[erlang-bugs] Bug in TFTP client implementation?

[Stefan Zegenhagen]

Dear all,

There might be a bug in erlang's TFTP client implementation that
prevents erlang to successfully transfer files over IPv6 with many
Microsoft(R) Windows TFTP servers.

Those TFTP servers (most of them are available free of charge) are
capable to bind to a single IP address only, not to an interface. The
problem arises when the server has statically configured IPv6 addresses
as well as automatically configured ones. Here's the step-by-step error
description:

     1. start the Windows TFTP server and bind to the static IPv6 server
        address
     2. call tftp:read_file(...) to download a file from the TFTP server
        using the static IPv6 server address
     3. transmission of packets is as follows:

<client IPv6> => <static server IPv6> : Write Request
<dynamic server IPv6> => <client IPv6> : Ack, Port #xxxx

<client IPv6> => <dynamic IPv6>:xxxx : Data block=0

It appears that the TFTP servers are unwilling (or unable) to respond
from the IPv6 address they have been contacted at.

Furthermore it seems that the erlang TFTP client uses the source IPv6 of
the received Ack packet in all further communication. Since the TFTP
server is not listening to its dynamic IPv6 address (from which it
always responds, though), the data blocks transmitted after the initial
successful "connection establishment" are never received by the TFTP
server.

I agree that the TFTP server should respond from the IPv6 address it was
contacted at. However, I also think it to be a questionable behaviour to
change to a different IP address for the actual data transfer.



Kind regards,

-- 
Dr. Stefan Zegenhagen

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