coredns/middleware/errors/errors.go

// Package errors implements an HTTP error handling middleware.
package errors

import (
	"fmt"
	"log"
	"runtime"
	"strings"
	"time"

	"github.com/miekg/coredns/middleware"

	"github.com/miekg/dns"
	"golang.org/x/net/context"
)

// ErrorHandler handles DNS errors (and errors from other middleware).
type ErrorHandler struct {
	Next      middleware.Handler
	LogFile   string
	Log       *log.Logger
	LogRoller *middleware.LogRoller
	Debug     bool // if true, errors are written out to client rather than to a log
}

func (h ErrorHandler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
	defer h.recovery(ctx, w, r)

	rcode, err := h.Next.ServeDNS(ctx, w, r)

	if err != nil {
		state := middleware.State{W: w, Req: r}
		errMsg := fmt.Sprintf("%s [ERROR %d %s %s] %v", time.Now().Format(timeFormat), rcode, state.Name(), state.Type(), err)

		if h.Debug {
			// Write error to response as a txt message instead of to log
			answer := debugMsg(rcode, r)
			txt, _ := dns.NewRR(". IN 0 TXT " + errMsg)
			answer.Answer = append(answer.Answer, txt)
			state.SizeAndDo(answer)
			w.WriteMsg(answer)
			return 0, err
		}
		h.Log.Println(errMsg)
	}

	return rcode, err
}

func (h ErrorHandler) recovery(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) {
	rec := recover()
	if rec == nil {
		return
	}

	state := middleware.State{W: w, Req: r}
	// Obtain source of panic
	// From: https://gist.github.com/swdunlop/9629168
	var name, file string // function name, file name
	var line int
	var pc [16]uintptr
	n := runtime.Callers(3, pc[:])
	for _, pc := range pc[:n] {
		fn := runtime.FuncForPC(pc)
		if fn == nil {
			continue
		}
		file, line = fn.FileLine(pc)
		name = fn.Name()
		if !strings.HasPrefix(name, "runtime.") {
			break
		}
	}

	// Trim file path
	delim := "/coredns/"
	pkgPathPos := strings.Index(file, delim)
	if pkgPathPos > -1 && len(file) > pkgPathPos+len(delim) {
		file = file[pkgPathPos+len(delim):]
	}

	panicMsg := fmt.Sprintf("%s [PANIC %s %s] %s:%d - %v", time.Now().Format(timeFormat), r.Question[0].Name, dns.Type(r.Question[0].Qtype), file, line, rec)
	if h.Debug {
		// Write error and stack trace to the response rather than to a log
		var stackBuf [4096]byte
		stack := stackBuf[:runtime.Stack(stackBuf[:], false)]
		answer := debugMsg(dns.RcodeServerFailure, r)
		// add stack buf in TXT, limited to 255 chars for now.
		txt, _ := dns.NewRR(". IN 0 TXT " + string(stack[:255]))
		answer.Answer = append(answer.Answer, txt)
		state.SizeAndDo(answer)
		w.WriteMsg(answer)
	} else {
		// Currently we don't use the function name, since file:line is more conventional
		h.Log.Printf(panicMsg)
	}
}

// debugMsg creates a debug message that gets send back to the client.
func debugMsg(rcode int, r *dns.Msg) *dns.Msg {
	answer := new(dns.Msg)
	answer.SetRcode(r, rcode)
	return answer
}

const timeFormat = "02/Jan/2006:15:04:05 -0700"