twisted names exception with getHostByName()

	jonojono reported
Trac ID	trac#8041
Type	defect
Created	2015-10-05 03:45:55Z

An exception is thrown when using twisted.names.client.getHostByName() on certain hostnames.

An example to reproduce:

# cat dns.py
...
@defer.inlineCallbacks
def test_dns(reactor, domain):
    result = yield client.getHostByName(domain, timeout=(1, 2))
    print result

if __name__ == '__main__':
    task.react(test_dns, sys.argv[1:])

# python dns.py weblogin.umich.edu
main function encountered error
Traceback (most recent call last):
  File "/twisted/names/client.py", line 398, in _lookup
    d = self.queryUDP([dns.Query(name, type, cls)], timeout)
  File "/twisted/names/client.py", line 299, in queryUDP
    d = self._query(used, queries, timeout[0])
  File "/twisted/names/client.py", line 264, in _query
    d = protocol.query(*args)
  File "/twisted/names/dns.py", line 2802, in query
    return self._query(queries, timeout, id, writeMessage)
--- <exception caught here> ---
  File "/twisted/names/dns.py", line 2680, in _query
    writeMessage(m)
  File "/twisted/names/dns.py", line 2800, in writeMessage
    self.writeMessage(m, address)
  File "/twisted/names/dns.py", line 2728, in writeMessage
    self.transport.write(message.toStr(), address)
  File "/twisted/internet/udp.py", line 286, in write
    "write() only accepts IP addresses, not hostnames")
twisted.internet.error.InvalidAddressError: 

I've observed this exception thrown on hostnames that have both CNAME and A records in the answer section. See the dig output for weblogin.umich.edu:

# dig weblogin.umich.edu

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.37.rc1.el6_7.4 <<>> weblogin.umich.edu
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 40349
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;weblogin.umich.edu.            IN      A

;; ANSWER SECTION:
weblogin.umich.edu.     178     IN      CNAME   weblogin.dsc.umich.edu.
weblogin.dsc.umich.edu. 1800    IN      A       141.211.21.150
weblogin.dsc.umich.edu. 1800    IN      A       141.211.7.200
weblogin.dsc.umich.edu. 1800    IN      A       141.211.7.210
weblogin.dsc.umich.edu. 1800    IN      A       141.211.21.149

;; AUTHORITY SECTION:
dsc.umich.edu.          1800    IN      NS      dns2.itd.umich.edu.
dsc.umich.edu.          1800    IN      NS      dns1.itd.umich.edu.

;; Query time: 33 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Mon Oct  5 03:41:39 2015
;; MSG SIZE  rcvd: 169

trac-id__8041 8041
type__defect defect
reporter__jonojono jonojono
priority__high high
milestone__None None
branch__ 
branch_author__ 
status__new new
resolution__None None
component__names names
keywords__None None
time__1444016755835561 1444016755835561
changetime__1460040926788527 1460040926788527
version__None None
owner__pawelmhm pawelmhm

Start with twisted/names/client.py around getHostByName() and follow the query and write path through twisted/names/dns.py. Reproduce the failure with the supplied dns.py example and weblogin.umich.edu hostname, then verify that handling an answer containing CNAME and A records no longer raises InvalidAddressError.