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Copyright (c) 1996 David Sacerdote All rights reserved.
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nc - arbitrary TCP and UDP connections and listens

nc -h

nc [-46dnrtuvz] [-i interval] [-P proxy_username] [-p port]
 [-s source_ip_address] [-T ToS] [-w timeout]
 [-X proxy_protocol] [-x proxy_address[:port]]
 hostname port_list

nc -l [-46Ddnrtuvz] [-i interval] [-T ToS] [hostname] port

nc -l [-46Ddnrtuvz] [-i interval] [-T ToS] -p port

nc -U [-Ddtvz] [-i interval] [-w timeout] path

nc -Ul [-46Ddktv] [-i interval] path

The nc (or netcat) utility is used for a variety of tasks associated with TCP or UDP. nc can open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, perform 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.

The nc command is often used for the following tasks:

The following options are supported: -4

-6 -D -d -h -i interval -k -l -n -P proxy_username -p port -r -s source_ip_address -T ToS -t -U -u -v -w timeout -X proxy_protocol -x proxy_address[:port] -z

The following operands are supported: hostname

path port

port_list

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, the command:

$ nc -l 1234

listens on port 1234 for a connection. On a second console (or a second machine), connect to the machine and port to which nc is listening:

$ nc 127.0.0.1 1234

There should now be a connection between the ports. Anything typed at the second console is 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 can be terminated using an EOF (Ctrl/d).

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 is 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 closes automatically.

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:

$ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80

This also displays the headers sent by the web server. They can be filtered, if necessary, by using a tool such as sed(1).

More complicated examples can be built up when the user knows the format of requests required by the server. As another example, an email can be submitted to an SMTP server using:

$ nc localhost 25 << EOF
HELO host.example.com
MAIL FROM: <user@host.example.com
RCTP TO: <user2@host.example.com
DATA
Body of email.
.
QUIT
EOF

It can 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 to initiate a connection.

In this example:

$ nc -z 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.

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-2.0-Sun_SSH_1.1
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84 Ready

One of the possible uses is to create simple services by using inetd(1M).

The following example creates a redirect from TCP port 8080 to port 80 on host realwww:

# cat << EOF >> /etc/services
wwwredir 8080/tcp # WWW redirect
EOF
# cat << EOF > /tmp/wwwredir.conf
wwwredir stream tcp nowait nobody /usr/bin/nc /usr/bin/nc -w 3 realwww 80
EOF
# inetconv -i /tmp/wwwredir.conf
wwwredir -> /var/svc/manifest/network/wwwredir-tcp.xml
Importing wwwredir-tcp.xml ...Done
# inetadm -l wwwredir/tcp
SCOPE NAME=VALUE
name="wwwredir"
endpoint_type="stream"
proto="tcp"
isrpc=FALSE
wait=FALSE
exec="/usr/bin/nc -w 3 realwww 80"
arg0="/usr/bin/nc"
user="nobody"
default bind_addr=""
default bind_fail_max=-1
default bind_fail_interval=-1
default max_con_rate=-1
default max_copies=-1
default con_rate_offline=-1
default failrate_cnt=40
default failrate_interval=60
default inherit_env=TRUE
default tcp_trace=TRUE
default tcp_wrappers=FALSE

To bind to a privileged port number nc needs to be granted the net_privaddr privilege. If Solaris Trusted Extensions are configured and the port nc should listen on is configured as a multi-level port nc also needs the net_bindmlp privilege.

Privileges can be assigned to the user or role directly, by specifying them in the account's default privilege set in user_attr(4). However, this means that any application that this user or role starts have these additional privileges. To only grant the privileges(5) when nc is invoked, the recommended approach is to create and assign an rbac(5) rights profile. See EXAMPLES for additional information.

Example 1 Using nc

Open a TCP connection to port 42 of host.example.com, using port 3141 as the source port, with a timeout of 5 seconds:

$ nc -p 3141 -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

Use a list of ports and port ranges for a port scan on various ports:

$ nc -z host.example.com 21-25,53,80,110-120,443

Create and listen on a Unix Domain Socket:

$ nc -lU /var/tmp/dsocket

Create and listen on a UDP socket with associated port 8888:

$ nc -u -l -p 8888

which is the same as:

$ nc -u -l 8888

Create and listen on a TCP socket with associated port 2222 and bind to address 127.0.0.1 only:

$ nc -l 127.0.0.1 2222

Connect to port 42 of host.example.com using 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(4) 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

To run nc with the smallest possible set of privileges as a user or role that has additional privileges (such as the default root account) it can be invoked using ppriv(1) as well. For example, limiting it to only run with the privilege to bind to a privileged port:

$ ppriv -e -sA=basic,!file_link_any,!proc_exec,!proc_fork,\e
!proc_info,!proc_session,net_privaddr nc -l 42

To allow a user or role to use only nc with the net_privaddr privilege, a rights profile needs to be created:

/etc/security/exec_attr
Netcat privileged:solaris:cmd:::/usr/bin/nc:privs=net_privaddr

/etc/security/prof_attr
Netcat privileged:::Allow nc to bind to privileged ports:help=None.html

Assigning this rights profile using user_attr(4) permits the user or role to run nc allowing it to listen on any port. To permit a user or role to use nc only to listen on specific ports a wrapper script should be specified in the rights profiles:

/etc/security/exec_attr
Netcat restricted:solaris:cmd:::/usr/bin/nc-restricted:privs=net_privaddr

/etc/security/prof_attr
Netcat restricted:::Allow nc to bind to privileged ports:help=None.html

and write a shell script that restricts the permissible options, for example, one that permits one to bind only on ports between 42 and 64 (non-inclusive):

/usr/bin/nc-restricted:

#!/bin/sh
[ $# -eq 1 ] && [ $1 -gt 42 -a $1 -lt 64 ] && /usr/bin/nc -l -p "$1"

This grants the extra privileges when the user or role invokes nc using the wrapper script from a profile shell. See pfsh(1), pfksh(1), pfcsh(1), and pfexec(1).

Invoking nc directly does not run it with the additional privileges, and neither does invoking the script without using pfexec or a profile shell.

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE

Interface Stability See below.

The package name is Committed. The command line syntax is Committed for the -4, -6, -l, -n, -p ,-u, and -w options and their arguments (if any). The name and port list arguments are Committed. The port range syntax is Uncommitted. The interface stability level for all other command line options and their arguments is Uncommitted.

cat(1), pfcsh(1), pfexec(1), pfksh(1), pfsh(1), ppriv(1), sed(1), ssh(1), telnet(1), inetadm(1M), inetconv(1M), inetd(1M), ssh_config(4), user_attr(4), attributes(5), privileges(5), rbac(5)

The original implementation of nc was written by Hobbit, hobbit@avian.org.

nc was rewritten with IPv6 support by Eric Jackson, ericj@monkey.org.

UDP port scans always succeeds, that is, reports the port as open, rendering the -uz combination of flags relatively useless.