I have this simple minimal 'working' example below that opens a connection to google every two seconds. When I run this script when I have a working internet connection, I get the Success message, and when I then disconnect, I get the Fail message and when I reconnect again I get the Success again. So far, so good.
However, when I start the script when the internet is disconnected, I get the Fail messages, and when I connect later, I never get the Success message. I keep getting the error:
urlopen error [Errno -2] Name or service not known
What is going on?
import urllib2, time
while True:
try:
print('Trying')
response = urllib2.urlopen('http://www.google.com')
print('Success')
time.sleep(2)
except Exception, e:
print('Fail ' + str(e))
time.sleep(2)
This happens because the DNS name "www.google.com" cannot be resolved. If there is no internet connection the DNS server is probably not reachable to resolve this entry.
It seems I misread your question the first time. The behaviour you describe is, on Linux, a peculiarity of glibc. It only reads "/etc/resolv.conf" once, when loading. glibc can be forced to re-read "/etc/resolv.conf" via the res_init()
function.
One solution would be to wrap the res_init()
function and call it before calling getaddrinfo()
(which is indirectly used by urllib2.urlopen()
.
You might try the following (still assuming you're using Linux):
import ctypes
libc = ctypes.cdll.LoadLibrary('libc.so.6')
res_init = libc.__res_init
# ...
res_init()
response = urllib2.urlopen('http://www.google.com')
This might of course be optimized by waiting until "/etc/resolv.conf" is modified before calling res_init()
.
Another solution would be to install e.g. nscd (name service cache daemon).
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