From d6931ec491dfad55d2043caddcef242939f536dc Mon Sep 17 00:00:00 2001
From: sleshep <9625413+sleshep@users.noreply.github.com>
Date: Wed, 26 Apr 2023 15:57:55 +0800
Subject: [PATCH] feat: support system dns

---
 config/config.go      |  4 +++-
 dns/system.go         | 39 +++++++++++++++++++++++++++++++++++++++
 dns/system_windows.go |  7 +++++++
 dns/util.go           |  8 ++++++++
 4 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 dns/system.go
 create mode 100644 dns/system_windows.go

diff --git a/config/config.go b/config/config.go
index 71f4087e..35929849 100644
--- a/config/config.go
+++ b/config/config.go
@@ -841,7 +841,7 @@ func parseHosts(cfg *RawConfig) (*trie.DomainTrie[resolver.HostValue], error) {
 				} else {
 					ips := make([]netip.Addr, 0)
 					for _, addr := range addrs {
-						if ipnet, ok := addr.(*net.IPNet); ok && !ipnet.IP.IsLoopback()&&!ipnet.IP.IsLinkLocalUnicast() {
+						if ipnet, ok := addr.(*net.IPNet); ok && !ipnet.IP.IsLoopback() && !ipnet.IP.IsLinkLocalUnicast() {
 							if ip, err := netip.ParseAddr(ipnet.IP.String()); err == nil {
 								ips = append(ips, ip)
 							}
@@ -938,6 +938,8 @@ func parseNameServer(servers []string, preferH3 bool) ([]dns.NameServer, error)
 		case "quic":
 			addr, err = hostWithDefaultPort(u.Host, "853")
 			dnsNetType = "quic" // DNS over QUIC
+		case "system":
+			dnsNetType = "system" // System DNS
 		default:
 			return nil, fmt.Errorf("DNS NameServer[%d] unsupport scheme: %s", idx, u.Scheme)
 		}
diff --git a/dns/system.go b/dns/system.go
new file mode 100644
index 00000000..2e52f90f
--- /dev/null
+++ b/dns/system.go
@@ -0,0 +1,39 @@
+//go:build !windows
+
+package dns
+
+import (
+	"fmt"
+	"os"
+	"regexp"
+)
+
+var (
+	// nameserver xxx.xxx.xxx.xxx
+	nameserverPattern = regexp.MustCompile(`nameserver\s+(?P<ip>\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})`)
+)
+
+func loadSystemResolver() (clients []dnsClient, err error) {
+	content, err := os.ReadFile("/etc/resolv.conf")
+	if err != nil {
+		err = fmt.Errorf("failed to read /etc/resolv.conf: %w", err)
+		return
+	}
+	nameservers := make([]string, 0)
+	for _, line := range nameserverPattern.FindAllStringSubmatch(string(content), -1) {
+		addr := line[1]
+		nameservers = append(nameservers, addr)
+	}
+	if len(nameservers) == 0 {
+		err = fmt.Errorf("no nameserver found in /etc/resolv.conf")
+		return
+	}
+	servers := make([]NameServer, 0, len(nameservers))
+	for _, addr := range nameservers {
+		servers = append(servers, NameServer{
+			Addr: fmt.Sprintf("%s:%d", addr, 53),
+			Net:  "udp",
+		})
+	}
+	return transform(servers, nil), nil
+}
diff --git a/dns/system_windows.go b/dns/system_windows.go
new file mode 100644
index 00000000..3334af96
--- /dev/null
+++ b/dns/system_windows.go
@@ -0,0 +1,7 @@
+//go:build windows
+
+package dns
+
+func loadSystemResolver() (clients []dnsClient, err error) {
+	return nil, errors.New("system resolver is not yet supported on Windows")
+}
diff --git a/dns/util.go b/dns/util.go
index bfd2e9ed..948687ac 100644
--- a/dns/util.go
+++ b/dns/util.go
@@ -79,6 +79,14 @@ func transform(servers []NameServer, resolver *Resolver) []dnsClient {
 		case "dhcp":
 			ret = append(ret, newDHCPClient(s.Addr))
 			continue
+		case "system":
+			clients, err := loadSystemResolver()
+			if err != nil {
+				log.Warnln("[DNS:system] load system resolver failed: %s", err.Error())
+				continue
+			}
+			ret = append(ret, clients...)
+			continue
 		case "quic":
 			if doq, err := newDoQ(resolver, s.Addr, s.ProxyAdapter, s.ProxyName); err == nil {
 				ret = append(ret, doq)