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Questions tagged [networking]

Managing and using network connectivity on a unix system

369
votes
24answers
519k views

How can I resolve a hostname to an IP address in a Bash script?

What's the most concise way to resolve a hostname to an IP address in a Bash script? I'm using Arch Linux.
322
votes
4answers
216k views

How does reverse SSH tunneling work?

As I understand this, firewalls (assuming default settings) deny all incoming traffic that has no prior corresponding outgoing traffic. Based on Reversing an ssh connection and SSH Tunneling Made ...
256
votes
16answers
779k views

How do I set my DNS when resolv.conf is being overwritten?

Most of the info I see online says to edit /etc/resolv.conf, but any changes I make there just get overridden. $ cat /etc/resolv.conf # Dynamic resolv.conf(5) file for glibc resolver(3) generated by ...
218
votes
12answers
579k views

What DNS servers am I using?

How can I check which DNS server am I using (in Linux)? I am using network manager and a wired connection to my university's LAN. (I am trying to find out why my domain doesn't get resolved)
127
votes
2answers
408k views

ifconfig command not found

I've just installed CentOS7 as a virtual machine on my mac (osx10.9.3 + virtualbox) .Running ifconfig returns command not found. Also running sudo /sbin/ifconfig returns commmand not found. I am root. ...
125
votes
2answers
135k views

Good detailed explanation of /etc/network/interfaces syntax?

I understood the very basic concept of how to use /etc/network/interfaces, but all I find online are examples, example after example, which I can copy-paste from. What I miss is an explanation of the ...
117
votes
4answers
239k views

How to make a machine accessible from the LAN using its hostname

Here are details of the machine I want to access using its hostname: $ hostname hostname $ cat /etc/hosts 127.0.0.1 localhost 127.0.1.1 hostname.company.local hostname It's a default Debian 6 (...
109
votes
20answers
24k views

What is the fastest way to send massive amounts of data between two computers? [closed]

This is a situation I am frequently in: I have a source server with a 320GB hard-drive inside of it, and 16GB of ram (exact specs available here, but as this is an issue I run into frequently on ...
86
votes
5answers
161k views

Determining what process is bound to a port

I know that using the command: lsof -i TCP (or some variant of parameters with lsof) I can determine which process is bound to a particular port. This is useful say if I'm trying to start ...
83
votes
3answers
62k views

Throttle the download speed of wget or curl while downloading

Is it possible to throttle (limit) the download speed of wget or curl ? Is it possible to change the throttle value while it is downloading ?
77
votes
4answers
306k views

How can I find available network interfaces?

This is in regard to linux, but if anyone knows of a general *nix method that would be good. I booted a system yesterday with an ethernet cable plugged in. "NetworkManager" is not installed, so once ...
76
votes
6answers
102k views

On-the-fly monitoring HTTP requests on a network interface?

For debugging purposes I want to monitor the http requests on a network interface. Using a naive tcpdump command line I get too much low-level information and the information I need is not very ...
71
votes
8answers
427k views

Connecting to wifi network through command line

I am trying to connect to my WEP network just using the command-line (Linux). I run: sudo iwconfig wlan0 mode Managed essid 'my_network' key 'xx:xx:... hex key, 26 digits' Then I try to obtain an ...
70
votes
7answers
97k views

Simple way to create a tunnel from one local port to another?

I have a development server, which is only accessible from 127.0.0.1:8000, not 192.168.1.x:8000. As a quick hack, is there a way to set up something to listen on another port (say, 8001) so that from ...
70
votes
6answers
148k views

How to limit network bandwidth?

We are hosting an application on remote server. We need to test it with a limited network bandwidth (for users with bad Internet access). Can I limit my internet bandwidth? For instance: 128 KB per ...
70
votes
3answers
98k views

Linux network troubleshooting and debugging

From time to time Linux and Unix users faced with various network problems. Many of these problems are presented here and at some others troubleshooting forums, but they are very concrete and contains ...
69
votes
10answers
35k views

Block network access of a process?

Is it possible to block the (outgoing) network access of a single process?
65
votes
7answers
266k views

How can I monitor all outgoing requests/connections from my machine?

My machine is a server so I want to ignore connections being made to my server (e.g. when someone visits my website). I want to see only connections/requests being made by my server to other places. ...
63
votes
2answers
294k views

Sending messages to another user

Is there any command to send messages through the Linux shell to other people on the same network? I'm using write user and then write the message itself. But there's any command that doesn't show my ...
63
votes
2answers
12k views

How does SSH connection survive a network restart?

From a Linux SSH shell, type /etc/init.d/network restart to restart the network service. I expect my SSH connection to die since the network service goes down. But it doesn't. Very cool. But how does ...
61
votes
2answers
133k views

How can I create a virtual ethernet interface on a machine without a physical adapter?

I have a Dell XPS 13 ultrabook which has a wifi nic, but no physical ethernet nic (wlan0, but no eth0). I need to create a virtual adapter for using Vagrant with NFS, but am finding that the typical ...
60
votes
20answers
108k views

How to get my own IP address and save it to a variable in a shell script?

How can I get my own IP address and save it to a variable in a shell script?
59
votes
6answers
102k views

Allowing a user to let listen to a port below 1024

I need to allow a user (different from root) to run a server listening on port 80. Is there any way to do this?
57
votes
3answers
34k views

Is it possible to connect to TCP port 0?

Listening to TCP port 0 allocates me a free port number on the system. But what happens when I try to connect to TCP port 0? The obvious answer is: "It doesn't work": $ nc localhost 0 ...
52
votes
1answer
124k views

Using ip addr instead of ifconfig reports “RTNETLINK answers: File exists” on Debian

I have a Debian system working as a wireless router with eth0 and wlan0. Now I added an additional network manually on eth1 with ifconfig: alix:~# ifconfig eth1 192.168.0.2 netmask 255.255.255.0 alix:...
51
votes
3answers
93k views

what is the meaning of 0.0.0.0 as a gateway?

Can anyone clarify gateway assignment for me? What is the difference between adding gateway as 0.0.0.0 and assigning a specific ip address as a gateway?
51
votes
1answer
44k views

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 ...
50
votes
2answers
48k views

Why is my ethernet interface called enp0s10 instead of eth0?

When I run ifconfig -a, I only get lo and enp0s10 interfaces, not the classical eth0 What does enp0s10 mean? Why is there no eth0?
50
votes
4answers
23k views

Can TCP provide more than 65535 ports?

Is it possible to setup a Linux system so that it provides more than 65,535 ports? The intent would be to have more than 65k daemons listening on a given system. Clearly there are ports being used ...
50
votes
10answers
118k views

CentOS no network interface after installation in VirtualBox

I have Ubuntu 12.10 on which I installed virtual box on which I setup Centos 6.4 i386. On Ubuntu I have: 3.5.0-17-generic kernel running. On virtual box I install Centos 6.4 and everything works fine,...
50
votes
4answers
268k views

How do I know what service is running on a particular port in linux? [duplicate]

I am trying to run weblogic server on my linux machine and I am getting the following error : ERROR: transport error 202: bind failed: Address already in use ERROR: JDWP Transport dt_socket failed to ...
49
votes
14answers
49k views

What's the easiest way to find an unused local port?

What's the easiest way to find an unused local port? Currently I'm using something similar to this: port=$RANDOM quit=0 while [ "$quit" -ne 1 ]; do netstat -a | grep $port >> /dev/null if ...
49
votes
6answers
178k views

Can't ifdown eth0 (main interface)

I can't ifdown an interface on Debian 6.0.5: user@box:/etc/network$ sudo ifdown eth0 && sudo ifup eth0 ifdown: interface eth0 not configured SIOCADDRT: File exists Failed to bring up eth0. ...
46
votes
4answers
89k views

VirtualBox: two network interfaces (NAT and host-only ones) in a Debian guest on Ubuntu

I created a Debian VM on VirtualBox with two interfaces: a NAT one (for accessing internet) and a host-only one. However, I do not know how to make both interfaces work at the same time. If I the ...
45
votes
5answers
157k views

Bring down and delete bridge interface that's up

How do I remove a bridge that has an IP address that was brought up manually and isn't in /etc/network/interfaces? $ ifconfig br100 br100 Link ...
42
votes
7answers
34k views

What is a socket?

Could someone explain to me what a socket is? I see it in many acronyms in context of SSL, etc. Also, why is it called a socket? Is it purely because it was what a name they invented? Or was it the ...
42
votes
5answers
41k views

How can I limit the bandwidth used by a process?

I have a CentOS 5.7 server that will be backing up its files nightly. I am concerned that visitors to the various sites that the server hosts will experience degraded performance while the backup is ...
42
votes
4answers
178k views

Why the default eth0 interface is down by default on CentOS?

I am already a little bit familiar with Linux distros like Debian or Ubuntu (yeah, very similar) but I wanted to try Red Hat based - CentOS 6.2 . I have installed it on my Windows 7 host in virtualBox ...
42
votes
2answers
50k views

How to identify a process which has no pid?

I have a process which listen to 2 ports : 45136/tcp and 37208/udp (actually I assume it is the same process). But netstat doesn't return any pid : netstat -antlp | grep 45136 tcp 0 0 0.0....
40
votes
3answers
46k views

Is there an easy way to programmatically extract IP address? [duplicate]

Is there an easy way to programmatically extract IP address, without tedious parsing of ifconfig? I would not mind simple command output processing using sed to do it but not processing multiline ...
39
votes
2answers
38k views

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 net.core....
38
votes
6answers
158k views

History of IP addresses that accesed a server via ssh

It has come to my attention that a server of mine has been hacked and infected with a known chinese botnet. It was a prototype/testing virtual machine with its own static IP(US address) so no harm ...
37
votes
1answer
7k views

Connecting to IP 0.0.0.0 succeeds. How? Why?

We are serving a port on localhost and want to check in another process if the port is available. Due to a bug in our code, it is actually trying to connect to the IP 0.0.0.0:<port>, and for ...
33
votes
7answers
49k views

How to recover a shell after a disconnection

Am doing some work on a remote CentOS 5.6 machine and my network keeps dropping. Is there a way that I can recover my hung sessions after I reconnect? EDIT: am doing some updating and installing with ...
32
votes
3answers
123k views

How to create a TCP listener?

Introduction: I have created a bash function that is able to check whether a port is available and increments it by 1 if false until a certain maximum port number. E.g., if port 500 is unavailable ...
32
votes
1answer
62k views

What -A INPUT -j REJECT --reject-with icmp-host-prohibited Iptables line does exactly?

I have been reading redhat iptables documentation but can't figure out what do the following lines do: ... -j REJECT **--reject-with icmp-host-prohibited** ... -j REJECT **--reject-with icmp-host-...
31
votes
3answers
31k views

How do I tell a script to wait for a process to start accepting requests on a port?

I need a command that will wait for a process to start accepting requests on a specific port. Is there something in linux that does that? while (checkAlive -host localhost -port 13000 == false) ...
31
votes
4answers
40k views

How to speed up X over SSH on a slow network connection?

Are there any specific recommendations on speeding up X applications over ssh on a slow network connection? In this specific case, I am accessing a server located in west coast from a laptop in east ...
31
votes
3answers
34k views

Limit outgoing bandwidth on an specific interface

I would like to know the easiest way to limit outgoing bandwidth for an interface. I want to cap the outgoing port speed so that file transfers between my servers don't fail during busy hours. What's ...
31
votes
10answers
72k views

Streaming to an Apple TV (3) from Linux

Alright, here is the setup: One apple tv 3 in the living room, connected to the network (wired). One pc running Linux (Arch Linux x64, AMD), containing all music, movies and series (wired). Now I ...