Use LRU cache for virtual DNS

This commit introduces an LRU cache for virtual DNS and fixes a bug
where the virtual DNS feature would cause connection mappings to time
out.

src/tun2proxy.rs

@@ -253,7 +253,7 @@ pub(crate) trait ConnectionManager {
     fn get_credentials(&self) -> &Option<Credentials>;
 }
 
-#[derive(Default, Clone, Debug)]
+#[derive(Default)]
 pub struct Options {
     virtdns: Option<VirtualDns>,
 }
@@ -423,11 +423,12 @@ impl<'a> TunToProxy<'a> {
         if let Some((connection, first_packet, _payload_offset, _payload_size)) =
             connection_tuple(frame)
         {
-            let resolved_conn = match &self.options.virtdns {
+            let resolved_conn = match &mut self.options.virtdns {
                 None => connection.clone(),
                 Some(virt_dns) => {
                     let ip = SocketAddr::try_from(connection.dst.clone()).unwrap().ip();
-                    match virt_dns.ip_to_name(&ip) {
+                    virt_dns.touch_ip(&ip);
+                    match virt_dns.resolve_ip(&ip) {
                         None => connection.clone(),
                         Some(name) => connection.to_named(name.clone()),
                     }
@@ -564,6 +565,10 @@ impl<'a> TunToProxy<'a> {
                 {
                     let socket = self.sockets.get_mut::<tcp::Socket>(socket_handle);
                     if socket.may_send() {
+                        if let Some(virtdns) = &mut self.options.virtdns {
+                            // Unwrapping is fine because every smoltcp socket is bound to an.
+                            virtdns.touch_ip(&IpAddr::from(socket.local_endpoint().unwrap().addr));
+                        }
                         consumed = socket.send_slice(event.buffer).unwrap();
                         state
                             .handler

src/virtdns.rs

@@ -1,3 +1,5 @@
+use hashlink::linked_hash_map::RawEntryMut;
+use hashlink::LruCache;
 use smoltcp::wire::Ipv4Cidr;
 use std::collections::{HashMap, LinkedList};
 use std::convert::{TryFrom, TryInto};
@@ -5,8 +7,8 @@ use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
 use std::str::FromStr;
 use std::time::{Duration, Instant};
 
-const DNS_TTL: u8 = 30; // TTL in DNS replies
-const MAPPING_TIMEOUT: u64 = 60; // Mapping timeout
+const DNS_TTL: u8 = 30; // TTL in DNS replies in seconds
+const MAPPING_TIMEOUT: u64 = 60; // Mapping timeout in seconds
 
 #[derive(Eq, PartialEq, Debug)]
 #[allow(dead_code, clippy::upper_case_acronyms)]
@@ -21,10 +23,13 @@ enum DnsClass {
     IN = 1,
 }
 
-#[derive(Clone, Debug)]
+struct NameCacheEntry {
+    name: String,
+    expiry: Instant,
+}
+
 pub struct VirtualDns {
-    ip_to_name: HashMap<IpAddr, String>,
-    expiry: LinkedList<(IpAddr, Instant)>,
+    lru_cache: LruCache<IpAddr, NameCacheEntry>,
     name_to_ip: HashMap<String, IpAddr>,
     network_addr: IpAddr,
     broadcast_addr: IpAddr,
@@ -38,11 +43,10 @@ impl Default for VirtualDns {
 
         Self {
             next_addr: start_addr.into(),
-            ip_to_name: Default::default(),
             name_to_ip: Default::default(),
-            expiry: Default::default(),
             network_addr: IpAddr::try_from(cidr.network().address().into_address()).unwrap(),
             broadcast_addr: IpAddr::try_from(cidr.broadcast().unwrap().into_address()).unwrap(),
+            lru_cache: LruCache::new_unbounded(),
         }
     }
 }
@@ -160,21 +164,43 @@ impl VirtualDns {
         }
     }
 
-    pub fn ip_to_name(&self, addr: &IpAddr) -> Option<&String> {
-        self.ip_to_name.get(addr)
+    // This is to be called whenever we receive or send a packet on the socket
+    // which connects the tun interface to the client, so existing IP address to name
+    // mappings to not expire as long as the connection is active.
+    pub fn touch_ip(&mut self, addr: &IpAddr) -> bool {
+        match self.lru_cache.get_mut(addr) {
+            None => false,
+            Some(entry) => {
+                entry.expiry = Instant::now() + Duration::from_secs(MAPPING_TIMEOUT);
+                true
+            }
+        }
+    }
+
+    pub fn resolve_ip(&mut self, addr: &IpAddr) -> Option<&String> {
+        match self.lru_cache.get(addr) {
+            None => None,
+            Some(entry) => Some(&entry.name),
+        }
     }
 
     fn allocate_ip(&mut self, name: String) -> Option<IpAddr> {
         let now = Instant::now();
-        while let Some((ip, expiry)) = self.expiry.front() {
-            if now > *expiry {
-                let name = self.ip_to_name.remove(ip).unwrap();
-                self.name_to_ip.remove(&name);
-                self.expiry.pop_front();
-            } else {
-                break;
+
+        // Building the to_remove list seems to be a bit clunky.
+        // But removing inside the loop does not immediately work due to borrow rules.
+        // TODO: Is there a better solution?
+        let mut to_remove = LinkedList::<IpAddr>::new();
+        for (ip, entry) in self.lru_cache.iter() {
+            if now > entry.expiry {
+                to_remove.push_back(*ip);
+                self.name_to_ip.remove(&entry.name);
+                continue;
             }
         }
+        for ip in to_remove {
+            self.lru_cache.remove(&ip);
+        }
 
         if let Some(ip) = self.name_to_ip.get(&name) {
             return Some(*ip);
@@ -183,15 +209,19 @@ impl VirtualDns {
         let started_at = self.next_addr;
 
         loop {
-            if let std::collections::hash_map::Entry::Vacant(e) =
-                self.ip_to_name.entry(self.next_addr)
+            if let RawEntryMut::Vacant(vacant) =
+                self.lru_cache.raw_entry_mut().from_key(&self.next_addr)
             {
-                e.insert(name.clone());
-                self.name_to_ip.insert(name, self.next_addr);
-                self.expiry.push_back((
+                let expiry = Instant::now() + Duration::from_secs(MAPPING_TIMEOUT);
+                vacant.insert(
                     self.next_addr,
-                    Instant::now() + Duration::from_secs(MAPPING_TIMEOUT),
-                ));
+                    NameCacheEntry {
+                        name: name.clone(),
+                        expiry,
+                    },
+                );
+                // e.insert(name.clone());
+                self.name_to_ip.insert(name, self.next_addr);
                 return Some(self.next_addr);
             }
             self.next_addr = Self::increment_ip(self.next_addr);