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Eliminate all useless allocation

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This commit is contained in:
Emil Hernvall
2020-06-18 02:23:42 +02:00
parent 45cbfa59f7
commit 4202cae860
7 changed files with 81 additions and 75 deletions
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59
chapter5.md
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@@ -171,11 +171,11 @@ impl DnsPacket {
/// It's useful to be able to pick a random A record from a packet. When we
/// get multiple IP's for a single name, it doesn't matter which one we
/// choose, so in those cases we can now pick one at random.
pub fn get_random_a(&self) -> Option<String> {
pub fn get_random_a(&self) -> Option<Ipv4Addr> {
self.answers
.iter()
.filter_map(|record| match record {
DnsRecord::A { ref addr, .. } => Some(addr.to_string()),
DnsRecord::A { addr, .. } => Some(*addr),
_ => None,
})
.next()
@@ -183,8 +183,9 @@ impl DnsPacket {
/// A helper function which returns an iterator over all name servers in
/// the authorities section, represented as (domain, host) tuples
fn get_ns<'a>(&'a self, qname: &'a str) -> impl Iterator<Item=(&'a str, &'a str)> {
self.authorities.iter()
fn get_ns<'a>(&'a self, qname: &'a str) -> impl Iterator<Item = (&'a str, &'a str)> {
self.authorities
.iter()
// In practice, these are always NS records in well formed packages.
// Convert the NS records to a tuple which has only the data we need
// to make it easy to work with.
@@ -196,17 +197,18 @@ impl DnsPacket {
.filter(move |(domain, _)| qname.ends_with(*domain))
}
/// When there is a NS record in the authorities section, there may also
/// be a matching A record in the additional section. This saves us
/// from doing a separate query to resolve the IP of the name server.
pub fn get_resolved_ns(&self, qname: &str) -> Option<String> {
/// We'll use the fact that name servers often bundle the corresponding
/// A records when replying to an NS query to implement a function that
/// returns the actual IP for an NS record if possible.
pub fn get_resolved_ns(&self, qname: &str) -> Option<Ipv4Addr> {
// Get an iterator over the nameservers in the authorities section
self.get_ns(qname)
// Now we need to look for a matching A record in the additional
// section. Since we just want the first valid record, we can just
// build a stream of matching records.
.flat_map(|(_, host)| {
self.resources.iter()
self.resources
.iter()
// Filter for A records where the domain match the host
// of the NS record that we are currently processing
.filter_map(move |record| match record {
@@ -214,19 +216,19 @@ impl DnsPacket {
_ => None,
})
})
.map(|addr| addr.to_string())
.map(|addr| *addr)
// Finally, pick the first valid entry
.next()
}
/// However, not all name servers are as that nice. In certain cases there won't
/// be any A records in the additional section, and we'll have to perform *another*
/// lookup in the midst of our first. For this, we introduce a method for
/// returning the hostname of an appropriate name server.
pub fn get_unresolved_ns(&self, qname: &str) -> Option<String> {
/// lookup in the midst. For this, we introduce a method for returning the host
/// name of an appropriate name server.
pub fn get_unresolved_ns<'a>(&'a self, qname: &'a str) -> Option<&'a str> {
// Get an iterator over the nameservers in the authorities section
self.get_ns(qname)
.map(|(_, host)| host.to_string())
.map(|(_, host)| host)
// Finally, pick the first valid entry
.next()
}
@@ -240,36 +242,35 @@ We move swiftly on to our new `recursive_lookup` function:
```rust
fn recursive_lookup(qname: &str, qtype: QueryType) -> Result<DnsPacket> {
// For now we're always starting with *a.root-servers.net*.
let mut ns = "198.41.0.4".to_string();
let mut ns = "198.41.0.4".parse::<Ipv4Addr>().unwrap();
// Since it might take an arbitrary number of steps, we enter an unbounded loop.
loop {
println!("attempting lookup of {:?} {} with ns {}", qtype, qname, ns);
// The next step is to send the query to the active server.
let ns_copy = ns.clone();
let ns_copy = ns;
let server = (ns_copy.as_str(), 53);
let response = lookup(qname, qtype.clone(), server)?;
let server = (ns_copy, 53);
let response = lookup(qname, qtype, server)?;
// If there are entries in the answer section, and no errors, we are done!
if !response.answers.is_empty() && response.header.rescode == ResultCode::NOERROR {
return Ok(response.clone());
return Ok(response);
}
// We might also get a `NXDOMAIN` reply, which is the authoritative name servers
// way of telling us that the name doesn't exist.
if response.header.rescode == ResultCode::NXDOMAIN {
return Ok(response.clone());
return Ok(response);
}
// Otherwise, we'll try to find a new nameserver based on NS and a corresponding A
// record in the additional section. If this succeeds, we can switch name server
// and retry the loop.
if let Some(new_ns) = response.get_resolved_ns(qname) {
ns = new_ns.clone();
ns = new_ns;
continue;
}
@@ -278,7 +279,7 @@ fn recursive_lookup(qname: &str, qtype: QueryType) -> Result<DnsPacket> {
// we'll go with what the last server told us.
let new_ns_name = match response.get_unresolved_ns(qname) {
Some(x) => x,
None => return Ok(response.clone()),
None => return Ok(response),
};
// Here we go down the rabbit hole by starting _another_ lookup sequence in the
@@ -289,14 +290,22 @@ fn recursive_lookup(qname: &str, qtype: QueryType) -> Result<DnsPacket> {
// Finally, we pick a random ip from the result, and restart the loop. If no such
// record is available, we again return the last result we got.
if let Some(new_ns) = recursive_response.get_random_a() {
ns = new_ns.clone();
ns = new_ns;
} else {
return Ok(response.clone());
return Ok(response);
}
}
}
```
This also requires a small change to the `lookup` function, as we need to
pass which server to use. We add a server parameter to the function signature,
and remove the hardcoded `server` variable we used in chapter 4:
```rust
fn lookup(qname: &str, qtype: QueryType, server: (Ipv4Addr, u16)) -> Result<DnsPacket> {
```
### Trying out recursive lookup
The only thing remaining is to change our `handle_query` function to use