chore: better resolv.conf parsing

dns/system_posix.go

@@ -3,25 +3,41 @@
 package dns
 
 import (
+	"bufio"
 	"fmt"
+	"net/netip"
 	"os"
-	"regexp"
+	"strings"
 )
 
-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})`)
-)
+const resolvConf = "/etc/resolv.conf"
 
 func dnsReadConfig() (servers []string, err error) {
-	content, err := os.ReadFile("/etc/resolv.conf")
+	file, err := os.Open(resolvConf)
 	if err != nil {
-		err = fmt.Errorf("failed to read /etc/resolv.conf: %w", err)
+		err = fmt.Errorf("failed to read %s: %w", resolvConf, err)
 		return
 	}
-	for _, line := range nameserverPattern.FindAllStringSubmatch(string(content), -1) {
-		addr := line[1]
-		servers = append(servers, addr)
+	defer func() { _ = file.Close() }()
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
+			// comment.
+			continue
+		}
+		f := strings.Fields(line)
+		if len(f) < 1 {
+			continue
+		}
+		switch f[0] {
+		case "nameserver": // add one name server
+			if len(f) > 1 {
+				if addr, err := netip.ParseAddr(f[1]); err == nil {
+					servers = append(servers, addr.String())
+				}
+			}
+		}
 	}
 	return
 }