All Questions
157
questions
1
vote
1
answer
54
views
Capture TCP stream with tcpdump/netcat
I have a hardware device that hosts two separate TCP modules. One is for receiving commands from other machines, while the other is a TCP stream server that sends data based on the commands received. (...
0
votes
0
answers
355
views
Minecraft server using ngrok for port forwarding stopped working
So I built a PC using pieces I had laying around to make into a Minecraft server. I installed Ubuntu on it (I think it's the latest version because I installed it like a week ago. I didn't manage to ...
2
votes
1
answer
769
views
How to explain sequences of FIN,ACK instead of FIN - FIN,ACK?
Context: newly installed Debian 12, I get a bunch of strange logs related to ssh:
root@square:~# journalctl -u ssh -f
May 07 11:13:00 yop-square sshd[766]: error: kex_exchange_identification: ...
- 3,263
0
votes
0
answers
74
views
Linux 6.5 netfilter NAT reuses TCP ports in TIME_WAIT status?
In my understanding, Linux (netfilter)'s NAT doesn't reuse TCP ports in TIME_WAIT status, but seemingly Linux >= 6.5 reuses TCP ports in TIME_WAIT, even if the destination is the same.
For example, ...
- 41
0
votes
0
answers
62
views
Linux Server - Bandwidth test doesn't change during TCP Tuning process
I'm new to network administration and have a question.
I am conducting research on whether it is possible to carry out TCP tuning using the genetic algorithm method. To do this, I wrote a Python ...
0
votes
1
answer
873
views
How do I run a permanent tunnel in localxpose?
I am able to create a localxpose tunnel in ubuntu with this command:
:loclx tunnel tcp
but when I close my local connection to my server, the tunnel closes with it.
Is there a way to keep a tunnel ...
0
votes
1
answer
780
views
is it possible to OpenVPN connection UDP through SSH tunnel?
Hi i search a lot on the Internet, especially in this site and read more post and articles about using OpenVPN connection through SSH tunnel but all tutorials about using TCP protocol to do that. in ...
- 101
0
votes
0
answers
37
views
Seeing several thousand retransmits every 10 sec. via iperf3 on TCP traffic through a propritary network bridge device
What is the most likely cause of TCP packet fastACKretransmits when testing with iperf3 on a small 5-7 node Linus AutoIP network running on a TI AM5728 board? Where and what setting should we check ...
0
votes
0
answers
43
views
Teardrop detected from TCP ACK from myself
I'm experimenting with linux (Debian 5.10) networking to allow me to connect two networks (eg. 192.168.0.0/24 with 172.17.0.0/16) together.
Setup
In my case it is a small server with one NIC, hosting ...
- 126
2
votes
1
answer
3k
views
How to force `telnet` (in TCP mode) to send bytes immediately?
In Linux, when I run telnet <address> <port> and then type things, it buffers data and only sends collected line when I hit Enter. Furthermore, it buffers up to 4096 characters and ...
- 2,885
1
vote
2
answers
735
views
socat TCP server bidirectional using sleep, non-interactive client returns without response. Server reports: I/O error
I have the following TCP server:
socat TCP-LISTEN:10000,fork,reuseaddr SYSTEM:'read -r msg && sleep 3 && echo "OK"'
When I execute (interactive mode):
socat - TCP:localhost:...
- 748
1
vote
2
answers
9k
views
TCP to virtual serial port with socat
I have a program (program 1) that has a TCP server listening on port 1234. I have confirmed that the server accepts connections and sends data with a utility str2str.
Program 2, on the same machine, ...
- 73
2
votes
1
answer
638
views
Python TCP socket connection between Linux and Windows10 not working as expected
I am trying to establish a TCP connection and sending packets over a python socket server-client. I am using Windows 10 as my main system and VM Kali-Linux on the other side
I used Windows as a client ...
- 47
1
vote
1
answer
255
views
Possible to use shell script that does loopback to localhost when receiving a packet?
I want to run a shell script, that sends a packet to localhost (loopback) when it receives a packet from anywhere.
I am using netcat.
nc -lp $port, and then echo test | nc localhost $port works.
But, ...
- 21
0
votes
0
answers
1k
views
ModuleNotFoundError: No module named 'pyModbusTCP'
I am trying to run a python file on bootup which will collect some data from another device.
I created a service in the /etc/systemd/system directory
[Unit]
Description = File which requests data from ...
- 33
0
votes
2
answers
1k
views
How to measure the latency to a webserver?
I wish to measure the TCP network latency from my Linux server, to a websocket URL.
I am using netperf like this:
netperf -H ws.the_websocket_url.com -l 5 -t TCP_STREAM -v 2 -- -o min_latency,...
- 219
0
votes
1
answer
74
views
New LAN from Linux PC
I've got one Linux PC connected to the Internet but I want to connect more devices to the PC using TCP/IP but I don't want the router to act as a DHCP Server for them. I mean, I don't want the router ...
2
votes
1
answer
669
views
Why does my virtual machine (Debian on Proxmox) not respond to a TCP SYN of another VM even though it arrives?
I got a very strange problem and I am out of ideas to solve it. I hope anyone over here has a helpful idea.
Some info about the virtual machines:
VM0: Debian 10 - 192.168.10.102 [+ 2 more interfaces]
...
- 41
0
votes
1
answer
565
views
How can I assign an ephemeral port range to a specific linux service and not the entire IP stack
For example: If I want sshd to listen on 22 but use ephemeral ports of tcp 50-60 for communication.
0
votes
1
answer
1k
views
Meaning of IPTABLE duplicate values
I'm new to IPTABLES and when I execute the following command
/sbin/iptables --list -n | grep 16381
I could see multiple entries as shown below :
ACCEPT tcp -- a.b.c.d 0.0.0.0/0 ...
0
votes
0
answers
423
views
Can TCP implementation send several packets without waiting an ACK for the first one?
I basically know who TCP works from sockets point of view, but I can't figure out some thing.
Suppose I have now 1000 bytes to send, it comes for TCP-stack internal buffer and gets sent. These bytes ...
- 195
0
votes
1
answer
541
views
How can I identity which process is sending data to a certain host on Linux?
As per title, I want to figure out the process responsible for certain traffic. I can see the traffic in wireshark, but I don't know which process it is.
Is there any straightforward way to do it? I'm ...
2
votes
1
answer
6k
views
Prometheus Blackbox Exporter TCP probe returns value of 0, even when port is open (Iperf3 server)
I have a strange problem that I've been digging into the last couple of days.
The setup
I have an Iperf3 server running in Docker on this internal address and port: 10.10.2.2:5201
I also have ...
- 172
4
votes
2
answers
245
views
linux service TCP Gateway with service starting
Is there a service (linux) that starts the service on an incoming TCP request (if it does not already exist) and then forwards it to the application? For example, for a docker container that would ...
- 63
0
votes
0
answers
94
views
Low-traffic network retransmissions and fails
I have an local network with generally very low traffic. On the network I have oscilloscope with LAN access, which I want to interrogate. When downloading samples (which come in blocks of a few ...
- 737
0
votes
1
answer
569
views
TCP Packet drops on application server
I have a monitoring application (Zabbix proxy) installed on RHEL 7.8 and since we have a very large environment, we have 2500+ agents connecting to this one server. We're seeing frequent errors while ...
- 101
0
votes
0
answers
229
views
How to allow only 100 half open SYN connections
I'm running Ubuntu and want to allow only 100 waiting SYN connections at a given time,
and drop the rest.
How can I configure iptables to implement this rule?
Or sysctl config to allow only 100, and ...
- 103
0
votes
1
answer
4k
views
Increase buffer size for a specific socket on Linux
I am using CentOS7 and I have to increase the send buffer size (SO_SNDBUF) on a socket listening on a specific port. The application that has the port opened does not natively support this.
Few things ...
2
votes
1
answer
1k
views
What would prevent a TCP server port (once closed), from being re-opened right away?
I have a server program that uses TCP.
Sometimes, I need to restart the program for updates, or any other reasons, but when I do, the program closes the server port, and then once it restarts, it ...
- 95
1
vote
0
answers
45
views
understand ss -to4 output for a jdbc connection
My application connects to a database over JDBC, and performs a lot of sequential inserts. There is only 1 JDBC connection opened by the application. As the performance is not as expected, I started ...
- 11
0
votes
1
answer
673
views
Sniff tcp connection and forward to TCP connection on other adapter
I have a system with two network adapters. One of the network adapters is connected to a port mirror, so it sees all the traffic in the network. I'd like to sniff that connection for a certain TCP ...
0
votes
1
answer
88
views
Attack on a Linux file server | What is the damage of this file contents?
TCP Networking
This file was uploaded using a loophole in a file uploading network program I was writing. The ability to access the server itself was too simple and the techniques I used resulted in ...
- 11
-3
votes
1
answer
83
views
Why does my IPTables changes not deny access from all IP addresses except CloudFlare’s ranges?
I am running apache on ubuntu 18.04. I used IPtables to deny access from all IP addressed except CloudFlare’s ranges and when I run iptables -L –line-number I get
1 ACCEPT tcp – 131.0.72.0/22 anywhere ...
0
votes
1
answer
466
views
Samsung Smart TV UA43N5300ARLXL/UA43N5300ARXXL unable to connect to internet over UFW firewall
I have a home network with a Raspberry Pi at the center running Pihole and UFW. The Pihole is both the DHCP as well as DNS server.
Ever since I started using Pi Hole I've not had any issues until ...
- 21
1
vote
0
answers
2k
views
SSH fails to connect from one computer but works from other computers
Problem
I have an external VPS server running Ubuntu 18.04 with openssh-server 7.6p1. I can connect via ssh to this server from many different networks except from one particular client. This ...
- 111
1
vote
2
answers
2k
views
Packet reaches OS but not the process
Sorry if my issue is a bit hard to summarize in the title. That is the best I could come up with.
TL;DR Version: How do I debug when packets reach OS, but not the destination process?
Explanation: I ...
- 123
1
vote
0
answers
592
views
How to enable ECN marking in IP header in Linux?
Is the ECN marking in the IP header is performed by default in Ubuntu (4.15.x kernel) or should the kernel be recompiled with a special option? I configured a RED ingress queue at an Ubuntu machine, ...
- 11
6
votes
1
answer
13k
views
Capture only TCP SYN-ACK packets with tcpdump
I'm trying to capture only TCP SYN-ACK packets, i.e. with both SYN and ACK bits set with:
tcpdump -vvvni eth0 tcp[tcpflags] == tcp-syn and tcp[tcpflags] == tcp-ack
but it gives such error:
tcpdump: ...
- 244
3
votes
1
answer
18k
views
Use netcat to listen on a port and send output from a command when a client connects
I have a Raspberry Pi (Debian Linux) connected to my LAN that can read data from some connected devices and output it to STDOUT.
Let's say the program is run on "Server" and I want to serve the ...
- 135
0
votes
0
answers
273
views
log tcp contakting connection with nftables
I Want to log TCP contracting Connexion using nftables(iptables)
I tried to write tcp flags in Rule, but when I acces to webpage of destination IP address I get just this Info in my logfile :
...
- 71
0
votes
2
answers
280
views
If I disable iptables its working. Which rule is making my websites a timeout?
I have a VPS with nginx but no website is working and timeout is sent. When I disable iptables everything is fine. But no idea which rule/s is taking the problem. Any help?
$sudo iptables -S
-P INPUT ...
- 1
0
votes
2
answers
592
views
how to run mincraft server on linux
I wish to run minecraft server om my linux mint 19.0, but I have some troubles, I can start it in local, and connect by 127.0.0.1:25565 but i can't run it on my ip in inthernet, 4 example my ip is 2....
- 101
1
vote
2
answers
357
views
Why Imx6q(Embedded Linux Device) ethernet communication is too slow?
I have tried several methods to test the ethernet communication of the Imx6 card.
1) When I ping in my computer from IMX6, the ping does not reach the IMX6. But when I ping to imx6 from the computer, ...
- 11
3
votes
1
answer
2k
views
Create a TCP half open connection with bash command
Is there a way to (easily) create a TCP half-open connection using a bash command with common Linux tooling?
I want to monitor availability of a remote server using such mechanism. I know it is also ...
- 151
0
votes
0
answers
216
views
Iptables configuration
Here is my iptables script. Can it be better ?
Thanks in advance !
#!/bin/bash
#iptables-restore < /etc/iptables.test.rules
iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables ...
- 1
2
votes
0
answers
307
views
How can I use the "location of socket in memory" field in /proc/net/tcp
I'm working on a proxy for linux(C++) that, amongst other functions, keeps track of tcp connections and associates them with the process's pid. To do that, I get the inode in /proc/net/tcp and then ...
1
vote
1
answer
2k
views
What is it happening if a port is constantly (since one or more days) in FIN_WAIT1?
I've a problem when I try to communicate with a server.
If I use the command line:
netstat -np 10.aaa.bbb.12
where 10.aaa.bbb.12 indicates the server address; I obtain an extract with the ...
- 123
0
votes
1
answer
320
views
Limiting nginx requests based on port
Is it possible to drop excessive connections on per port basis?
Let's say I 've multiple virtual hosts in Nginx listening on different ports. I want to set different rules for different ports.
For ...
- 1,335
1
vote
1
answer
3k
views
Using lsof and fuser I can't find the process listening on a strange port
I've read other posts about finding the process ID and what's listening, but sadly couldn't get my issue solved. So, apologize if I missed some info about it.
Running netstat I found a port that I ...
- 13
2
votes
1
answer
722
views
What does a suspended TCP process in linux exactly do and do not?
Regarding TCP connections using NetGui (and wireshark for analyzing):
I set a TCP connection between two PCs using the command nc -p 11111 etc... for the server, and the corresponding one for the ...
- 35