,
|
|
recursion_available: true,
|
|
questions: 1,
|
|
answers: 1,
|
|
authoritative_entries: 0,
|
|
resource_entries: 0
|
|
}
|
|
DnsQuestion {
|
|
name: "google.com",
|
|
qtype: A
|
|
}
|
|
A {
|
|
domain: "google.com",
|
|
addr: 216.58.209.110,
|
|
ttl: 79
|
|
}
|
|
```
|
|
|
|
The next chapter covers implementing a richer set of record types: [Chapter 3 - Adding more Record Types](/chapter3.md)
|