I am trying to follow a number of recipes which use netcat or ideally knc (kerberos netcat) to create a proxy; either to look at what is being piped through the proxy for debugging or to create an kerberos authenticated link over which I can tunnel some java based tcp server-to-server traffic.
Taking this example http://notes.tweakblogs.net/blog/7955/using-netcat-to-build-a-simple-tcp-proxy-in-linux.html the pattern seems to be something like:
mkfifo fifo
nc -l -p 8080 <fifo | nc tweakers.net 80 >fifo
Yet that gives the error:
usage: nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port]
[-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_version]
[-x proxy_address[:port]] [hostname] [port[s]]
Yet if I run ether side of the pipe alone I get no error:
nc -l -p 8080 <fifo
nc tweakers.net 80 >fifo
What am I missing to get this to work?
I am running Red Hat Enterprise Linux Server release 6.2 (Santiago) either bash or ksh.
Annoyingly it was the -p option which was tripping things up. The following command runs fine:
nc -l 8080 <fifo | nc somehost.com 80 >fifo
Then in another shell doing
telnet localhost 8080
get /
works fine.
Related
netcat command that installed on Ubuntu 20.04.01 by default has -X option, but netcat 0.7.1 compiled from source code does not have -X option. What is the difference?
netcat that installed on Ubuntu 20.04.01:
foo#foo:~/data/tmp$ netcat --help
netcat: invalid option -- '-'
usage: nc [-46CDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl]
[-m minttl] [-O length] [-P proxy_username] [-p source_port]
[-q seconds] [-s source] [-T keyword] [-V rtable] [-W recvlimit] [-w timeout]
[-X proxy_protocol] [-x proxy_address[:port]] [destination] [port]
netcat 0.7.1 compiled from the source code:
foo#foo:~/data/tmp$ ./netcat/bin/netcat --help
GNU netcat 0.7.1, a rewrite of the famous networking tool.
Basic usages:
connect to somewhere: ./netcat/bin/netcat [options] hostname port [port] ...
listen for inbound: ./netcat/bin/netcat -l -p port [options] [hostname] [port] ...
tunnel to somewhere: ./netcat/bin/netcat -L hostname:port -p port [options]
Mandatory arguments to long options are mandatory for short options too.
Options:
-c, --close close connection on EOF from stdin
-e, --exec=PROGRAM program to exec after connect
-g, --gateway=LIST source-routing hop point[s], up to 8
-G, --pointer=NUM source-routing pointer: 4, 8, 12, ...
-h, --help display this help and exit
-i, --interval=SECS delay interval for lines sent, ports scanned
-l, --listen listen mode, for inbound connects
-L, --tunnel=ADDRESS:PORT forward local port to remote address
-n, --dont-resolve numeric-only IP addresses, no DNS
-o, --output=FILE output hexdump traffic to FILE (implies -x)
-p, --local-port=NUM local port number
-r, --randomize randomize local and remote ports
-s, --source=ADDRESS local source address (ip or hostname)
-t, --tcp TCP mode (default)
-T, --telnet answer using TELNET negotiation
-u, --udp UDP mode
-v, --verbose verbose (use twice to be more verbose)
-V, --version output version information and exit
-x, --hexdump hexdump incoming and outgoing traffic
-w, --wait=SECS timeout for connects and final net reads
-z, --zero zero-I/O mode (used for scanning)
Remote port number can also be specified as range. Example: '1-1024'
The netcat from Ubuntu is Open BSD one :
$ netcat -help
OpenBSD netcat (Debian patchlevel 1.206-1ubuntu1)
And you compiled the GNU version:
$ ./netcat/bin/netcat --help
GNU netcat 0.7.1, a rewrite of the famous networking tool.
This post discusses the differences between both.
I am trying to understand why netcat listener isn't working in my Kali Linux VM. From what I understand,I open a terminal and open the port.
nc -l 155
Then, I open another terminal within my VM and use the following command to connect to that port number.
nc 127.0.0.1 155 (loopback IP address and same port number)
It was unsuccessful and since I am just a newbie in this field, I was hoping to get some assistance on this issue. However, I found a new way to execute this command but I am not understanding the logic behind why this new way works and not the original method that I learned in class. Thank you for your help in advance!
First of all, to elevate your self from newbie status, you have to understand what errors mean. "it was unsuccessful" is an insufficient description of your results for any real debugging. Probably, what happened was a valuable clue to the issue - you should have included that information. Furthermore, you really have to get your commands in the quetsion exactly right. Don't say you did one thing, then post a screenshot of something else happening. I'm not sure what the -e is supposed to be doing, but I don't find any record of it in my osx implementation or online man pages.
Different builds or implementations of netcat could differ, but from what I'm seeing from a netcat on my osx box, -p is not the right way to specify destination port.
$ nc localhost -p 1055
nc: missing hostname and port
usage: nc [-46AacCDdEFhklMnOortUuvz] [-K tc] [-b boundif] [-i interval] [-p source_port] [--apple-delegate-pid pid] [--apple-delegate-uuid uuid]
[-s source_ip_address] [-w timeout] [-X proxy_version]
[-x proxy_address[:port]] [hostname] [port[s]]
-p specifies source port. You don't usually need to specify this. Furthermore, you can't have a source and destination of the socket on the same box on the same port. Usually source port doesn't need to be specified.
Finally, ports under 1024 can only be allocated as root. Like most linux professionals, I don't run anything as root unless I really have to, so I changed to 1055 for this demonstration. One nc each in a termina window, typing messages in one print out the other side. Observe:
$ nc -l 1055
hi world
hi yourself, world!
$ nc localhost 1055
hi world
hi yourself, world!
server: nc -l ${port} > ${file}
local: nc ${ip} -z ${port} < ${file}
I am using netcat utility on linux to receive outputs from a program on a windows machine. My problem being that the program on the windows machine does not always give an output.
How can i check that either a connection has been made to netcat ?
What i am doing till now is "nc -l -v 9103 > output" then i check the size of output, the problem this poses is that netcat only write to a file after a certain buffer size has been reached or a new line char is encountered, so some cases evne though a connection has been made the file size is detected as zero.
How can i check if someone has made a connection with netcat.
I tried using
nc -l -v -e someprog.exe 9103 > output
but my netcat doesnt seem to support this
below are the options i have
$ nc -h
usage: nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port]
[-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_version]
[-x proxy_address[:port]] [hostname] [port[s]]
Command Summary:
-4 Use IPv4
-6 Use IPv6
-D Enable the debug socket option
-d Detach from stdin
-h This help text
-i secs Delay interval for lines sent, ports scanned
-k Keep inbound sockets open for multiple connects
-l Listen mode, for inbound connects
-n Suppress name/port resolutions
-p port Specify local port for remote connects
-r Randomize remote ports
-s addr Local source address
-T ToS Set IP Type of Service
-C Send CRLF as line-ending
-t Answer TELNET negotiation
-U Use UNIX domain socket
-u UDP mode
-v Verbose
-w secs Timeout for connects and final net reads
-X proto Proxy protocol: "4", "5" (SOCKS) or "connect"
-x addr[:port] Specify proxy address and port
-z Zero-I/O mode [used for scanning]
Port numbers can be individual or ranges: lo-hi [inclusive]
verbose mode will write connectivity to stderr, and you can redirect stderr to a file, the verbose log has something like
connect to [xxx] from [xxxx]
try
nc -l -v -p 9103 -k 1> output 2>connect.log
and monitor connect.log for connectivity
if you don't use -k , netcat quits after 1st connection.
If you can upgrade your copy of netcat: the modern versions (1.10, for one) have an option to execute a program (or a shell command) upon connect. Otherwise, you can make the netcat think it runs in a terminal (to disable buffering of stdout), by using for example script (it just saves everything on stdin/stdout/stderr in the given file). Or use logging features of screen and tmux.
I have a webserver WWW1 and a front-facing proxy PRX. I use SSH ProxyCommand to connect to WWW1's internal IP (private IP) via PRX (private+public IP). For some connections (not all) I see a network connection left open after I'm finished. These add up!
~/.ssh/config
Host *
ServerAliveInterval 5
ControlMaster auto
ControlPath ~/.ssh/master-%r#%h:%p
Host WWW1 WWW2 WWW3
User foo
ProxyCommand ssh -q -a -x PRX nc %h 22
IdentityFile ~/.ssh/id_foo_WWWx
On PRX, lsof | grep WWW1:ssh shows 124 open connections at the moment. On WWW1, the same command shows 243 open connections. There are similar open connections for WWW2, WWW3 etc.
WWW1 and PRX are Debian. Client connections are coming from a mix of Debian, Ubuntu and OSX10.6. I use Emacs Tramp but this has no special configuration (AFAIK) outside of my ~/.ssh/config.
I'm concerned about running out of internal ports, and ideally I want these connections to clean themselves up without intervention. Ideally by configuring them to kill themselves off; failing that a command I can kill old processes with is fine!
A better way would be to use the -W option of SSH, so you could put
ProxyCommand ssh -q -a -x PRX -W %h:22
instead of
ProxyCommand ssh -q -a -x PRX nc %h 22
This way you get rid of dependence on nc too.
Don't know whether it matters but I use nc -w 1 %h %p
Is there a way to quickly bind to a TCP port/ip address and simply print out all information to STDOUT? I have a simple debugging solution which writes things to 127.0.0.1:4444 and I'd like to be able to simply bind up a port from bash and print everything that comes across. Is there an easy way to do this?
$ nc -k -l 4444 > filename.out
see nc(1)
Just because you asked how to do it in bash, though netcat answer is very valid:
$ exec 3<>/dev/tcp/127.0.0.1/4444
$ cat <&3
That is working as you expecting:
nc -k -l 4444 |bash
and then you
echo "ls" >/dev/tcp/127.0.0.1/4444
then you see the listing performed by bash.
[A Brief Security Warning]
Of course if you leave a thing like this running on your computer, you have a wide open gateway for all kinds of attacks because commands can be sent from any user account on any host in your network. This implements no security (authentication, identification) whatsoever and sends all transmitted commands unencrypted over the network, so it can very easily be abused.
Adding an answer using ncat that #Freedom_Ben alluded to:
ncat -k -l 127.0.0.1 4444
and explanation of options from man ncat:
-k, --keep-open Accept multiple connections in listen mode
-l, --listen Bind and listen for incoming connections