Possible bug in ncat 6.01?

[Jonas Wielicki <nmap-dev () sotecware net>]

Hi folks,

We tried to use our favourite local network file transfer mechanism (tar
| ncat), when we noticed the following odd behaviour. It seems like ncat
is randomly dropping data. To reproduce, it is sufficient on my machine
to do the following:

1. Open two shells, A and B.
2. On shell A, run:
    dd if=/dev/zero bs=1024k count=128 2>/dev/null | \
    tee >(wc -c) | \
    ncat --listen 1337 --send-only
3. On shell B, run:
    ncat ::1 1337 | wc -c
4. Notice how the lengths differ. Not only that, you'll get different
lengths on the receiving side on each run, making ncat a nice PRNG.
Maybe you need to increase the count parameter of dd, we found that the
effect is more prominent on huge data sizes, although we were able to
consistently observe it with 1 MByte of data via a gigabit link. On
loopback the effect does not occur stablely with 1 MByte or even 16
MBytes of data, the transfer is successful sometimes.

Running wireshark on the connection reveals that the packets did not get
emitted by the interface initially (tested using the gbit link,
wireshark being on the sender site).

The same test runs fine and reliable using a small C program (source
attached; sender-side only, as receiving sides ncat seems to work) or
python script doing the same task, which is why I would exclude kernel,
firewall and/or hardware issues for now (that is, if ncat isn't doing
something fancy).

Actually, I was able to find out that if ncat is not listening but only
connecting, no loss happens either. However, there is no correlation
between the time ncat has been idle before accepting the connection and
the amount of data loss.

$ ncat --version
Ncat: Version 6.01 ( http://nmap.org/ncat )
$ nmap --version
Nmap version 6.01 ( http://nmap.org )
Platform: x86_64-redhat-linux-gnu
Compiled with: liblua-5.1.4 openssl-1.0.0j libpcre-8.12 libpcap-1.1.1
nmap-libdnet-1.12 ipv6
Compiled without:
$ uname -a
Linux zion 3.4.9-1.fc16.x86_64 #1 SMP Wed Aug 15 20:45:23 UTC 2012
x86_64 x86_64 x86_64 GNU/Linux

If any further information is needed, don't hesitate to ask back!

best regards,
Jonas Wielicki

Attachment: test-listen.c
Description:

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