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 configure a reverse SSH connection to the connecting computer?

I'm going to be deploying a number of machines in the near future which will be behind routers. It won't be feasible to set up dynamic DNS on each router and port forwarding, so is there a way I can ...
42
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1answer
48k views

How does tcp-keepalive work in ssh?

I am trying to code a shell-script that uses a ssh-connection for doing "heartbeats". I want to terminate the client- and server-side of that connection after a certain timeout (after the connection ...
32
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7answers
43k 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
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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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2answers
3k 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, ...
11
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4answers
15k 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 ...
10
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1answer
2k views

D-Bus authentication and authorization

I'm trying to set up remote access to D-Bus, and I don't understand how authentication and authorization are (not) working. I have a D-Bus server listening on an abstract socket. $ echo ...
28
votes
1answer
18k views

Why are the first 1024 ports restricted to the root user only?

This is more idle curiosity than anything else. A friend of mine asked me 'which port range is it that only root can use under Linux?' I told him 0-1024 were restricted. Then he asked my why it was so ...
21
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2answers
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Orphaned connections in CLOSE_WAIT state

I've got a SLES machine that accumulates TCP connections in a CLOSE_WAIT state for what appears to be forever. These descriptors eventually suck up all available memory. At the moment, I've got 3037 ...
19
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1answer
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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 - ...
20
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2answers
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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 ...
12
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3answers
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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
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1answer
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socat reliable file transfer over TCP

I am aware of disadvantages of "wait->stop" design, that usually is proposed with netcat: server$ cat test.dat | nc -q 10 -l -p 7878 client$ nc -w 10 remotehost 7878 > out.dat (It's not reliable ...
14
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5answers
62k 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 ...
5
votes
3answers
17k 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? ...
15
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4answers
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TCP dies on a Linux laptop

Once in several days I have the following problem. My laptop (Debian testing) suddenly becomes unable to work with TCP connections to the internet. The following things continue working fine: UDP ...
10
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3answers
9k views

Using in/out named pipes for a TCP connection

I've been fiddling with getting this to work for a while now, so I suspect some sort of fundamental misunderstanding about how pipes work is the root cause of my troubles. My goal is to initiate a ...
6
votes
2answers
877 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 ...
12
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2answers
22k views

Getting current TCP connection count on a system

Instead of doing wc -l /proc/net/tcp, is there a faster way of doing it? I just need a total count of tcp connections.
6
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4answers
657 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 ...
3
votes
2answers
3k views

SSH tunnel to router periodically hangs. TCP problem?

Something weird I've noticed since upgrading my office router to the Buffalo WZR-HP-G300NH. When I remote in using an SSH tunnel, the tunnel will "freeze" pretty often. The freeze lasts 1-2 minutes ...
5
votes
1answer
3k 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 ...
3
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2answers
304 views

Distinguish TCP server from client within netstat output

I have a socket connection between a client process and a server process, both running on my machine. There are two entries shown for the connection as, sudo netstat -ntp | grep 56442 tcp 1 ...
2
votes
1answer
98 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 ...
2
votes
1answer
2k views

Why can't I use strace with rsh?

If I run rsh, it works, but prints some strange “Connection refused” messages at the beginning: $ rsh localhost pwd connect to address 127.0.0.1 port 544: Connection refused Trying krb4 rsh... ...
1
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2answers
137 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 ...
0
votes
1answer
30 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 ...