fix: fallback to hickory_resolver's default config if reading /etc/resolv.conf fails

src/dns/hickory.rs

@@ -1,11 +1,13 @@
 //! DNS resolution via the [hickory-resolver](https://github.com/hickory-dns/hickory-dns) crate
 
 use hickory_resolver::{
-    config::LookupIpStrategy, lookup_ip::LookupIpIntoIter, ResolveError, TokioResolver,
+    config::{LookupIpStrategy, ResolverConfig},
+    lookup_ip::LookupIpIntoIter,
+    name_server::TokioConnectionProvider,
+    TokioResolver,
 };
 use once_cell::sync::OnceCell;
 
-use std::fmt;
 use std::net::SocketAddr;
 use std::sync::Arc;
 
@@ -24,14 +26,11 @@ struct SocketAddrs {
     iter: LookupIpIntoIter,
 }
 
-#[derive(Debug)]
-struct HickoryDnsSystemConfError(ResolveError);
-
 impl Resolve for HickoryDnsResolver {
     fn resolve(&self, name: Name) -> Resolving {
         let resolver = self.clone();
         Box::pin(async move {
-            let resolver = resolver.state.get_or_try_init(new_resolver)?;
+            let resolver = resolver.state.get_or_init(new_resolver);
 
             let lookup = resolver.lookup_ip(name.as_str()).await?;
             let addrs: Addrs = Box::new(SocketAddrs {
@@ -51,23 +50,17 @@ impl Iterator for SocketAddrs {
 }
 
 /// Create a new resolver with the default configuration,
-/// which reads from `/etc/resolve.conf`. The options are
-/// overridden to look up for both IPv4 and IPv6 addresses
+/// which reads from `/etc/resolve.conf`. If reading `/etc/resolv.conf` fails,
+/// it fallbacks to hickory_resolver's default config.
+/// The options are overridden to look up for both IPv4 and IPv6 addresses
 /// to work with "happy eyeballs" algorithm.
-fn new_resolver() -> Result<TokioResolver, HickoryDnsSystemConfError> {
-    let mut builder = TokioResolver::builder_tokio().map_err(HickoryDnsSystemConfError)?;
+fn new_resolver() -> TokioResolver {
+    let mut builder = TokioResolver::builder_tokio().unwrap_or_else(|_| {
+        TokioResolver::builder_with_config(
+            ResolverConfig::default(),
+            TokioConnectionProvider::default(),
+        )
+    });
     builder.options_mut().ip_strategy = LookupIpStrategy::Ipv4AndIpv6;
-    Ok(builder.build())
-}
-
-impl fmt::Display for HickoryDnsSystemConfError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str("error reading DNS system conf for hickory-dns")
-    }
-}
-
-impl std::error::Error for HickoryDnsSystemConfError {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        Some(&self.0)
-    }
+    builder.build()
 }