Posts Tagged ‘DHCP’

How to filter dhcp traffic between two networks running separate DHCP servers to prevent IP assignment issues and MAC duplicate addresses

Tuesday, February 8th, 2022

how-to-filter-dhcp-traffic-2-networks-running-2-separate-dhcpd-servers-to-prevent-ip-assignment-conflicts-linux
Tracking the Problem of MAC duplicates on Linux routers
 

If you have two networks that see each other and they're not separated in VLANs but see each other sharing a common netmask lets say 255.255.254.0 or 255.255.252.0, it might happend that there are 2 dhcp servers for example (isc-dhcp-server running on 192.168.1.1 and dhcpd running on 192.168.0.1 can broadcast their services to both LANs 192.168.1.0.1/24 (netmask 255.255.255.0) and Local Net LAN 192.168.1.1/24. The result out of this is that some devices might pick up their IP address via DHCP from the wrong dhcp server.

Normally if you have a fully controlled little or middle class home or office network (10 – 15 electronic devices nodes) connecting to the LAN in a mixed moth some are connected via one of the Networks via connected Wifi to 192.168.1.0/22 others are LANned and using static IP adddresses and traffic is routed among two ISPs and each network can see the other network, there is always a possibility of things to go wrong. This is what happened to me so this is how this post was born.

The best practice from my experience so far is to define each and every computer / phone / laptop host joining the network and hence later easily monitor what is going on the network with something like iptraf-ng / nethogs  / iperf – described in prior  how to check internet spepeed from console and in check server internet connectivity speed with speedtest-cliiftop / nload or for more complex stuff wireshark or even a simple tcpdump. No matter the tools network monitoring is only part on solving network issues. A very must have thing in a controlled network infrastructure is defining every machine part of it to easily monitor later with the monitoring tools. Defining each and every host on the Hybrid computer networks makes administering the network much easier task and  tracking irregularities on time is much more likely. 

Since I have such a hybrid network here hosting a couple of XEN virtual machines with Linux, Windows 7 and Windows 10, together with Mac OS X laptops as well as MacBook Air notebooks, I have followed this route and tried to define each and every host based on its MAC address to pick it up from the correct DHCP1 server  192.168.1.1 (that is distributing IPs for Internet Provider 1 (ISP 1), that is mostly few computers attached UTP LAN cables via LiteWave LS105G Gigabit Switch as well from DHCP2 – used only to assigns IPs to servers and a a single Wi-Fi Access point configured to route incoming clients via 192.168.0.1 Linux NAT gateway server.

To filter out the unwanted IPs from the DHCPD not to propagate I've so far used a little trick to  Deny DHCP MAC Address for unwanted clients and not send IP offer for them.

To give you more understanding,  I have to clear it up I don't want to have automatic IP assignments from DHCP2 / LAN2 to DHCP1 / LAN1 because (i don't want machines on DHCP1 to end up with IP like 192.168.0.50 or DHCP2 (to have 192.168.1.80), as such a wrong IP delegation could potentially lead to MAC duplicates IP conflicts. MAC Duplicate IP wrong assignments for those older or who have been part of administrating large ISP network infrastructures  makes the network communication unstable for no apparent reason and nodes partially unreachable at times or full time …

However it seems in the 21-st century which is the century of strangeness / computer madness in the 2022, technology advanced so much that it has massively started to break up some good old well known sysadmin standards well documented in the RFCs I know of my youth, such as that every electronic equipment manufactured Vendor should have a Vendor Assigned Hardware MAC Address binded to it that will never change (after all that was the idea of MAC addresses wasn't it !). 
Many mobile devices nowadays however, in the developers attempts to make more sophisticated software and Increase Anonimity on the Net and Security, use a technique called  MAC Address randomization (mostly used by hackers / script kiddies of the early days of computers) for their Wi-Fi Net Adapter OS / driver controlled interfaces for the sake of increased security (the so called Private WiFi Addresses). If a sysadmin 10-15 years ago has seen that he might probably resign his profession and turn to farming or agriculture plant growing, but in the age of digitalization and "cloud computing", this break up of common developed network standards starts to become the 'new normal' standard.

I did not suspected there might be a MAC address oddities, since I spare very little time on administering the the network. This was so till recently when I accidently checked the arp table with:

Hypervisor:~# arp -an
192.168.1.99     5c:89:b5:f2:e8:d8      (Unknown)
192.168.1.99    00:15:3e:d3:8f:76       (Unknown)

..


and consequently did a network MAC Address ARP Scan with arp-scan (if you never used this little nifty hacker tool I warmly recommend it !!!)
If you don't have it installed it is available in debian based linuces from default repos to install

Hypervisor:~# apt-get install –yes arp-scan


It is also available on CentOS / Fedora / Redhat and other RPM distros via:

Hypervisor:~# yum install -y arp-scan

 

 

Hypervisor:~# arp-scan –interface=eth1 192.168.1.0/24

192.168.1.19    00:16:3e:0f:48:05       Xensource, Inc.
192.168.1.22    00:16:3e:04:11:1c       Xensource, Inc.
192.168.1.31    00:15:3e:bb:45:45       Xensource, Inc.
192.168.1.38    00:15:3e:59:96:8e       Xensource, Inc.
192.168.1.34    00:15:3e:d3:8f:77       Xensource, Inc.
192.168.1.60    8c:89:b5:f2:e8:d8       Micro-Star INT'L CO., LTD
192.168.1.99     5c:89:b5:f2:e8:d8      (Unknown)
192.168.1.99    00:15:3e:d3:8f:76       (Unknown)

192.168.x.91     02:a0:xx:xx:d6:64        (Unknown)
192.168.x.91     02:a0:xx:xx:d6:64        (Unknown)  (DUP: 2)

N.B. !. I found it helpful to check all available interfaces on my Linux NAT router host.

As you see the scan revealed, a whole bunch of MAC address mess duplicated MAC hanging around, destroying my network topology every now and then 
So far so good, the MAC duplicates and strangely hanging around MAC addresses issue, was solved relatively easily with enabling below set of systctl kernel variables.
 

1. Fixing Linux ARP common well known Problems through disabling arp_announce / arp_ignore / send_redirects kernel variables disablement

 

Linux answers ARP requests on wrong and unassociated interfaces per default. This leads to the following two problems:

ARP requests for the loopback alias address are answered on the HW interfaces (even if NOARP on lo0:1 is set). Since loopback aliases are required for DSR (Direct Server Return) setups this problem is very common (but easy to fix fortunately).

If the machine is connected twice to the same switch (e.g. with eth0 and eth1) eth2 may answer ARP requests for the address on eth1 and vice versa in a race condition manner (confusing almost everything).

This can be prevented by specific arp kernel settings. Take a look here for additional information about the nature of the problem (and other solutions): ARP flux.

To fix that generally (and reboot safe) we  include the following lines into

 

Hypervisor:~# cp -rpf /etc/sysctl.conf /etc/sysctl.conf_bak_07-feb-2022
Hypervisor:~# cat >> /etc/sysctl.conf

# LVS tuning
net.ipv4.conf.lo.arp_ignore=1
net.ipv4.conf.lo.arp_announce=2
net.ipv4.conf.all.arp_ignore=1
net.ipv4.conf.all.arp_announce=2

net.ipv4.conf.all.send_redirects=0
net.ipv4.conf.eth0.send_redirects=0
net.ipv4.conf.eth1.send_redirects=0
net.ipv4.conf.default.send_redirects=0

Press CTRL + D simultaneusly to Write out up-pasted vars.


To read more on Load Balancer using direct routing and on LVS and the arp problem here


2. Digging further the IP conflict / dulicate MAC Problems

Even after this arp tunings (because I do have my Hypervisor 2 LAN interfaces connected to 1 switch) did not resolved the issues and still my Wireless Connected devices via network 192.168.1.1/24 (ISP2) were randomly assigned the wrong range IPs 192.168.0.XXX/24 as well as the wrong gateway 192.168.0.1 (ISP1).
After thinking thoroughfully for hours and checking the network status with various tools and thanks to the fact that my wife has a MacBook Air that was always complaining that the IP it tried to assign from the DHCP was already taken, i"ve realized, something is wrong with DHCP assignment.
Since she owns a IPhone 10 with iOS and this two devices are from the same vendor e.g. Apple Inc. And Apple's products have been having strange DHCP assignment issues from my experience for quite some time, I've thought initially problems are caused by software on Apple's devices.
I turned to be partially right after expecting the logs of DHCP server on the Linux host (ISP1) finding that the phone of my wife takes IP in 192.168.0.XXX, insetad of IP from 192.168.1.1 (which has is a combined Nokia Router with 2.4Ghz and 5Ghz Wi-Fi and LAN router provided by ISP2 in that case Vivacom). That was really puzzling since for me it was completely logical thta the iDevices must check for DHCP address directly on the Network of the router to whom, they're connecting. Guess my suprise when I realized that instead of that the iDevices does listen to the network on a wide network range scan for any DHCPs reachable baesd on the advertised (i assume via broadcast) address traffic and try to connect and take the IP to the IP of the DHCP which responds faster !!!! Of course the Vivacom Chineese produced Nokia router responded DHCP requests and advertised much slower, than my Linux NAT gateway on ISP1 and because of that the Iphone and iOS and even freshest versions of Android devices do take the IP from the DHCP that responds faster, even if that router is not on a C class network (that's invasive isn't it??). What was even more puzzling was the automatic MAC Randomization of Wifi devices trying to connect to my ISP1 configured DHCPD and this of course trespassed any static MAC addresses filtering, I already had established there.

Anyways there was also a good think out of tthat intermixed exercise 🙂 While playing around with the Gigabit network router of vivacom I found a cozy feature SCHEDULEDING TURNING OFF and ON the WIFI ACCESS POINT  – a very useful feature to adopt, to stop wasting extra energy and lower a bit of radiation is to set a swtich off WIFI AP from 12:30 – 06:30 which are the common sleeping hours or something like that.
 

3. What is MAC Randomization and where and how it is configured across different main operating systems as of year 2022?

Depending on the operating system of your device, MAC randomization will be available either by default on most modern mobile OSes or with possibility to have it switched on:

  • Android Q: Enabled by default 
  • Android P: Available as a developer option, disabled by default
  • iOS 14: Available as a user option, disabled by default
  • Windows 10: Available as an option in two ways – random for all networks or random for a specific network

Lately I don't have much time to play around with mobile devices, and I do not my own a luxury mobile phone so, the fact this ne Androids have this MAC randomization was unknown to me just until I ended a small mess, based on my poor configured networks due to my tight time constrains nowadays.

Finding out about the new security feature of MAC Randomization, on all Android based phones (my mother's Nokia smartphone and my dad's phone, disabled the feature ASAP:


4. Disable MAC Wi-Fi Ethernet device Randomization on Android

MAC Randomization creates a random MAC address when joining a Wi-Fi network for the first time or after “forgetting” and rejoining a Wi-Fi network. It Generates a new random MAC address after 24 hours of last connection.

Disabling MAC Randomization on your devices. It is done on a per SSID basis so you can turn off the randomization, but allow it to function for hotspots outside of your home.

  1. Open the Settings app
  2. Select Network and Internet
  3. Select WiFi
  4. Connect to your home wireless network
  5. Tap the gear icon next to the current WiFi connection
  6. Select Advanced
  7. Select Privacy
  8. Select "Use device MAC"
     

5. Disabling MAC Randomization on MAC iOS, iPhone, iPad, iPod

To Disable MAC Randomization on iOS Devices:

Open the Settings on your iPhone, iPad, or iPod, then tap Wi-Fi or WLAN

 

  1. Tap the information button next to your network
  2. Turn off Private Address
  3. Re-join the network


Of course next I've collected their phone Wi-Fi adapters and made sure the included dhcp MAC deny rules in /etc/dhcp/dhcpd.conf are at place.

The effect of the MAC Randomization for my Network was terrible constant and strange issues with my routings and networks, which I always thought are caused by the openxen hypervisor Virtualization VM bugs etc.

That continued for some months now, and the weird thing was the issues always started when I tried to update my Operating system to the latest packetset, do a reboot to load up the new piece of software / libraries etc. and plus it happened very occasionally and their was no obvious reason for it.

 

6. How to completely filter dhcp traffic between two network router hosts
IP 192.168.0.1 / 192.168.1.1 to stop 2 or more configured DHCP servers
on separate networks see each other

To prevent IP mess at DHCP2 server side (which btw is ISC DHCP server, taking care for IP assignment only for the Servers on the network running on Debian 11 Linux), further on I had to filter out any DHCP UDP traffic with iptables completely.
To prevent incorrect route assignments assuming that you have 2 networks and 2 routers that are configurred to do Network Address Translation (NAT)-ing Router 1: 192.168.0.1, Router 2: 192.168.1.1.

You have to filter out UDP Protocol data on Port 67 and 68 from the respective source and destination addresses.

In firewall rules configuration files on your Linux you need to have some rules as:

# filter outgoing dhcp traffic from 192.168.1.1 to 192.168.0.1
-A INPUT -p udp -m udp –dport 67:68 -s 192.168.1.1 -d 192.168.0.1 -j DROP
-A OUTPUT -p udp -m udp –dport 67:68 -s 192.168.1.1 -d 192.168.0.1 -j DROP
-A FORWARD -p udp -m udp –dport 67:68 -s 192.168.1.1 -d 192.168.0.1 -j DROP

-A INPUT -p udp -m udp –dport 67:68 -s 192.168.0.1 -d 192.168.1.1 -j DROP
-A OUTPUT -p udp -m udp –dport 67:68 -s 192.168.0.1 -d 192.168.1.1 -j DROP
-A FORWARD -p udp -m udp –dport 67:68 -s 192.168.0.1 -d 192.168.1.1 -j DROP

-A INPUT -p udp -m udp –sport 67:68 -s 192.168.1.1 -d 192.168.0.1 -j DROP
-A OUTPUT -p udp -m udp –sport 67:68 -s 192.168.1.1 -d 192.168.0.1 -j DROP
-A FORWARD -p udp -m udp –sport 67:68 -s 192.168.1.1 -d 192.168.0.1 -j DROP


You can download also filter_dhcp_traffic.sh with above rules from here


Applying this rules, any traffic of DHCP between 2 routers is prohibited and devices from Net: 192.168.1.1-255 will no longer wrongly get assinged IP addresses from Network range: 192.168.0.1-255 as it happened to me.


7. Filter out DHCP traffic based on MAC completely on Linux with arptables

If even after disabling MAC randomization on all devices on the network, and you know physically all the connecting devices on the Network, if you still see some weird MAC addresses, originating from a wrongly configured ISP traffic router host or whatever, then it is time to just filter them out with arptables.

## drop traffic prevent mac duplicates due to vivacom and bergon placed in same network – 255.255.255.252
dchp1-server:~# arptables -A INPUT –source-mac 70:e2:83:12:44:11 -j DROP


To list arptables configured on Linux host

dchp1-server:~# arptables –list -n


If you want to be paranoid sysadmin you can implement a MAC address protection with arptables by only allowing a single set of MAC Addr / IPs and dropping the rest.

dchp1-server:~# arptables -A INPUT –source-mac 70:e2:84:13:45:11 -j ACCEPT
dchp1-server:~# arptables -A INPUT  –source-mac 70:e2:84:13:45:12 -j ACCEPT


dchp1-server:~# arptables -L –line-numbers
Chain INPUT (policy ACCEPT)
1 -j DROP –src-mac 70:e2:84:13:45:11
2 -j DROP –src-mac 70:e2:84:13:45:12

Once MACs you like are accepted you can set the INPUT chain policy to DROP as so:

dchp1-server:~# arptables -P INPUT DROP


If you later need to temporary, clean up the rules inside arptables on any filtered hosts flush all rules inside INPUT chain, like that
 

dchp1-server:~#  arptables -t INPUT -F

Deny DHCP Address by MAC on Linux

Thursday, October 8th, 2020

Deny DHCP addresses by MAC ignore MAC to not be DHCPD leased on GNU / Linux howto

I have not blogged for a long time due to being on a few weeks vacation and being in home with a small cute baby. However as a hardcore and a bit of dumb System administrator, I have spend some of my vacation and   worked on bringing up the the www.pc-freak.net and the other Websites hosted as a high availvailability ones living on a 2 Webservers running on a Master to Master MySQL Replication backend database, this is oll hosted on  servers, set to run as a round robin DNS hosts on 2 servers one old Lenove ThinkCentre Edge71 as well as a brand new real Lenovo server Lenovo ThinkServer SD350 with 24 CPUs and a 32 GB of RAM
To assure Internet Connectivity is having a good degree of connectivity and ensure websites hosted on both machines is not going to die if one of the 2 pair configured Fiber Optics Internet Providers Bergon.NET has some Issues, I've rented another Internet Provider Line is set bought from the VIVACOM Mobile Fiber Internet provider – that is a 1 Gigabit Fiber Optics Line.
Next to that to guarantee there is no Database, Webserver, MailServer, Memcached and other running services did not hit downtimes due to Electricity power outage, two Powerful Uninterruptable Power Supplies (UPS)  FPS Fortron devices are connected to the servers each of which that could keep the machine and the connected switches and Servers for up to 1 Hour.

The machines are configured to use dhcpd to distributed IP addresses and the Main Node is set to distribute IPs, however as there is a local LAN network with more of a personal Work PCs, Wireless Devices and Testing Computers and few Virtual machines in the Network and the IPs are being distributed in a consequential manner via a ISC DHCP server.

As always to make everything work properly hence, I had again some a bit weird non-standard requirement to make some of the computers within the Network with Static IP addresses and the others to have their IPs received via the DHCP (Dynamic Host Configuration Protocol) and add some filter for some of the Machine MAC Addresses which are configured to have a static IP addresses to prevent the DHCP (daemon) server to automatically reassign IPs to this machines.

After a bit of googling and pondering I've done it and some of the machines, therefore to save others the efforts to look around How to set Certain Computers / Servers Network Card MAC (Interfaces) MAC Addresses  configured on the LAN network to use Static IPs and instruct the DHCP server to ingnore any broadcast IP addresses leases – if they're to be destined to a set of IGNORED MAcs, I came up with this small article.

Here is the DHCP server /etc/dhcpd/dhcpd.conf from my Debian GNU / Linux (Buster) 10.4

 

option domain-name "pcfreak.lan";
option domain-name-servers 8.8.8.8, 8.8.4.4, 208.67.222.222, 208.67.220.220;
max-lease-time 891200;
authoritative;
class "black-hole" {
    match substring (hardware, 1, 6);
    ignore booting;
}
subclass "black-hole" 18:45:91:c3:d9:00;
subclass "black-hole" 70:e2:81:13:44:11;
subclass "black-hole" 70:e2:81:13:44:12;
subclass "black-hole" 00:16:3f:53:5d:11;
subclass "black-hole" 18:45:9b:c6:d9:00;
subclass "black-hole" 16:45:93:c3:d9:09;
subclass "black-hole" 16:45:94:c3:d9:0d;/etc/dhcpd/dhcpd.conf
subclass "black-hole" 60:67:21:3c:20:ec;
subclass "black-hole" 60:67:20:5c:20:ed;
subclass "black-hole" 00:16:3e:0f:48:04;
subclass "black-hole" 00:16:3e:3a:f4:fc;
subclass "black-hole" 50:d4:f5:13:e8:ba;
subclass "black-hole" 50:d4:f5:13:e8:bb;
subnet 192.168.0.0 netmask 255.255.255.0 {
        option routers                  192.168.0.1;
        option subnet-mask              255.255.255.0;
}
host think-server {
        hardware ethernet 70:e2:85:13:44:12;
        fixed-address 192.168.0.200;
}
default-lease-time 691200;
max-lease-time 891200;
log-facility local7;

To spend you copy paste efforts a file with Deny DHCP Address by Mac Linux configuration is here
/home/hipo/info
Of course I have dumped the MAC Addresses to omit a data leaking but I guess the idea behind the MAC ADDR ignore is quite clear

The main configuration doing the trick to ignore a certain MAC ALenovo ThinkServer SD350ddresses that are reachable on the Connected hardware switch on the device is like so:

class "black-hole" {
    match substring (hardware, 1, 6);
    ignore booting;
}
subclass "black-hole" 18:45:91:c3:d9:00;


The Deny DHCP Address by MAC is described on isc.org distribution lists here but it seems the documentation on the topic on how to Deny / IGNORE DHCP Addresses by MAC Address on Linux has been quite obscure and limited online.

As you can see in above config the time via which an IP is freed up and a new IP lease is done from the server is severely maximized as often DHCP servers do use a max-lease-time like 1 hour (3600) seconds:, the reason for increasing the lease time to be to like 10 days time is that the IPs in my network change very rarely so it is a waste of CPU cycles to do a frequent lease.

default-lease-time 691200;
max-lease-time 891200;


As you see to Guarantee resolving works always as expected I have configured – Google Public DNS and OpenDNS IPs

option domain-name-servers 8.8.8.8, 8.8.4.4, 208.67.222.222, 208.67.220.220;


One hint to make is, after setting up all my desired config in the standard config location /etc/dhcp/dhcpd.conf it is always good idea to test configuration before reloading the running dhcpd process.

 

root@pcfreak: ~# /usr/sbin/dhcpd -t
Internet Systems Consortium DHCP Server 4.4.1
Copyright 2004-2018 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Config file: /etc/dhcp/dhcpd.conf
Database file: /va/home/hipo/infor/lib/dhcp/dhcpd.leases
PID file: /var/run/dhcpd.pid
 

That's all folks with this sample config the IPs under subclass "black-hole", which are a local LAN Static IP Addresses will never be offered leasess anymore from the ISC DHCP.
Hope this stuff helps someone, enjoy and in case if you need a colocation of a server or a website hosting for a really cheap price on this new set High Availlability up described machines open an inquiry on https://web.www.pc-freak.net.

 

How to scan for DHCP available servers in a Network range on Linux and FreeBSD

Thursday, December 8th, 2011

GNU / Linux and FreeBSD had a nifty little program (tool) called dhcping . dhcpingsend a DHCP request to DHCP server to see if it’s up and running. dhcping is also able to send a request to DHCP servers on a whole network range and therefore it can e asily be used as a scanner to find any available DHCP servers in a network.
This makes dhcping a nmap like scanner capable to determine if dhcp servers are in a network 😉
To scan an an entire network range with dhclient and find any existing DHCP servers:

noah:~# dhcping -s 255.255.255.255 -r -v
Got answer from: 192.168.2.1
received from 192.168.2.1, expected from 255.255.255.255
no answer

In above’s output actually my Dlink wireless router returns answer to the broadcast DHCP LEASE UDP network requests of dhcping .
On a networks where there is no DHCP server available, the requests dhcping -s 255.255.255.255 -r -v returns:

noah:~# dhcping -s 255.255.255.255 -r -v
no answer

This article was inspired by a post, I’ve red by a friend (Amridikon), so thx goes to him.

 

How to renew IP address, Add Routing and flush DNS cache on Windows XP / Vista / 7

Friday, November 25th, 2011

There are two handy Windows commands which can be used to renew IP address or flush prior cached DNS records which often create problems with resolving hosts.

1. To renew the IP address (fetch address from DHCP server) C:> ipconfig /release
C:> ipconfig /renew

In above cmd ipconfig /release will de-assign the IP address configured on all Windows LAN and Wireless interfaces, whether ipconfig /renew will send request for IP address to the DNS server.

To unassign and assign again IP address from DHCP server only for a particular LAN or WLAN card:

C:> ipconfig /release LAN
C:> ipconfig /renew LAN
C:> ipconfig /release WLAN
C:> ipconfig /renew WLAN

2. Adding specific routing to Windows

Windows has a Route command similar by syntax to Linux’s route command.
To add routing via a specific predefined IP addresses on Windows the commands should be something like:

C:> Route add 192.168.40.0 mask 255.255.255.0 192.168.41.253
C:> Route add 0.0.0.0 mask 0.0.0.0 192.168.41.254
The first command adds IP 192.168.40.0 in the network of 255 hosts to be routed via 192.168.41.253
The second one adds 192.168.41.254 as a default gateway for all outbound traffic from the Windows host.
To make permanent routing -p switch is used.
3. To clear Windows DNS cache (flush DNS cached records) C:> ipconfig /flushdns
This will clear all IP records corresponding to hostnames previously cached on the Windows host. Using ipconfig /flushdns is especially handy when IP address for a specific DNS host is changed. Flushing the Windows DNS cache can save us a lot of waiting before the domain example.com starts resolving to the new IP address let’s say 1.2.3.4 instead of the old one 2.2.2.2

How to connect to WiFi network using console or terminal on GNU / Linux

Wednesday, November 2nd, 2011

iwlist-screenshot-howto-connect-to-wifi-in-text-console-linux-wpasupplicant

Sometimes its useful to connect to Wireless Networks using console . The reasons for that might be many, one possible reason is to be able to debug, Wireless connection failures or simply omit the use of the many available GUI wifi connection programs.

As a first step before connecting in terminal is to look up for the wifi networks available for connection, this is done with cmd:

linux:~# iwlist wlan0 scanning
wlan0 Scan completed :
Cell 01 - Address: 00:24:01:90:8F:38
Channel:7
Frequency:2.442 GHz (Channel 7)
Quality=70/70 Signal level=-39 dBm
Encryption key:on
ESSID:"magdanoz"
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s
Bit Rates:6 Mb/s; 9 Mb/s; 12 Mb/s; 48 Mb/s; 18 Mb/s
24 Mb/s; 36 Mb/s; 54 Mb/s
Mode:Master
Extra:tsf=000000034f5c786b
Extra: Last beacon: 68ms ago
IE: Unknown: 00086D616764616E6F7A
IE: Unknown: 010482848B96
IE: Unknown: 030107
IE: Unknown: 32080C1218602430486C
IE: Unknown: CC0700CC020000018A
IE: Unknown: CC0700CC0300000100
IE: WPA Version 1
Group Cipher : TKIP
Pairwise Ciphers (2) : TKIP CCMP
Authentication Suites (1) : PSK
IE: IEEE 802.11i/WPA2 Version 1
Group Cipher : TKIP
Pairwise Ciphers (2) : TKIP CCMP
Authentication Suites (1) : PSK
Cell 02 - Address: 00:1E:2A:60:5E:DC
Channel:1
...

To just list the ESSID s of the wifi networks:

linux:~# iwlist wlan0 scanning|grep -i 'essid'
ESSID:"magdanoz"
ESSID:"default"
ESSID:"todorov"
ESSID:"BTC-ADSL"
ESSID:"Zahari"
ESSID:"Drago"

1. Connecting to Open Wireless Network

Now from the above output it is clear 6 wifi networks are available for connection. The default wifi network from the list is an Open network (e.g. without pass). To connect to it I use cmd:

linux:~# /sbin/iwconfig wlan0 essid 'default'
linux:~# /sbin/iwconfig wlan0 key open

After connected to configure IP, Gateway and DNS from a DHCP server running on the WIFI router, dhclient cmd is used:

linux:~# /sbin/dhclient wlan0

2. Connecting to WEP 64bit / 128bit encrypted network

linux:~# /sbin/iwconfig wlan0 key 1234-5678-9101-1213

3. Connecting to WPA / WPA2 encrypted wifi network

To connect to WPA or WPA2 encrypted network its necessery to have installed wpasupplicant package. The name of the package might vary in different distributions on Debian and Ubuntu, the name of the package is wpasupplicant, on Fedora, CentOS and RHEL the package that has to be in is wpa_supplicant :
After having installed the wpa_supplicant to connect to the network with ESSID name magdanoz , wpa_passphrase is used first:

linux:~# /usr/bin/wpa_passphrase magdanoz Secret_Wifi_Password | tee -a /etc/wpa_supplicant.conf
network={
ssid="magdanoz"
#psk="Secret_Wifi_Password"
psk=6f7590250c4496ff7bf8dd25199ac3bb5a614d4bc069975aa884bcf084da73bc
}

As you see in above command the secret password key is generated printed on the screen and then added to /etc/wpa_supplicant.conf , necessery to establish the wireless connection with wpa_supplicant with cmd:

linux:~# /sbin/wpa_supplicant wpa_supplicant -d wext -i wlan0 -c /etc/wpa_supplicant.conf -B

-d wext instructs wpa_supplicant to use (Linux wireless extension driver).
-B tells wpa_supplicant to background the connection to prevent the wireless connection to drop off, if the console / terminal from which it is launched gets closed.

In case of succesful connection with wpa_supplicant , once again IP, Gateway and DNS is configured fetching the settings from the wifi hotspot dhcp server:

linux:~# /sbin/dhclient wlan0

General information about the wireless network and info related to the established connection can be obtained with /usr/bin/iwconfig :

linux:~# /sbin/iwconfig
lo no wireless extensions.
eth0 no wireless extensions.
wlan0 IEEE 802.11abg ESSID:"magdanoz"
Mode:Managed Frequency:2.442 GHz Access Point: 00:24:00:90:8F:38
Bit Rate=54 Mb/s Tx-Power=15 dBm
Retry long limit:7 RTS thr:off Fragment thr:off
Encryption key:off
Power Management:off
Link Quality=70/70 Signal level=-39 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0

To configure the exact channel over which the wireless connection will be established again is done with iwconfig, for instance to configure wlan0 wifi connection established to be on wifi channel 7:

linux:~# /sbin/iwconfig wlan0 channel 11

By default iwconfig is set to automatically set the channel based on connected network ESSID , if the channel is modified to some specific number to revert it back use:

linux:~# /sbin/iwconfig wlan0 channel auto