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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High Instances of Zero Window Messages

On my web servers I am seeing a high rate (106 over ~13 seconds or 300,000 packets) of zero window update messages sent form my web servers to my database servers during peak traffic. Firmware is ...
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VMXNET3 performance on Linux on ESX 5.0

What kind of performance can I expect from VMXNET3 adapter when running two Linux guests on the same VMware ESX private network defined between those guests? I use VMXNET3 adapter for communicating ...
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1answer
5k views

Block all incoming DNS requests EXCEPT from IPs x,y,

I want to block all incoming requests on my two DNS servers APART FROM certain IP addresses e.g. IP of 1.2.3.4 will be allowed to make requests but NOBODY else will. How do you do this with iptables? ...
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1answer
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Is ECN (Explicit Congestion Notification) turned on by default on Windows Server 2012?

In a company network I noticed that all examined Windows Server 2012 machines had ECN turned on: > netsh interface tcp show global TCP Global Parameters -------------------------------------------...
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1answer
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TCP/IP routing: accessing a LAN device with wrong static IP configuration

I forgot to change the static IP of a network device before moving it from my datacenter network to my home LAN. Since it isn't configured properly for my LAN network, I'm not sure how to access it. ...
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How to change receive queue length of a network interface in Linux?

I have a server running Linux (kernel 2.6.18) which is dropping incoming network packets drastically. I thought it is suffering from this because the length of the receive queue of that interface is ...
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2answers
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FreeBSD Listen Queue Overflows - can't increase max queue size

I have a decently high trafficked FreeBSD Nginx server, and I'm starting to get a large number of listen queue overflows: [root@svr ~]# netstat -sp tcp | fgrep listen 80361931 listen queue overflows [...
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Why Packet Loss AFTER tcpdump has logged the packet?

We encounter some strange packet loss and want to know the reason for this. We have an imageserver and a server for stressing the imageserver. Both are located in the same datacenter First we run a ...
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430 views

TCP packet filter based on TCP sequence

I need to build a test environment with a very precise packet loss based on specific TCP sequence or, optionally, on payload content. I couldn't figure out how to do it with tc or iptables. What tool ...
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2answers
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How to achieve multiple NFS/TCP connections to the same server?

I'm trying to maximize the NFS/TCP bandwidth between a Linux client and a single NFS server over a 10Gbps link. Although each NFS mount has its own device (major/minor), everything comes down to a ...
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2answers
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syn/ack sequence number confusion

I was looking at some random traffic in wireshark and came across this (using relative seq/ack numbers): 1. myIP -> 74.125.227.96 [SYN] seq=0 2. 74.125.227.96 -> myIP ...
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3answers
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Karmic Koala (Ubuntu): enable remote x clients through TCP

It seems every version of Ubuntu has a different way to enable remote connections to the local X server. I'm asking this question, while I do my own research: how can I enable the X server listening ...
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FTP client/server failing on switching to PASV mode

Update The ISP said that they recently added the area where this computer is located to their IPv6 network. Coincidentally, ever since the day they added that area to their IPv6 network, no FTP ...
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2answers
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Why is iptables not dropping packets?

I have the following rule in our iptables config file /etc/sysconfig/iptables -A INPUT -s 84.23.99.97 -j DROP And when I do iptables --list I get the following Chain INPUT (policy ACCEPT) target ...
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1answer
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Rotating outgoing IPs using iptables

I am trying to rotate outgoing IPs using iptables. I want to rotate outgoing connections between three IPs, one by one. Here is what I'm doing with iptables: root@server:~# iptables -t nat -I ...
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1answer
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Does a growing number of TIME_WAIT connections affect server performance?

I have 2 servers, both running RHEL 6, one for Apache 2.2 and one for memcached (memcached-1.4.4-3). I noticed an increase in the number of TIME_WAIT connections on the Apache server netstat -n | ...
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1answer
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option http-server-close on HAProxy haproxy-1.5-dev19 will result in Connection: close header sent in response

I basically have the following issue: whenever I add option http-server-close to the haproxy config (either in frontend or backend section), haproxy will send a Connection: close header in the ...
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2answers
746 views

Server 2012: SYN, no ACKs, on TCP ports higher than 6000

Seems ridiculous to read it back out loud, but I'm at a loss here. Server 2012 Standard. Runs a daemon for an accounting package on TCP 12502; at some point, the server stopped accepting connections ...
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1answer
480 views

Bad Mumble control channel performance in KVM guest

I'm running a Mumble server (Murmur) on a Debian Wheezy Beta 4 (x86_64) KVM guest which runs on a Debian Wheezy Beta 4 (x86_64) KVM hypervisor. The guest machines are attached to a bridge device on ...
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0answers
152 views

Inconsistency in TCP and HTTP connection management

I observe following scenario in WireShark: The problem I see here is that after receiving [FIN, ACK] from server (4090) and acknowledging it (4092), http client attempts to send another HTTP ...
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1answer
95 views

Is it possible to run 2 concurrent tcpdump with different options?

I need to run 2 concurrent tcpdump commands with different arguments/options. Why ? Because we wrote some long long scripts compatible with following options : tcpdump -ixenbr0 -s 400 -n -A 'port sip ...
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1answer
130 views

Ubuntu 14.04 TCP established hash table entries

A bit of background I am running two servers under high traffic one with ubuntu 12.04 (linux 3.2.0-69-generic) and one with ubuntu 14.04 (linux 3.13.0-52-generic). I am now trying to secure both. ...
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180 views

How can I tell when a TCP connection was created on Linux?

How can I figure out at what time a given (as in currently-open) TCP connection was created?
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3answers
144 views

SMTP server force outbound traffic

I'm having some trouble with setting up a SMTP server on a windows server 2003 machine, while trying to send emails using PHP through the internet, but to domain email addresses. Common info: ...
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Is the TCP checksum over the payload?

I was somehow convinced that the TCP checksum was a header checksum (similar to the IP checksum), but I'm now having doubts reading the Wikipedia article: The checksum field is the 16 bit one's ...
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1answer
449 views

Is tcp_keepalive application dependant on Linux?

According to http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/index.html , the kernel keepalive function occurs at the OS level, and doesn't require an application to explicitly call it. However I also read ...
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windows firewall , some ports not listening

I've added an inbound connection rule for my windows 8 firewall on windows server 2012: I've restarted the Windows Firewall. When I check that port using this program, it says that it is not ...
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46 views

What happens when connecting to port in TIME_WAIT state?

I'm trying to understand the effect of TIME_WAIT on subsequent TCP connections. I understand how TIME_WAIT behaves when net.ipv4.tcp_tw_reuse = 1, as explained by this answer. But what happens when ...
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1answer
123 views

REDSOCKS goes 'out of file descriptors' when trying to redirect all outgoing (TCP) traffic from my Linux machine

I'm trying to redirect all outgoing TCP traffic from my linux box to the remote SOCKS5 proxy server. Currently I'm using the following rule in the OUTPUT chain: sudo iptables -t nat -A OUTPUT -p tcp ...
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2answers
246 views

Is an Apache use TCP port on each connection?

Our system admin said "Apache use a TCP port on each connection" so A server can serve 65534 (MAX) simultaneously. is it Right? I think Apache use only 80 or 443. But , he use many file descriptor (...