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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Port Mirroring in Fedora?

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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1answer
187 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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2answers
350 views

Real-World Use of TCP_DEFER_ACCEPT?

I was perusing the Apache httpd manual online and came across a directive for enabling this. Found a description in the man page for tcp: TCP_DEFER_ACCEPT (since Linux 2.4) Allow a ...
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1answer
88 views

Linux fedora - httpd package and router, how to make it work?

I installed HTTPD package to setup private web server in my house on one of the computers. The server is up and running, but I can't access the server from outside by using my external IP address. I ...
6
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3answers
74 views

How does one capture traffic on virtual interfaces?

I would like to capture traffic on Linux virtual interfaces, for debugging purposes. I have been experimenting with veth, tun and dummy interface types; on all three, I am having trouble getting ...
12
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1answer
922 views

Why is my TCP throughput much greater than UDP throughput?

I haven't done anything unusual to my hardware or kernel configurations (all default settings, fresh OS install, Linux kernel 3.11 TCP/IP stack) and I'm averaging about 3.83 million messages per ...
3
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1answer
61 views

How to use netperf on specific NIC port?

I have two servers, SERVER and CLIENT. Each server has its own onboard 1 GbE ports, and its own NIC with a 10 GbE port. SERVER and CLIENT are interconnected on both a 1 GbE switch (administrative ...
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2answers
89 views

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 ...
2
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1answer
254 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 ...
2
votes
1answer
272 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 ...
1
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1answer
31 views

How can I tell which ports have been used for reverse SSH tunnels?

I have two machines, client and server. client has created a reverse-tunnel on server with something like ssh -R 12345:localhost:12345 server. When logged into server how could I find ports which ...
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3answers
54 views

tcp6 in the output netstat

What does column 'tcp6' mean on output netstat? Please anyone explain the follow output of netstat: tcp6 0 0 dmz.local.net:www 5.140.235.6%14631:49964 ESTABLISHED 21393/apache2 What does tcp6 mean? ...
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2answers
503 views

How do I find out what file Apache is serving when a port is hit?

I'm working on a server someone else set up. The server is CentOS 4.6 running Apache/2.0.59 (httpd). They have an update service setup on port 8443. I know it is working but I need to find out what ...
2
votes
3answers
104 views

How to record an interactice socat TCP/TLS session?

Socat is great for interactively testing line based human readable protocols like HTTP or IMAP. For example: $ socat -d -d READLINE,history=$HOME/s.hist openssl:host:port,crnl,cafile=some.ca For ...
2
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1answer
30 views

How does the kernel know which file descriptor to write data to after fork() in a concurrent server?

Basic concurrent client/server architecture: There's a main loop listening for requests on a port (for example 3000), after accepting the connection the server spawns a child process that ends up ...
1
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1answer
56 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
66 views

Command for finding open ports

On my linux servers I have this alias set: alias ns="sudo lsof -Pi|grep -v ESTABL|grep -v CLOSED|grep -v -e '->' | tr -s ' ' '\t' | awk '{print $3 "\t" $8 "\t" $5 "\t" $9 "\t" $1}'| column ...
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6answers
14k views

Linux: allowing an user to listen to a port below 1024

I need to allow an user (different from root) to run a server listening on port 80. Is there any way to do this?
6
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2answers
188 views

what's the format spec for `ss -D `?

Background of the question: I am trying to sum up the total bandwidth, but ss -i cleverly converted numbers to more readable format like Kbps/s or Mbps/s, which is not accurate. So does ss -o with ...
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0answers
93 views

graph TCP connection performance based on `ss -i`

A nearby question has brought to my attention the ss utility on Linux, another utility to investigate sockets. It would appear that it may be a great tool to monitor the performance of, say, an ssh ...
0
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1answer
62 views

Strange network problem (ssh) after hardware switch - packet loss?

My machine crashed (would not start) and so I pulled my harddrives and memory (which was not stock) and brought to another identical Dell optiplex 755 which I found in the office. Now everything seems ...
2
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2answers
132 views

How do I share stdout between multiple processes?

I want to receive live TCP stream, and make it readable by another processes at the same time, without saving it. For example, 111.222.233.244:1234 streams actual time. Server supports only one ...
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2answers
68 views

What ports are essential and should generally be allowed on a Linux system?

I'm running a desktop computer, but one with high threat risk. I've blocked all ports using a firewall except: HTTP, HTTPS, DNS, one port for torrent client. Do any common processes in distros ...
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0answers
31 views

Is it possible to move established connections on one port, to another without dropping data?

this problem has been bugging me for a couple of days now. I can't figure out a good enough solution using iptables without having dropped packets. So here is the back story. Say I have a client ...
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1answer
224 views

Is it better to set -j REJECT or -j DROP in iptables?

There's an example of iptables rules on archlinux wiki: # Generated by iptables-save v1.4.18 on Sun Mar 17 14:21:12 2013 *filter :INPUT DROP [0:0] :FORWARD DROP [0:0] :OUTPUT ACCEPT [0:0] :TCP - ...
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4answers
8k views

Logging outgoing connections as they happen

Is there a way to log to file all the outgoing connections that a process creates? I am aware of netstat but that seems to be more of a snapshot of a point in time rather than something that runs and ...
3
votes
1answer
129 views

Linux:How to tweak TSO and GSO from proc window / sysctl

OS:CentOS 5 I am doing some TCP optimization on my Linux box and want to put ON tcp segmentation offload and generic segmentation offload. I am doing it through ethtool Here is what I am doing: ...
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0answers
197 views

How to redirect/forward between a tun/tap device and a socket using linux tools?

I have a socks5 proxy that is bind to eth0. It opens two ports, Port1 is used to connect to remote (for example a http server), Port2 is to listen for clients to connect. Then I created a tun device. ...
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3answers
8k views

How do I reserve ports for my application?

How do I reserve a list of ports for my custom applications? To be specific, the product I'm creating has a lot of processes and a lot of intercommunication between them. The problem I'm having is ...
5
votes
2answers
7k views

How to configure SVN/SSH with SSH on non standard port?

I have configured SSH to be on port 20000. When I try: svn co svn+ssh://server.com:20000/home/svn/proj1 proj1 --username jm I get svn: To better debug SSH connection problems, remove the -q ...
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2answers
128 views

I want a static IP address for my (network? or computer?)

Me and two friends have been making a video game. We've thought about download hosting a bit, but found that Mediafire, Box, and those sorts of file-hosting sites' maximum free file-upload size is far ...
3
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1answer
1k views

Run nmap via Tor

Is it possible to runnmap via Tor? When I googled around, I got the impression that Tor uses Polipo / Privoxy, which are socks5 proxies. So any TCP / UDP aware applications should be able to use them ...
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0answers
83 views

irc connection timeout problem

I am using tcpredir to connect to freenode server via port 6665 for some firewall difficulties: server:~# redir --lport=6665 --cport=6665 --caddr=irc.freenode.net and recently i faced a problem, i ...
0
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0answers
63 views

How to make Network alive but speed low, slow for upload, download?

I have own application written (Python, Java) which does upload/download for streaming. Now in my local office network and home network setup i can't simulate slow/low latency network (as a result i ...
1
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1answer
113 views

`cat /dev/ttyACM` loses data when piped through netcat

I'm reading data from /dev/ttyACM0 using cat /dev/ttyACM0 on a Raspberry Pi. The data comes from a device, which sends lines in the form 15.00 1316013 15 where the first number is a value, the ...
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votes
3answers
1k views

Simple way to create a tunnel from one local port to another?

I have a development server, which is only accessible from 127.0.0.1:8000, not 192.168.1.x:8000. As a quick hack, is there a way to set up something to listen on another port (say, 8001) so that from ...
7
votes
4answers
8k views

How to check whether firewall opened for a port but not listening on the port

We will be deploying a new application to a Server and the application will be listening on port 8443. We have asked Network team to open the firewall for the port 8443 on that server before deploying ...
10
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1answer
1k views

Kernel socket structure and TCP_DIAG

I'm working on a software which connects to a Real Time data server (using TCP) and I have some connections dropping. My guess is that the clients do not read the data coming from the server fast ...
5
votes
1answer
4k views

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 ...
2
votes
2answers
174 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 ...
3
votes
1answer
569 views

What does net.ipv4.tcp_app_win do?

I can't figure out why do these variables coexist in Linux. The information from man tcp says (Ubuntu 12.04): For tcp_adv_win_scale: tcp_adv_win_scale (integer; default: 2; since Linux 2.4) ...
0
votes
1answer
54 views

How do I verify that the Linux kernel is not freezing when uploading or downloading large data over the ethernet interface?

How to verify if the Linux kernel is doing what it's saying for network interfaces? I have one network interface eth0 which has hardware support to do 1GB full-duplex. I have prepared 2 box. Now ...
4
votes
4answers
1k views

How does the server find out what client port to send to?

As I understand it this is what happens when a client makes a connection request: The server will be bound to a particular port number. The port number is always bound to a listening process. Since ...
0
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0answers
77 views

How to run Varnish together with Solr/Jetty on different ports?

I am trying to make Varnish work on my server but it seems that both configurations use port 8080 so Apache doesn't start. Anyway, then I check the used ports, this is what I get for netstat -tulpn| ...
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5answers
2k views

How can I tell if a TCP port is open or not? [closed]

I am trying to implement socket programming in C. When I try to connect from a client to a server (Ubuntu), it shows an error like "connection failed". So I think the problem is with the port. I am ...
4
votes
3answers
493 views

ssh will sporadically hang temporarily on fast connection

I am using Ubuntu 13.04 on my laptop, connected to my router at home. When working from home, I will ssh into the servers on campus, through a vpn, with X11 forwarding. ssh -X ...
4
votes
2answers
2k views

Performance of unix sockets vs TCP ports

For example on php-fpm: #listen = 127.0.0.1:9000 listen = /var/run/php-fpm/php-fpm.sock Is there any major performance differences between using unix socket-based listeners over TCP ports? (Not ...
16
votes
5answers
22k views

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 ...
3
votes
1answer
91 views

detect process that executes portscans

I have to administer an ubuntu 12.04 server, where there seem to go portscans to the whole subnet. How can I find out the process that does the scans? pstree init─┬─acpid ...
2
votes
1answer
703 views

Using tcpdump to extract NFS RPC contents

Fairly simple question... I'm running tcpdump and trying to analyze the contents of the TCP packets between server/client. I see the "GETATTR" RPC being received, which is great! However, I want to ...