OK, we all know how to use PING to test connectivity to an IP address. What I need to do is something similar but test if my outbound request to a given IP Address as well as a specif port (in the present case 1775) is successful. The test should be performed preferably from the command prompt.
Here is a small site I made allowing to test any outgoing port. The server listens on all TCP ports available.
http://portquiz.net
telnet portquiz.net XXXX
If there is a server running on the target IP/port, you could use Telnet. Any response other than "can't connect" would indicate that you were able to connect.
To automate the awesome service portquiz.net, I did write a bash script :
NB_CONNECTION=10
PORT_START=1
PORT_END=1000
for (( i=$PORT_START; i<=$PORT_END; i=i+NB_CONNECTION ))
do
iEnd=$((i + NB_CONNECTION))
for (( j=$i; j<$iEnd; j++ ))
do
#(curl --connect-timeout 1 "portquiz.net:$j" &> /dev/null && echo "> $j") &
(nc -w 1 -z portquiz.net "$j" &> /dev/null && echo "> $j") &
done
wait
done
If you're testing TCP/IP, a cheap way to test remote addr/port is to telnet to it and see if it connects. For protocols like HTTP (port 80), you can even type HTTP commands and get HTTP responses.
eg
Command IP Port
Telnet 192.168.1.1 80
The fastest / most efficient way I found to to this is with nmap and portquiz.net described here: http://thomasmullaly.com/2013/04/13/outgoing-port-tester/ This scans to top 1000 most used ports:
# nmap -Pn --top-ports 1000 portquiz.net
Starting Nmap 6.40 ( http://nmap.org ) at 2017-08-02 22:28 CDT
Nmap scan report for portquiz.net (178.33.250.62)
Host is up (0.072s latency).
rDNS record for 178.33.250.62: electron.positon.org
Not shown: 996 closed ports
PORT STATE SERVICE
53/tcp open domain
80/tcp open http
443/tcp open https
8080/tcp open http-proxy
Nmap done: 1 IP address (1 host up) scanned in 4.78 seconds
To scan them all (took 6 sec instead of 5):
# nmap -Pn -p1-65535 portquiz.net
The bash script example of #benjarobin for testing a sequence of ports did not work for me so I created this minimal not-really-one-line (command-line) example which writes the output of the open ports from a sequence of 1-65535 (all applicable communication ports) to a local file and suppresses all other output:
for p in $(seq 1 65535); do curl -s --connect-timeout 1 portquiz.net:$p >> ports.txt; done
Unfortunately, this takes 18.2 hours to run, because the minimum amount of connection timeout allowed integer seconds by my older version of curl is 1. If you have a curl version >=7.32.0 (type "curl -V"), you might try smaller decimal values, depending on how fast you can connect to the service. Or try a smaller port range to minimise the duration.
Furthermore, it will append to the output file ports.txt so if run multiple times, you might want to remove the file first.
Related
I have an autossh tunnel set up over which I am sending something that needs an uninterrupted connection for a couple dozen minutes. However, I noticed that every 10 minutes the SSH tunnel managed by autossh is killed and recreated.
This is not due to an inactive connection, as there is active communication happening through that channel.
The command used to set up the tunnel was:
autossh -C -f -M 9910 -N -L 6969:127.0.0.1:12345 remoteuser#example.com
In my case the problem was a clash of the monitoring ports on the remote server. There are multiple servers all autossh-ing to the single central server and two of those "clients" used the same monitoring port (-M).
The default interval in which autossh tries to communicate over the monitoring channel is 600 seconds, 10 minutes. When autossh starts up, it does not verify that it could open the remote monitoring port. Everything will look fine until the time when autossh tries to check that the connection is open - and it fails. At that point the SSH tunnel will be forcibly killed and recreated.
A good way to check if this is your case as well is change the default timeout using the AUTOSSH_POLL environment variable:
AUTOSSH_POLL=10 autossh -C -f -M 9910 -N -L 6969:127.0.0.1:12345 remoteuser#example.com
I'm trying to use tcpdump to identify which IP address a particular person is coming from but I'm not seeing the HTTP commands as various web site show. I've used the following to set up tcpdump:
nohup tcpdump -i eth0 -P in -nn -n -tttt -w /home/tcpdump/port80.log -C 100 -W 50 "port 80" > /home/tcpdump/nohup.log 2>&1 &
And I'm peridoically checking the file using
tcpdump -r port80.log00 -n -nn -A
I'm connecting to the following URL from a web browser:
http://10.10.0.50?test
And I was expecting to see a bunch of HTTP "GET" commands but tcpdump doesn't seem to be showing me the incoming messages. Instead I just get something like
16:21:35.708250 IP 10.10.0.222.55924 > 10.10.0.50.80: Flags [S], seq 1869638484, win 8192, options [mss 1340,nop,nop,sackOK], length 0
E..0#H#....\
..
.2.t.PopkT....p. ........<....
Looking at other info on using tcpdump for HTTP logging I'm should be seeing the "GET" command after the first few bytes of garbage. There's no actual web server running - I'm only interested in seeing the incoming request as a test, hence the "?test" on the end to help me search the logs for the right thing. I don't see that that's the issue tho'.
Any help very gratefully received.
Is there is a way to check if localhost is making ftp connection to other server?
The requirement is like this: Local host -> serverA
Remote server --> serverB.
Need to check if serverA is making ftp connection to serverB.
So whenever serverA is making ftp connection to serverB, how to get notified.
I tried like this: ps -ef | grep -i ftp; however since ps process too would get notified, so can't make this use in shell script, is there any better way which checks if serverA is making ftp connections to serverB, and if so, get notified / logs to a file.
Thanks
Your problem of "ps -ef | grep -i ftp" also reporting the 'ps' process is resulting from grep searching the string "ftp". This would also hit a lot of other processes which also have the word 'ftp' in it's command line.
To fix that check if you have the procps tools "pgrep" and "pkill" installed. They are very helpful for 'grepping' processes and running commandlines.
To solve your initial problem you might check if you have the 'ss' (show sockets from iproute2 packages) command installed.
It's output might be useful (11.22.33.44 is you local IP 130.133.3.130 the remote):
root:sigkill:~/# ss -p|cat
State Recv-Q Send-Q Local Address:Port Peer Address:Port
[...]
ESTAB 0 0 11.22.33.44:43681 130.133.3.130:ftp users:(("ftp",19729,4),("ftp",19729,3))
[...]
There are a few approaches that you could take:
You could poll running processes for ftp. This wouldn't catch other FTP clients (if you care about that), and it wouldn't catch very short ftp sessions that slip between polls.
If your system supports execution logging, you could log all executions of ftp. Again, this wouldn't catch other FTP clients.
You could watch for outbound connections on port 21/tcp using some mechanism provided by your system (for instance, on Linux, use an iptables rule that matches outbound FTP connections to any servers that you care about and logs them using the LOG target). This would catch all connections regardless of client, but tracking down the process and user would be a little more complicated.
You can use $ grep ftp /etc/services to list the current ftp connections.
$ grep ftp /etc/services
ftp-data 20/tcp
ftp-data 20/udp
...
ftp 21/tcp
ftp 21/udp fsp fspd
...
sftp 115/tcp
sftp 115/udp
...
ftp-data 20/sctp # FTP
ftp 21/sctp # FTP
...
ftps-data 989/tcp # ftp protocol, data, over TLS/SSL
ftps-data 989/udp # ftp protocol, data, over TLS/SSL
ftps 990/tcp # ftp protocol, control, over TLS/SSL
ftps 990/udp # ftp protocol, control, over TLS/SSL
Use netstat to see the open connections. e.g., For simple FTP...
$ netstat -tan | grep \:21
tcp 0 0 0.0.0.0:21 0.0.0.0:* LISTEN
tcp 0 0 :::21 :::* LISTEN
I'm trying to write the linux client script for a simple port knocking setup. My server has iptables configured to require a certain sequence of TCP SYN's to certain ports for opening up access. I'm able to successfully knock using telnet or manually invoking netcat (Ctrl-C right after running the command), but failing to build an automated knock script.
My attempt at an automated port knocking script consists simply of "nc -w 1 x.x.x.x 1234" commands, which connect to x.x.x.x port 1234 and timeout after one second. The problem, however, seems to be the kernel(?) doing automated SYN retries. Most of the time more than one SYN is being send during the 1 second nc tries to connect. I've checked this with tcpdump.
So, does anyone know how to prevent the SYN retries and make netcat simply send only one SYN per connection/knock attempt? Other solutions which do the job are also welcome.
Yeah, I checked that you may use nc too!:
$ nc -z example.net 1000 2000 3000; ssh example.net
The magic comes from (-z: zero-I/O mode)...
You may use nmap for port knocking (SYN). Just exec:
for p in 1000 2000 3000; do
nmap -Pn --max-retries 0 -p $p example.net;
done
try this (as root):
echo 1 > /proc/sys/net/ipv4/tcp_syn_retries
or this:
int sc = 1;
setsockopt(sock, IPPROTO_TCP, TCP_SYNCNT, &sc, sizeof(sc));
You can't prevent the TCP/IP stack from doing what it is expressly designed to do.
I noticed a strange behaviour working with netcat and UDP. I start an instance (instance 1) of netcat that listens on a UDP port:
nc -lu -p 10000
So i launch another instance of netcat (instance 2) and try to send datagrams to my process:
nc -u 127.0.0.1 10000
I see the datagrams. But if i close instance 2 and relaunch again netcat (instance 3):
nc -u 127.0.0.1 10000
i can't see datagrams on instance 1's terminal. Obsiously the operating system assigns a different UDP source port at the instance 3 respect to instance 2 and the problem is there: if i use the same instance'2 source port (example 50000):
nc -u -p 50000 127.0.0.1 10000
again the instance 1 of netcat receives the datagrams. UDP is a connection less protocol so, why? Is this a standard netcat behaviour?
When nc is listening to a UDP socket, it 'locks on' to the source port and source IP of the first packet it receives. Check out this trace:
socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP) = 3
setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
bind(3, {sa_family=AF_INET, sin_port=htons(10000), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
recvfrom(3, "f\n", 2048, MSG_PEEK, {sa_family=AF_INET, sin_port=htons(52832), sin_addr=inet_addr("127.0.0.1")}, [16]) = 2
connect(3, {sa_family=AF_INET, sin_port=htons(52832), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
Here you can see that it created a UDP socket, set it for address reuse, and bound it to port 10,000. As soon as it received its first datagram (from port 52,832), it issued a connect system call 'connecting' it to the 127.0.0.1:52,832. For UDP, a connect rejects all packets that don't match the IP and port in the connect.
Use the -k option:
nc -l -u -k 0.0.0.0 10000
-k means keep-alive, that netcat keeps listening after each connection
-u means UDP
-l listening on port 10000
Having given up on netcat on my OS version this is pretty short and gets the job done:
#!/usr/bin/ruby
# Receive UDP packets bound for a port and output them
require 'socket'
require 'yaml'
unless ARGV.count == 2
puts "Usage: #{$0} listen_ip port_number"
exit(1)
end
listen_ip = ARGV[0]
port = ARGV[1].to_i
u1 = UDPSocket.new
u1.bind(listen_ip, port)
while true
mesg, addr = u1.recvfrom(100000)
puts mesg
end
As the accepted answer explains, ncat appears not to support --keep-open with the UDP protocol. However, the error message which it prints hints at a workaround:
Ncat: UDP mode does not support the -k or --keep-open options, except with --exec or --sh-exec. QUITTING.
Simply adding --exec /bin/cat allows --keep-open to be used. Both input and output will be connected to /bin/cat, with the effect of turning it an "echo server" because whatever the client sends will be copied back to it.
To do something more useful with the input, we can use the shell's redirection operators (thus requiring --sh-exec instead of --exec). To see the data on the terminal, this works:
ncat -k -l -u -p 12345 --sh-exec "cat > /proc/$$/fd/1"
Caveat: the above example sends data to the stdout of ncat's parent shell, which could be confusing if combined with additional redirections. To simply append all output to a file is more straightforward:
ncat -k -l -u -p 12345 --sh-exec "cat >> ncat.out"