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Questions tagged [tcp]

TCP stands for Transmission Control Protocol and is one of the core protocols of the Internet Protocol Suite. TCP complements the Internet Protocol (IP), and therefore the entire suite is commonly referred to as TCP/IP.

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SYN packets never responded to with SYN/ACK

I'm having an issue with a tcp handshake that I can't find the answer to anywhere else. I have a program running on my local device that posts data to port 50000. In a terminal on the same machine, I ...
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TCP KeepAlives with NAT?

We have a Network set up like this: MQTT Client -- TCP Forward (Netscaler) -- TLS Termination + TCP Forwarding (HAProxy) -- MQTT Server For several reasons we needed to turn on TCP KeepAlives in ...
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What are TCP Port 5445 and UDP Port 3935 used for on Windows Server 2016?

I'm facing a dilemma, I built 2 physical servers and for some reason for all the IP addresses assigned to that server it's listening on Port 5445 (TCP) and also 3935 (UDP). Applying the same ...
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Can I capture full TCP packet content with haproxy without knowing length of the packet?

I know I can capture 6 bytes length packet using this config: global log /dev/log local0 debug log /dev/log local1 notice chroot /var/lib/haproxy stats socket /...
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TCP retransmissions inside vlan

I want to monitor a service from another machine. Both machines are connected to a VLAN. Both machines can ping and also establish connections to each other. So connectivity in general seems fine. ...
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Is it possible to watch the tcp connection packets on a connect call from the command line in linux?

Basically I want to do something like strace ssh user@host and instead of seeing the system calls, I want to watch the tcp connection packets as the connection is established. I'm having this weird ...
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Strange connection timeouts on server when connecting with multiple linux clients on multiple domains.

Here is what happens: When trying to load a page on one of our websites using 2 or more linux clients from the same ip the page seemingly randomly does not seem to load. Looking at the network tab in ...
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44 views

What exactly happens when a TCP segment is sent to a broadcast address?

Does the segment get delivered -- and the recipients are supposed to drop it? Does the segment get dropped at the (layer 3) switch/router? Or something completely different? Explanations or "RTFM!" ...
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“connection reset” occurs in client side and “client prematurely closed connection” showed in Nginx log

I was haunted recently by a problem that some of our customers were complaining that occasionally they could not open the web pages and web browser showed "the connection is reset". This issue did ...
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2answers
69 views

/proc/sys/net/netfilter/nf_conntrack_count extreme drop when reading /proc/net/nf_conntrack

I have very busy Web servers, and wanted to introduce some analysis to see what kind of traffic present. Namely, total number of all connections, number of time waits, established connections, udp and ...
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ngrok specifiy forwarded ports

In ngrok, I can connect to my local host like this: http://123abc.ngrok.io -> localhost:55914 If I want to connect to say http://123abc.ngrok.io:2001, will that be routed to localhost:2001 or ...
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Round-robin outgoing TCP requests between multiple Ethernet adapters on Windows

I have a Windows Server Standard with 3 Ethernet adapters. I am running a C# application that makes TCP connections to another server. Right now they are all made via the adapter that has a default ...
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How can I see a tcp sessions properties (congestion window size, current slow start threshold value, etc.) on Windows?

On linux you can run ss -i dst 10.0.0.1 and get the following: Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port tcp ESTAB 0 0 10.172.44.11:46525 ...
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SSH Tunnel - channel 3: open failed: administratively prohibited: open failed

ssh -D 9090 user@host when I try to request web pages through socks5 (127.0.0.1:9090) I get: channel 3: open failed: administratively prohibited: open failed server sshd_config: #MaxStartups 10:30:...
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Netstat showing python services listening to multiple ports

In my search to reduce the vulnerabilities of a server, I used netstat to check which ports are listening and stumbled upon a cascade of python services listening to a large number of ports, as shown: ...
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92 views

Can a TCP packet don't reach the 7th OSI layer

I'm facing a situation where a client establish a TCP connection via telnet (or netcat) in order to connect to a linux server. The server must then send back a banner containing some text info... In ...
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1answer
32 views

Remove tcp entry from netstat -a

I have a rather strange issue and no matter what I do or search, I cannot rectify it and have now decided to seek help here initially. I have a cluster of servers, about 10 clustered together. A few ...
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1answer
86 views

Running tcpdump starts ssh flood

I have runtime Linux running in a PLC. My development machine is running Ubuntu 14.04. The PLC and the development machine are connected through a five port switch. I ssh to PLC from my development ...
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Socket usage with Linux

I am wondering, does anyone know if there is a setting within linux that designates the max amount of time a socket is allowed open? I ask this because for whatever reason, no matter if there is data ...
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1answer
148 views

Connection refused for Java HTTP Server running on localhost

How would you investigate the following issue? On a specific server; any HTTP-Requests sent to a Java Server running on localhost fails with "Connection refused". (See the Java code here) In the ...
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Server does not respond to SYN [duplicate]

When I launch an FTP transfer from a client machine, it fails because it cannot connect to the server. With tcpdump I can see that the client sends a SYN to the server, and I can also see that the ...
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107 views

why linux assigns ephemeral ports out of its range

I have a server running mysql, ssh and other services. I want to see the range for ephemeral ports available for the OS running: cat /proc/sys/net/ipv4/ip_local_port_range which returns 32768 - ...
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lsof displays two connections from one port

I'm following lsof output for my process COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME AppName 79899 heximal 7u IPv4 0x6c18d5501d2e17c9 0t0 TCP 10.3.2.8:...
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2answers
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some servers won't reply to SYN if client is on VPN

We have some ubuntu 18.04 servers (both physical and VMs) that will ignore pings and TCP connections coming from clients connected to our intranet via VPN. It is only fresh installs of 18.04 that are ...
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117 views

How to debug IIS 8.5 TCP resets?

I have a Win 2012 R2 / IIS 8.5 server that has been running a handful of public-facing production websites for 2 years. Today, for about 6 hours, it started randomly dropping port 80 requests (but ...
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1answer
69 views

How to turn an ethernet port into a passive listener?

I want to capture ethernet packets with my raspberry pi's ethernet port. I know I could create an ethernet bridge between two ethernet ports on the raspberry and analyze the packets internally with ...
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NGINX server not responding to new connections intermittently

I have a web server set up and it usually responds extremely fast (<100ms) and on repeated connections even with keepalive_timeout set to 0 has no issues connecting. However, occasionally on ...
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What is a TCP stream in a Nginx connection?

I'm working with Nginx and I'm searching what is a TCP stream. I haven't found relevant definition on the web, so please maybe someone has any answer to provide us ?
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For many different TCP hosts, how can I differentiate and route them from the same external IP?

I realize I may be asking the impossible here -- At the moment I use too many elastic IPs, and they cost money. Not only do they cost money...but in a disaster scenario, it takes time to reattach 25 -...
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Cisco ASA 5515 - huge amount of long running idle connections

I've got a Cisco ASA 5515 and it's come to my attention that the connections usage is quite high. Here's the result of running show resource usage summary Resource Current Peak ...
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123 views

What is the difference between net.core.netdev_max_backlog and net.ipv4.tcp_max_syn_backlog

Several docs on the net describe these options as following: net.core.netdev_max_backlog: Maximum number of packets, queued on the INPUT side, when the interface receives packets faster than ...
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Analyze Packet Loss Bursts

I need to analyze packet loss bursts in TCP streams. Given a number X a packet lost burst is when more then X consecutive packets got lost. So I have a pcap file and need to count how many bursts ...
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321 views

In Linux server - how to check if established TCP port connection supports TLS 1.2

I have a Linux server that has an established TCP port connection to a client. Can I somehow check if the connection on this port supports TLS 1.2 with any of the networking commands on a basic Linux ...
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Why a TCP connection fail faster on Linux closed ports than on Windows ones?

While trying the simple exploit described here I noticed that it doesn't works on Linux while it works on Windows (tried with several browsers). Apparently this is because, on Linux, the operating ...
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Is it possible for packets sent using TCP to ever arrive with different data? [closed]

Sometimes in my networking library when I send a packet the data arrives different than when I sent it. I assumed that TCP guarenteed exact delivery. Is this true? Or must there be something wrong in ...
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iPhone OS 11_3 http connection flood

I have been trying to limit max TCP connections to port 80 of my server coming from the same IP. I have used iptables for this task: -A INPUT -p tcp --dport 80 -m conntrack --ctstate ESTABLISHED -m ...
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All TCP packets have the PSH flag set - who / what would be responsible for that?

On a server->client TCP stream, all packets from the server have the PSH flag set. Is good old Nagle responsible for this flag being set?
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1answer
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“edit” TCP header in a incoming traffic

I have three set of servers which will involve for the current data flow. Server A ----> Server B ---> Remote Server/Storage I am downloading/push files from Server A to B, and then server B will ...
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reasons for fastpath sockets to fail?

I have a windows 2012 server that refuse to open TCP fastpath connections. The code that opens it do all the possible tests in every step on the road and it doesn't have any errors (it also works on ...
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1answer
116 views

Set static TCP port in SQL Server unattended installation

I have an unattended installation of SQL Server 2016 using the .INI answer file. In my answer file, I enable TCP like this: ; Specify 0 to disable or 1 to enable the TCP/IP protocol. TCPENABLED="1" ...
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Logstash losing connection to Elasticsearch nodes

We're running Logstash on a server which is pushing logs into an Elasticsearch cluster. In the Logstash logs we see it losing connection to the Elasticsearch servers with a connection reset error. We ...
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1answer
23 views

Getting number of established TCP connections per unit time in Amazon Linux (CentOS-based)

I'm running a server in AWS on the Amazon Linux operating system (it's a variant of CentOS I've heard), and I'd like to count the number of TCP connections that get ESTABLISHED per unit time to a ...
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How to set up a persistent TCP gender-changer proxy?

I have a provider (A) that wants to send us data through an incoming TCP connection. Unfortunately the consuming service (B) cannot receive inbound TCP connections. Also it does not have a static IP, ...
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52 views

Remote Client is unable to connect to TCP Server on VPS (Getting 10060 error)

I have searched all over the internet for the solution. Unfortunately, I could not get any working solution yet. Here is the story. I am running TCP server for small communication application on the ...
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40 views

What happens when UDP and TCP are sharing a link?

The question that I have is about TCP fairness: Say one Host ist transmitting data via UDP, while a second Host is transmitting data via TCP. Both of these Hosts use the same Ethernet connection. ...
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Packet Loss Analysis of a UDP upload in dependence of shared link capacity

I consider two hosts: Host1 is sending UDP packets (between 1000 and 1500 Bytes) to Host2 and at the same time Host2 is sending UDP packets (also between 1000 and 1500 Bytes) to Host1. The ...
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1answer
72 views

Netstat output contains several connections with the same ip and port

Netstat output (netstat -an) contains several connections with the same foreign (client) ip and port, for example: TCP server_ip:49421 client_ip:4000 TIME_WAIT TCP server_ip:52772 ...
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198 views

SFTP slow with high latency

Server has a 1Gbps uplink speed, local PC has a download speed of 100Mbps. Latency is 89ms. SFTP speed while downloading a single file from the server to the PC maxes out at 2.5MB/s, even though ...
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1answer
166 views

Haproxy Passthrough SSL and http logs?

I am running a proxy service, thus I can only use TCP passthrough, or users would get certificate warnings. Unfortunately very little is known in tcp logs, and I want to ensure no illegal activities ...
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174 views

tcpdump TCP Header offset 13

I am trying to understand how tcpdump works and trying to read TCP header control flags SYN,ACK etc. After researching online I came to know that the control flags are available at offset 13 and I ...