nc —
arbitrary
TCP and UDP connections and listens
| nc |
[-46cDdFhklNnrStUuvz]
[-C
certfile]
[-e
name]
[-H
hash]
[-I
length]
[-i
interval]
[-K
keyfile]
[-M
ttl]
[-m
minttl]
[-O
length]
[-o
staplefile]
[-P
proxy_username]
[-p
source_port]
[-R
CAfile]
[-s
source]
[-T
keyword]
[-V
rtable]
[-W
recvlimit]
[-w
timeout]
[-X
proxy_protocol]
[-x
proxy_address[:port]]
[-Z
peercertfile]
[destination]
[port] |
The
nc (or
netcat)
utility is used for just about anything under the sun involving TCP, UDP, or
UNIX-domain sockets. It can open TCP connections, send
UDP packets, listen on arbitrary TCP and UDP ports, do 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, as
telnet(1) does with some.
Common uses include:
- simple TCP proxies
- shell-script based HTTP
clients and servers
- network daemon testing
- a SOCKS or HTTP ProxyCommand
for ssh(1)
- and much, much more
The options are as follows:
-
-
- -4
- Forces nc to use IPv4
addresses only.
-
-
- -6
- Forces nc to use IPv6
addresses only.
-
-
- -C
certfile
- Specifies the filename from which the public key part of
the TLS certificate is loaded, in PEM format. May only be used with
TLS.
-
-
- -c
- If using a TCP socket to connect or listen, use TLS.
Illegal if not using TCP sockets.
-
-
- -D
- Enable debugging on the socket.
-
-
- -d
- Do not attempt to read from stdin.
-
-
- -e
name
- Specify the name that must be present in the peer
certificate when using TLS. Illegal if not using TLS.
-
-
- -F
- Pass the first connected socket using
sendmsg(2) to stdout and
exit. This is useful in conjunction with -X
to have nc perform connection setup with a
proxy but then leave the rest of the connection to another program (e.g.
ssh(1) using the
ssh_config(5)
ProxyUseFdpass option).
-
-
- -H
hash
- Specifies the required hash string of the peer certificate
when using TLS. The string format required is that used by
tls_peer_cert_hash(3).
Illegal if not using TLS, and may not be used with -T noverify.
-
-
- -h
- Prints out nc help.
-
-
- -I
length
- Specifies the size of the TCP receive buffer.
-
-
- -i
interval
- Specifies a delay time interval between lines of text sent
and received. Also causes a delay time between connections to multiple
ports.
-
-
- -K
keyfile
- Specifies the filename from which the private key is loaded
in PEM format. May only be used with TLS.
-
-
- -k
- Forces nc to stay listening
for another connection after its current connection is completed. It is an
error to use this option without the -l
option. When used together with the -u
option, the server socket is not connected and it can receive UDP
datagrams from multiple hosts.
-
-
- -l
- Used to specify that nc should
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
-p, -s, or
-z options. Additionally, any timeouts
specified with the -w option are
ignored.
-
-
- -M
ttl
- Set the TTL / hop limit of outgoing packets.
-
-
- -m
minttl
- Ask the kernel to drop incoming packets whose TTL / hop
limit is under minttl.
-
-
- -N
- shutdown(2)
the network socket after EOF on the input. Some servers require this to
finish their work.
-
-
- -n
- Do not do any DNS or service lookups on any specified
addresses, hostnames or ports.
-
-
- -O
length
- Specifies the size of the TCP send buffer.
-
-
- -o
staplefile
- Specifies the filename from which to load data to be
stapled during the TLS handshake. The file is expected to contain an OCSP
response from an OCSP server in DER format. May only be used with TLS and
when a certificate is being used.
-
-
- -P
proxy_username
- Specifies a username to present to a proxy server that
requires authentication. If no username is specified then authentication
will not be attempted. Proxy authentication is only supported for HTTP
CONNECT proxies at present.
-
-
- -p
source_port
- Specifies the source port nc
should use, subject to privilege restrictions and availability. It is an
error to use this option in conjunction with the
-l option.
-
-
- -R
CAfile
- Specifies the filename from which the root CA bundle for
certificate verification is loaded, in PEM format. Illegal if not using
TLS. The default is /etc/ssl/cert.pem.
-
-
- -r
- Specifies that source and/or destination ports should be
chosen randomly instead of sequentially within a range or in the order
that the system assigns them.
-
-
- -S
- Enables the RFC 2385 TCP MD5 signature option.
-
-
- -s
source
- Specifies the IP of the interface which is used to send the
packets. For UNIX-domain datagram sockets,
specifies the local temporary socket file to create and use so that
datagrams can be received. It is an error to use this option in
conjunction with the -l option.
-
-
- -T
keyword
- Change IPv4 TOS value or TLS options. For TLS options
keyword may be one of
tlsall; which allows the use of all
supported TLS protocols and ciphers,
noverify; which disables certificate
verification; noname, which disables
certificate name checking; clientcert,
which requires a client certificate on incoming connections; or
muststaple, which requires the peer to
provide a valid stapled OCSP response with the handshake. It is illegal to
specify TLS options if not using TLS.
For IPv4 TOS value keyword may be one of
critical,
inetcontrol,
lowdelay,
netcontrol,
throughput,
reliability, or one of the DiffServ Code
Points: ef,
af11 ... af43,
cs0 ... cs7; or a number in either hex or
decimal.
-
-
- -t
- Causes 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.
-
-
- -U
- Specifies to use UNIX-domain
sockets.
-
-
- -u
- Use UDP instead of the default option of TCP. For
UNIX-domain sockets, use a datagram socket instead
of a stream socket. If a UNIX-domain socket is
used, a temporary receiving socket is created in
/tmp unless the
-s flag is given.
-
-
- -V
rtable
- Set the routing table to be used.
-
-
- -v
- Have nc give more verbose
output.
-
-
- -W
recvlimit
- Terminate after receiving
recvlimit packets from the network.
-
-
- -w
timeout
- Connections which cannot be established or are idle timeout
after timeout seconds. The
-w flag has no effect on the
-l option, i.e.
nc will listen forever for a connection, with
or without the -w flag. The default is no
timeout.
-
-
- -X
proxy_protocol
- Requests that nc should use
the specified protocol when talking to the proxy server. Supported
protocols are “4” (SOCKS v.4), “5” (SOCKS v.5) and
“connect” (HTTPS proxy). If the protocol is not specified,
SOCKS version 5 is used.
-
-
- -x
proxy_address[:port]
- Requests that nc should
connect to destination 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 HTTPS). An
IPv6 address can be specified unambiguously by enclosing
proxy_address in square brackets.
-
-
- -Z
peercertfile
- Specifies the filename in which the peer supplied
certificates will be saved in PEM format. May only be used with TLS.
-
-
- -z
- Specifies that nc should just
scan for listening daemons, without sending any data to them. It is an
error to use this option in conjunction with the
-l option.
destination can be a numerical IP address or a
symbolic hostname (unless the
-n option is
given). In general, a destination must be specified, unless the
-l option is given (in which case the local host
is used). For
UNIX-domain sockets, a destination is
required and is the socket path to connect to (or listen on if the
-l option is given).
port can be a specified as a numeric port
number, or as a service name. Ports may be specified in a range of the form
nn-mm. In general, a destination port must be specified, unless the
-U option is given.
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:
$ nc -l 1234
nc is now listening on port 1234 for a connection.
On a second console (or a second machine), connect to the machine and port
being listened on:
$ nc 127.0.0.1 1234
There should now be a connection between the ports. Anything typed at the second
console will be 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 may be terminated using
an
EOF (‘^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 will be
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 -N host.example.com 1234 <
filename.in
After the file has been transferred, the connection will close 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:
$ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
Note that this also displays the headers sent by the web server. They can be
filtered, using a tool such as
sed(1), if necessary.
More complicated examples can be built up when the user knows the format of
requests required by the server. As another example, an email may 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 may 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 initiate a
connection. For 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-1.99-OpenSSH_3.6.1p2
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84 Ready
Open a TCP connection to port 42 of host.example.com, using port 31337 as the
source port, with a timeout of 5 seconds:
$ nc -p 31337 -w 5 host.example.com
42
Open a TCP connection to port 443 of www.google.ca, and negotiate TLS. Check for
a different name in the certificate for validation.
$ nc -v -c -e adsf.au.doubleclick.net
www.google.ca 443
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 stream socket:
$ nc -lU /var/tmp/dsocket
Connect to port 42 of host.example.com via 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
cat(1),
ssh(1)
Original implementation by *Hobbit*
<
hobbit@avian.org>.
Rewritten with IPv6 support by
Eric Jackson
<
ericj@monkey.org>.
UDP port scans using the
-uz combination of flags
will always report success irrespective of the target machine's state.
However, in conjunction with a traffic sniffer either on the target machine or
an intermediary device, the
-uz combination could
be useful for communications diagnostics. Note that the amount of UDP traffic
generated may be limited either due to hardware resources and/or configuration
settings.
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