ssl connection incorrectly reset when using NAT

[exg]

After upgrading VirtualBox from version 5.0.26 to 5.0.28 on OS X 10.11.6, I noticed that ssl connections created in python with urllib2.urlopen are incorrectly reset on a Debian 8 guest with a single network interface in NAT mode. I attached a minimal python script that almost always fails with the following traceback:

gzip: stdin: unexpected end of file tar: Unexpected EOF in archive tar: Error is not recoverable: exiting now Failed Traceback (most recent call last):

File "./test.py", line 12, in <module>

shutil.copyfileobj(xact, pipe)

File "/usr/lib/python2.7/shutil.py", line 49, in copyfileobj

buf = fsrc.read(length)

File "/usr/lib/python2.7/socket.py", line 380, in read

data = self._sock.recv(left)

File "/usr/lib/python2.7/httplib.py", line 602, in read

s = self.fp.read(amt)

File "/usr/lib/python2.7/socket.py", line 380, in read

data = self._sock.recv(left)

File "/usr/lib/python2.7/ssl.py", line 714, in recv

return self.read(buflen)

File "/usr/lib/python2.7/ssl.py", line 608, in read

v = self._sslobj.read(len or 1024)

socket.error: [Errno 104] Connection reset by peer

I also attached the tcpdump output on both the host and the guest. The issue seems to only occur when the network traffic goes through the host thunderbolt ethernet adapter. If I switch to Wi-Fi, the issue does not occur. Moreover, I am unable to reproduce the problem with curl or wget.

[exg]

testcase