read() returns too few data (TAP-mode: MTU != Ethernet Frame Size)

[Stefan Braun]

In TAP-Mode I had the problem that larger Ping packets and TCP connections after establishing didn't work over TAP<->pytap<->MyTestScript<->pytap<->TAP.

I found the problem in the read()-method of pytap.py. This code works in TAP-mode (I don't know the consequences in TUN-mode):
...
ETHERNET_HEADER_SIZE = 18
...
def read(self):
        data = os.read(self.__fd__, self.mtu + ETHERNET_HEADER_SIZE)
        return data

According to http://de.wikipedia.org/wiki/Maximum_Transmission_Unit#Beispiel_Ethernet and
http://de.wikipedia.org/w/index.php?title=Datei:Ethernetpaket.svg&filetimestamp=20090225134531
the Ethernet frame size is 18 Byte longer than the MTU (with activated VLAN it's 22 Bytes longer, but in Linux VLAN isn't activated as default).

Files:
Modified pytap.py: http://pastebin.com/gMB8zUfj
My current test project (Ethernet-tunnel via TCP): http://pastebin.com/SLpeRngY

[Dominik George (natureshadow)]

Thank you for the report, Stefan.

(Unfortunately Launchpad doesn't e-mail me on new bug reports anymore so I didn't see this earlier ....).

I have implemented a fix that only affects TAP mode, becuase TUN moder is not affected by this.

Please note that development has moved to http://git.proparity.net/redmine/projects/pytap .