netcat
NC(1) BSD General Commands Manual NC(1) nc — arbitrary TCP and UDP connections and listens
see also :
cat - ssh
Synopsis
nc
[-46bCDdhklnrStUuvZz]
[-I length]
[-i interval]
[-O length]
[-P proxy_username]
[-p source_port]
[-q seconds]
[-s source]
[-T toskeyword]
[-V rtable]
[-w timeout]
[-X proxy_protocol] [
-x proxy_address[
:port] ] [destination] [port]
add an example, a script, a trick and tips
examples
source
netcat -i 1 -l -p 9764 -u | netcat -v -u
192.168.0.116 9764
source
Sending file via netcat
Start another instance of netcat on computer B. Just do what you
did on computer A, but serve it from B. Give the new server a new
port.
source
Sending text file contents to server using netcat?
If you are using the GNU version of netcat then you can use the
-c flag to close the connection on EOF.
-c, --close close connection on EOF from stdin
If you are using the original version of the tool then you can
use the -q flag.
-q secs quit after EOF on stdin and delay of secs
An example for the original version is:
cat file.txt | nc -u -q 0 127.0.0.1 5144
I have add "-q 0" to your original command. This closes the
connection after the file has been sent.
source
Use netcat as a proxy to log traffic
Use socat, you don't need the pipes and fifos
Open a TCP connection to port 42 of host.example.com, using port
31337 as the source port, with a timeout of 5 seconds:
$ nc -p 31337 -w 5 host.example.com 42
Open a UDP connection to port 53 of host.example.com:
$ nc -u host.example.com 53
Open a TCP connection to port 42 of host.example.com using
10.1.2.3 as the IP for the local end of the connection:
$ nc -s 10.1.2.3 host.example.com 42
Create and listen on a UNIX-domain stream socket:
$ nc -lU /var/tmp/dsocket
Connect to port 42 of host.example.com via an HTTP proxy at
10.2.3.4, port 8080. This example could also be used by ssh(1);
see the ProxyCommand directive in ssh_config(5) for more
information.
$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
The same example again, this time enabling proxy authentication
with username ’’ruser’’ if the proxy requires it:
$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
source
Monitoring incoming requests on a web hosting server
I think its not possible. You have to ask the godaddy server to
do the same for you and if allowed they will surely do it for
you.
source
rlwrap netcat localhost 23753
numerorandom /teste.txt
source
echo "Chatting using
'netcat' through 'TCP' port $PORT "
netcat -l $PORT
source
How can I force netcat to send my input immediately, not just on newlines?
Use the "eof" key, CtrlD. When pressed in
the middle of a line, it will give to netcat everything that has
been input at that point.
source
Use xargs to send requests to the same server in parallel with netcat
Use echo -n or you will get 5 connections instead
four, one of them with no data. If in the server this is not
checked, maybe it is causing your problem.
echo -n A,B,C,D | xargs -d, -I{} -P 4 -n 1 echo {} | nc localhost 7000
Moreover, using a netcat as listening server, it appears to work
right:
zhen@sydow:~$ nc -l 7000
A
D
B
C
And also it seems to be parallel, I see out-of-order echo.
source
Netcat stops listening for UDP traffic
So there are multiple things called netcat; ubuntu even has
/etc/alternatives symbolic-link-hackery for it.
I think part of your problem is that UDP doesn't do sessions;
I've copied in part of the file
/usr/share/doc/netcat-traditional/README.gz below which does a
pretty good job of explaining.
UDP connections are opened instead of TCP when -u is specified.
These aren't really "connections" per se since UDP is a
connectionless protocol, although netcat does internally use
the "connected UDP socket" mechanism that most kernels support.
Although netcat claims that an outgoing UDP connection is
"open" immediately, no data is sent until something is read
from standard input. Only thereafter is it possible to
determine whether there really is a UDP server on the other
end, and often you just can't tell. Most UDP protocols use
timeouts and retries to do their thing and in many cases won't
bother answering at all, so you should specify a timeout and
hope for the best. You will get more out of UDP connections if
standard input is fed from a source of data that looks like
various kinds of server requests.
OK so maybe that's not all that great of an explanation but it's
what I could find.
If you haven't yet, you might want to experiment with any netcat
options you can find that would have to do with waiting... have
you experimented with:
-
using -l as well as -u to ensure you're in "listening" mode
-
-vv to see exactly what's happening
-
-q -1 ...which should "wait forever" even after receiving EOF
(hopefully, listening again?)
source
How to pipe the output of a linux command back into the predecesssing command
Named pipes are your friend!
mkfifo ncloop
nc -l 8090 < ncloop | (read METHOD URI PROTOCOL ; echo "method: $METHOD" ; echo "path: $URI" ; echo "prot: $PROTOCOL") > ncloop
(Tested in OS X 10.8.2 but should work on any Unix.)
source
Unix bidirectional pipe on commandline
Netcat documentation says
To obtain a hex dump file of the data sent either way, use "-o
logfile". The dump lines begin with "<" or ">" to
respectively indicate "from the net" or "to the net", and
contain the total count per direction, and hex and ascii
representations of the traffic. Capturing a hex dump naturally
slows netcat down a bit, so don't use it where speed is
critical.
source
Why is "nc -l xxxx" not opening a port?
I ran the verbose mode -v and got a clue back:
4444: inverse host lookup failed: Unknown server error : Connection timed out`
listening on [any] 41579 ...`
so I tried specifying a port with -p like this
nc -lvp 4444 and it works:
listening on [any] 4444 ...
Obviously I need to use -p with -l with
this version of nc.
I'd love it if anyone could tell my why there is this difference.
Am I running an old version? (-h reports
v1.10-38)
source
How can I verify that a 1TB file transferred correctly?
Two options:
Use sha1sum
sha1sum foo.box
In some circumstances sha1sum is faster.
Use rsync
It will take longer to transfer, but rsync verifies that the file
arrived intact.
From the rsync man page
Note that rsync always verifies that each transferred file was
correctly reconstructed on the receiving side by checking a
whole-file checksum that is generated as the file is
transferred...
source
Listen to port continuously and dump data to file
You must add an option to nc. The option depends on the
version of nc you are running. For instance, in my case
(Kubuntu), the option is -k. From the man page,
-k Forces nc to stay listening for another connection after its
current connection is completed. It is an error to use this
option without the -l option.
I cannot promise -k works (unless you are n Ubuntu, of
course), you will have to search for something similar. For
instance, on my Debian, -k does not exist, but you find:
-q seconds after EOF on stdin, wait the specified number of
seconds and then quit. If seconds is negative, wait forever
Edit:
To check that it works, on one pc:
nc -k -l 0.0.0.0 10000 > out.txt
On a second pc:
echo Hello | nc IP_address_of_first_pc 10000
Issue the command above a few times, then interrupt the
nc command on pc1, check number of lines in out.txt.
source
Can I pipe/redirect a console application through netcat so it can be used remotely?
Well, as documented in Wikipedia and netcat
documentation, there is a -e option that causes it
to spawn (execute) a program upon receipt of a connection,
attaching the socket to stdin, stdout, and stderr of the
process. Example usage:
nc -l
-pport_number-eprogram_name
Examples commonly show /bin/sh or bash
being used as the program_name. Use of this option
is discouraged because it basically opens an anonymous,
passwordless access portal into your machine. Of course,
this is mitigated by using a program with less power than the
shell (one that doesn’t have the ability to escape to a shell),
minimizing your use of it, and keeping it a secret.
Nonetheless, the original developers of netcat felt
strongly enough that this option was a bad idea that they
disabled it by default, and conditioned it under the
“GAPING_SECURITY_HOLE” compilation option. This is
mentioned briefly in this NetCat Tutorial and other netcat
documentation.
A Google search led me to discussions of this issue on other
Stack Exchange sites: Stack Overflow
and Server
Fault. Multiple contributors offered the following
technique to do the same thing without using the -e
option (i.e., in a version of netcat that has the
-e option disabled):
On the server:
mkfifopipe_name
nc -l
-pport_number<pipe_name
| program_name>pipe_name
On the client:
nc server_machine_name port_number
A couple of notes:
- On some versions of
netcat, -l
implies -p, so you should say just -l
and then the port number.
- You might want to wrap your solution in a
while
true loop.
source
Netcat stops when in a detached session
netcat wants to send its stdin across the socket and print out
the response, so it expects to have them both connected and
active. I don't know a good way of creating a command that waits
forever, but you could try
sleep 86400000 | nc localhost 3000 > test.txt &
description
The nc (or netcat)
utility is used for just about anything under the sun
involving TCP, UDP, or UNIX-domain sockets. It can open TCP
connections, send UDP packets, listen on arbitrary TCP and
UDP ports, do port scanning, and deal with both IPv4 and
IPv6. Unlike telnet(1), nc scripts nicely, and
separates error messages onto standard error instead of
sending them to standard output, as telnet(1) does with
some.
Common uses
include:
•
simple TCP proxies
•
shell-script based HTTP clients
and servers
•
network daemon testing
•
a SOCKS or HTTP ProxyCommand
for ssh(1)
•
and much, much more
The options are
as follows:
-4
Forces
nc to use IPv4 addresses only.
-6
Forces
nc to use IPv6 addresses only.
-b
Allow
broadcast.
-C
Send CRLF as
line-ending.
-D
Enable
debugging on the socket.
-d
Do not attempt
to read from stdin.
-h
Prints out
nc help.
-I
length
Specifies the size of the TCP
receive buffer.
-i
interval
Specifies a delay time interval
between lines of text sent and received. Also causes a delay
time between connections to multiple ports.
-k
Forces
nc to stay listening for another connection after its
current connection is completed. It is an error to use this
option without the -l option.
-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.
-n
Do not do any
DNS or service lookups on any specified addresses, hostnames
or ports.
-O
length
Specifies the size of the TCP
send buffer.
-P
proxy_username
Specifies a username to present
to a proxy server that requires authentication. If no
username is specified then authentication will not be
attempted. Proxy authentication is only supported for HTTP
CONNECT proxies at present.
-p
source_port
Specifies the source port
nc should use, subject to privilege restrictions and
availability.
-q
seconds
after EOF on stdin, wait the
specified number of seconds and then quit. If seconds
is negative, wait forever.
-r
Specifies that
source and/or destination ports should be chosen randomly
instead of sequentially within a range or in the order that
the system assigns them.
-S
Enables the RFC
2385 TCP MD5 signature option.
-s
source
Specifies the IP of the
interface which is used to send the packets. For UNIX-domain
datagram sockets, specifies the local temporary socket file
to create and use so that datagrams can be received. It is
an error to use this option in conjunction with the
-l option.
-T
toskeyword
Change IPv4 TOS value.
toskeyword may be one of critical,
inetcontrol, lowcost, lowdelay,
netcontrol, throughput, reliability, or
one of the DiffServ Code Points: ef, af11 ...
af43, cs0 ... cs7; or a number in either hex or
decimal.
-t
Causes
nc to send RFC 854 DON’T and WON’T
responses to RFC 854 DO and WILL requests. This makes it
possible to use nc to script telnet sessions.
-U
Specifies to
use UNIX-domain sockets.
-u
Use UDP instead
of the default option of TCP. For UNIX-domain sockets, use a
datagram socket instead of a stream socket. If a UNIX-domain
socket is used, a temporary receiving socket is created in
/tmp unless the -s flag is given.
-V
rtable
Set the routing table to be
used. The default is 0.
-v
Have nc
give more verbose output.
-w
timeout
Connections which cannot be
established or are idle timeout after timeout
seconds. The -w flag has no effect on the
-l option, i.e. nc will listen forever
for a connection, with or without the -w flag.
The default is no timeout.
-X
proxy_protocol
Requests that nc should
use the specified protocol when talking to the proxy server.
Supported protocols are ’’4’’ (SOCKS
v.4), ’’5’’ (SOCKS v.5) and
’’connect’’ (HTTPS proxy). If the
protocol is not specified, SOCKS version 5 is used.
-x
proxy_address[
:port]
Requests that nc should
connect to destination using a proxy at
proxy_address and port. If port is not
specified, the well-known port for the proxy protocol is
used (1080 for SOCKS, 3128 for HTTPS).
-Z
DCCP mode.
-z
Specifies that
nc should just scan for listening daemons, without
sending any data to them. It is an error to use this option
in conjunction with the -l option.
destination
can be a numerical IP address or a symbolic hostname (unless
the -n option is given). In general, a
destination must be specified, unless the -l
option is given (in which case the local host is used). For
UNIX-domain sockets, a destination is required and is the
socket path to connect to (or listen on if the
-l option is given).
port can
be a single integer or a range of ports. Ranges are in the
form nn-mm. In general, a destination port must be
specified, unless the -U option is given.
caveats
UDP port scans using the -uz combination of flags will
always report success irrespective of the target machine’s state.
However, in conjunction with a traffic sniffer either on the
target machine or an intermediary device, the -uz
combination could be useful for communications diagnostics. Note
that the amount of UDP traffic generated may be limited either
due to hardware resources and/or configuration settings.
BSD July 18, 2013 BSD
client server model
It is quite simple to build a very basic client/server model
using nc. On one console, start nc listening on a
specific port for a connection. For example:
$ nc -l 1234
nc is now listening on port 1234 for a connection. On a
second console (or a second machine), connect to the machine and
port being listened on:
$ nc 127.0.0.1 1234
There should now be a connection between the ports. Anything
typed at the second console will be concatenated to the first,
and vice-versa. After the connection has been set up, nc
does not really care which side is being used as a ’server’ and
which side is being used as a ’client’. The connection may be
terminated using an EOF (’^D’).
There is no -c or -e option in this netcat, but you
still can execute a command after connection being established by
redirecting file descriptors. Be cautious here because opening a
port and let anyone connected execute arbitrary command on your
site is DANGEROUS. If you really need to do this, here is an
example:
On ’server’ side:
$ rm -f /tmp/f; mkfifo /tmp/f
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 >
/tmp/f
On ’client’ side:
$ nc host.example.com 1234
$ (shell prompt from host.example.com)
By doing this, you create a fifo at /tmp/f and make nc listen at
port 1234 of address 127.0.0.1 on ’server’ side, when a ’client’
establishes a connection successfully to that port, /bin/sh gets
executed on ’server’ side and the shell prompt is given to
’client’ side.
When connection is terminated, nc quits as well. Use
-k if you want it keep listening, but if the command quits
this option won’t restart it or keep nc running. Also
don’t forget to remove the file descriptor once you don’t need it
anymore:
$ rm -f /tmp/f
data transfer
The example in the previous section can be expanded to build a
basic data transfer model. Any information input into one end of
the connection will be output to the other end, and input and
output can be easily captured in order to emulate file transfer.
Start by using nc to listen on a specific port, with
output captured into a file:
$ nc -l 1234 > filename.out
Using a second machine, connect to the listening nc
process, feeding it the file which is to be transferred:
$ nc host.example.com 1234 < filename.in
After the file has been transferred, the connection will close
automatically.
port scanning
It may be useful to know which ports are open and running
services on a target machine. The -z flag can be used to
tell nc to report open ports, rather than initiate a
connection. Usually it’s useful to turn on verbose output to
stderr by use this option in conjunction with -v option.
For example:
$ nc -zv host.example.com 20-30
Connection to host.example.com 22 port [tcp/ssh] succeeded!
Connection to host.example.com 25 port [tcp/smtp] succeeded!
The port range was specified to limit the search to ports 20 -
30, and is scanned by increasing order.
You can also specify a list of ports to scan, for example:
$ nc -zv host.example.com 80 20 22
nc: connect to host.example.com 80 (tcp) failed: Connection
refused
nc: connect to host.example.com 20 (tcp) failed: Connection
refused
Connection to host.example.com port [tcp/ssh] succeeded!
The ports are scanned by the order you given.
Alternatively, it might be useful to know which server software
is running, and which versions. This information is often
contained within the greeting banners. In order to retrieve
these, it is necessary to first make a connection, and then break
the connection when the banner has been retrieved. This can be
accomplished by specifying a small timeout with the -w
flag, or perhaps by issuing a "QUIT" command to the server:
$ echo "QUIT" | nc host.example.com 20-30
SSH-1.99-OpenSSH_3.6.1p2
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84 Ready
talking to servers
It is sometimes useful to talk to servers ’’by hand’’ rather than
through a user interface. It can aid in troubleshooting, when it
might be necessary to verify what data a server is sending in
response to commands issued by the client. For example, to
retrieve the home page of a web site:
$ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
Note that this also displays the headers sent by the web server.
They can be filtered, using a tool such as sed(1), if necessary.
More complicated examples can be built up when the user knows the
format of requests required by the server. As another example, an
email may be submitted to an SMTP server using:
$ nc [-C] localhost 25 << EOF
HELO host.example.com
MAIL FROM:<user[:at:]host.example[:dot:]com>
RCPT TO:<user2[:at:]host.example[:dot:]com>
DATA
Body of email.
.
QUIT
EOF
see also
cat , ssh
authors
Original implementation by
*Hobbit* ⟨ hobbit[:at:]avian[:dot:]org⟩ .
Rewritten with IPv6 support by Eric Jackson
<ericj[:at:]monkey[:dot:]org>.
Modified for Debian port by Aron Xu ⟨
aron[:at:]debian[:dot:]org⟩ .