feat: support system dns

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
+}