Add support for prefetching

4 files changed, 16 insertions, 2 deletions

diff --git a/README.md b/README.md
index 4aeb9d6..fbbf057 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,8 @@ configure.
 
 * **Control**: Filter unwanted content at the DNS-level. Similar to
   [Pi-hole](https://github.com/pi-hole/pi-hole).
-* **Fast**: Efficient filtering and caching of DNS requests.
+* **Fast**: Efficient filtering and caching of DNS requests. With pre-fetching
+  enabled, cached requests will never block waiting for the upstream resolver.
 * **Reliable**: Built with Go and [miekg/dns](https://github.com/miekg/dns) - a
   mature DNS library.
 * **Secure**: Protect your DNS requests from snooping and tampering using [DNS
diff --git a/cmd/zdns/main.go b/cmd/zdns/main.go
index fef8455..03fdb26 100644
--- a/cmd/zdns/main.go
+++ b/cmd/zdns/main.go
@@ -94,7 +94,11 @@ func (c *cli) run() {
 	client := dnsutil.NewClient(config.Resolver.Protocol, config.Resolver.Timeout, config.DNS.Resolvers...)
 
 	// Cache
-	cache := cache.New(config.DNS.CacheSize, nil)
+	var cclient *dnsutil.Client
+	if config.DNS.CachePrefetch {
+		cclient = client
+	}
+	cache := cache.New(config.DNS.CacheSize, cclient)
 	sigHandler.OnClose(cache)
 
 	// DNS server
diff --git a/config.go b/config.go
index e74413f..3d581f1 100644
--- a/config.go
+++ b/config.go
@@ -25,6 +25,7 @@ type DNSOptions struct {
 	Listen          string
 	Protocol        string `toml:"protocol"`
 	CacheSize       int    `toml:"cache_size"`
+	CachePrefetch   bool   `toml:"cache_prefetch"`
 	HijackMode      string `toml:"hijack_mode"`
 	hijackMode      int
 	RefreshInterval string `toml:"hosts_refresh_interval"`
diff --git a/zdnsrc b/zdnsrc
index 449a2b3..35daa0b 100644
--- a/zdnsrc
+++ b/zdnsrc
@@ -14,6 +14,14 @@
 #
 # cache_size = 4096
 
+# Cache pre-fetching.
+#
+# If enabled, cached entries will be re-resolved asynchronously. Note that this
+# may lead to slightly stale entries, but cached requests will never block
+# waiting for the upstream resolver.
+#
+# cache_prefetch = false
+
 # Upstream DNS servers to use when answering queries.
 #
 # The default is Cloudflare DNS servers, which support DNS-over-TLS.