NC(1) User Commands NC(1)

nc — arbitrary TCP and UDP connections and listens

nc -h

nc [-46dnrStuvz] [-i interval] [-M ttl] [-m minttl] [-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 [-46DdnrStuvz] [-i interval] [-M ttl] [-m minttl] [-T ToS] [hostname] port

nc -l [-46DdnrStuvz] [-i interval] [-M ttl] [-m minttl] [-T ToS] -p port

nc -U [-Ddtvz] [-i interval] [-w timeout] -p 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:

Force nc to use IPv4 addresses only.
Force nc to use IPv6 addresses only.
Enable debugging on the socket.
Do not attempt to read from stdin.
Print nc help.
interval
Specify a delay time of interval between lines of text sent and received. This option also causes a delay time between connections to multiple ports.
Force nc to listen for another connection after its current connection is closed.

It is an error to use this option without the -l option.

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 -s or -z options. Additionally, any timeout specified with the -w option is ignored.

Set the default IPv4 TTL or IPv6 hop count that should be used in outgoing packets. This corresponds to the IP_TTL and IPV6_UNICAST_HOPS socket options.
Set the minimum IPv4 TTL or IPv6 hop count that must be present on a packet for a packet to be received. This corresponds to the IP_MINTTL and IPV6_MINHOPCOUNT socket options.
Do not do any naming or service lookups on any addresses, hostnames, or ports.

Use of this option means that hostname and port arguments are restricted to numeric values.

If used with -v option all addresses and ports are printed in numeric form, in addition to the restriction imposed on the arguments. This option does not have any effect when used in conjunction with the -U option.

proxy_username
Specify a username (proxy_username) to present to a proxy server that requires authentication. If proxy_username is not specified, authentication is not attempted. Proxy authentication is only supported for HTTP CONNECT proxies at present.

It is an error to use this option in conjunction with the -l option.

port
When used without -l option, specify the source port nc should use, subject to privilege restrictions and availability. When used with the -l option, set the listen port.

This option can be used with -l option only provided global port argument is not specified.

Choose source or destination ports randomly instead of sequentially within a range or in the order that the system assigns them.

It is an error to use this option in conjunction with the -l option.

Enables the RFC 2385 TCP MD5 signature option.

In order for packets to be sent or received in conjunction with this option, a security association that matches the traffic must also be created using tcpkey(8).

source_ip_address
Specify the IP of the interface which is used to send the packets.

It is an error to use this option in conjunction with the -l option.

ToS
Specify IP Type of Service (ToS) or IPv6 traffic class for the connection. Valid values are the tokens: lowdelay, throughput, reliability, or any decimal or hexadecimal integer between 0 and 255.
Cause 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.
Specify the use of Unix Domain Sockets. If you specify this option without -l, it becomes an AF_UNIX client. If you specify this option with the -l option, a AF_UNIX server is created.

Use of this option requires that a single argument of a valid Unix domain path has to be provided to nc, not a host name or port.

Use UDP instead of the default option of TCP.
Specify verbose output.
timeout
Silently close the connection if a connection and stdin are idle for more than timeout seconds.

This option has no effect on the -l option, that is, nc listens forever for a connection, with or without the -w flag. The default is no timeout.

proxy_protocol
Use the specified protocol when talking to the proxy server. Supported protocols are 4 (SOCKS v.4), 5 (SOCKS v.5) and connect (HTTP proxy). If the protocol is not specified, SOCKS v.5 is used.

It is an error to use this option in conjunction with the -l option.

proxy_address[&: port]
Request connection to hostname 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 HTTP).

It is an error to use this option in conjunction with the -l option.

Scan for listening daemons, without sending any data to them.

It is an error to use this option in conjunction with the -l option.

The following operands are supported:

hostname
Specify host name.

hostname can be a numerical IP address or a symbolic hostname (unless the -n option is specified).

In general, hostname must be specified, unless the -l option is given or -U is used (in which case the argument is a path). If hostname argument is specified with -l option then port argument must be given as well and nc tries to bind to that address and port. If hostname argument is not specified with -l option then nc tries to listen on a wildcard socket for given port.

path
Specify pathname.
port | port_list
Specify port.

port_list can be specified as single integers, ranges or combinations of both. Specify ranges in the form of nn-mm. The port_list must have at least one member, but can have multiple ports/ranges separated by commas.

In general, a destination port must be specified, unless the -U option is given, in which case a Unix Domain Socket path must be specified instead of hostname.

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\r\n\r\n" | 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>
RCPT 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(8).

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 Trusted Extensions are configured and the port nc should listen on is configured as a multi-level port nc also needs the privilege.

Privileges can be assigned to the user or role directly, by specifying them in the account's default privilege set in user_attr(5). However, this means that any application that this user or role starts have these additional privileges. To only grant the privileges(7) when nc is invoked, the recommended approach is to create and assign an rbac(7) rights profile. See EXAMPLES for additional information.

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(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

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,\
    !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(5) 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:

#!/bin/ksh

(( $# == 1 )) || exit 1
(( $1 > 42 && $1 < 64 )) || exit 1
exec /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 or a profile shell.

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 . The interface stability level for all other command line options and their arguments is

cat(1), pfcsh(1), pfexec(1), pfksh(1), pfsh(1), ppriv(1), sed(1), ssh(1), telnet(1), ssh_config(5), user_attr(5), attributes(7), privileges(7), rbac(7), inetadm(8), inetconv(8), inetd(8), tcpkey(8)

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 succeed, that is, report the port as open, rendering the -uz combination of flags relatively useless.

April 27, 2024 OmniOS