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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
It is an error to use this option without the -l option.
-l
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.
-n
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.
-P proxy_username
It is an error to use this option in conjunction with the -l option.
-p port
This option can be used with -l option only provided global port argument is not specified.
-r
It is an error to use this option in conjunction with the -l option.
-s source_ip_address
It is an error to use this option in conjunction with the -l option.
-T ToS
-t
-U
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.
-u
-v
-w timeout
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.
-X proxy_protocol
It is an error to use this option in conjunction with the -l option.
-x proxy_address[:port]
It is an error to use this option in conjunction with the -l option.
-z
It is an error to use this option in conjunction with the -l option.
The following operands are supported:
hostname
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
port
port_list
port_list can be specified as single integers or ranges. Specify ranges in the form of nn-mm. The port_list must have at least one member, but can have multiple members separated by spaces or ranges.
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/1.0r | 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
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,\ !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:
| |||||||||
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.
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