nc and dd not working

Posted on 2011-09-24
Last Modified: 2012-05-12

Receiving server :

 nc -l -p 1234 | dd of=/dev/sda

netstat -ant
tcp        0      0  *               LISTEN

Sending Server :

dd if=/dev/hda | nc 1234

but it does not do any transfer ..

while investigating i can see it connects ..

example :

After typing both command in both server ..

if i press Ctrl + c  ( to break the connection)

now if i do netstat in receiving server

tcp        0      0        ESTABLISHED

i can see there is establish connection , its only after pressing "control C"

when i will press Again Control +c.
then receiving server

 nc -l -p 1234 | dd of=/dev/sda
128+0 records in
128+0 records out
65536 bytes (66 kB) copied, 96.2275 s, 0.7 kB/s

so the connection establisehd after pressing Control +c Second time .. data does not transfer..

is there any bug or something ??

this command works :

Sending server :  cat | nc -vv -l -p 3333

Receving server : nc 3333 >

both server has same version of nc running ..

any workway round will be really usefull


Question by:fosiul01
  • 2
LVL 20

Assisted Solution

by:Daniel McAllister
Daniel McAllister earned 250 total points
ID: 36594179
From the man page for nc:
-l      Used to specify that nc should listen for an incoming connection
             rather than initiate a connection to a remote host.  It is an
             error to use this option in conjunction with the -p, -s, or -z
             options.  Additionally, any timeouts specified with the -w option
             are ignored.

So, the correct syntax for the listening endpoint (which apparently has an IP of would be:
nc -l 1234 | dd of=/dev/sda

and the correct syntax for the sending endpoint would be:
dd if=/dev/sda | nc 1234

NOTE: This will make the TCP network connection work -- I make no argument or guarantee that the dd portion of this will correctly copy an entire disk from point a to point b... personally, I would use an rsync command, or in a pinch, a tar tunneled through an ssh (or an nc) command.

Just my initial thoughts looking at this... let me know if it still doesn't work!


PS: I am assuming as well that the listening endpoint is either not using a firewall (iptables) or has a rule in place for port 1234....

LVL 29

Author Comment

ID: 36597514
hi Sorry did not see the post ..

if i use live cd example
knoppix in both server then i use command

Receiving server :

 nc -l -p 1234 | dd of=/dev/sda

netstat -ant
tcp        0      0  *               LISTEN

Sending Server :

dd if=/dev/hda | nc 1234

it works perfectly ..

but lenny its not working ..
LVL 68

Accepted Solution

woolmilkporc earned 250 total points
ID: 36598541
Hi again fosiul,

if I remember well we had the same case some time ago, and again if I remember well, I told you to use
the "-w" option at the sending end to have the connection terminated  after 1 second of network inactivity (i.e. after transfer is complete)..

Background: What you describe is quite normal netcat behaviour. It will never come to and end if you don't set a timeout or terminate it via Ctl-C.

You can verify this by not redirecting the output at the receiving (listening) end. You'll see the output appear immediately on the screen, but you will not see netcat terminate if you don't use the timeout (or Ctl-C).


LVL 29

Author Comment

ID: 36954775

yes, just remembered about that post..

i will try with that again to see how it works.. but because i did not have time .. i had to take both server offline then use live CD and then use dd + netcat to copy data

its works fine with live cd (knopxis)


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