Hello!
On Tue, Oct 13, 2020 at 01:08:57PM +0000, Fabian Flückiger wrote:
> I am using NGINX as a mailproxy and recently discovered, that wireshark
> detects IMAP-LOGIN messages sent by nginx as "Malformed".
>
> The message contains: Line: 3 LOGIN {18}\r\n (HEX: 0000 33 20 4c 4f 47 49
> 4e 20 7b 31 38 7d 0d 0a)
This certainly isn't malformed (though incomplete). Most likely,
Wireshark fails to recognize IMAP string literal correctly.
> Some imap servers react to this with "BAD UNKNOWN Command" and close the
> connection.
Define "some imap servers".
> Here the full communication between nginx and backend:
>
> 3 LOGIN {18}
>
> + go ahead
>
> <redacted user> {9}
>
> + go ahead
>
> <redacted pass>
>
> 3 OK LOGIN completed
>
> 4 CAPABILITY
>
>
> BAD UNKNOWN Command
>
>
> any ideas?
The error seems to be returned to the "4 CAPABILITY" command.
This is not a command nginx sends, rather something it proxies
from the client.
Nevertheless, this seems to be incorrect behaviour of the backend
server: the CAPABILITY command looks perfectly valid. Moreover, even
for unrecognized commands correct response would be "* BAD ...",
that is, an untagged response, prefixed with the token "*".
--
Maxim Dounin
http://mdounin.ru/
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