net: can not unmarshal DNS over UDP message which is more than 512 octets long and indicates TC=0

[netroby]

cannot unmarshal DNS message
dial tcp: lookup g-ec4.images-amazon.com on 127.0.0.1:53: cannot unmarshal DNS message

$drill g-ec4.images-amazon.com     
;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 4883
;; flags: qr rd ra ; QUERY: 1, ANSWER: 12, AUTHORITY: 0, ADDITIONAL: 0 
;; QUESTION SECTION:
;; g-ec4.images-amazon.com. IN  A

;; ANSWER SECTION:
g-ec4.images-amazon.com.    399 IN  CNAME   ec4.images-amazon.chinacache.net.
ec4.images-amazon.chinacache.net.   126 IN  CNAME   joyo-amazon.cncssr.chinacache.net.
joyo-amazon.cncssr.chinacache.net.  1627    IN  CNAME   hpcc-page.cncssr.chinacache.net.
hpcc-page.cncssr.chinacache.net.    60  IN  A   119.188.138.29
hpcc-page.cncssr.chinacache.net.    60  IN  A   119.188.138.24
hpcc-page.cncssr.chinacache.net.    60  IN  A   123.130.123.6
hpcc-page.cncssr.chinacache.net.    60  IN  A   218.58.209.106
hpcc-page.cncssr.chinacache.net.    60  IN  A   218.60.108.135
hpcc-page.cncssr.chinacache.net.    60  IN  A   218.24.17.40
hpcc-page.cncssr.chinacache.net.    60  IN  A   222.163.198.56
hpcc-page.cncssr.chinacache.net.    60  IN  A   221.204.171.42
hpcc-page.cncssr.chinacache.net.    60  IN  A   221.204.23.16

;; AUTHORITY SECTION:

;; ADDITIONAL SECTION:

;; Query time: 0 msec
;; SERVER: 127.0.0.1
;; WHEN: Mon Dec 21 08:59:15 2015
;; MSG SIZE  rcvd: 690

[netroby]

the source code to dial a tcp connection

c, err = net.Dial(network, addr)

[mikioh]

I think you already have the answer. Please take a look at the output of drill.

;; SERVER: 127.0.0.1
;; WHEN: Mon Dec 21 08:59:15 2015
;; MSG SIZE rcvd: 690

I suppose that the DNS server your are running should behave like the following:

WHILE
        READ QueryMessage from UDP endpoint
        IF QueryMessage exists THEN
                PARSE QueryMessage to extract Header, Questions, OptRR
                IF Header and Questions look fine THEN
                        COMPOSE ResponseMessage with Answers
                        IF ResponseMessageLength <= 512 THEN
                                WRITE ResponseMessage to UDP endpoint
                        ELSE
                                IF OptRR exists THEN
                                        RECOMPOSE ResponseMessage using OptRR
                                        WRITE ResponseMessage to UDP endpoint
                                ELSE
                                        RECOMPOSE ResponseMessage to be <= 512
                                        SET TC in ResponseMessage to 1
                                        WRITE ResponseMessage to UDP endpoint
                                ENDIF
                        ENDIF
                ELSE
                        COMPOSE ResponseMessage with ErrorCode
                        WRITE ResponseMessage to UDP endpoint
                ENDIF
        ENDIF
        READ QueryMessage from TCP endpoint
        IF QueryMessage exists THEN
                PARSE QueryMessage to extract Header, Questions, OptRR
                IF Header and Questions look fine THEN
                        COMPOSE ResponseMessage with Answers
                        WRITE ResponseMessage to TCP endpoint
                ELSE
                        COMPOSE ResponseMessage with ErrorCode
                        WRITE ResponseMessage to TCP endpoint
                ENDIF
        ENDIF
ENDWHILE

Closing as "working as intended." FWIW, we already have #13356.

[mikioh]

@netroby,

I think you may follow https://golang.org/wiki/Questions.