The Transport Control Protocol (TCP) is a low-level part of the protocol stack used by the Internet. It sits directly above the IP layer and is responsible for making sure that dropped packets are resent.

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iptables port-mirroring

I need to be able to get a copy of packets received on port 8001 to port 8002. I have tried the following but I get an error that --tee is undefined. sudo iptables -t nat -A PREROUTING -p TCP -s ...
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6answers
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Determining what process is bound to a port

I know that using the command: lsof -i TCP (or some variant of parameters with lsof) I can determine which process is bound to a particular port. This is useful say if I'm trying to start ...
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54 views

SSH randomly stops working, and any other request from network related services

I'm pulling my hair out right now, since I don't know where else to look, what else to do, to find out why the SSH stops working suddenly. The ssh-server is on the same network as the client, but it's ...
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different ways how Linux TCP/IP stack terminates connection

I have a Linux machine which connects to SSH server with SSH client(OpenSSH_4.1p1). SSH client machine has IP address 10.10.11.1 and SSH server has IP address 10.10.10.1. SSH client has kernel version ...
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1answer
92 views

Why are these connections not classified as “established” under conntrack?

I have the following rule for iptables: -A OUTPUT -m conntrack --ctstate ESTABLISHED -j ACCEPT such that outgoing connections not classified as established will be logged. Occasionally, the ...
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34 views

Socat execute commands on remote

I am playing around with socat and wanted to execute commands to hosts that connect to my IP PORT running socat. In this case I am trying to dump a file on localhost and this is how I am trying to ...
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2answers
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nc: bind failed: Address already in use

I am trying to execute nc command from a script , my script is executing nc command on different ports of Destination using the same source port. e.g: nc -p 8140 -z -v -n 10.X.X.9 9090 nc -p 8140 -z ...
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1answer
44 views

enable echo service and linux security loophole issues

in order to enable the echo service we need to add the echo lines as the following example : more /etc/inetd.conf echo stream tcp nowait root internal echo dgram udp wait root ...
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1answer
63 views

How can I create Local Loopback on Linux (Red Hat or Debian)?

How can I do the same on Linux (RedHat or Debian)? I need to create local loopback as shown herein: Performing Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces. Excerpt ...
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0answers
46 views

How do I find out how long a socket is in CLOSE_WAIT?

I have a server relatively with a high number of sockets in CLOSE_WAIT I want to know the age of each socket, to know if it's a real problem (sockets not getting closed) or that the sockets are ...
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0answers
13 views

Capturing data from a Fluke 1620a via Bash /dev/tcp file descriptor

I am currently trying to develop a script that will connect to a Fluke DewK 1620a Themo-Hygrometer and pull the current temperature and humidity readings from the device. While these devices have a ...
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2answers
26 views

Capturing data from a Fluke 1620a via netcat

I am trying to pull temperature and humidity data from Fluke DewK 1620a thermo-hygrometers and write the temp/humidity readings to a log. I can connect to the device via netcat with a simple 'nc ...
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1answer
12 views

Itcpdump output - why is it showing an Ethernet status?

tcpdump -i mlan0 -n tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on mlan0, link-type EN10MB (Ethernet), capture size 262144 bytes Why is this ...
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2answers
88 views

Redirect stdin and stdout to ports

I'd like to take a program P that reads from stdin & writes to stdout, but connect it to nc or whatever such that it reads from a certain port and outputs to another port. # The reading is easy, ...
3
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1answer
662 views

Port Mirroring in Fedora? [closed]

I have set up an application that listens constantly to a port for network packets, and I would like to create another one that receives the same network packets ( for use as a test bed). Getting the ...
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2answers
30 views

If I send SYN / ACK with one IP and other packets with another, what happens?

I'm learning about UNIX and network programming. I'm not yet sure how to implement this. But I'm wondering: If I forge IP packets of a TCP connection to a server, so that the SYN / ACK packets use ...
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1answer
35 views

Relay server for generic TCP traffic?

Here's my situation: I have a computer (Mac) on a school network that I want to port forward TCP 5222 to for ejabberd, but of course I can't. I have a server at home (Mac with FreeBSD VM) that I can ...
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1answer
34 views

bound, but not listening process

python code for simple daemon import socket import time import signal s=socket.socket() s.bind(('127.0.1.1', 20201)) def stop(*a, **k): global work work = False ...
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1answer
26 views

Start a service on a network request (socket activation)

I have a program that under normal activation listens on some port. I don't want the program running continuously. Is there a "quick and dirty" way to wrap the application in a shell script or ...
2
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2answers
1k views

Redirect the default HTTP port to an application

On my CentOS server at mydomain.com, I have an svn web application running on port 8880. I want to hide the port: at this moment, if anybody wants to access, they need to access through ...
2
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2answers
67 views

How to find out how many TCP retransmits are occurring?

I am running Debian Wheezy on an ARMHF platform and do most of the communications over fairly unreliable cell phone networks. Is there a way to read the no. of re-transmits the TCP stack has done ...
2
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1answer
85 views

What are the semantics of getting a EADDRINUSE when no listening socket is bound, but connections are open

Consider a parent process which completes a socket/bind/accept, and will fork children with that socket open for them to communicate with, while the parent continues accepting connections. That parent ...
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1answer
270 views

Networking issue in IBM x3650 after install Ubuntu 14.04.1

I have a problem making TCP connections after a fresh Ubuntu 14.04 installation. First of all, after booting the interface is always off. In my case the interface is manually configured and named em2. ...
2
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1answer
141 views

nc -l in background closes immediately (nc -l 1234 &)

When I put this nc -l 12345 >nc_out in a shell script, run it and then connect from other shell using telnet, it allows me to type some text and have it end up in nc_out. But if I start nc in ...
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1answer
299 views

Is there a way to see if process listening on TCP port receives a message?

According to tcpdump, my server receives the following TCP packet: 12:52:29.603233 00:19:e2:9e:df:f0 00:16:3e:6a:25:3f, ethertype IPv4 (0x0800), length 74: 10.10.10.65.38869 192.168.215.82.22: ...
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3answers
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Can TCP provide more than 65535 ports?

Is it possible to setup a Linux system so that it provides more than 65,535 ports? The intent would be to have more than 65k daemons listening on a given system. Clearly there are ports being used ...
2
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1answer
47 views

Discovering the Cause of tcp rcvpruned?

I get a few rcv pruned packets on my load balancers a day and I would like to know what the cause is: However, based on my tcp_mem vs tcp_mem usage I don't understand why some packets are being ...
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2answers
108 views

Is this kernel warning a major problem that needs my attention?

Our Linux based Thecus N12000 NAS recently experienced this message in its dmesg log. [2014-05-21 11:34:56] ------------[ cut here ]------------ [2014-05-21 11:34:56] WARNING: at ...
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1answer
1k views

Meaning of bash -i >& /dev/tcp/HOST/PORT 0>&1?

I tried this bash -i >& /dev/tcp/HOST/PORT 0>&1, but it does not seem to work behind firewall. What does this code actually do, which ports are forwarded and could it work behind ...
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0answers
29 views

Can spawn-fcgi be user for tcp/udp server

Spawn-fcgi by lighty labs It only uses unix socket functions socket(), bind(), connect(), listen(). Is it possible to use it not for FastCGI server, but TCP or UDP server?
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1answer
50 views

Down single connection

There are many connections to the external interface(s) on the specified port e.g. ![netstat |grep 1521][1] tcp….1540….…0 172.17.40.33:36791……..172.17.40.90:1521…….…..ESTABLISHED tcp….1540….…0 ...
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0answers
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Exclude a loopback interface or address from processes listening on the “any” address

I'm trying to create a setup where I can connect to some service running on a host only reachable through a SOCKS proxy by the host's actual name and the port the service actually uses (so that ...
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2answers
171 views

Drop TCP packets and prevent TCP retransmission

I have an application that sends TCP data to a server. My server is made to intentionally drop the packets sent by the application, and as a result the client retransmits the packets. I need to ...
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2answers
59 views

How can I determine what program or user connected to a particular port after the fact?

I am examining the egress IPTABLES log of my computer for yesterday and notice the following: IN= OUT=eth0 SRC=192.168.1.1 DST=69.46.36.10 LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=12345 DF PROTO=TCP ...
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2answers
211 views

How to troubleshooting failing ping to router on a linux box?

I have a home network 192.168.0.x from which I am posting this message, on this network I have a CentOS7 host 192.168.0.3 which is running a DNS server for the network. I can ssh into 192.168.0.3 I ...
3
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1answer
64 views

Force application to use specific port on Fedora machine

I have an application on my fedora machine that sends SIP TCP requests to the server. The application chooses a random source port to send the requests. Is there some way I can force the application ...
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5answers
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How to close ports in Linux?

I have some question in closing port, I think I got some strange things. When I use execute nmap --top-ports 10 192.168.1.1 it shows that 23/TCP port is open. But when I execute nmap --top-ports ...
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0answers
92 views

Rsync - Corrupted MAC on input

Background I have a script on my NAS which is a Synology DS212J which is running on a special distro where uname -a gives me Linux DS212J 2.6.32.12 #4493 Wed Jul 16 11:37:16 CST 2014 armv5tel ...
4
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6answers
701 views

How do I work out which port to log in on with SSH?

I have an Ubuntu 10.04 server setup remotely that I setup a while back. While I recorded the username and password, I seem to have been clever and changed the usual ssh port from 22 to... something ...
0
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1answer
15 views

tcpdump Filter with certain file types?

I use the unix command tcpdump with these extensions -A -s0 -ien0 port 80 I know how to add, for example a new host, that would be and \ host random.com Can I sniff for certain filetypes? I have ...
4
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2answers
3k views

iptables port forwarding

I have a CentOS server with Java/J2EE(Tomcat) installed on TCP port 8080. I have two interfaces, eth0 and lo. I need to forward all incoming connection on TCP port 80 to 8080. I tried doing the ...
4
votes
1answer
210 views

Can iptables be used to monitor TCP timeouts/resets system-wide?

I'm trying to determine if an application's connection to a database is internal to the application or due to a networking event. This seems like something netfilter would have system-wide visibility ...
3
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1answer
116 views

Allow non-root unmodifiable sofware to bind to ports 80 & 443

The company I'm in uses the Java Secure Application Manager to "secure" non-internal connections their webmail and electronic collaboration software. Aside from using expired certificates, and ...
4
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1answer
342 views

How to investigate a random reset on TCP client port connected via loopback interface to a server

I have a test suite which initializes a server on the localhost and then runs thousands of stress tests which include starting and stopping clients connecting to this local server on same machine. I ...
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1answer
52 views

Analyzing network traffic

I want to get a good overview what kind of network communication is going on on my computer. One way would be starting wireshark, which is fine for very low-level problems but is quite cumbersome to ...
2
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1answer
328 views

Kill any service running at a specific port

I am trying to write a script that kills service running at a specific port. This is my script: a=$(ps ax | grep 4990 | grep java | awk '{print $1}') kill -9 $a It's a java program. This script ...
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246 views

tcpdump not capturing http or tcp/ssl traffic

I have been trying to learn tcpdump and I am using this command to attempt to monitor my network: sudo tcpdump -I -i en1 But this gives me a bunch of stuff I don't want, so I used this version to ...
0
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3answers
918 views

How to get info about given webserver FAST?

$ time nmap -q -sV -P0 -p80 google.com | fgrep '80/tcp open' 80/tcp open http Google httpd 2.0 (GFE) real 0m6.563s user 0m0.280s sys 0m0.016s $ Are there any faster methods to get info ...
2
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1answer
139 views

how to connect ssh 'with' specified port?

I know how to connect 'to' a certain port when ssh'ing. ssh user@remotehostip -p XXX but is there a way to establish a ssh 'with' a certain port? I mean, is there a way to specify the port which ...
2
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2answers
114 views

“tcpdump” to capture the last packets

I am monitoring a certain port, because my application uses that port, it seems that the connection drops at random times, I want to see what are the last packets passing through before the connection ...