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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create listening port on IPV6 WITHOUT netcat

I wish to be able to listen to a port (eventually creating a bind shell) on an IPV6 address. I have tried using the netcat installed on the linux (debian) machine I am trying to bind. sadly it seems ...
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27 views

unbind port of crashed program

is there any way to unbind a port that was previously binded by a program that crashed at some time? I don't like to reboot the development-machine every hour.
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25 views

Are source IPs included in Linux's Ephemeral Port Uniqueness?

From http://aleccolocco.blogspot.com/2008/11/ephemeral-ports-problem-and-solution.html (2008) Linux shares the assigned list of ephemeral ports across all local IPs for unconnected sockets. ...
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25 views

iptables rules to access/mount a remote NFS server

Actually when iptables rules are not cleared, trying to mount a remote NFS share failed with a timeout. After a reset of iptables rules, i'm allowed to mount properly the NFS remote server. What are ...
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13 views

iperf3 inconsistent results between Windows and Linux configuration

I have recently started to use Iperf3 and am struggling to comprehend its behavior. My problem stems from the fact that I am getting different results between Linux and Windows OSs. More particularly,...
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42 views

Why does reading /proc/net/tcp take a long time ss is quick to return the same sort of data about the connections table?

On Linux 2.6.32, if you read (for example, cat) /proc/net/tcp, on a very loaded system (for example, a web server), you will find that reading 100K entries takes over 30 seconds. That's a long time. ...
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40 views

Iptables Help redirecting specific TCP/UDP Ports to VPN (tun1)

So I have a linux based router and have a vpn client set up to route traffic through the vpn and isp per iptables commands i am attempting to route specific tcp and all udp ports through the vpn only ...
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27 views

Port netcat request info to bash script

I have this super simple TCP server that takes requests and returns the output of handler.sh: #!/bin/bash while true ; do nc -l -p 1234 -e handler.sh; done I'd like to port the requested path to ...
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788 views

How to terminate a TCP connection established by bash itself?

I have used exec 3<>/dev/tcp/192.168.0.101/6435 to establish a TCP connection with 192.168.0.101:6435. And I have received as well as sent a few messages with the pipe command. Now, I want to ...
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1answer
43 views

TCP over TUN: SYN/ACK is received, but no ACK is generated, and socket remains in SYN_SENT state

I am tunneling an outgoing TCP connection over a TUN interface set up by a program I wrote. The TUN interface basically just relays the packets to/from a proxy server, with minimal modifications (...
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32 views

Cannot telnet outside network to open port on OEL 6 machine

I have an OEL 6 server that locally is assigned IP 10.200.15.79. It is hosting a business application that uses port 1031 to send and receive information through TCP/IP. To allow this on the OEL ...
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1answer
76 views

Transfer Size limit for SCP, SFTP, RSYNC?

Is there a maximum size limit to the size of collective size of the files that are to be transferred in one go for the above protocol. My intent is to know if for example after 1 GB any of these ...
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1answer
29 views

How do I configure VirtualBox snapshots running server processes to restore correctly?

I'm running VirtualBox 5.0.20 on an OS X host. There's a variety of guest systems, including Ubuntu, CentOS, Windows and OS X. For each of these systems, I created a live snapshot (i.e. while the VM ...
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1answer
37 views

Can you send a TCP packet with RST flag set using IPTABLES as a way to trick NMAP into thinking a port is closed?

I am learning Nmap and a thought occurred to me with regards to a SYN scan... A SYN scan sends an empty TCP packet with the SYN flag set to illicit a response from the target of either RST, ...
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1answer
115 views

SSH connection giving Connection refused error

I am using CentOs. I can SSH from local machine to server A. I can also SSH from local machine to server B. But I cannot SSH from server A to server B: $ ssh -p <portB> root@B root@B's ...
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1answer
84 views

How do I configure Docker containers to have unique IP addresses that are not the default ones?

How do I configure Docker containers to have unique IP addresses that are not the default ones? The Docker containers will run Apache or some web service. These Docker containers will share one host ...
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99 views

How do I get Consul to work so I can get Docker Overlay to work?

I am running CentOS 7 with Docker. I am trying to install Consul on the Docker server. Consul is unzipped. The binary file works for showing the version. I ran this: echo $CONSUL_RPC_ADDR This ...
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1answer
61 views

netcat daemon for calling functions in sh script

I'm new to shell programming and I have created a script that opens a connection to a server of mine. I want to have this script listen for an input from a client node and use that to run a function. ...
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39 views

What does TCPRcvCoalesce, TCPAutoCorking and TCPHystartTrainCwnd in netstat mean?

We have some RHEL7 Apache reverse proxies that experienced a performance degradation event. After a few hours, restarting Apache restored performance to normal levels. We are trying to determine the ...
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56 views

Why would all our CLOSE_WAIT connections have 24 bytes in the receive queue?

We have an Apache reverse proxy that is having connectivity issues with the back-end. This results in timeout errors such as this: [Thu Apr 21 12:12:26.031207 2016] [proxy_http:error] [pid 64620:tid ...
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1answer
39 views

log script output to file and terminal at the same time

I have a simple tcpnotification receiver script that logs notifications. At first I just printed the notifications in terminal but then changed script so that notifications are logged in a file. But ...
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90 views

Connecting to server on localhost with same source and destination port

Here is simple server listening to port 80 of localhost: nc -4 --listen 127.0.0.1 80 Here is the client to connect to server on localhost using source port same as destination port of server: nc -4 ...
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2answers
127 views

Add TCP congestion control variant to Linux Ubuntu

I want to test different variants of TCP in Linux Ubuntu. I have Ubuntu 14.04 LTS with Kernel version 3.14. When I check the available congestion control algorithm using the following command sysctl ...
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why does nmap shows same ports sometimes as filtered and sometimes as closed?

When I'm scanning port range 4050-4060 of an smart metering device all ports are reported as closed: root@abc:~/rvm/dlms# nmap -p 4050-4060 192.168.1.242 Starting Nmap 6.47 ( http://nmap.org ) at ...
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70 views

Bash reading from /dev/tcp, without external binaries

I'm trying to read files with bash tcp socket, So on one host I do, nc -l 8888 < /etc/passwd And on another machine, echo </dev/tcp/127.0.0.1/8888 But it didn't read the file as expected, ...
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17 views

Several TCP Window Scale values depending on interface?

Is it possible that TCP Window Scale value set by default on a socket (not raw) depends on the interface used? Where are these values stored?
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100 views

TCP SYN,ACK Retransmissions

The following network trace was recorded on a Raspberry PI running Debian Linux. It seems the Raspberry is not seeing the data packet #36995 for some reason; it is just stupidly repeating his SYN,ACK ...
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134 views

How to pass received data from netcat to another script as argument?

I have simple bash nc script: #!/bin/bash nc -k -l 127.0.0.1 4444 > filename.out which listens 4444 port for TCP connection. Instead of redirecting received data to filename.out I would like, if ...
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2answers
48 views

Count Bytes by Ports

Using RHEL/CentOS 6.7 -- I want to be be able to know byte counts on IPv4 ports. We have thousands.. I would think counting via IP Tables for that would be a bit unweldy and slow the firewall a wee ...
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55 views

reading the output of netstat for tcp

If I ssh root@server -R 5901:localhost:5900 and netstat -an I get: Active Internet connections (servers and established) │ Proto Recv-Q Send-Q Local Address ...
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1answer
393 views

socat/netcat UDP to TCP

I am trying to bridge a connection between UDP and TCP. The UDP - must listen on port. The TCP - must connect to a server. The purpose of the connection is to control VLC player. I tried netcat and ...
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136 views

Apache Load Balancer Timeout

We have two httpd servers running on RHEL 6 behind a hardware load balancer We are getting timeout errors in our httpd error log. [Fri Feb 19 15:06:49.823372 2016] [proxy_http:error] [pid 15760] (...
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235 views

Lots of TIME_WAIT in netstat on apache server

What would be the reason for many TIME_WAIT here? Even we have very few ESTABLISHED, SYN connection. [root@www ~]# netstat -natp | grep "210.XX.XX.XX:80" | awk '{print $6}' | sort | uniq -c 1 ...
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1answer
55 views

ubuntu IPTables

I started hosting my game-server on Ubuntu Linux 14.04 I am having an issues ، some enemies/bug abusers log in 2+ characters in the same second then they spam a certain spell/word that lags/stops my ...
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46 views

Netid in ss output is confusing

I'm on an Ubuntu 14.04 x64 machine. I have something running on port 51236 and I want to know whether it's TCP or UDP. So I ran ss -an | grep 51236 and got the following output. tcp ESTAB ...
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186 views

Half-close netcat connection

How can I have a netcat connection terminate the sending half of a TCP connection if its input reaches EOF? I have a (non-standard) TCP service which reads all its input (i.e. until the client sends ...
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71 views

nginx server never resolves

I have set up an nginx server which is a proxy for port 80 to http://127.0.0.1:3000, but the request never gets through. It just infinitely loads. I have tried sudo /sbin/iptables -I INPUT -p tcp -m ...
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55 views

Cannot access server outside of server`

I have a node server listening on port 80. If I send a request to it like this: request('http://127.0.0.1:80',function(err,res,data) {console.log(data)}) It works. It returns what I would expect. ...
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21 views

Changing the Retransmisson Timeout (RTO) estimation in Fedora

Retransmission Timeout(RTO) in a TCP plays a vital role in the internet. There are many algorithms proposed for calculating the Retransmission timeout. RFC 6298 is used for the estimation of RTO. I ...
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40 views

WARNING: at net/ipv4/tcp.c:1654 tcp_recvmsg

Our Linux based device (3.7.2) experiencing following issue when we try to do FTP or SCP over WiFi Connection. Same will work on Ethernet Connection or on TFTP. Here is the dmesg log for this issue: ...
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205 views

How to know if port 25 is blocked?

I am running Debian Jessie 8.1. The VPS provider says that port 25 is open. However, when I installed Rainloop and am not able to use it with the System users. Under the RainLoop admin panel I went ...
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1answer
872 views

Can nmap display only hosts with specific ports open?

Can nmap list all hosts on the local network that have both SSH and HTTP open? To do so, I can run something like: nmap 192.168.1.1-254 -p22,80 --open However, this lists hosts that have ANY of ...
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2answers
49 views

Unable to connect remotely to custom application

I've built an application that listens on the localhost address and a specific port, lets say port 11001. Its on a remote machine with CentOS. I currently have a python script on my local machine that ...
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1answer
128 views

pfsense tcp connection between openvpn and lan is broken

I am trying to connect openvpn clients into an internal lan, so they can access a http server. pfSense as well as my http server are running on a virtual machine of my home proxmox node. Both are ...
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is it possible to read a process's incoming TCP RecvQ?

I have a process running which has a number of incoming TCP connections, all of which show a non-zero RecvQ bit for some reason the process has not gone to read the incoming data. netstat -nap ...
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150 views

Netstat Listen Queues

What information can I gather from netstat listen queues? Looking at the man page I can see that using "-L" in netstat it shows me the size of listen queues. It tells me information about each ...
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26 views

Can't get websocket- answer from client browser in gawk

I want to make websocket server with gawk. I can pass handshake and send many messages to client, but when i'm trying to listen host to get client's dataframe gawk stops and cant do it. What i'm doing ...
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how to remove Queue in send_Q-part of netstat?

I have a tcp-server or may be it is lazy tcp-server because sometime the program dose not take the data from socket and i see some number is send_Q/recv_Q column of 'netstat -tn' command. is there a ...
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1answer
126 views

Delay FIN-ACK sent on linux

For a project I need to delay the FIN-ACK sent in an SSL connection on Linux. Is there any way to do this? I can send an https request by using curl https://www.google.com, but how do I modify the TCP ...
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4answers
1k views

Writing a TCP server in bash [duplicate]

So, I recently found out about how you can send TCP packets through the /dev/tcp/hostname/port file, but my main problem here is how I receive them. I want to make a bash script that can host a TCP ...