customizing "system" DNS server with config key "dns"

Cargo.toml

@@ -173,4 +173,4 @@ daemonize = "0.4"
 byteorder = "1.3"
 env_logger = "0.9"
 byte_string = "1.0"
-tokio = { version = "1", features = ["net", "time", "macros", "io-util"]}
+tokio = { version = "1", features = ["net", "time", "macros", "io-util"] }

README.md

@@ -590,6 +590,8 @@ Example configuration:
     // Value could be IP address of DNS server, for example, "8.8.8.8".
     // DNS client will automatically request port 53 with both TCP and UDP protocol.
     //
+    // - system, uses system provided API (`getaddrinfo` on *NIX)
+    //
     // It also allows some pre-defined well-known public DNS servers:
     // - google (TCP, UDP)
     // - cloudflare (TCP, UDP)

crates/shadowsocks-service/src/config.rs

@@ -1736,6 +1736,8 @@ impl Config {
     /// 2. Pre-defined. Like `google`, `cloudflare`
     pub fn set_dns_formatted(&mut self, dns: &str) -> Result<(), Error> {
         self.dns = match dns {
+            "system" => DnsConfig::System,
+
             #[cfg(feature = "trust-dns")]
             "google" => DnsConfig::TrustDns(ResolverConfig::google()),