chore: cleanup system dns code

dns/system_posix.go

@@ -0,0 +1,27 @@
+//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 dnsReadConfig() (servers []string, err error) {
+	content, err := os.ReadFile("/etc/resolv.conf")
+	if err != nil {
+		err = fmt.Errorf("failed to read /etc/resolv.conf: %w", err)
+		return
+	}
+	for _, line := range nameserverPattern.FindAllStringSubmatch(string(content), -1) {
+		addr := line[1]
+		servers = append(servers, addr)
+	}
+	return
+}