03 Networking

Network Configuration

Ubuntu ships with a number of graphical utilities to configure your network devices. This lesson is geared toward server administrators and will focus on managing your network on the command line.

Ethernet Interfaces

Ethernet interfaces are identified by the system using the naming convention of ethX, where X represents a numeric value. The first Ethernet interface is typically identified as eth0, the second as eth1, and all others should move up in numerical order.

Identify Ethernet Interfaces

To quickly identify all available Ethernet interfaces, you can use the ifconfig command as shown below.

student@ubuntu:~$ ifconfig -a | grep eth

eth0 Link encap:Ethernet HWaddr 00:0c:29:81:03:f8

student@ubuntu:~$

Another application that can help identify all network interfaces available to your system is the lshw command. In the example below, lshw shows a single Ethernet interface with the logical name of eth0 along with bus information, driver details and all supported capabilities.

student@ubuntu:~$ sudo lsh -class network

sudo: lsh: command not found

student@ubuntu:~$ sudo lshw -class network

*-network

description: Ethernet interface

product: 79c970 [PCnet32 LANCE]

vendor: Hynix Semiconductor (Hyundai Electronics)

physical id: 1

bus info: pci@0000:02:01.0

logical name: eth0

version: 10

serial: 00:0c:29:81:03:f8

width: 32 bits

clock: 33MHz

capabilities: bus_master rom ethernet physical logical

configuration: broadcast=yes driver=pcnet32 driverversion=1.35 ip=192.168.1.102 latency=64 link=yes maxlatency=255 mingnt=6 multicast=yes

resources: irq:19 ioport:2000(size=128) memory:dc400000-dc40ffff

student@ubuntu:~$

Click here for more info in lshw command

Ethernet Interface logical names

Interface logical names are configured in the file /etc/udev/rules.d/70-persistent-net.rules. If you would like control which interface receives a particular logical name, find the line matching the interfaces physical MAC address and modify the value of NAME=ethX to the desired logical name. Reboot the system to commit your changes.

Ethernet Interface Settings

ethtool is a program that displays and changes Ethernet card settings such as auto-negotiation, port speed, duplex mode, and Wake-on-LAN.

It is not installed by default, but is available for installation in the repositories.

student@ubuntu:~$ sudo apt-get install ethtool

Reading package lists... Done

Building dependency tree

Reading state information... Done

The following packages were automatically installed and are no longer required:

linux-headers-3.5.0-23 linux-headers-3.5.0-23-generic

Use 'apt-get autoremove' to remove them.

The following NEW packages will be installed:

ethtool

0 upgraded, 1 newly installed, 0 to remove and 43 not upgraded.

Need to get 91.6 kB of archives.

After this operation, 293 kB of additional disk space will be used.

Get:1 http://us.archive.ubuntu.com/ubuntu/ precise/main ethtool i386 1:3.1-1 [91.6 kB]

Fetched 91.6 kB in 1s (62.1 kB/s)

Selecting previously unselected package ethtool.

(Reading database ... 199610 files and directories currently installed.)

Unpacking ethtool (from .../ethtool_1%3a3.1-1_i386.deb) ...

Processing triggers for man-db ...

Setting up ethtool (1:3.1-1) ...

student@ubuntu:~$

Changes made with the ethtool command are temporary and will be lost after a reboot.

If you would like to retain settings, simply add the desired ethtool command to a pre-up statement in the interface configuration file /etc/network/interfaces.

The following is an example of how the interface identified as eth0 could be permanently configured with a port speed of 1000Mb/s running in full duplex mode.

auto eth0

iface eth0 inet dhcp

pre-up /sbin/ethtool -s eth0 speed 1000 duplex full

Exercice

Set this configuration to your hosts /etc/network/interfaces file.

Solution

We will use a text editor called gedit to modify the /etc/network/interfaces where the configuration for the interfaces is stored.

The editor needs to be opened with super user rights in order to be able to write changes to the above file.

student@ubuntu:~$ sudo gedit

[sudo] password for student:

It will open the gedit text editor

save the file and restart the server for the changes to take effect.

IP Addressing

The following section describes the process of configuring your systems IP address and default gateway needed for communicating on a local area network and the Internet.

Temporary IP Address assignement

For temporary network configurations, you can use standard commands such as ip, ifconfig and route, which are also found on most other GNU/Linux operating systems. These commands allow you to configure settings which take effect immediately, however they are not persistent and will be lost after a reboot.

To temporarily configure an IP address, you can use the ifconfig command in the following manner. Just modify the IP address and subnet mask to match your network requirements.

student@ubuntu:~$ sudo ifconfig eth0 192.168.1.102 netmask 255.255.255.0

[sudo] password for student:

To configure a default gateway, you can use the route command in the following manner. Modify the default gateway address to match your network requirements.

student@ubuntu:~$ sudo route add default gw 192.168.1.1 eth0

To verify your default gateway configuration, you can use the route command in the following manner.

student@ubuntu:~$ route -n

Kernel IP routing table

Destination Gateway Genmask Flags Metric Ref Use Iface

0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0

0.0.0.0 192.168.1.1 0.0.0.0 UG 100 0 0 eth0

169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 eth0

192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0

student@ubuntu:~$

If you no longer need this configuration and wish to purge all IP configuration from an interface, you can use the ip command with the flush option as shown below.

ip addr flush eth0

Dynamic IP Address Assignement (DHCP Client)

set this parameter at the interfaces file /etc/network/interfaces

for static IP address assignement

auto eth0 iface eth0 inet static

for dhcp assigned IP address

auto eth0 iface eth0 inet dhcp

By adding an interface configuration as shown above, you can manually enable the interface through the ifup command which initiates the DHCP process via dhclient.

sudo ifup eth0

To manually disable the interface, you can use the ifdown command, which in turn will initiate the DHCP release process and shut down the interface.

sudo ifdown eth0

Static IP Address Assignement

o configure your system to use a static IP address assignment, add the static method to the inet address family statement for the appropriate interface in the file /etc/network/interfaces. The example below assumes you are configuring your first Ethernet interface identified as eth0. Change the address, netmask, and gateway values to meet the requirements of your network

auto eth0

iface eth0 inet static

address 192.168.1.122

netmask 255.255.255.0

gateway 192.168.1.1

Loopback interface

The loopback interface is identified by the system as lo and has a default IP address of 127.0.0.1. It can be viewed using the ifconfig command.

student@ubuntu:~$ ifconfig lo

lo Link encap:Local Loopback

inet addr:127.0.0.1 Mask:255.0.0.0

inet6 addr: ::1/128 Scope:Host

UP LOOPBACK RUNNING MTU:16436 Metric:1

RX packets:1138 errors:0 dropped:0 overruns:0 frame:0

TX packets:1138 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:0

RX bytes:86270 (86.2 KB) TX bytes:86270 (86.2 KB)

By default, there should be two lines in /etc/network/interfaces responsible for automatically configuring your loopback interface. It is recommended that you keep the default settings unless you have a specific purpose for changing them. An example of the two default lines are shown below.

auto lo

iface lo inet loopback

Name Resolution

Name resolution as it relates to IP networking is the process of mapping IP addresses to hostnames, making it easier to identify resources on a network. The following section will explain how to properly configure your system for name resolution using DNS and static hostname records.

DNS Client configuration

Traditionally, the file /etc/resolv.conf was a static configuration file that rarely needed to be changed or automatically changed via DCHP client hooks. Nowadays, a computer can switch from one network to another quite often and the resolvconf framework is now being used to track these changes and update the resolver's configuration automatically. It acts as an intermediary between programs that supply nameserver information and applications that need nameserver information. Resolvconf gets populated with information by a set of hook scripts related to network interface configuration. The most notable difference for the user is that any change manually done to /etc/resolv.conf will be lost as it gets overwritten each time something triggers resolvconf. Instead, resolvconf uses DHCP client hooks, and /etc/network/interfaces to generate a list of nameservers and domains to put in /etc/resolv.conf, which is now a symlink:

student@ubuntu:/etc$ ls resol*

resolv.conf

resolvconf:

interface-order resolv.conf.d update.d update-libc.d

student@ubuntu:/etc$

To configure the resolver, add the IP addresses of the nameservers that are appropriate for your network in the file /etc/network/interfaces. The resulting file might look like the following:

iface eth0 inet static address 192.168.3.3 netmask 255.255.255.0 gateway 192.168.3.1 dns-search example.com dns-nameservers 192.168.3.45 192.168.8.10

The search option can also be used with multiple domain names so that DNS queries will be appended in the order in which they are entered. For example, your network may have multiple sub-domains to search; a parent domain of example.com, and two sub-domains, sales.example.com and dev.example.com.

If you have multiple domains you wish to search, your configuration might look like the following:

iface eth0 inet static address 192.168.3.3 netmask 255.255.255.0 gateway 192.168.3.1 dns-search example.com sales.example.com dev.example.com dns-nameservers 192.168.3.45 192.168.8.10

If you try to ping a host with the name of server1, your system will automatically query DNS for its Fully Qualified Domain Name (FQDN) in the following order:

• 1. server1.example.com

  2. server1.sales.example.com

  3. server1.dev.example.com

If no matches are found, the DNS server will provide a result of notfound and the DNS query will fail.

Static hostnames

Static hostnames are locally defined hostname-to-IP mappings located in the file /etc/hosts. Entries in the hosts file will have precedence over DNS by default. This means that if your system tries to resolve a hostname and it matches an entry in /etc/hosts, it will not attempt to look up the record in DNS. In some configurations, especially when Internet access is not required, servers that communicate with a limited number of resources can be conveniently set to use static hostnames instead of DNS.

The following is an example of a hosts file where a number of local servers have been identified by simple hostnames, aliases and their equivalent Fully Qualified Domain Names (FQDN's).

127.0.0.1 localhost 127.0.1.1 ubuntu-server 10.0.0.11 server1.example.com server1 vpn 10.0.0.12 server2.example.com server2 mail 10.0.0.13 server3.example.com server3 www 10.0.0.14 server4.example.com server4 file

Exercice

Show the hosts file in yout server

127.0.0.1 localhost

127.0.1.1 ubuntu

# The following lines are desirable for IPv6 capable hosts

::1 ip6-localhost ip6-loopback

fe00::0 ip6-localnet

ff00::0 ip6-mcastprefix

ff02::1 ip6-allnodes

ff02::2 ip6-allrouters

Exercice

Add an entry 127.0.2.1 myserver in the hosts file.

Try to ping myserver

Bridging

Bridging multiple interfaces is a more advanced configuration, but is very useful in multiple scenarios. One scenario is setting up a bridge with multiple network interfaces, then using a firewall to filter traffic between two network segments. Another scenario is using bridge on a system with one interface to allow virtual machines direct access to the outside network. The following example covers the latter scenario.

Before configuring a bridge you will need to install the bridge-utils package. To install the package, in a terminal enter:

sudo apt-get install bridge-utils

Next, configure the bridge by editing /etc/network/interfaces:

auto lo iface lo inet loopback auto br0 iface br0 inet static address 192.168.0.10 network 192.168.0.0 netmask 255.255.255.0 broadcast 192.168.0.255 gateway 192.168.0.1 bridge_ports eth0 bridge_fd 9 bridge_hello 2 bridge_maxage 12 bridge_stp off

Now restart networking to enable the bridge interface:

sudo /etc/init.d/networking restart

TCP/IP

TCP/IP Introduction

The two protocol components of TCP/IP deal with different aspects of computer networking. Internet Protocol, the "IP" of TCP/IP is a connectionless protocol which deals only with network packet routing using the IP Datagram as the basic unit of networking information. The IP Datagram consists of a header followed by a message. The Transmission Control Protocol is the "TCP" of TCP/IP and enables network hosts to establish connections which may be used to exchange data streams. TCP also guarantees that the data between connections is delivered and that it arrives at one network host in the same order as sent from another network host.

Format of an IP Datagramm

TCP/IP Configuration

The TCP/IP protocol configuration consists of several elements which must be set by editing the appropriate configuration files, or deploying solutions such as the Dynamic Host Configuration Protocol (DHCP) server which in turn, can be configured to provide the proper TCP/IP configuration settings to network clients automatically. These configuration values must be set correctly in order to facilitate the proper network operation of your lynix system.

The common configuration elements of TCP/IP and their purposes are as follows:

• • IP address The IP address is a unique identifying string expressed as four decimal numbers ranging from zero (0) to two-hundred and fifty-five (255), separated by periods, with each of the four numbers representing eight (8) bits of the address for a total length of thirty-two (32) bits for the whole address. This format is called dotted quad notation.

  • Netmask The Subnet Mask (or simply, netmask) is a local bit mask, or set of flags which separate the portions of an IP address significant to the network from the bits significant to the subnetwork. For example, in a Class C network, the standard netmask is 255.255.255.0 which masks the first three bytes of the IP address and allows the last byte of the IP address to remain available for specifying hosts on the subnetwork.

  • Network Address The Network Address represents the bytes comprising the network portion of an IP address. For example, the host 12.128.1.2 in a Class A network would use 12.0.0.0 as the network address, where twelve (12) represents the first byte of the IP address, (the network part) and zeroes (0) in all of the remaining three bytes to represent the potential host values. A network host using the private IP address 192.168.1.100 would in turn use a Network Address of 192.168.1.0, which specifies the first three bytes of the Class C 192.168.1 network and a zero (0) for all the possible hosts on the network.

  • Broadcast Address The Broadcast Address is an IP address which allows network data to be sent simultaneously to all hosts on a given subnetwork rather than specifying a particular host. The standard general broadcast address for IP networks is 255.255.255.255, but this broadcast address cannot be used to send a broadcast message to every host on the Internet because routers block it. A more appropriate broadcast address is set to match a specific subnetwork. For example, on the private Class C IP network, 192.168.1.0, the broadcast address is 192.168.1.255. Broadcast messages are typically produced by network protocols such as the Address Resolution Protocol (ARP) and the Routing Information Protocol (RIP).

  • Gateway Address A Gateway Address is the IP address through which a particular network, or host on a network, may be reached. If one network host wishes to communicate with another network host, and that host is not located on the same network, then a gateway must be used. In many cases, the Gateway Address will be that of a router on the same network, which will in turn pass traffic on to other networks or hosts, such as Internet hosts. The value of the Gateway Address setting must be correct, or your system will not be able to reach any hosts beyond those on the same network.

  • Nameserver Address Nameserver Addresses represent the IP addresses of Domain Name Service (DNS) systems, which resolve network hostnames into IP addresses. There are three levels of Nameserver Addresses, which may be specified in order of precedence: The Primary Nameserver, the Secondary Nameserver, and the Tertiary Nameserver. In order for your system to be able to resolve network hostnames into their corresponding IP addresses, you must specify valid Nameserver Addresses which you are authorized to use in your system's TCP/IP configuration. In many cases these addresses can and will be provided by your network service provider, but many free and publicly accessible nameservers are available for use, such as the Level3 (Verizon) servers with IP addresses from 4.2.2.1 to 4.2.2.6.

IP Routing

IP routing is a means of specifying and discovering paths in a TCP/IP network along which network data may be sent. Routing uses a set of routing tables to direct the forwarding of network data packets from their source to the destination, often via many intermediary network nodes known as routers. There are two primary forms of IP routing: Static Routing and Dynamic Routing.

Static routing involves manually adding IP routes to the system's routing table, and this is usually done by manipulating the routing table with the route command. Static routing enjoys many advantages over dynamic routing, such as simplicity of implementation on smaller networks, predictability (the routing table is always computed in advance, and thus the route is precisely the same each time it is used), and low overhead on other routers and network links due to the lack of a dynamic routing protocol. However, static routing does present some disadvantages as well. For example, static routing is limited to small networks and does not scale well. Static routing also fails completely to adapt to network outages and failures along the route due to the fixed nature of the route.

Dynamic routing depends on large networks with multiple possible IP routes from a source to a destination and makes use of special routing protocols, such as the Router Information Protocol (RIP), which handle the automatic adjustments in routing tables that make dynamic routing possible. Dynamic routing has several advantages over static routing, such as superior scalability and the ability to adapt to failures and outages along network routes. Additionally, there is less manual configuration of the routing tables, since routers learn from one another about their existence and available routes. This trait also eliminates the possibility of introducing mistakes in the routing tables via human error. Dynamic routing is not perfect, however, and presents disadvantages such as heightened complexity and additional network overhead from router communications, which does not immediately benefit the end users, but still consumes network bandwidth.

TCP and UDP

TCP is a connection-based protocol, offering error correction and guaranteed delivery of data via what is known as flow control. Flow control determines when the flow of a data stream needs to be stopped, and previously sent data packets should to be re-sent due to problems such as collisions, for example, thus ensuring complete and accurate delivery of the data. TCP is typically used in the exchange of important information such as database transactions.

The User Datagram Protocol (UDP), on the other hand, is a connectionless protocol which seldom deals with the transmission of important data because it lacks flow control or any other method to ensure reliable delivery of the data. UDP is commonly used in such applications as audio and video streaming, where it is considerably faster than TCP due to the lack of error correction and flow control, and where the loss of a few packets is not generally catastrophic.

ICMP

The Internet Control Messaging Protocol (ICMP) is an extension to the Internet Protocol (IP) as defined in the Request For Comments (RFC) #792 and supports network packets containing control, error, and informational messages. ICMP is used by such network applications as the ping utility, which can determine the availability of a network host or device. Examples of some error messages returned by ICMP which are useful to both network hosts and devices such as routers, include Destination Unreachable and Time Exceeded.

Deamons

Daemons are special system applications which typically execute continuously in the background and await requests for the functions they provide from other applications. Many daemons are network-centric; that is, a large number of daemons executing in the background on an Ubuntu system may provide network-related functionality. Some examples of such network daemons include the Hyper Text Transport Protocol Daemon (httpd), which provides web server functionality; the Secure SHell Daemon (sshd), which provides secure remote login shell and file transfer capabilities; and the Internet Message Access Protocol Daemon (imapd), which provides E-Mail services.

Dynamic Host Configuration Protocol (DHCP)

The Dynamic Host Configuration Protocol (DHCP) is a network service that enables host computers to be automatically assigned settings from a server as opposed to manually configuring each network host. Computers configured to be DHCP clients have no control over the settings they receive from the DHCP server, and the configuration is transparent to the computer's user.

The most common settings provided by a DHCP server to DHCP clients include:

• • IP address and netmask

  • IP address of the default-gateway to use

  • IP adresses of the DNS servers to use

However, a DHCP server can also supply configuration properties such as:

• • Host Name

  • Domain Name

  • Time Server

  • Print Server

The advantage of using DHCP is that changes to the network, for example a change in the address of the DNS server, need only be changed at the DHCP server, and all network hosts will be reconfigured the next time their DHCP clients poll the DHCP server. As an added advantage, it is also easier to integrate new computers into the network, as there is no need to check for the availability of an IP address. Conflicts in IP address allocation are also reduced.

A DHCP server can provide configuration settings using the following methods:

Manual allocation (MAC address)

This method entails using DHCP to identify the unique hardware address of each network card connected to the network and then continually supplying a constant configuration each time the DHCP client makes a request to the DHCP server using that network device. This ensures that a particular address is assigned automatically to that network card, based on it's MAC address.

Dynamic allocation (address pool)

In this method, the DHCP server will assign an IP address from a pool of addresses (sometimes also called a range or scope) for a period of time or lease, that is configured on the server or until the client informs the server that it doesn't need the address anymore. This way, the clients will be receiving their configuration properties dynamically and on a "first come, first served" basis. When a DHCP client is no longer on the network for a specified period, the configuration is expired and released back to the address pool for use by other DHCP Clients. This way, an address can be leased or used for a period of time. After this period, the client has to renegociate the lease with the server to maintain use of the address.

Automatic allocation

Using this method, the DHCP automatically assigns an IP address permanently to a device, selecting it from a pool of available addresses. Usually DHCP is used to assign a temporary address to a client, but a DHCP server can allow an infinite lease time.

The last two methods can be considered “automatic” because in each case the DHCP server assigns an address with no extra intervention needed. The only difference between them is in how long the IP address is leased, in other words whether a client's address varies over time. Ubuntu is shipped with both DHCP server and client. The server is dhcpd (dynamic host configuration protocol daemon). The client provided with Ubuntu is dhclient and should be installed on all computers required to be automatically configured. Both programs are easy to install and configure and will be automatically started at system boot.

Instalation

To install dhcp server

student@ubuntu:~$ sudo apt-get install isc-dhcp-server

[sudo] password for student:

Reading package lists... Done

Building dependency tree

Reading state information... Done

The following packages were automatically installed and are no longer required:

linux-headers-3.5.0-23 linux-headers-3.5.0-23-generic

Use 'apt-get autoremove' to remove them.

Suggested packages:

isc-dhcp-server-ldap

The following NEW packages will be installed:

isc-dhcp-server

0 upgraded, 1 newly installed, 0 to remove and 43 not upgraded.

Need to get 428 kB of archives.

After this operation, 1,040 kB of additional disk space will be used.

Get:1 http://us.archive.ubuntu.com/ubuntu/ precise-updates/main isc-dhcp-server i386 4.1.ESV-R4-0ubuntu5.6 [428 kB]

Fetched 428 kB in 2s (148 kB/s)

Preconfiguring packages ...

Selecting previously unselected package isc-dhcp-server.

(Reading database ... 199646 files and directories currently installed.)

Unpacking isc-dhcp-server (from .../isc-dhcp-server_4.1.ESV-R4-0ubuntu5.6_i386.deb) ...

Processing triggers for man-db ...

Processing triggers for ureadahead ...

Setting up isc-dhcp-server (4.1.ESV-R4-0ubuntu5.6) ...

Generating /etc/default/isc-dhcp-server...

isc-dhcp-server start/running, process 4453

isc-dhcp-server6 stop/pre-start, process 4514

Configuration

TO configure the DHCP Server we need to change the content of

/etc/dhcp/dhcpd.conf

Below there is a sample content of this file

# minimal sample /etc/dhcp/dhcpd.conf default-lease-time 600; #seconds max-lease-time 7200; subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.150 192.168.1.200; option routers 192.168.1.254; option domain-name-servers 192.168.1.1, 192.168.1.2; option domain-name "mydomain.example"; }

After changing the config file you have to restart the host to apply the changes to dhcp server.

Time Synchronization with NTP

NTP is a TCP/IP protocol for synchronising time over a network. Basically a client requests the current time from a server, and uses it to set its own clock.

Behind this simple description, there is a lot of complexity - there are tiers of NTP servers, with the tier one NTP servers connected to atomic clocks, and tier two and three servers spreading the load of actually handling requests across the Internet. Also the client software is a lot more complex than you might think - it has to factor out communication delays, and adjust the time in a way that does not upset all the other processes that run on the server. But luckily all that complexity is hidden from you!

Ubuntu uses ntpdate and ntpd.

ntpdate

Ubuntu comes with ntpdate as standard, and will run it once at boot time to set up your time according to Ubuntu's NTP server.

ntpdate -s ntp.ubuntu.com

Commande date shows us the current system time

student@ubuntu:~$ ntpdate -s ntp.ubuntu.com

student@ubuntu:~$ date

Wed Mar 13 04:58:57 CET 2013

student@ubuntu:~$

ntpd - ntp daemon

To install ntp daemon

sudo apt-get install ntp

NTPD Configuration

Edit /etc/ntp.conf to add/remove server lines. By default these servers are configured:

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board # on 2011-02-08 (LP: #104525). See http://www.pool.ntp.org/join.html for # more information. server 0.ubuntu.pool.ntp.org server 1.ubuntu.pool.ntp.org server 2.ubuntu.pool.ntp.org server 3.ubuntu.pool.ntp.org

After changing the config file you have to reload the ntpd:

sudo /etc/init.d/ntp reload

View status

Use ntpq to see to see more info:

# sudo ntpq -p remote refid st t when poll reach delay offset jitter ============================================================================== +stratum2-2.NTP. 129.70.130.70 2 u 5 64 377 68.461 -44.274 110.334 +ntp2.m-online.n 212.18.1.106 2 u 5 64 377 54.629 -27.318 78.882 *145.253.66.170 .DCFa. 1 u 10 64 377 83.607 -30.159 68.343 +stratum2-3.NTP. 129.70.130.70 2 u 5 64 357 68.795 -68.168 104.612 +europium.canoni 193.79.237.14 2 u 63 64 337 81.534 -67.968 92.792