DNS check

dns.go

@@ -1,5 +1,123 @@
 package cachet
 
+import (
+	"net"
+
+	"strings"
+
+	"regexp"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/miekg/dns"
+)
+
+type DNSAnswer struct {
+	Regex  string
+	regexp *regexp.Regexp
+	Exact  string
+}
+
 type DNSMonitor struct {
 	AbstractMonitor `mapstructure:",squash"`
+
+	// IP:port format or blank to use system defined DNS
+	DNS string
+
+	// A(default), AAAA, MX, ...
+	Question string
+	question uint16
+
+	Answers []DNSAnswer
+}
+
+func (monitor *DNSMonitor) Validate() []string {
+	errs := monitor.AbstractMonitor.Validate()
+
+	if len(monitor.DNS) == 0 {
+		config, _ := dns.ClientConfigFromFile("/etc/resolv.conf")
+		if len(config.Servers) > 0 {
+			monitor.DNS = net.JoinHostPort(config.Servers[0], config.Port)
+		}
+	}
+
+	if len(monitor.DNS) == 0 {
+		monitor.DNS = "8.8.8.8:53"
+	}
+
+	if len(monitor.Question) == 0 {
+		monitor.Question = "A"
+	}
+	monitor.Question = strings.ToUpper(monitor.Question)
+
+	monitor.question = findDNSType(monitor.Question)
+	if monitor.question == 0 {
+		errs = append(errs, "Could not look up DNS question type")
+	}
+
+	for i, a := range monitor.Answers {
+		if len(a.Regex) > 0 {
+			monitor.Answers[i].regexp, _ = regexp.Compile(a.Regex)
+		}
+	}
+
+	return errs
+}
+
+func (monitor *DNSMonitor) test() bool {
+	m := new(dns.Msg)
+	m.SetQuestion(dns.Fqdn(monitor.Target), monitor.question)
+	m.RecursionDesired = true
+
+	c := new(dns.Client)
+	r, _, err := c.Exchange(m, monitor.DNS)
+	if err != nil {
+		logrus.Warnf("DNS error: %v", err)
+		return false
+	}
+
+	if r.Rcode != dns.RcodeSuccess {
+		return false
+	}
+
+	for _, check := range monitor.Answers {
+		found := false
+		for _, answer := range r.Answer {
+			found = matchAnswer(answer, check)
+			if found {
+				break
+			}
+		}
+
+		if !found {
+			logrus.Warnf("DNS check failed: %v. Not found in any of %v", check, r.Answer)
+			return false
+		}
+	}
+
+	return true
+}
+
+func findDNSType(t string) uint16 {
+	for rr, strType := range dns.TypeToString {
+		if t == strType {
+			return rr
+		}
+	}
+
+	return 0
+}
+
+func matchAnswer(answer dns.RR, check DNSAnswer) bool {
+	fields := []string{}
+	for i := 0; i < dns.NumField(answer); i++ {
+		fields = append(fields, dns.Field(answer, i+1))
+	}
+
+	str := strings.Join(fields, " ")
+
+	if check.regexp != nil {
+		return check.regexp.Match([]byte(str))
+	}
+
+	return str == check.Exact
 }