go: net: "cannot unmarshal DNS message" for SRV lookup

$ go version
go version go1.13.1 darwin/amd64

Does this issue reproduce with the latest release?

Yep.

What operating system and processor architecture are you using (go env)?

$ go env
➜  ~ go env
GO111MODULE=“”
GOARCH=“amd64”
GOBIN=“/Users/$USER/go/bin”
GOCACHE=“/Users/$USER/Library/Caches/go-build”
GOENV=“/Users/$USER/Library/Application Support/go/env”
GOEXE=“”
GOFLAGS=“”
GOHOSTARCH=“amd64”
GOHOSTOS=“darwin”
GONOPROXY=“”
GONOSUMDB=“”
GOOS=“darwin”
GOPATH=“/Users/$USER/go”
GOPRIVATE=“”
GOPROXY=“https://proxy.golang.org,direct”
GOROOT=“/usr/local/go”
GOSUMDB=“sum.golang.org”
GOTMPDIR=“”
GOTOOLDIR=“/usr/local/go/pkg/tool/darwin_amd64”
GCCGO=“gccgo”
AR=“ar”
CC=“clang”
CXX=“clang++”
CGO_ENABLED=“1”
GOMOD=“”
CGO_CFLAGS=“-g -O2”
CGO_CPPFLAGS=“”
CGO_CXXFLAGS=“-g -O2”
CGO_FFLAGS=“-g -O2”
CGO_LDFLAGS=“-g -O2”
PKG_CONFIG=“pkg-config”
GOGCCFLAGS=“-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wg/8x33rs4j5d7bgr5z58_4ql0m0000gn/T/go-build827220434=/tmp/go-build -gno-record-gcc-switches -fno-common”

What did you do?

I compiled the following program and executed the resultant program on an AWS EC2 instance passing the URI specified by the package vendor’s documentation.

package main

import (
	"context"
	"flag"
	"log"
	"time"

	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
	"go.mongodb.org/mongo-driver/mongo/readpref"
)

func main() {
	uri := flag.String("uri", "mongodb://localhost:27017", "URI of the mongo server of interest")
	flag.Parse()
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	client, err := mongo.Connect(ctx, options.Client().ApplyURI(*uri))
	if err != nil {
		log.Fatalf("connection error: %s", err)
	}
	if err := client.Ping(ctx, readpref.Primary()); err != nil {
		log.Fatalf("pinging error: %s", err)
	}
	log.Println("exiting")
}

Compiled on my MacBook Pro using different go versions, all the way to gotip version

go version devel +ccb95b6492 Fri Feb 21 05:04:06 2020 +0000 darwin/amd64

Compiled using GOOS=linux GOARCH=amd64 go build . since I’m building for an Ubuntu 18.04 EC2 instance.

I then scped the binary to the remote machine.

What did you expect to see?

Using the mongo client binary to connect succeeds with proper DNS resolution. So, I expected that…

What did you see instead?

After sshing to the remote box I executed

./mongoConnectTest -uri=mongodb+srv://$USER:$PASSWORD@$HOST/$TESTDB

and received the following error.

2020/02/21 17:28:56 connection error: error parsing uri: lookup $HOST on 127.0.0.53:53: cannot unmarshal DNS message

Notes

I’m using a DNS seedlist provided by an SRV record to access a cluster of machines exposing a mongo interface to me. The documentation for that DNS resolution process by which a list of hostname:port pairs and connection options are acquired is here.

This problem seems related to net’s inability to parse the resolved DNS entries. So, I’ve included the relevant DNS records below.

SRV record

ubuntu@ip-172-31-32-188:~$ dig SRV _mongodb._tcp.staging-vduzy.mongodb.net

; <<>> DiG 9.11.3-1ubuntu1.11-Ubuntu <<>> SRV _mongodb._tcp.staging-vduzy.mongodb.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4811
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;_mongodb._tcp.staging-vduzy.mongodb.net. IN SRV

;; ANSWER SECTION:
_mongodb._tcp.staging-vduzy.mongodb.net. 60 IN SRV 0 0 27017 staging-shard-00-02-vduzy.mongodb.net.
_mongodb._tcp.staging-vduzy.mongodb.net. 60 IN SRV 0 0 27017 staging-shard-00-00-vduzy.mongodb.net.
_mongodb._tcp.staging-vduzy.mongodb.net. 60 IN SRV 0 0 27017 staging-shard-00-01-vduzy.mongodb.net.

;; Query time: 9 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Fri Feb 21 17:39:47 UTC 2020
;; MSG SIZE  rcvd: 206

TXT record

ubuntu@ip-172-31-32-188:~$ dig TXT staging-vduzy.mongodb.net

; <<>> DiG 9.11.3-1ubuntu1.11-Ubuntu <<>> TXT staging-vduzy.mongodb.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 20788
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;staging-vduzy.mongodb.net.	IN	TXT

;; ANSWER SECTION:
staging-vduzy.mongodb.net. 60	IN	TXT	"authSource=admin&replicaSet=Staging-shard-0"

;; Query time: 23 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Fri Feb 21 17:40:15 UTC 2020
;; MSG SIZE  rcvd: 110