Managing and using network connectivity on a unix system

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148
votes
17answers
205k 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.
117
votes
3answers
75k 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 ...
100
votes
13answers
312k 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 ...
93
votes
19answers
16k views

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

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 ...
56
votes
8answers
108k 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)
54
votes
2answers
8k 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 ...
49
votes
4answers
26k 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 ?
45
votes
4answers
88k 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 ...
40
votes
5answers
75k 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 ...
39
votes
1answer
47k 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 ...
37
votes
7answers
12k views

Block network access of a process?

Is it possible to block the (outgoing) network access of a single process?
34
votes
6answers
65k views

How to limit network bandwidth?

Problem: We are hosting an application on remote server. We need to test it with a limited network bandwidth (for users with bad Internet access). Question: Can I limit my internet bandwidth? For ...
33
votes
7answers
43k views

Linux: allowing an user to listen to a port below 1024

I need to allow an user (different from root) to run a server listening on port 80. Is there any way to do this?
33
votes
6answers
33k 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 ...
31
votes
13answers
36k 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?
30
votes
5answers
24k 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 ...
30
votes
4answers
8k 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 ...
30
votes
10answers
63k 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 ...
29
votes
3answers
64k views

Can't ifdown eth0 (main interface)

I can't ifdown an interface on a Debian 6.0.5 box; user@box:/etc/network$ sudo ifdown eth0 && sudo ifup eth0 ifdown: interface eth0 not configured SIOCADDRT: File exists Failed to bring up ...
29
votes
1answer
18k 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 ...
27
votes
3answers
9k 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 ...
27
votes
2answers
23k 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 ...
26
votes
4answers
84k 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 ...
26
votes
2answers
101k 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 ...
26
votes
4answers
133k 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 ...
26
votes
4answers
22k 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 ...
26
votes
1answer
837 views

Why does it take up to several minutes to clean a listening TCP port after a program dies?

If I kill a program that is listening on a TCP port, it takes up to several minutes until the port is reclaimed by the system and usable again. I've seen several Q/A mentioning this phenomenon, but ...
25
votes
2answers
76k 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. ...
24
votes
1answer
62k 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 ...
24
votes
3answers
34k 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 the are very concrete and contains ...
24
votes
3answers
44k 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 ...
23
votes
4answers
89k 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 ...
23
votes
5answers
9k views

What is the need for rsync server in daemon mode

I don't understand the need for an rsync server in daemon mode. What are the benefits from it if I can use rsync with SSH or telnet?
22
votes
2answers
62k 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 ...
22
votes
6answers
49k 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 kernal running. On virtual box I install centos 6.4 and everything works ...
22
votes
2answers
10k views

Is there an easy way to programmatically extract IP address?

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 ...
22
votes
5answers
17k 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 ...
21
votes
3answers
33k 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?
21
votes
2answers
68k views

Orphaned connections in CLOSE_WAIT state

I've got a SLES machine that accumulates TCP connections in a CLOSE_WAIT state for what appears to be forever. These descriptors eventually suck up all available memory. At the moment, I've got 3037 ...
20
votes
6answers
9k 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 ...
20
votes
2answers
26k 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 ...
20
votes
4answers
46k 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 ...
20
votes
3answers
906 views

Limiting a specific shell's internet bandwidth usage

I want update my linux in one shell but by default wget or axel in updater use all the bandwidth. How can I limit the speed in this shell? I want other shells to have a fair share, and to limit ...
20
votes
4answers
31k views

Can I create a virtual ethernet interface named eth0?

I am running Arch based on the Linux 3.10.5-1 kernel. The system uses the new de-facto naming conventions of ethernet interfaces enp*s* and wlp* etc. This is a problem however, as my educational ...
20
votes
3answers
13k 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 ...
19
votes
3answers
8k views

Why are TCP/IP sockets considered “open files”?

I need some assistance grasping what I'm sure is a fundamental concept in Linux: the limit for open files. Specifically, I'm confused on why open sockets can count towards the total number of "open ...
19
votes
3answers
26k views

How to find what other machines are connected to the local network

How can I see a list of all machines that are available on the LAN I'm am part of.
19
votes
7answers
2k views

Why compute checksums of downloaded files?

I often see a checksum given next to a file available for download. The purpose of this practice eludes me. It is obviously to detect corrupt files, but what could be the cause of this corruption and ...
18
votes
5answers
17k views

Sending audio through network

This is a rather crazy idea. I am planing to set up a configuration that would send audio being played on my laptop to my home server via local network so that the sound is played on the server ...
17
votes
6answers
86k 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. ...