Async DNS seeder lookups (#1662)

* replace to_socket_addrs
* refactor `resolve()` into `resolve_host()`
* use `resolve_host()` to resolve config peers
* add DNS_LOOKUP_TIMEOUT constant
* don't block the main thread in initialize

zebra-network/src/config.rs

@@ -1,9 +1,4 @@
-use std::{
-    collections::HashSet,
-    net::{SocketAddr, ToSocketAddrs},
-    string::String,
-    time::Duration,
-};
+use std::{collections::HashSet, net::SocketAddr, string::String, time::Duration};
 
 use zebra_chain::parameters::Network;
 
@@ -37,19 +32,55 @@ pub struct Config {
 }
 
 impl Config {
-    fn parse_peers<S: ToSocketAddrs>(peers: HashSet<S>) -> HashSet<SocketAddr> {
-        peers
+    /// Concurrently resolves `peers` into zero or more IP addresses, with a timeout
+    /// of a few seconds on each DNS request.
+    ///
+    /// If DNS resolution fails or times out for all peers, returns an empty list.
+    async fn parse_peers(peers: &HashSet<String>) -> HashSet<SocketAddr> {
+        use futures::stream::StreamExt;
+        let peer_addresses = peers
             .iter()
-            .flat_map(|s| s.to_socket_addrs())
-            .flatten()
-            .collect()
+            .map(|s| Config::resolve_host(s))
+            .collect::<futures::stream::FuturesUnordered<_>>()
+            .concat()
+            .await;
+
+        if peer_addresses.is_empty() {
+            tracing::warn!(
+                ?peers,
+                ?peer_addresses,
+                "empty peer list after DNS resolution"
+            );
+        };
+        peer_addresses
     }
 
     /// Get the initial seed peers based on the configured network.
-    pub fn initial_peers(&self) -> HashSet<SocketAddr> {
+    pub async fn initial_peers(&self) -> HashSet<SocketAddr> {
         match self.network {
-            Network::Mainnet => Config::parse_peers(self.initial_mainnet_peers.clone()),
-            Network::Testnet => Config::parse_peers(self.initial_testnet_peers.clone()),
+            Network::Mainnet => Config::parse_peers(&self.initial_mainnet_peers).await,
+            Network::Testnet => Config::parse_peers(&self.initial_testnet_peers).await,
+        }
+    }
+
+    /// Resolves `host` into zero or more IP addresses.
+    ///
+    /// If `host` is a DNS name, performs DNS resolution with a timeout of a few seconds.
+    /// If DNS resolution fails or times out, returns an empty list.
+    async fn resolve_host(host: &str) -> HashSet<SocketAddr> {
+        let fut = tokio::net::lookup_host(host);
+        let fut = tokio::time::timeout(crate::constants::DNS_LOOKUP_TIMEOUT, fut);
+
+        match fut.await {
+            Ok(Ok(ips)) => ips.collect(),
+            Ok(Err(e)) => {
+                tracing::info!(?host, ?e, "DNS error resolving peer IP address");
+                HashSet::new()
+            }
+            Err(e) => {
+                tracing::info!(?host, ?e, "DNS timeout resolving peer IP address");
+                HashSet::new()
+            }
         }
     }
 }

zebra-network/src/constants.rs

@@ -108,6 +108,15 @@ lazy_static! {
     }.expect("regex is valid");
 }
 
+/// The timeout for DNS lookups.
+///
+/// [6.1.3.3 Efficient Resource Usage] from [RFC 1123: Requirements for Internet Hosts]
+/// suggest no less than 5 seconds for resolving timeout.
+///
+/// [RFC 1123: Requirements for Internet Hosts] https://tools.ietf.org/rfcmarkup?doc=1123
+/// [6.1.3.3  Efficient Resource Usage] https://tools.ietf.org/rfcmarkup?doc=1123#page-77
+pub const DNS_LOOKUP_TIMEOUT: Duration = Duration::from_secs(5);
+
 /// Magic numbers used to identify different Zcash networks.
 pub mod magics {
     use super::*;

zebra-network/src/peer_set/initialize.rs

@@ -135,16 +135,19 @@ where
 
     let listen_guard = tokio::spawn(listen(config.listen_addr, listener, peerset_tx.clone()));
 
+    // 2. Initial peers, specified in the config.
     let initial_peers_fut = {
-        let initial_peers = config.initial_peers();
+        let config = config.clone();
         let connector = connector.clone();
-        let tx = peerset_tx.clone();
-
-        // Connect the tx end to the 3 peer sources:
-        add_initial_peers(initial_peers, connector, tx)
+        let peerset_tx = peerset_tx.clone();
+        async move {
+            let initial_peers = config.initial_peers().await;
+            // Connect the tx end to the 3 peer sources:
+            add_initial_peers(initial_peers, connector, peerset_tx).await
+        }
+        .boxed()
     };
 
-    // 2. Initial peers, specified in the config.
     let add_guard = tokio::spawn(initial_peers_fut);
 
     // 3. Outgoing peers we connect to in response to load.