The Transport Control Protocol (TCP) is a low-level part of the protocol stack used by the Internet. It sits directly above the IP layer and is responsible for making sure that dropped packets are resent.

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php mail not working in rhel server?

I have installed roundcube mail and when it tried to use installer which uses php mail it is loading for 5 to 10 minutes and then telling me 'ok' but no mail comes to me. smtp_port is 25 host is ...
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Route incoming network requests for a given port to different applications

I have an application that listens for connections on a port, but I occasionally take it down for updates and the like. I'd like a way to configure the system such that if that program is running, ...
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Why doesn't Linux use the IANA Ephemeral port range?

According to Wikipedia The Internet Assigned Numbers Authority (IANA) suggests the range 49152 to 65535 for dynamic or private ports. Many Linux kernels use the port range 32768 to 61000. Even ...
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Books/Resources which explain TCP/IP kernel implementation [closed]

I only know about: 1) Internetworking with TCP/IP: Vol.II, Design, Implementation, and Internals 2) TCP/IP Illustrated, Vol. 2: The Implementation but these are quite dated. I am particularly ...
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How to measure the time when a packet arrives?

When the packet arrives the NIC generates an interrupt and the kernel then takes over. I want to measure the time at which a packet of a particular TCP flow arrives. How do I do this in Linux? This ...
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Why does this simple Perl client not work?

I am fairly new to IPC with sockets and expect this issue is a simple one to resolve. I'm trying to understand why I cannot get this example script from perldoc perlipc to run on my Ubuntu setup: ...
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Linux Networking routing to virtual ip addresses from a different subnet

New to linux networking, had a question about routing to virtual ip addresses from a different subnet. I have three servers (they can all ping each other): A 10.10.51.182 (connected to router ...
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6k views

Kill TIME_WAIT sockets immediately?

Is there anyway to kill those TIME_WAIT sockets immediately ? e.g output of netstat: tcp 0 0 127.0.0.1:8080 127.0.0.1:41500 TIME_WAIT - tcp 0 ...
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IPTables and transparent proxies

I have set up a local, transparent TCP proxy on localhost. I want to redirect ALL TCP traffic to this proxy, so it can handle it and nothing "leaks out," circumventing the proxy. I need to use ...
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Using in/out named pipes for a TCP connection

I've been fiddling with getting this to work for a while now, so I suspect some sort of fundamental misunderstanding about how pipes work is the root cause of my troubles. My goal is to initiate a ...
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3answers
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How can I read lines at a fixed speed?

I need to read a large log file and send it over a local network using (netbsd) netcat between two VMs on the same host workstation. I know that netcat has an interval, but as far as I can tell, the ...
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/dev/tcp not present in Linux

I am porting C/pro*c code from UNIX to Linux. The code is: #define __NFDBIT (8 * sizeof(unsigned long)) #define __FD_SETSIZ 1024 #define __FDSET_LONG (__FD_SETSIZ/__NFDBIT) typedef struct ...
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User associated with a socket

I use ss -p to see TCP sockets information. But process info are not printed for some sockets. Below: $ ss -p4 State Recv-Q Send-Q Local Address:Port Peer Address:Port ESTAB 0 0 ...
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Faster alternatives to lsof

I want to extract the process id of a certain process in order to shut it down. The process is a local web server using a certain port (localhost:3000), and I am currently doing it like this to ...
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1answer
429 views

How to limit all tcp traffic to the network interface the session was initiated on?

I have an embedded linux system with two network interfaces (lan0, wlan0) that are configured to the same subnet. A server on this system accepts tcp connections on both interfaces. How I can assure ...
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3answers
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Trying to Open a Port using CentOS

(I'm new to this!) I'm running a virtual server running CentOS. I'm trying to run a server that listens on port 37760. The code I'm running executes successfully, so I run netstat -lptu To show a ...
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1answer
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How does tcp-keepalive work in ssh?

I am trying to code a shell-script that uses a ssh-connection for doing "heartbeats". I want to terminate the client- and server-side of that connection after a certain timeout (after the connection ...
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3answers
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What is the formula to determine how much memory a socket consumes under Linux?

I'm doing some capacity planning and I was wondering if there is a formula that I could use to predict (from a memory standpoint) how many TCP connections I could handle on my server. At the moment, ...
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3k views

How to run tcptraceroute utility on UNIX machine?

How can I run the tcptraceroute utility on a UNIX machine? I want to run a TCP traceroute for port 80.
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643 views

How can I trigger a script when a certain port becomes available for requests?

I want to trigger a script when port 3000 is opened by a process. The process is a Rails development server, and I just want to be notified when it finishes spinning up and port 3000 starts accepting ...
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socat reliable file transfer over TCP

I am aware of disadvantages of "wait->stop" design, that usually is proposed with netcat: server$ cat test.dat | nc -q 10 -l -p 7878 client$ nc -w 10 remotehost 7878 > out.dat (It's not reliable ...
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minimal TCP MSS in Linux

The TCP MSS in Linux must be at least 88 (include/net/tcp.h): /* Minimal accepted MSS. It is (60+60+8) - (20+20). */ #define TCP_MIN_MSS 88U My question is: where did they come up with ...
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what level of the network stack does tcpdump get its info from?

As I was trying in vain to fix a faulty ethernet controller here, one thing I tried was running tcpdump on the machine. I found it interesting that tcpdump was able to detect that some of the ICMP ...
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2answers
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Solaris Command to Stop Listening on a Port

I have a legacy application (that I do not control) that occasionally crashes without properly closing the ports it is listening on. Is there a Solaris command to stop listening on a given port, ...
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2k views

How to do “netcat” relays properly so that I have more control?

How to use the netcat to have relays? Like following I tried but not working: cd /tmp mknod backpipe p nc -l -p 7007 0<backpipe | nc 192.168.1.101 9001 | tee backpipe I want to receive data on ...
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644 views

port redirection with PF under OpenBSD

i need to do a setting a bit unusual. I have 3 machines with public Ips. I want to use one of them as a load balancer for the 2 others. My machine >> machine1 who redirect the connection to machine ...
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SSH tunnel to router periodically hangs. TCP problem?

Something weird I've noticed since upgrading my office router to the Buffalo WZR-HP-G300NH. When I remote in using an SSH tunnel, the tunnel will "freeze" pretty often. The freeze lasts 1-2 minutes ...
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how to check rx ring, max_backlog, and max_syn_backlog size

Quite often in the course of troubleshooting and tuning things I find myself thinking about the following Linux kernel settings: net.core.netdev_max_backlog net.ipv4.tcp_max_syn_backlog ...
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How can I configure a reverse SSH connection to the connecting computer?

I'm going to be deploying a number of machines in the near future which will be behind routers. It won't be feasible to set up dynamic DNS on each router and port forwarding, so is there a way I can ...
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How long is a TCP local socket address that has been bound unavailable after closing?

On Linux (my live servers are on RHEL 5.5 - the LXR links below are to the kernel version in that), man 7 ip says: A TCP local socket address that has been bound is unavailable for some time after ...
24
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1answer
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Why are the first 1024 ports restricted to the root user only?

This is more idle curiosity than anything else. A friend of mine asked me 'which port range is it that only root can use under Linux?' I told him 0-1024 were restricted. Then he asked my why it was so ...
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What tools do I need to intercept and change an incoming packet on a WP2-Personal wifi hub

Which tools should I research into to intercept a plain text packet, edit it and then continue it on it's way. I am using Ubuntu 11.04 and Backtrack 5; my wifi is WPA2-person encrypted. I need to edit ...
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How to fake the source-port of a TCP packet?

Is there an easy way (or is it possible at all) to rewrite a TCP packet as it leaves the machine? For example if I run telnet binfalse.de 22 it will search for an unused port to leave the machine. ...
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1answer
719 views

Bind unprivileged application to privileged port on Mac OS X

I'd like to create launchd.plist file for an httpd server. Problem is - when the httpd is run as privileged user it daemonizes, which is not allowed by launchd.plist manual page. If I don't run it as ...
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3answers
510 views

Port monitoring on GNU/Linux based NAT router

I am using a Ubuntu machine as a NAT router. How can I find out the following: the ports on which the LAN machines are listening or communicating (both tcp and udp); which local machines have ...
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1answer
180 views

vnc over ssh port assignment

At the office I have on the servers a vnc server running. In Windows I use putty to connect to the server, I log in and then use realvnc to see the desktop. But I cannot get this to work on Linux. I ...
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1answer
1k views

Need further explanation on TIME_WAIT

Hi Unix/Linux Gurus out there! I need a solid proof that TIME_WAIT (a lot of it actually) is the real culprit in the slowdown in one of our servers. The server is hosted on Parallels Baremetal ...
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1answer
629 views

Select YeAH-TCP like congestion control algorithm to configure kernel

I'm configuring/compiling the 2.6.37 kernel and I want select YeAH-TCP like default congestion control algorithm but, although I enable this option (CONFIG_TCP_CONG_YEAH), it doesn't show in the ...