Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Programming > Python > result sorting in socket.getaddrinfo?

Reply
Thread Tools

result sorting in socket.getaddrinfo?

 
 
Bernhard Schmidt
Guest
Posts: n/a
 
      10-09-2004
Hello,

sorry for bothering, I'm not a programmer and I don't do much python,
I'm more a networking guy trying to get his favourite linux distribution
to update through the shiny new protocol IPv6 again (for those who are
interested, I'm talking about Gentoo Linux)

Gentoo's portage system is implemented in python calling rsync to sync
with a mirror server. There are rotations (metahostnames with many
address records) where portage has to decide to which IP it wants to
sync.

Basically the program needs a list of all available IP addresses and
will cycle through those until the sync is finished successfully.

The old code looked like that

| ips = socket.gethostbyname(hostname)[2]

if you test this for example with rsync.de.gentoo.org as hostname you
will get a list of addresses that changes its order with every call.
This behaviour is used for loadbalancing and failover through the
servers.

Now to support IPv6 addresses one has to use socket.getaddrinfo. This is
my current try (don't laugh ):

| ipsockets = socket.getaddrinfo(hostname,None,0,socket.SOCK_STR EAM)
| for socket in ipsockets:
| if (socket[0]==10):
| ips.append('[' + socket[4][0] + ']')
| else:
| ips.append(socket[4][0])

Big problem: The result of getaddrinfo() and therefor of ips is sorted
in some whay. If you again do

| >>> socket.getaddrinfo("rsync.de.gentoo.org",None,0,so cket.SOCK_STREAM);

and have a closer look at the resulting list you will observe two
things (at least I do on my box)

a) The two IPv6 addresses ([2001:andsoon]) are always in front of the
IPv4 addresses. This is expected behaviour and is consistent with
most applications/stacks supporting IPv6

b) The records within one address family (IPv4 or IPv6) are not really
in randomized order. I called it several hundred times now and the
order of the IPv6 records is always

'2001:638:500:101::21', '2001:7b0:11ff:1::1:1'

even worse, there are 15 IPv4 records in that list, and I have so far
seen only two of them at the beginning of the list.

When I debug the on-wire format of the DNS queries I can see that the
resolver server indeed answers with randomized order, so the sorting
seems to appear either somewhere in Python or somewhere in the
glibc.

The consequence of this would be, that the two servers in front of the
list would be hammered with traffic and the others idle around.

1.) Is it possible to change this behaviour?

2.) If not, does someone have a code snippet available for randomizing
the resulting list or another idea how to solve this?

Python 2.3.3 (#1, Jun 4 2004, 00:57:34)
[GCC 3.3.2 20031218 (Gentoo Linux 3.3.2-r5, propolice-3.3-7)] on linux2

[ some time later ]
Gnah, I just found a way so even I being a non-programmer (especially
regarding C) could test this ... using OpenSSH I verified that C
programs also suffer from this problem, it's not pythons fault.
Disregard question one above

Thanks a lot
Bernhard
 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
i = 10; result = ++i - --i; How result become ZERO Lakshmi Sreekanth C Programming 52 09-23-2010 07:41 AM
Re: i = 10; result = ++i - --i; How result become ZERO Mr. Buffoon C Programming 4 09-23-2010 03:01 AM
Is the result of valid dynamic cast always equal to the result ofcorrespondent static cast? Pavel C++ 7 09-18-2010 11:35 PM
simulation result is correct but synthesis result is not correct J.Ram VHDL 7 12-03-2008 01:26 PM
1. Ruby result: 101 seconds , 2. Java result:9.8 seconds, 3. Perl result:62 seconds Michael Tan Ruby 32 07-21-2005 03:23 PM



Advertisments