Strange "loopback device" actions on Mac OS X Leopard

Question

Hi all,
I want to explain my little problem to you, hoping someone can give me an answer:
from about a week I'm trying to develop an application that use pylibpcap (a python extension of pcap library) to sniff packets on a pc on ethernet and examine udp packets sniffed.
This python program works very well on LAN, on two different computers (1Gnu/Linux and 1Mac), but if I try to make it work on loopback device (both server program and client program), some strange things happen.
The client simply do this:
*bunnybox:~ alex$ python*
*Python 2.5.1 (r251:54863, Oct 5 2007, 21:08:09)*
*GCC 4.0.1 (Apple Inc. build 5465)] on darwin*
*Type "help", "copyright", "credits" or "license" for more information.*
*>>> import socket*
*>>> s=socket.socket(socket.AF_INET, socket.SOCK_DGRAM)*
*>>> s.connect(("127.0.0.1",23))*
*>>> s.send("try to send this")*
16
Listening on loopback device (lo0) with Wireshark (a Sniffer Program), it returns (this is the exported log):
*No. Time Source Destination Protocol Info*
*11 2.840622 127.0.0.1 127.0.0.1 UDP Source port: 49256 Destination port: telnet [UDP CHECKSUM INCORRECT]*
*Frame 11 (48 bytes on wire, 48 bytes captured)*
Null/Loopback
*Internet Protocol, Src: 127.0.0.1 (127.0.0.1), Dst: 127.0.0.1 (127.0.0.1)*
*User Datagram Protocol, Src Port: 49256 (49256), Dst Port: telnet (23)*
*Data (16 bytes)*
*0000 74 72 79 20 74 6f 20 73 65 6e 64 20 74 68 69 73 try to send this*
*As you can see, there is a problem on UDP checksum.*
But if I try the same command on a different computer on my LAN,
for example: a Gnu/Linux Box of Address: 192.168.1.20 (Mac OS X has address: 192.168.1.100),
this is the python code:
*alex@boxxolo ~ $ python*
*Python 2.4.4 (#1, Oct 28 2007, 15:48:55)*
*GCC 4.1.2 (Gentoo 4.1.2)] on linux2*
*Type "help", "copyright", "credits" or "license" for more information.*
*>>> import socket*
*>>> s=socket.socket(socket.AF_INET, socket.SOCK_DGRAM)*
*>>> s.connect(("192.168.1.100",23))*
*>>> s.send("try to send this")*
16
and this is the result sniffed by Wireshark on Mac Os X Leopard Computer (this is the exported log):
*No. Time Source Destination Protocol Info*
*4953 57.558319 192.168.1.20 192.168.1.100 UDP Source port: filenet-tms Destination port: telnet*
*Frame 4953 (60 bytes on wire, 60 bytes captured)*
*Ethernet II, Src: 00:e0:18:29:74:28 (00:e0:18:29:74:28), Dst: 00:19:e3:43:85:e8 (00:19:e3:43:85:e8)*
*Internet Protocol, Src: 192.168.1.20 (192.168.1.20), Dst: 192.168.1.100 (192.168.1.100)*
*User Datagram Protocol, Src Port: filenet-tms (32768), Dst Port: telnet (23)*
*Data (16 bytes)*
*0000 74 72 79 20 74 6f 20 73 65 6e 64 20 74 68 69 73 try to send this*
As you can see, no errors on UDP Checksum, datagram correctly received and no problems.
*NOTE: don't care about port 23, because no telnet or other services are listening on it, it's only a closed port.*
I don't know if it's a bug of loopback device (lo0),
or a bug of Python's Socket, or pcap problem..
Hope someone can give me an answer,
PS: If this is not an appropriate place to publish this type of question, please tell me where I can.
Have a nice day,
Alessandro.

Answer 1

Hi,
Have you receive some answers about your problem because I have the same ...
Thanks,
Leyoda