panic in goroutine, rmsg is nil

[bradleyjkemp]

I'm getting a nil panic from this line:

dnsr/resolver.go

Line 374 in aa0c407

if rmsg.Rcode == dns.RcodeNameError {

Because this occurs in a goroutine spawned by this library, it's not recoverable

Full trace

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x1066173]

goroutine 22668 [running]:
github.com/domainr/dnsr.(*Resolver).exchangeIP(0xc0011de090, {0x481cb28, 0xc003b10c30}, {0xc0055ffa28, 0x13}, {0xc01925a140, 0xe}, {0xc0055ff668, 0x12}, {0x1f3bc0a, ...}, ...)
   /root/.cache/go-build/github.com/domainr/[email protected]/resolver.go:374 +0x6b3
github.com/domainr/dnsr.(*Resolver).exchange(0xc0011de090, {0x481cb28, 0xc003b10c30}, {0xc0055ffa28, 0x13}, {0xc0055ff668, 0x12}, {0x1f3bc0a, 0x3}, 0x1)
   /root/.cache/go-build/github.com/domainr/[email protected]/resolver.go:294 +0x185
github.com/domainr/dnsr.(*Resolver).iterateParents.func1({0xc0055ffa28?, 0xc001f8aa20?})
   /root/.cache/go-build/github.com/domainr/[email protected]/resolver.go:235 +0x89
created by github.com/domainr/dnsr.(*Resolver).iterateParents in goroutine 19692
   /root/.cache/go-build/github.com/domainr/[email protected]/resolver.go:234 +0x9ae

I've not managed to reliably reproduce it but I'm reasonably sure the bug comes from these lines

dnsr/resolver.go

Lines 354 to 359 in aa0c407

	conn, err := dialer.DialContext(ctx, "tcp", addr)
	if err == nil {
	dconn := &dns.Conn{Conn: conn}
	rmsg, dur, err = client.ExchangeWithConnContext(ctx, &qmsg, dconn)
	conn.Close()
	}

The err return from client.ExchangeWithConnContext is never checked because it's been shadowed by line 354

This means that resolvers with dnsr.WithTCPRetry() can panic if:

1. UDP lookup is truncated
2. TCP connection succeeds but lookup fails

In this case, the error check is ineffective at guarding against nil rmsg values because the error from the TCP lookup has been lost to the shadowed err var

dnsr/resolver.go

Lines 369 to 371 in aa0c407

	if err != nil {
	return nil, err
	}

[cee-dub]

Thanks for the report and digging in to investigate!

[cee-dub]

It does appear there are possible gaps in error handling in this area:

dnsr/resolver.go

Lines 337 to 369 in aa0c407

	conn, err := dialer.DialContext(ctx, "udp", addr)
	var rmsg *dns.Msg
	var dur time.Duration
	if err == nil {
	dconn := &dns.Conn{Conn: conn}
	rmsg, dur, err = client.ExchangeWithConnContext(ctx, &qmsg, dconn)
	conn.Close()
	}
	if r.tcpRetry && rmsg != nil && rmsg.MsgHdr.Truncated {
	// Since we are doing another query, we need to recheck the deadline
	if dl, ok := ctx.Deadline(); ok {
	if start.After(dl.Add(-TypicalResponseTime)) { // bail if we can't finish in time (start is too close to deadline)
	return nil, ErrTimeout
	}
	client.Timeout = dl.Sub(start)
	}
	// Retry with TCP
	conn, err := dialer.DialContext(ctx, "tcp", addr)
	if err == nil {
	dconn := &dns.Conn{Conn: conn}
	rmsg, dur, err = client.ExchangeWithConnContext(ctx, &qmsg, dconn)
	conn.Close()
	}
	}
	select {
	case <-ctx.Done(): // Finished too late
	logCancellation(host, &qmsg, rmsg, depth, dur, client.Timeout)
	return nil, ctx.Err()
	default:
	logExchange(host, &qmsg, rmsg, depth, dur, client.Timeout, err) // Log hostname instead of IP
	}
	if err != nil {