Problems sending/attaching files in email using Linux

[francisgirard]

Hi All,

CrashedAgain -> yeah I tried that one both ways - renaming a jpg with .txt and renaming a .txt with jpg (in case it was some type of filter blocking). the renamed jpg still did not pass and the renamed txt file still passed. So, my guess was that it is the actual content of the file that does not pass. Txt and doc do not have the same tructure as binaries like zip, exe, pdf, jpg.

Green1 -> thanks for the tips and the test. I see it is not that common factor, I have received your mail attachment so obviously it is not gmail blocking it.


On the same line as above, since the files that are causing trouble are binaries (txt and doc pass without trouble)someone from the office actually suggested that it might be related to MIME settings but I am not sure where I can find those in Linux... Maybe one of you would know...

Thanks again for all your help

[CrashedAgain]

Hi All,

CrashedAgain -> yeah I tried that one both ways - renaming a jpg with .txt and renaming a .txt with jpg (in case it was some type of filter blocking). the renamed jpg still did not pass and the renamed txt file still passed. So, my guess was that it is the actual content of the file that does not pass. Txt and doc do not have the same tructure as binaries like zip, exe, pdf, jpg.

I'm surprised at that result; I always thought mail programs just sent attached files 'blind' without trying to read or identify them (else how would spies be able to send encrypted secret messages?). Maybe that's a thought...have you made any changes to encryption or mime policies? Mine are all just default.

[Steven2ic]

Aha ! Someone else with exactly the same problem I am having with SMTP !

Running Knoppix 3.6, installed on my hard disk, with the 2.4.27 kernel.
KPPP for dial-up.
The PC doesn't have much memory - only 96 Mb RAM, with a 500 Mb swap partition.
I have tried the following e-mail programs, with exactly the same symptoms: Thunderbird, Netscape mail, Sylpheed, Mutt, Mozilla Mail. What's the common thread for all of these ?
I have tried both my local ISP SMTP server (which requires no authentication), and gmail's SMTP server (which requires authentication).

txt files go thru. most jpg files get stuck. most pdf files also get stuck. There seems to be a size dependency - very small jpg and pdf files go thru, but somewhere around 30 kb, they get stuck.

I did a snoop with Ethereal - it gives the impression that the ISP SMTP server suddenly stops acking the message body packets. Not sure I believe that - Seems more likely that the packets stop going out the ppp interface.

None of these problems occur running Win 98SE with Outllook Express on the same PC (it's dual boot).

What other experiments or diagnostics can I try ?

[Steven2ic]

Here's a fix, but it's not pretty....

There are a few things going on here:

1) The SMTP servers seem to be losing a large percentage of packets, causing the Linux TCP to retransmit. The servers are also slow to ACK packets. I have no idea why this occurs - but it is common to both the gmail SMTP server and my local ISP SMTP server with my 19.2 kb/s dial-up connection. I also have no idea why this problem is so much worse with binary attachments than text attachments.

2) The Linux TCP algorithm is adapting to the SMTP server packet-drop and slow ACK problems by increasing the amount of time between retransmissions, up to a maximum of 120 seconds. The TCP throughput drops to virtually zero and the e-mail gets "stuck".

The fix to the problem is to force the Linux maximum TCP retransmission timer to a much smaller value - like a few seconds. This probably violates some RFC, but it gets the job done !

I ported the patch at http://www.jankratochvil.net/project/tcp_rto to the 2.4.27 kernel source, and rebuilt and installed this kernel. Once that is done, you can tweak the maximum TCP retransmission timer to something like 3 seconds:

echo 300 > /proc/sys/net/ipv4/tcp_rto_max

Put this in a rc*.d script so it will get set with each reboot.