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

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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0answers
43 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 ...
8
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2answers
1k views

Where are my TIME_WAITs on Mac OS X?

No TIME_WAITs on Mac OS X Normally, when a TCP connection is closed, the socket on the side where close() is called first is left in the TIME_WAIT state. When one of the peers is a Mac OS X (Lion) ...
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1answer
23 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 ...
8
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4answers
9k views

tool for splitting pcap files by TCP connection?

Is there tool to split a packet capture file (in pcap format) into separate files for each TCP connection ? (other than a home grown shell script which probably needs to run twice over the capture...)...
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1answer
32 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
1k views

How to increase nginx connections

I am facing the problem of nginx max connection issues. I am not able to increase max active connections(520) made by nginx. Some time it is using whole writing (516) operation connections. and due to ...
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1answer
72 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 ...
2
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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
28 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 ...
2
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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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22 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
1k views

Squid3 CONNECT imap.gmail.com:993 TCP_DENIED:NONE

I have the following configuration under squid.conf. However, when my Blackberry email client attempts to fetch new emails, etc. Squid access log show the following error: imap.gmail.com "CONNECT ...
1
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0answers
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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3answers
5k views

Configuring keepalives with TOAD client communication

I have a scenario where long-idle sessions of TOADSoft client communication with the Oracle server seem to be torn down as they timeout on an intermediate state-full firewall. These sessions could ...
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1answer
34 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 ...
0
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1answer
44 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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0answers
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 ...
28
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8answers
35k views

Why would a server not send a SYN/ACK packet in response to a SYN packet

Lately, we've become aware of a TCP connection issue that is mostly limited to mac and Linux users who browse our websites. From the user perspective, it presents itself as a really long connection ...
2
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2answers
3k views

many different TCP timestamps through NAT device cause server to drop packets (PAWS)

Various workstations behind the client side Network Address Translation (NAT) firewall are sending timestamp information on the TCP packet to our server. The packets from many workstations arrive ...
3
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1answer
1k views

No response to some SYN packets when timestamps are enabled

I have a TCP server listening on a machine ("the server") running Ubuntu 12.04.3 (kernel 3.8.0-31-generic). It receives connections from 2 different client machines. Machine A is running Ubuntu 12.04....
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11answers
37k views

What is the difference between UDP and TCP?

My router has two protocols (and a "both" option) that I can select when setting up port forwarding: UDP and TCP. What is the difference between these two protocols and when would you select one over ...
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2answers
35 views

Bad throughput for single tcp connections

I have a server with a Gigabit Uplink, and testing with iperf3 with 100 parallel connections i get at least 600 MBit/s, depending on the other server (i tried some public test servers). But when i run ...
0
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0answers
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 ...
2
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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 ...
1
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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. ...
1
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1answer
90 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 . ...
3
votes
4answers
949 views

How can private IPV4 addresses get past iptables NAT (tcp RST,FIN)

I've got a router performing simple NAT translation using iptables iptables -t nat -o -j MASQUERADE This works fine almost all of the time except for one particular case where some TCP RST and FIN ...
7
votes
5answers
3k views

Netcat fails to start in listening mode

I'm using CentOS 6.7 (Final) system, and when I try to run nc in listening mode it prints the following: # nc -l 1234 nc: Protocol not available The port is not bound. I tried other port numbers ...
0
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1answer
49 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 ...
12
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1answer
9k views

Proxy with netcat forever

I am proxying a VNC TCP server port with netcat. The proxy machine runs linux. This is the comand I use: mkfifo backpipe nc -l 5902 0<backpipe | nc 10.1.1.116 5902 1>backpipe 10.1.1.116 is ...
0
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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) ...
0
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0answers
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 ...
42
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9answers
19k views

Windows TCP Window Scaling Hitting plateau too early

Scenario: We have a number of Windows clients regularly uploading large files (FTP/SVN/HTTP PUT/SCP) to Linux servers that are ~100-160ms away. We have 1Gbit/s synchronous bandwidth at the office and ...
0
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0answers
33 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 ...
6
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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 ...
0
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0answers
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). ...
0
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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 ...
0
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0answers
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 ...
0
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1answer
1k views

How to enable TCP Vegas on Centos 5.4?

I'm trying to enable TCP Vegas congestion control on Centos 5.4 box. According to Vegas home page it is as simple as echo 1 > /proc/sys/net/ipv4/tcp_vegas_cong_avoid but this file does not exist. I ...
0
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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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votes
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 ...
0
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0answers
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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0answers
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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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 ...
0
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1answer
54 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?
1
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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....
0
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1answer
70 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.