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.\" Portions Copyright (c) 2009, Sun Microsystems, Inc. All Rights Reserved.
.\" Copyright 2024 Oxide Computer Company
.\"
.Dd April 27, 2024
.Dt NC 1
.Os
.Sh NAME
.Nm nc
.Nd arbitrary TCP and UDP connections and listens
.Sh SYNOPSIS
.Nm
.Fl h
.Nm
.Op Fl 46dnrStuvz
.Op Fl i Ar interval
.Op Fl M Ar ttl
.Op Fl m Ar minttl
.Op Fl P Ar proxy_username
.Op Fl p Ar port
.Op Fl s Ar source_ip_address
.Op Fl T Ar ToS
.Op Fl w Ar timeout
.Op Fl X Ar proxy_protocol
.Op Fl x Ar proxy_address Ns Op &: Ar port
.Ar hostname
.Ar port_list
.Nm
.Fl l
.Op Fl 46DdnrStuvz
.Op Fl i Ar interval
.Op Fl M Ar ttl
.Op Fl m Ar minttl
.Op Fl T Ar ToS
.Op Ar hostname
.Ar port
.Nm
.Fl l
.Op Fl 46DdnrStuvz
.Op Fl i Ar interval
.Op Fl M Ar ttl
.Op Fl m Ar minttl
.Op Fl T Ar ToS
.Fl p Ar port
.Nm
.Fl U
.Op Fl Ddtvz
.Op Fl i Ar interval
.Op Fl w Ar timeout
.Fl p Ar path
.Nm
.Fl Ul
.Op Fl 46Ddktv
.Op Fl i Ar interval
.Ar path
.Sh DESCRIPTION
The
.Nm
.Po
or
.Nm netcat
.Pc
utility is used for a variety of tasks associated with TCP or UDP.
.Nm
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
.Xr telnet 1 ,
.Nm
scripts nicely, and separates error messages onto standard error instead of
sending them to standard output.
.Pp
The
.Nm
command is often used for the following tasks:
.Bl -bullet -width Ds
.It
simple TCP proxies
.It
shell-script based HTTP clients and servers
.It
network daemon testing
.It
a SOCKS or HTTP ProxyCommand for
.Xr ssh 1
.El
.Sh OPTIONS
The following options are supported:
.Bl -tag -width Ds
.It Fl 4
Force
.Nm
to use IPv4 addresses only.
.It Fl 6
Force
.Nm
to use IPv6 addresses only.
.It Fl D
Enable debugging on the socket.
.It Fl d
Do not attempt to read from
.Dv stdin .
.It Fl h
Print
.Nm
help.
.It Fl i Ar interval
Specify a delay time of
.Ar interval
between lines of text sent and received.
This option also causes a delay time between connections to multiple ports.
.It Fl k
Force
.Nm
to listen for another connection after its current connection is closed.
.Pp
It is an error to use this option without the
.Fl l
option.
.It Fl l
Listen for an incoming connection rather than initiate a connection to a remote
host.
.Pp
It is an error to use this option in conjunction with the
.Fl s
or
.Fl z
options.
Additionally, any
.Ar timeout
specified with the
.Fl w
option is ignored.
.It Fl M
Set the default IPv4 TTL or IPv6 hop count that should be used in outgoing
packets.
This corresponds to the
.Dv IP_TTL
and
.Dv IPV6_UNICAST_HOPS
socket options.
.It Fl m
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
.Dv IP_MINTTL
and
.Dv IPV6_MINHOPCOUNT
socket options.
.It Fl n
Do not do any naming or service lookups on any addresses, hostnames, or ports.
.Pp
Use of this option means that
.Ar hostname
and
.Ar port
arguments are restricted to numeric values.
.Pp
If used with
.Fl 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
.Fl U
option.
.It Fl P Ar proxy_username
Specify a username
.Po
.Ar proxy_username
.Pc
to present to a proxy server that requires authentication.
If
.Ar proxy_username
is not specified, authentication is not attempted.
Proxy authentication is only supported for HTTP CONNECT proxies at present.
.Pp
It is an error to use this option in conjunction with the
.Fl l
option.
.It Fl p Ar port
When used without
.Fl l
option, specify the source port
.Nm
should use, subject to privilege restrictions and availability.
When used with the
.Fl l
option, set the listen port.
.Pp
This option can be used with
.Fl l
option only provided global port argument is not specified.
.It Fl r
Choose source or destination ports randomly instead of sequentially within a
range or in the order that the system assigns them.
.Pp
It is an error to use this option in conjunction with the
.Fl l
option.
.It Fl S
Enables the
.%T RFC 2385
TCP MD5 signature option.
.Pp
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
.Xr tcpkey 8 .
.It Fl s Ar source_ip_address
Specify the IP of the interface which is used to send the packets.
.Pp
It is an error to use this option in conjunction with the
.Fl l
option.
.It Fl T Ar ToS
Specify IP Type of Service
.Pq ToS
or IPv6 traffic class for the connection.
Valid values are the tokens:
.Cm lowdelay ,
.Cm throughput ,
.Cm reliability ,
or any decimal or hexadecimal integer between 0 and 255.
.It Fl t
Cause
.Nm
to send
.%T RFC 854
.Dq DON'T
and
.Dq WON'T
responses to
.%T RFC 854
.Dq DO
and
.Dq WILL
requests.
This makes it possible to use
.Nm
to script telnet sessions.
.It Fl U
Specify the use of Unix Domain Sockets.
If you specify this option without
.Fl l ,
it becomes an
.Dv AF_UNIX
client.
If you specify this option with the
.Fl l
option, a
.Dv AF_UNIX
server is created.
.Pp
Use of this option requires that a single argument of a valid Unix domain path
has to be provided to
.Nm ,
not a host name or port.
.It Fl u
Use UDP instead of the default option of TCP.
.It Fl v
Specify verbose output.
.It Fl w Ar timeout
Silently close the connection if a connection and
.Dv stdin
are idle for more than
.Ar timeout
seconds.
.Pp
This option has no effect on the
.Fl l
option, that is,
.Nm
listens forever for a connection, with or without the
.Fl w
flag.
The default is no timeout.
.It Fl X Ar proxy_protocol
Use the specified protocol when talking to the proxy server.
Supported protocols are 4
.Pq SOCKS v.4 ,
5
.Pq SOCKS v.5
and connect
.Pq HTTP proxy .
If the protocol is not specified, SOCKS v.5 is used.
.Pp
It is an error to use this option in conjunction with the
.Fl l
option.
.It Fl x Ar proxy_address Ns Op &: Ar port
Request connection to
.Ar hostname
using a proxy at
.Ar proxy_address
and
.Ar port .
If
.Ar port
is not specified, the well-known port for the proxy protocol is used
.Pq 1080 for SOCKS, 3128 for HTTP .
.Pp
It is an error to use this option in conjunction with the
.Fl l
option.
.It Fl z
Scan for listening daemons, without sending any data to them.
.Pp
It is an error to use this option in conjunction with the
.Fl l
option.
.El
.Sh OPERANDS
The following operands are supported:
.Bl -tag -width Ds
.It Ar hostname
Specify host name.
.Pp
.Ar hostname
can be a numerical IP address or a symbolic hostname
.Po
unless the
.Fl n
option is specified
.Pc .
.Pp
In general,
.Ar hostname
must be specified, unless the
.Fl l
option is given or
.Fl U
is used
.Pq in which case the argument is a path .
If
.Ar hostname
argument is specified with
.Fl l
option then
.Ar port
argument must be given as well and
.Nm
tries to bind to that address and port.
If
.Ar hostname
argument is not specified with
.Fl l
option then
.Nm
tries to listen on a wildcard socket for given
.Ar port .
.It Ar path
Specify pathname.
.It Ar port | port_list
Specify port.
.Pp
.Ar port_list
can be specified as single integers, ranges or combinations of both.
Specify ranges in the form of nn-mm.
The
.Ar port_list
must have at least one member, but can have multiple ports/ranges separated by
commas.
.Pp
In general, a destination port must be specified, unless the
.Fl U
option is given, in which case a Unix Domain Socket path must be specified
instead of
.Ar hostname .
.El
.Sh USAGE
.Ss Client/Server Model
It is quite simple to build a very basic client/server model using
.Nm .
On one console, start
.Nm
listening on a specific port for a connection.
For example, the command:
.Pp
.Dl $ nc -l 1234
.Pp
listens on port 1234 for a connection.
On a second console
.Pq or a second machine ,
connect to the machine and port to which
.Nm
is listening:
.Pp
.Dl $ nc 127.0.0.1 1234
.Pp
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,
.Nm
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
.Pq Ctrl/d .
.Ss 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 is output to the other
end, and input and output can be easily captured in order to emulate file
transfer.
.Pp
Start by using
.Nm
to listen on a specific port, with output captured into a file:
.Pp
.Dl $ nc -l 1234 > filename.out
.Pp
Using a second machine, connect to the listening
.Nm
process, feeding it the file which is to be transferred:
.Pp
.Dl $ nc host.example.com 1234 < filename.in
.Pp
After the file has been transferred, the connection closes automatically.
.Ss 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.
.Pp
For example, to retrieve the home page of a web site:
.Pp
.Dl $ echo -n \&"GET / HTTP/1.0\er\en\er\en\&" | nc host.example.com 80
.Pp
This also displays the headers sent by the web server.
They can be filtered, if necessary, by using a tool such as
.Xr sed 1 .
.Pp
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:
.Bd -literal -offset indent
$ 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
.Ed
.Ss Port Scanning
It can be useful to know which ports are open and running services on a target
machine.
The
.Fl z
flag can be used to tell
.Nm
to report open ports, rather than to initiate a connection.
.Pp
In this example:
.Bd -literal -offset indent
$ 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!
.Ed
.Pp
The port range was specified to limit the search to ports 20 - 30.
.Pp
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
.Fl w
flag, or perhaps by issuing a QUIT command to the server:
.Bd -literal -offset indent
$ 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
.Ed
.Ss inetd Capabilities
One of the possible uses is to create simple services by using
.Xr inetd 8 .
.Pp
The following example creates a redirect from TCP port 8080 to port 80 on host
realwww:
.Bd -literal -offset indent
# 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
.Ed
.Ss Privileges
To bind to a privileged port number
.Nm
needs to be granted the
.Sy net_privaddr
privilege.
If Trusted Extensions are configured and the port
.Nm
should listen on is configured as a multi-level port
.Nm
also needs the
.Sy net_bindmlp
privilege.
.Pp
Privileges can be assigned to the user or role directly, by specifying them in
the account's default privilege set in
.Xr user_attr 5 .
However, this means that any application that this user or role starts have
these additional privileges.
To only grant the
.Xr privileges 7
when
.Nm
is invoked, the recommended approach is to create and assign an
.Xr rbac 7
rights profile.
See
.Sx EXAMPLES
for additional information.
.Sh EXAMPLES
Open a TCP connection to port 42 of host.example.com, using port 3141 as the
source port, with a timeout of 5 seconds:
.Pp
.Dl $ nc -p 3141 -w 5 host.example.com 42
.Pp
Open a UDP connection to port 53 of host.example.com:
.Pp
.Dl $ nc -u host.example.com 53
.Pp
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:
.Pp
.Dl $ nc -s 10.1.2.3 host.example.com 42
.Pp
Use a list of ports and port ranges for a port scan on various ports:
.Pp
.Dl $ nc -z host.example.com 21-25,53,80,110-120,443
.Pp
Create and listen on a Unix Domain Socket:
.Pp
.Dl $ nc -lU /var/tmp/dsocket
.Pp
Create and listen on a UDP socket with associated port 8888:
.Pp
.Dl $ nc -u -l -p 8888
.Pp
which is the same as:
.Pp
.Dl $ nc -u -l 8888
.Pp
Create and listen on a TCP socket with associated port 2222 and bind to address
127.0.0.1 only:
.Pp
.Dl $ nc -l 127.0.0.1 2222
.Pp
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
.Xr ssh 1 .
See the
.Cm ProxyCommand
directive in
.Xr ssh_config 5
for more information.
.Pp
.Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
.Pp
The same example again, this time enabling proxy authentication with username
ruser if the proxy requires it:
.Pp
.Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
.Pp
To run
.Nm
with the smallest possible set of privileges as a user or role that has
additional privileges
.Pq such as the default root account
it can be invoked using
.Xr ppriv 1
as well.
For example, limiting it to only run with the privilege to bind to a privileged
port:
.Bd -literal -offset indent
$ ppriv -e -sA=basic,!file_link_any,!proc_exec,!proc_fork,\e
    !proc_info,!proc_session,net_privaddr nc -l 42
.Ed
.Pp
To allow a user or role to use only
.Nm
with the
.Sy net_privaddr
privilege, a rights profile needs to be created:
.Pp
.Pa /etc/security/exec_attr:
.Dl Netcat privileged:solaris:cmd:::/usr/bin/nc:privs=net_privaddr
.Pa /etc/security/prof_attr
.Dl Netcat privileged:::Allow nc to bind to privileged ports:help=None.html
.Pp
Assigning this rights profile using
.Xr user_attr 5
permits the user or role to run
.Nm
allowing it to listen on any port.
To permit a user or role to use
.Nm
only to listen on specific ports a wrapper script should be specified in the
rights profiles:
.Pp
.Pa /etc/security/exec_attr
.Dl Netcat restricted:solaris:cmd:::/usr/bin/nc-restricted:privs=net_privaddr
.Pa /etc/security/prof_attr
.Dl Netcat restricted:::Allow nc to bind to privileged ports:help=None.html
.Pp
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:
.Bd -literal -offset indent
#!/bin/ksh

(( $# == 1 )) || exit 1
(( $1 > 42 && $1 < 64 )) || exit 1
exec /usr/bin/nc -l -p "$1"
.Ed
.Pp
This grants the extra privileges when the user or role invokes
.Nm
using the wrapper script from a profile shell.
See
.Xr pfsh 1 ,
.Xr pfksh 1 ,
.Xr pfcsh 1 ,
and
.Xr pfexec 1 .
.Pp
Invoking
.Nm
directly does not run it with the additional privileges, and neither does
invoking the script without using
.Sy pfexec
or a profile shell.
.Sh INTERFACE STABILITY
The command line syntax is
.Sy Committed
for the
.Fl 4 ,
.Fl 6 ,
.Fl l ,
.Fl n ,
.Fl p ,
.Fl u ,
and
.Fl w
options and their arguments
.Pq if any .
The
.Ar name
and
.Ar port
list arguments are
.Sy Committed .
The port range syntax is
.Sy Uncommitted .
The interface stability level for all other command line options and their
arguments is
.Sy Uncommitted.
.Sh SEE ALSO
.Xr cat 1 ,
.Xr pfcsh 1 ,
.Xr pfexec 1 ,
.Xr pfksh 1 ,
.Xr pfsh 1 ,
.Xr ppriv 1 ,
.Xr sed 1 ,
.Xr ssh 1 ,
.Xr telnet 1 ,
.Xr ssh_config 5 ,
.Xr user_attr 5 ,
.Xr attributes 7 ,
.Xr privileges 7 ,
.Xr rbac 7 ,
.Xr inetadm 8 ,
.Xr inetconv 8 ,
.Xr inetd 8 ,
.Xr tcpkey 8
.Sh AUTHORS
The original implementation of
.Nm
was written by
.An Hobbit Aq Mt hobbit@avian.org
.Pp
.Nm
was rewritten with IPv6 support by
.An -nosplit
.An Eric Jackson Aq Mt ericj@monkey.org
.Sh NOTES
UDP port scans always succeed, that is, report the port as open, rendering the
.Fl uz
combination of flags relatively useless.