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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why linux reuse 'time_wait' port?

As I know, tcp port in 'time_wait' stat cann't be used. However, in my experiment, server reuses the 'time_wait' port? Why? Firstly, in client machine, type command ehco 40000 40001 > /proc/sys/...
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11 views

SYN/ACK dropped by Nexus 7000 series

We have a Cisco Nexus 7000 series switch sitting behind a Palo Alto firewall that does not accept the SYN/ACK packet sent back from LDAP connection. Any other packets are processed just fine. When ...
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18 views

Two identical SYN packets sent to two different ips when connecting to port 80 on my VPS [on hold]

I can't connect to my VPS (IP = V) from my home machine (IP = H); I can connect easilly from other IPs (office for example); Here is what I see when I do a TCPdump: 14:45:54.686642 IP H.49558 > V....
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64 views

Higher TCP latencies in the latest versions of Linux

In my research group, we recently upgraded the OS on our machines from Red Hat 6.2 to Debian 8.3 and observed that the TCP round trip time through the integrated Intel 1G NICs between our machines had ...
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1answer
26 views

WIndows is not following TcpAckFrequency settings

This is a wierd one. I have set the tcpackfrequency to 2 and windows does not seem to honor it no matter what I do. I would need your help with this. My current template is Internet and congestion ...
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1answer
35 views

How to block 111 udp port via iptables?

I've got an issue related to block 111 port only for udp. For tcp it was blocked without any problem. This port is used by application rpcbind. I test it via netcat. This is my iptables. It contains ...
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1answer
75 views

tcp source port is not randomly

When we trying to get a file from a web-server, sometimes access is not possible. After checking the communication with tcpdump, we saw that the Source-machine is randomly choosing a source-port for ...
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1answer
11 views

What happens when a Server sends a data packet to the Client when the later (Client) is on TCP FIN-WAIT-1 state?

According to TCP/IP Guide, when a TCP session is closed by the Client, it will send a packet with FIN set to the Server and transition to FIN-WAIT-1, however it says: In this state the client can ...
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1answer
29 views

Unable to establish network connection between two servers

I have 7 Ubuntu 14.04.4 servers running in EC2. One server is hosting memcached (port 11211) and the other 6 are clients. Of the 6 clients, 5 are able to connect and one is not (see note). I've ...
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23 views

Troubleshooting connection timeouts and SYN_RECV

My DigitalOcean droplet fails very often to connect to different resources, like making a REST call with curl or establish SSH connection. Every time it happens, I can see sockets with SYN_RECV state ...
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1answer
52 views

Why isn't net.ipv4.tcp_rfc1337 enabled by default?

The tcp_rfc1337 setting seems to have a solution for TIME-WAIT Assassination. The first problem is that old duplicate data may be accepted erroneously in new connections, leading to the sent data ...
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20 views

Cannot connect to anything above port 6000

I have an instance of Windows Server 2012 R2 running on Amazon Web Services. I am not able to make a TCP connection to the server on any port higher than 6000 for some reason. All ports below 6000 ...
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1answer
36 views

Run shell script on the event of “possible SYN flooding”

Since it doesn't appear to be possible to get "netstat" stats at the time I got an SYN flooding, I'd like to write a script that gets all the stats I need (top IPs, used memory, etc) and write to a ...
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1answer
46 views

Handle “possible SYN flooding on port 443. Sending cookies.”

My website constantly has several users online all the time. The server uses Apache/PHP, database and Memcached. On normal use, the application works well and fast. However, the server appears to ...
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14 views

Is a tcp port number somehow used as a pointer to memory? [migrated]

I'm trying to understand ports on a low-level. This isn't a use-case specific question (off-topic) but I tried google with no luck so I'm posting here. Ports are just numbers used to access a ...
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27 views

Redirect TCP traffic from interface wlan0 (AP mode) to localhost:8080?

Here I want to redirect all traffic heading to all_IP:80 from wlan0 ( AP mode, as a hotspot, in a router) to my local TCP listener, listening on localhost:8080 OS : debian I used these command to do ...
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1answer
29 views

TCP retransmission delays - lost acks

Maybe somebody will be able to help me out with this. I'm trying to find out if there is anything that can be optimized server-side to reduce delays in case of packet loss. Environment: Windows 2012 ...
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0answers
10 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? [migrated]

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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1answer
55 views

Close all _existing_ TCP connections on some ports using IPTables?

Say I have a quick network partition test that I'd like to run, like disconnecting two halves of a ReDiS cluster from each other, and I want to use IPTables to temporarily disconnect one group of ...
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1answer
91 views

Identify the ip to which an UDP port established connection

I'm trying to obtain the ip to which Rocketleague.exe has established connection on a UDP port. I've tried with netstat -p udp but for some strange reason it does not show any active udp connections . ...
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1answer
16 views

Syslog-ng can close connection

I have a server that collect secure/auth logs in my datacenter from all other server, it runs syslog-ng 3.2.5. It's configured to receive tcp connection, here is the conf: tcp(ip(0.0.0.0) port(514) ...
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29 views

initial connections refused - connection warm up?

I've got two firewalled VMs connected to the same vLAN but I see RST/ACK responses on initial connection attempts in a while only to succeed on immediate following attempts, like some sort of cache ...
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1answer
51 views

SYN flooding on port 443 while nginx reload

While reloading nginx, I started getting errors in messages log "possible SYN flooding on port 443", and it seems like nginx becomes completely irresponsive at that time (quite for a while), cause ...
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35 views

Alghough TCP SYN Backlog size is set to 128 but in practical it is limited to 16

as you know in ubuntu, tcp syn backlog size is 128 by default. i tried to syn flood my ubuntu linux by hping3. to do that, i send 100 spoofed tcp syn packets per second to the ubuntu and meanwhile i ...
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50 views

Duplicating and combining transmissions over TCP in Windows

I have a program, on socket S1, which I would like to send a message over TCP to socket, S2, the same message is then somehow sent to two my devices on S3 and S4 (they both listen to unique sockets). ...
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2answers
247 views

Can Squid be used as “TLS termination proxy” to encrypt TCP connections using client certificates?

Abstract I need an encrypted TCP connection from multiple clients to a single port over the internet. Can this be realized with Squid? Concrete situation We use a monitoring and client management ...
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1answer
31 views

Establishing TCP Connections by hping3

when a client sends 10 TCP SYN packets by hping3(not spoofing), server responds and sends 10 TCP SYN+ACK packets. now client should answer to server and Establish the connection by sending 10 ACK ...
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1answer
46 views

Need help in understanding the packet analysis(wireshark) [closed]

The snapshot capture below contains a single HTTP request to a web server, in which the client web browser requests some files from server, and the server returns an HTTP/1.1 200 (OK) response which ...
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1answer
124 views

Any tool for forking/teeing/sharing a TCP connection to the remote server? [closed]

Could down voter please post your reason, as it's not obvious to me? Here's my problem. I'm building an automation program that talks to a remote server (think of it as a chat server or MUD game ...
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16 views

What is the default value for TCP accept queue length on Linux?

I have read this article but it doesn't mention where the default accept queue length can be found (it does mention tcp_max_syn_backlog though, but it's different). I'm working on a Java 8 project ...
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27 views

Large payload - connection get's Hung with CLOSE_WAIT or LAST_ACK state

We have a webservice client which posts data to a 3rd party Webservice. We are having an issue when the payload is a bit large (Around 600KB) xml. Once we pay the process hangs forever. The Netstat -...
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18 views

Infinite loop of RST/ACK and ACK

I have web servers (Apache-Tomcat) at IDC to serve https requests from Android and iOS clients. Suddenly I noticed traffic to web servers was dramatically increased (10Mbps to 80Mbps). So I tried to ...
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2answers
42 views

Wireshark Packet Capture Data Data ACK Confusion

I understand how acks work and windowing works. What I am not getting is why am I seeing the following behavior in packet captures Client Server data1-----> data2-----> <--------ack ...
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1answer
56 views

How to setup linux to retransmit TCP packets by reducing the packet size like OS X

On OS X I noticed that for packets which are not ACKed, the system retransmits them with reduced packet size. This can be seen in the image below. Is it possible to enable the same behavior in ...
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1answer
14 views

Time until a TCP socket closes if the connection is lost

How long will a TCP socket remain open if there's a loss of a network connection between the source and the destination?
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34 views

Why sar ETCP shows TCP retransmission but wireshark tcp.analysis.retransmission doesn't

I am trying to investigate the cause of TCP retransmission. When I use sadc and sar, it shows there are tcp retransmission; but when I use wireshark on all of the server's interfaces, it doesn't show ...
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1answer
73 views

What's the default TCP window size of iperf3?

Does anyone know what's the default TCP window size for iperf3? It is not mentioned in the manual and I also can't find it in the output, when running a test.
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25 views

Can't transmit from multiple virtual vlan id interfaces on same interface

I'm creating virtual interfaces of vlan id on same interface, i.e: eth0 - the main interface with ip 0.0.0.0 . eth0.1 - tagged interface under eth0 with ip 10.0.0.1 . eth0.2 - tagged interface ...
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2answers
87 views

How does TCP offload NIC cause the TCP checksum to be invalid?

A tcpdump pcap exported and being investigated on another machine with wireshark is showing a lot of invalid TCP checksum messages. This is a known and documented phenomenon when using TCP offload ...
0
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1answer
52 views

Linux server failing to send SYN/ACK after receiving SYN remotely, works locally

Problem: A server process can be accessed by a device on the LAN or by the server itself, but it cannot be accessed outside the LAN (port forwarding is configured correctly as the server does receive ...
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1answer
60 views

How to set the maximum TCP receive window size in Linux?

I want to limit the rate of every TCP connection. Can I set the maximum TCP receive window size in Linux? With iptables + tc can only limit IP source. The parameters net.core.rmem_max and net.core....
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2answers
39 views

Count number of incoming connection on a port - Linux

We have a server which listens on port X. The server has a large number of clients, from time to time the process gets hung, I am seeing SYN flooding messages in the log. I have been trying to tune ...
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1answer
57 views

TCP connection RST after FIN, ACK

I have a situation that would like to clarify with the experts here. I am no network expert so maybe it's normal, but i rather ask. We are trying to diagnose a problem between two servers, both are ...
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3answers
68 views

Do clients typically implement failover/load-balancing on multiple A records?

Typically, load balancers like Amazon's Elastic Load Balancers use a DNS record set with multiple A records to provide multiple load balancer instances which can handle traffic to requesting endpoints:...
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71 views

multiplexing a TCP connection through a dumb shell connection?

I am connecting to a VPN whose gateway is sufficiently misconfigured that basically the only useful thing I can do with it is to ssh to a specific Linux host set up for my usage inside the VPN, and ...
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25 views

stunnel keeps stopping with WSAECONNREFUSED given

Right so bit of background to start with I've got 2 stunnel installs on 2 different servers and basically using ensemble and stunnel to bounce a message around with ssl cert. Here is the flow. Where ...
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54 views

TCP Balancer active/standby active/passive

I have two Servers, Server A,ServerB. running as TCP socket server. I want to a active/standby or /active/passive failover. that means: 1:Server A and Server B Running, route ALL connection to ...
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166 views

Nginx log to syslog on TCP port

I need to redirect Nginx access and error logs to a remote syslog server. From http://nginx.org/en/docs/syslog.html I saw that I can do: error_log syslog:server=192.168.1.1; However I need to ...
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1answer
86 views

TcpTimedWaitDelay is missing in Windows Server 2012

Suddenly my server log shows: An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full And I googled and found these links as ...
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1answer
43 views

PCI Scan failing at TCP Source Port Pass Firewall testing

I'm trying to get PCI scan PASS for my website, Scan failing at this step. The host responded 4 times to 4 TCP SYN probes sent to destination port 20 using source port 80. However, it did not ...