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<chapter id="networking" status="review">
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<title>Networking</title>
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Networks consist of two or more devices, such as computer systems, printers,
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and related equipment which are connected by either physical cabling or
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wireless links for the purpose of sharing and distributing information among
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the connected devices.
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This section provides general and specific information pertaining
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to networking, including an overview of network concepts and detailed
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discussion of popular network protocols.
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<sect1 id="network-configuration" status="review">
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<title>Network Configuration</title>
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Ubuntu ships with a number of graphical utilities to configure your
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network devices. This document is geared toward server administrators
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and will focus on managing your network on the command line.
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<sect2 id="ethernet-interfaces" status="review">
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<title>Ethernet Interfaces</title>
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Ethernet interfaces are identified by the system using the naming convention of
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<emphasis role="italix">ethX</emphasis>, where <emphasis role="italic">X</emphasis>
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represents a numeric value. The first Ethernet interface is typically identified
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as <emphasis role="italic">eth0</emphasis>, the second as
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<emphasis role="italic">eth1</emphasis>, and all others should move up in
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<sect3 id="identify-ethernet-interfaces" status="review">
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<title>Identify Ethernet Interfaces</title>
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To quickly identify all available Ethernet interfaces, you can use the
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<application>ifconfig</application> command as shown below.
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<userinput>ifconfig -a | grep eth</userinput>
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eth0 Link encap:Ethernet HWaddr 00:15:c5:4a:16:5a
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Another application that can help identify all network interfaces available to your system
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is the <application>lshw</application> command. In the example below, <application>lshw</application>
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shows a single Ethernet interface with the logical name of <emphasis role="italic">eth0</emphasis>
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along with bus information, driver details and all supported capabilities.
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<userinput>sudo lshw -class network</userinput>
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description: Ethernet interface
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product: BCM4401-B0 100Base-TX
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vendor: Broadcom Corporation
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bus info: pci@0000:03:00.0
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serial: 00:15:c5:4a:16:5a
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capabilities: (snipped for brevity)
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configuration: (snipped for brevity)
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resources: irq:17 memory:ef9fe000-ef9fffff
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<sect3 id="ethernet-interface-names" status="review">
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<title>Ethernet Interface Logical Names</title>
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Interface logical names are configured in the file
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<filename>/etc/udev/rules.d/70-persistent-net.rules.</filename> If you would
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like control which interface receives a particular logical name, find the line
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matching the interfaces physical MAC address and modify the value of
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<emphasis role="italic">NAME=ethX</emphasis> to the desired logical name.
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Reboot the system to commit your changes.
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SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:15:c5:4a:16:5a", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"
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SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="00:15:c5:4a:16:5b", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"
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<sect3 id="ethernet-interface-settings" status="review">
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<title>Ethernet Interface Settings</title>
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<application>ethtool</application> is a program that displays and changes Ethernet
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card settings such as auto-negotiation, port speed, duplex mode, and Wake-on-LAN. It
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is not installed by default, but is available for installation in the repositories.
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<userinput>sudo apt-get install ethtool</userinput>
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The following is an example of how to view supported features and configured
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settings of an Ethernet interface.
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<userinput>sudo ethtool eth0</userinput>
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Supported ports: [ TP ]
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Supported link modes: 10baseT/Half 10baseT/Full
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100baseT/Half 100baseT/Full
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1000baseT/Half 1000baseT/Full
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Supports auto-negotiation: Yes
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Advertised link modes: 10baseT/Half 10baseT/Full
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100baseT/Half 100baseT/Full
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1000baseT/Half 1000baseT/Full
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Advertised auto-negotiation: Yes
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Transceiver: internal
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Current message level: 0x000000ff (255)
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Changes made with the <application>ethtool</application> command are temporary
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and will be lost after a reboot. If you would like to retain settings, simply add
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the desired <application>ethtool</application> command to a <emphasis role="italic">pre-up</emphasis>
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statement in the interface configuration file <filename>/etc/network/interfaces</filename>.
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The following is an example of how the interface identified as <emphasis role="italic">eth0</emphasis>
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could be permanently configured with a port speed of 1000Mb/s running in full duplex mode.
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iface eth0 inet static
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pre-up /usr/sbin/ethtool -s eth0 speed 1000 duplex full
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Although the example above shows the interface configured to use the
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<emphasis role="italic">static</emphasis> method, it actually works with other
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methods as well, such as DHCP. The example is meant to demonstrate only proper
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placement of the <emphasis role="italic">pre-up</emphasis> statement in relation
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to the rest of the interface configuration.
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<sect2 id="ip-addressing" status="review">
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<title>IP Addressing</title>
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The following section describes the process of configuring your systems IP address
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and default gateway needed for communicating on a local area network and the
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<sect3 id="temp-ip-assignment" status="review">
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<title>Temporary IP Address Assignment</title>
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For temporary network configurations, you can use standard commands
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such as <application>ip</application>, <application>ifconfig</application>
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and <application>route</application>, which are also found on most other
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GNU/Linux operating systems. These commands allow you to configure settings
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which take effect immediately, however they are not persistent and will
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be lost after a reboot.
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To temporarily configure an IP address, you can use the <application>ifconfig</application>
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command in the following manner. Just modify the IP address and subnet mask to match your
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network requirements.
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<userinput>sudo ifconfig eth0 10.0.0.100 netmask 255.255.255.0</userinput>
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To verify the IP address configuration of <application>eth0</application>,
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you can use the <application>ifconfig</application> command in the following manner.
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<userinput>ifconfig eth0</userinput>
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eth0 Link encap:Ethernet HWaddr 00:15:c5:4a:16:5a
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inet addr:10.0.0.100 Bcast:10.0.0.255 Mask:255.255.255.0
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inet6 addr: fe80::215:c5ff:fe4a:165a/64 Scope:Link
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UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
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RX packets:466475604 errors:0 dropped:0 overruns:0 frame:0
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TX packets:403172654 errors:0 dropped:0 overruns:0 carrier:0
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collisions:0 txqueuelen:1000
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RX bytes:2574778386 (2.5 GB) TX bytes:1618367329 (1.6 GB)
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To configure a default gateway, you can use the <application>route</application>
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command in the following manner. Modify the default gateway address to match
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your network requirements.
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<userinput>sudo route add default gw 10.0.0.1 eth0</userinput>
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To verify your default gateway configuration, you can use the <application>route</application>
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command in the following manner.
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<userinput>route -n</userinput>
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Kernel IP routing table
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Destination Gateway Genmask Flags Metric Ref Use Iface
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10.0.0.0 0.0.0.0 255.255.255.0 U 1 0 0 eth0
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0.0.0.0 10.0.0.1 0.0.0.0 UG 0 0 0 eth0
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If you require DNS for your temporary network configuration, you can add DNS server
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IP addresses in the file <filename>/etc/resolv.conf</filename>. The example below
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shows how to enter two DNS servers to <filename>/etc/resolv.conf</filename>, which
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should be changed to servers appropriate for your network. A more lengthy description
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of DNS client configuration is in a following section.
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If you no longer need this configuration and wish to purge all IP configuration from
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an interface, you can use the <application>ip</application> command with the flush option
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<userinput>ip addr flush eth0</userinput>
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Flushing the IP configuration using the <application>ip</application> command does not clear the
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contents of <filename>/etc/resolv.conf</filename>. You must remove or modify those entries manually.
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<sect3 id="dynamic-ip-addressing" status="review">
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<title>Dynamic IP Address Assignment (DHCP Client)</title>
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To configure your server to use DHCP for dynamic address assignment, add the
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<emphasis role="italic">dhcp</emphasis> method to the inet address family statement
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for the appropriate interface in the file <filename>/etc/network/interfaces</filename>.
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The example below assumes you are configuring your first Ethernet interface identified as
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<emphasis role="italic">eth0</emphasis>.
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By adding an interface configuration as shown above, you can manually enable the
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interface through the <application>ifup</application> command which initiates the
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DHCP process via <application>dhclient</application>.
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<userinput>sudo ifup eth0</userinput>
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To manually disable the interface, you can use the <application>ifdown</application>
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command, which in turn will initiate the DHCP release process and shut down the
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<userinput>sudo ifdown eth0</userinput>
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<sect3 id="static-ip-addressing" status="review">
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<title>Static IP Address Assignment</title>
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To configure your system to use a static IP address assignment, add the
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<emphasis role="italic">static</emphasis> method to the inet address family statement
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for the appropriate interface in the file <filename>/etc/network/interfaces</filename>.
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The example below assumes you are configuring your first Ethernet interface identified as
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<emphasis role="italic">eth0</emphasis>. Change the <emphasis role="italic">address</emphasis>,
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<emphasis role="italic">netmask</emphasis>, and <emphasis role="italic">gateway</emphasis>
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values to meet the requirements of your network.
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iface eth0 inet static
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netmask 255.255.255.0
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By adding an interface configuration as shown above, you can manually enable the
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interface through the <application>ifup</application> command.
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<userinput>sudo ifup eth0</userinput>
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To manually disable the interface, you can use the <application>ifdown</application>
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<userinput>sudo ifdown eth0</userinput>
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<sect3 id="loopback-interface" status="review">
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<title>Loopback Interface</title>
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The loopback interface is identified by the system as <emphasis role="italic">lo</emphasis>
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and has a default IP address of 127.0.0.1. It can be viewed using the ifconfig command.
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<userinput>ifconfig lo</userinput>
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lo Link encap:Local Loopback
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inet addr:127.0.0.1 Mask:255.0.0.0
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inet6 addr: ::1/128 Scope:Host
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UP LOOPBACK RUNNING MTU:16436 Metric:1
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RX packets:2718 errors:0 dropped:0 overruns:0 frame:0
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TX packets:2718 errors:0 dropped:0 overruns:0 carrier:0
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collisions:0 txqueuelen:0
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RX bytes:183308 (183.3 KB) TX bytes:183308 (183.3 KB)
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By default, there should be two lines in <filename>/etc/network/interfaces</filename>
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responsible for automatically configuring your loopback interface. It is recommended that you
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keep the default settings unless you have a specific purpose for changing them. An example of
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the two default lines are shown below.
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iface lo inet loopback
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<sect2 id="name-resolution" status="review">
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<title>Name Resolution</title>
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Name resolution as it relates to IP networking is the process of mapping IP addresses
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to hostnames, making it easier to identify resources on a network. The following section
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will explain how to properly configure your system for name resolution using DNS and static
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<sect3 id="dns-client-configuration" status="review">
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<title>DNS Client Configuration</title>
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To configure your system to use DNS for name resolution, add the IP addresses of the DNS
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servers that are appropriate for your network in the file <filename>/etc/resolv.conf</filename>.
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You can also add an optional DNS suffix search-lists to match your network domain names.
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Below is an example of a typical configuration of <filename>/etc/resolv.conf</filename>
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for a server on the domain "<emphasis role="italic">example.com</emphasis>" and using
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two public DNS servers.
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The <emphasis role="italic">search</emphasis> option can also be used with multiple domain names
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so that DNS queries will be appended in the order in which they are entered. For example,
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your network may have multiple sub-domains to search; a parent domain of
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<emphasis role="italic">example.com</emphasis>, and two sub-domains,
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<emphasis role="italic">sales.example.com</emphasis> and
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<emphasis role="italic">dev.example.com</emphasis>.
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If you have multiple domains you wish to search, your configuration might look like the following.
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search example.com sales.example.com dev.example.com
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If you try to ping a host with the name of <emphasis role="italic">server1</emphasis>, your system
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will automatically query DNS for its Fully Qualified Domain Name (FQDN) in the following order:
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server1<emphasis role="bold">.example.com</emphasis>
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server1<emphasis role="bold">.sales.example.com</emphasis>
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server1<emphasis role="bold">.dev.example.com</emphasis>
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If no matches are found, the DNS server will provide a result of
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<emphasis role="italic">notfound</emphasis> and the DNS query will fail.
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<sect3 id="static-hostnames" status="review">
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<title>Static Hostnames</title>
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Static hostnames are locally defined hostname-to-IP mappings located in the file <filename>/etc/hosts</filename>.
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Entries in the <filename>hosts</filename> file will have precedence over DNS by default. This means that if your
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system tries to resolve a hostname and it matches an entry in /etc/hosts, it will not attempt to look up the
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record in DNS. In some configurations, especially when Internet access is not required, servers that
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communicate with a limited number of resources can be conveniently set to use static hostnames instead of DNS.
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The following is an example of a <filename>hosts</filename> file where a number of local servers
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have been identified by simple hostnames, aliases and their equivalent Fully Qualified Domain Names (FQDN's).
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127.0.1.1 ubuntu-server
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10.0.0.11 server1 vpn server1.example.com
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10.0.0.12 server2 mail server2.example.com
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10.0.0.13 server3 www server3.example.com
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10.0.0.14 server4 file server4.example.com
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In the above example, notice that each of the servers have been given aliases in addition to their
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proper names and FQDN's. <emphasis role="italic">Server1</emphasis> has been mapped to the name
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<emphasis role="italic">vpn</emphasis>, <emphasis role="italic">server2</emphasis> is referred
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to as <emphasis role="italic">mail</emphasis>, <emphasis role="italic">server3</emphasis> as
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<emphasis role="italic">www</emphasis>, and <emphasis role="italic">server4</emphasis> as
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<emphasis role="italic">file</emphasis>.
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<sect3 id="name-service-switch-config" status="review">
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<title>Name Service Switch Configuration</title>
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The order in which your system selects a method of resolving hostnames to IP addresses is
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controlled by the Name Service Switch (NSS) configuration file <filename>/etc/nsswitch.conf</filename>.
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As mentioned in the previous section, typically static hostnames defined in the systems
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<filename>/etc/hosts</filename> file have precedence over names resolved from DNS. The following
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is an example of the line responsible for this order of hostname lookups in the file
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<filename>/etc/nsswitch.conf</filename>.
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hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4
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<emphasis role="bold">files</emphasis> first tries to resolve static hostnames located in
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<filename>/etc/hosts</filename>.
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<emphasis role="bold">mdns4_minimal</emphasis> attempts to resolve the name using Multicast DNS.
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<emphasis role="bold">[NOTFOUND=return]</emphasis> means that any response of
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<emphasis role="italic">notfound</emphasis> by the preceding
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<emphasis role="italic">mdns4_minimal</emphasis> process should be treated as
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authoritative and that the system should not try to continue hunting for an answer.
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<emphasis role="bold">dns</emphasis> represents a legacy unicast DNS query.
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<emphasis role="bold">mdns4</emphasis> represents a Multicast DNS query.
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To modify the order of the above mentioned name resolution methods, you can
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simply change the <emphasis role="italic">hosts:</emphasis> string to the value
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of your choosing. For example, if you prefer to use legacy Unicast DNS versus
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Multicast DNS, you can change the string in <filename>/etc/nsswitch.conf</filename>
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hosts: files dns [NOTFOUND=return] mdns4_minimal mdns4
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<sect2 id="bridging" status="review">
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<title>Bridging</title>
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Bridging multiple interfaces is a more advanced configuration, but is very useful in multiple scenarios.
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One scenario is setting up a bridge with multiple network interfaces, then using a firewall to filter traffic
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between two network segments. Another scenario is using bridge on a system with one interface to allow virtual
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machines direct access to the outside network. The following example covers the latter scenario.
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Before configuring a bridge you will need to install the <application>bridge-utils</application> package. To install the
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package, in a terminal enter:
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<command>sudo apt-get install bridge-utils</command>
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Next, configure the bridge by editing <filename>/etc/network/interfaces</filename>:
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iface lo inet loopback
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iface br0 inet static
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netmask 255.255.255.0
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broadcast 192.168.0.255
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Enter the appropriate values for your physical interface and network.
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Now restart networking to enable the bridge interface:
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<command>sudo /etc/init.d/networking restart</command>
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The new bridge interface should now be up and running. The <application>brctl</application> provides useful information
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about the state of the bridge, controls which interfaces are part of the bridge, etc. See <command>man brctl</command>
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for more information.
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<sect2 id="network-config-resources" status="review">
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<title>Resources</title>
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The <ulink url="https://help.ubuntu.com/community/Network">Ubuntu Wiki Network page</ulink> has
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links to articles covering more advanced network configuration.
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The <ulink url="http://manpages.ubuntu.com/manpages/&distro-short-codename;/en/man5/interfaces.5.html">interfaces man page</ulink> has
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details on more options for <filename>/etc/network/interfaces</filename>.
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The <ulink url="http://manpages.ubuntu.com/manpages/&distro-short-codename;/en/man8/dhclient.8.html">dhclient man page</ulink> has
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details on more options for configuring DHCP client settings.
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For more information on DNS client configuration see the
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<ulink url="http://manpages.ubuntu.com/manpages/&distro-short-codename;/en/man5/resolver.5.html">resolver man page</ulink>. Also, Chapter
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6 of O'Reilly's <ulink url="http://oreilly.com/catalog/linag2/book/ch06.html">Linux Network Administrator's Guide</ulink> is
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a good source of resolver and name service configuration information.
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For more information on <emphasis>bridging</emphasis> see the
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<ulink url="http://manpages.ubuntu.com/manpages/&distro-short-codename;/en/man8/brctl.8.html">brctl man page</ulink> and the Linux Foundation's
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<ulink url="http://www.linuxfoundation.org/en/Net:Bridge">Net:Bridge</ulink> page.
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<sect1 id="tcpip" status="review">
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<title>TCP/IP</title>
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The Transmission Control Protocol and Internet Protocol (TCP/IP) is a standard
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set of protocols developed in the late 1970s by the Defense Advanced Research
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Projects Agency (DARPA) as a means of communication between different types of
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computers and computer networks. TCP/IP is the driving force of the Internet,
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and thus it is the most popular set of network protocols on Earth.
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<sect2 id="tcpip-introduction" status="review">
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<title>TCP/IP Introduction</title>
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The two protocol components of TCP/IP deal with different aspects of computer networking.
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<emphasis>Internet Protocol</emphasis>, the "IP" of TCP/IP is a
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connectionless protocol which deals only with network packet routing using the <emphasis
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role="italics">IP Datagram</emphasis> as the basic unit of networking information. The
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IP Datagram consists of a header followed by a message. The <emphasis>
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Transmission Control Protocol</emphasis> is the "TCP" of TCP/IP and enables network hosts
631
to establish connections which may be used to exchange data streams. TCP also guarantees
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that the data between connections is delivered and that it arrives at one network host in
633
the same order as sent from another network host.
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<sect2 id="tcpip-configuration" status="review">
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<title>TCP/IP Configuration</title>
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The TCP/IP protocol configuration consists of several elements which must be set by
640
editing the appropriate configuration files, or deploying solutions such as the Dynamic
641
Host Configuration Protocol (DHCP) server which in turn, can be configured to provide the
642
proper TCP/IP configuration settings to network clients automatically. These
643
configuration values must be set correctly in order to facilitate the proper network
644
operation of your Ubuntu system.
647
The common configuration elements of TCP/IP and their purposes are as follows:
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<emphasis role="bold">IP address</emphasis> The IP address is a unique
652
identifying string expressed as four decimal numbers ranging from zero (0)
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to two-hundred and fifty-five (255), separated by periods,
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with each of the four numbers representing eight (8) bits of the address for a
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total length of thirty-two (32) bits for the whole address. This format is called
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<emphasis>dotted quad
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notation</emphasis>.</para>
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<emphasis role="bold">Netmask</emphasis> The Subnet Mask (or simply,
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<emphasis>netmask</emphasis>) is a local bit mask, or set of flags
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which separate the portions of an IP address significant to the network from the
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bits significant to the <emphasis>subnetwork</emphasis>. For example,
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in a Class C network, the standard netmask is 255.255.255.0 which masks the first
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three bytes of the IP address and allows the last byte of the IP address to
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remain available for specifying hosts on the subnetwork.
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<emphasis role="bold">Network Address</emphasis> The Network Address represents the
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bytes comprising the network portion of an IP address. For example, the host 12.128.1.2
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in a Class A network would use 12.0.0.0 as the network address, where twelve (12)
675
represents the first byte of the IP address, (the network part) and zeroes (0)
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in all of the remaining three bytes to represent the potential host values. A network
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host using the private IP address 192.168.1.100
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would in turn use a Network Address of 192.168.1.0, which specifies the first three
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bytes of the Class C 192.168.1 network and a zero (0) for all the possible hosts on the
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<emphasis role="bold">Broadcast Address</emphasis> The Broadcast Address is an IP address
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which allows network data to be sent simultaneously to all hosts on a given subnetwork rather
687
than specifying a particular host. The standard general broadcast address for IP networks is
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255.255.255.255, but this broadcast address cannot be used to send a broadcast message to
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every host on the Internet because routers block it. A more appropriate broadcast address
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is set to match a specific subnetwork. For example, on the private Class C IP network,
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192.168.1.0, the broadcast address is 192.168.1.255. Broadcast messages are
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typically produced by network protocols such as the Address Resolution Protocol (ARP) and the
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Routing Information Protocol (RIP).
698
<emphasis role="bold">Gateway Address</emphasis> A Gateway Address is the IP address through which
699
a particular network, or host on a network, may be reached. If one network host wishes to communicate
700
with another network host, and that host is not located on the same network, then a
701
<emphasis>gateway</emphasis> must be used. In many cases, the Gateway Address will be that
702
of a router on the same network, which will in turn pass traffic on to other networks or hosts, such as
703
Internet hosts. The value of the Gateway Address setting must be correct, or your system will not be able
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to reach any hosts beyond those on the same network.
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<emphasis role="bold">Nameserver Address</emphasis> Nameserver Addresses represent the IP addresses of
710
Domain Name Service (DNS) systems, which resolve network hostnames into IP addresses. There are three levels of
711
Nameserver Addresses, which may be specified in order of precedence: The
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<emphasis>Primary</emphasis>
713
Nameserver, the <emphasis>Secondary</emphasis> Nameserver, and the
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<emphasis>Tertiary</emphasis>
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Nameserver. In order for your system to be able to resolve network hostnames into their
716
corresponding IP addresses, you must specify valid Nameserver Addresses which you are authorized to use
717
in your system's TCP/IP configuration. In many cases these addresses can and will be provided by your
718
network service provider, but many free and publicly accessible nameservers are available for use, such as
719
the Level3 (Verizon) servers with IP addresses from
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4.2.2.1 to 4.2.2.6. </para>
723
The IP address, Netmask, Network Address, Broadcast Address, and Gateway Address
724
are typically specified via the appropriate directives in the file <filename>/etc/network/interfaces</filename>. The Nameserver Addresses are typically specified via <emphasis>nameserver</emphasis>
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directives in the file <filename>/etc/resolv.conf</filename>. For more information,
726
view the system manual page for <filename>interfaces</filename> or <filename>resolv.conf</filename> respectively, with the following commands typed at a terminal prompt:
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Access the system manual page for <filename>interfaces</filename> with the following command:
734
<command>man interfaces</command>
738
Access the system manual page for <filename>resolv.conf</filename> with the following command:
742
<command>man resolv.conf</command>
749
<sect2 id="ip-routing" status="review">
750
<title>IP Routing</title>
752
IP routing is a means of specifying and discovering paths in a TCP/IP network along which
753
network data may be sent. Routing uses a set of <emphasis>routing tables</emphasis>
754
to direct the forwarding of network data packets from their source to the destination, often
755
via many intermediary network nodes known as <emphasis>routers</emphasis>.
756
There are two primary forms of
757
IP routing: <emphasis>Static Routing</emphasis> and
758
<emphasis>Dynamic Routing.</emphasis>
761
Static routing involves manually adding IP routes to the system's routing table, and this is usually
762
done by manipulating the routing table with the <application>route</application> command. Static routing enjoys
763
many advantages over dynamic routing, such as simplicity of implementation on smaller networks,
764
predictability (the routing table is always computed in advance, and thus the route is precisely the
765
same each time it is used), and low overhead on other routers and network links due to the lack of a
766
dynamic routing protocol. However, static routing does present some disadvantages as well. For example,
767
static routing is limited to small networks and does not scale well. Static routing also fails completely
768
to adapt to network outages and failures along the route due to the fixed nature of the route.
771
Dynamic routing depends on large networks with multiple possible IP routes from a source to a destination
772
and makes use of special routing protocols, such as the Router Information Protocol (RIP), which handle
773
the automatic adjustments in routing tables that make dynamic routing possible. Dynamic routing
774
has several advantages over static routing, such as superior scalability and the ability to adapt
775
to failures and outages along network routes. Additionally, there is less manual configuration of the
776
routing tables, since routers learn from one another about their existence and available routes. This trait
777
also eliminates the possibility of introducing mistakes in the routing tables via human error.
778
Dynamic routing is not perfect, however, and presents disadvantages such as heightened complexity and
779
additional network overhead from router communications, which does not immediately benefit the end users,
780
but still consumes network bandwidth.
783
<sect2 id="tcp-and-udp" status="review">
784
<title>TCP and UDP</title>
786
TCP is a connection-based protocol, offering error correction and guaranteed delivery of
787
data via what is known as <emphasis>flow control</emphasis>. Flow control
788
determines when the flow of a data stream needs to be stopped, and previously sent data
789
packets should to be re-sent due to problems such as <emphasis>collisions</emphasis>,
790
for example, thus ensuring complete and accurate delivery of the data. TCP is typically
791
used in the exchange of important information such as database transactions.
794
The User Datagram Protocol (UDP), on the other hand, is a <emphasis>connectionless</emphasis>
795
protocol which seldom deals with the transmission of important data because it lacks flow
796
control or any other method to ensure reliable delivery of the data. UDP is commonly used
797
in such applications as audio and video streaming, where it is considerably faster than
798
TCP due to the lack of error correction and flow control, and where the loss of a few packets
799
is not generally catastrophic.
802
<sect2 id="icmp" status="review">
805
The Internet Control Messaging Protocol (ICMP) is an extension to the Internet Protocol (IP) as defined
806
in the Request For Comments (RFC) #792 and supports network packets containing control, error, and
807
informational messages. ICMP is used by such network applications as the <application>ping</application>
808
utility, which can determine the availability of a network host or device. Examples of some error messages
809
returned by ICMP which are useful to both network hosts and devices such as routers, include
810
<emphasis>Destination Unreachable</emphasis> and <emphasis>Time Exceeded</emphasis>.
813
<sect2 id="daemons" status="review">
814
<title>Daemons</title>
816
Daemons are special system applications which typically execute continuously in the background and
817
await requests for the functions they provide from other applications. Many daemons are network-centric; that is,
818
a large number of daemons executing in the background on an Ubuntu system may provide network-related functionality.
819
Some examples of such network daemons include the <emphasis>Hyper Text Transport Protocol Daemon</emphasis>
820
(httpd), which provides web server functionality; the <emphasis>Secure SHell Daemon</emphasis> (sshd), which
821
provides secure remote login shell and file transfer capabilities; and the <emphasis>Internet Message Access
822
Protocol Daemon</emphasis> (imapd), which provides E-Mail services.
825
<sect2 id="tcpip-resources" status="review">
826
<title>Resources</title>
831
There are man pages for <ulink url="http://manpages.ubuntu.com/manpages/&distro-short-codename;/en/man7/tcp.7.html">TCP</ulink> and
832
<ulink url="http://manpages.ubuntu.com/manpages/&distro-short-codename;/man7/ip.7.html">IP</ulink> that contain more useful information.
837
Also, see the <ulink url="http://www.redbooks.ibm.com/abstracts/gg243376.html">TCP/IP Tutorial and Technical Overview</ulink>
843
Another resource is O'Reilly's <ulink url="http://oreilly.com/catalog/9780596002978/">TCP/IP Network Administration</ulink>.
850
<sect1 id="dhcp" status="review">
851
<title>Dynamic Host Configuration Protocol (DHCP)</title>
853
The Dynamic Host Configuration Protocol (DHCP) is a network service that enables
854
host computers to be automatically assigned settings from a server as opposed to
855
manually configuring each network host. Computers configured to be DHCP clients have
856
no control over the settings they receive from the DHCP server, and the
857
configuration is transparent to the computer's user.
860
The most common settings provided by a DHCP server to DHCP clients include:
862
<itemizedlist spacing="compact">
864
<para>IP-Address and Netmask</para>
874
However, a DHCP server can also supply configuration properties such as:
876
<itemizedlist spacing="compact">
878
<para>Host Name</para>
881
<para>Domain Name</para>
884
<para>Default Gateway</para>
887
<para>Time Server</para>
890
<para>Print Server</para>
894
The advantage of using DHCP is that changes to the network, for example a change
895
in the address of the DNS server, need only be changed at the DHCP server, and all
896
network hosts will be reconfigured the next time their DHCP clients poll the
897
DHCP server. As an added advantage, it is also easier to integrate new computers
898
into the network, as there is no need to check for the availability of an IP
899
address. Conflicts in IP address allocation are also reduced.
902
A DHCP server can provide configuration settings using two methods:
906
<term>MAC Address</term>
909
This method entails using DHCP to identify the unique hardware address
910
of each network card connected to the network and then continually
911
supplying a constant configuration each time the DHCP client makes a
912
request to the DHCP server using that network device.
917
<term>Address Pool</term>
920
This method entails defining a pool (sometimes also called a range or
921
scope) of IP addresses from which DHCP clients are supplied their
922
configuration properties dynamically and on a "first come, first served"
923
basis. When a DHCP client is no longer on the network for a specified
924
period, the configuration is expired and released back to the address
925
pool for use by other DHCP Clients.
931
Ubuntu is shipped with both DHCP server and client. The server is
932
<application>dhcpd</application> (dynamic host configuration protocol daemon).
933
The client provided with Ubuntu is <application>dhclient</application> and should
934
be installed on all computers required to be automatically configured. Both
935
programs are easy to install and configure and will be automatically started at
938
<sect2 id="dhcp-installation" status="review">
939
<title>Installation</title>
941
At a terminal prompt, enter the following command to install
942
<application>dhcpd</application>:
945
<command>sudo apt-get install dhcp3-server</command>
948
You will probably need to change the default configuration
949
by editing /etc/dhcp3/dhcpd.conf to suit your needs and particular configuration.
952
You also need to edit /etc/default/dhcp3-server to specify the interfaces dhcpd
953
should listen to. By default it listens to eth0.
956
NOTE: dhcpd's messages are being sent to syslog. Look there for
957
diagnostics messages.
960
<sect2 id="dhcp-configuration" status="review">
961
<title>Configuration</title>
963
The error message the installation ends with might be a little confusing, but the
964
following steps will help you configure the service:
967
Most commonly, what you want to do is assign an IP address randomly. This can be
968
done with settings as follows:
971
# Sample /etc/dhcpd.conf
972
# (add your comments here)
973
default-lease-time 600;
975
option subnet-mask 255.255.255.0;
976
option broadcast-address 192.168.1.255;
977
option routers 192.168.1.254;
978
option domain-name-servers 192.168.1.1, 192.168.1.2;
979
option domain-name "mydomain.example";
981
subnet 192.168.1.0 netmask 255.255.255.0 {
982
range 192.168.1.10 192.168.1.100;
983
range 192.168.1.150 192.168.1.200;
987
This will result in the DHCP server giving a client an IP address from the range
988
192.168.1.10-192.168.1.100 or 192.168.1.150-192.168.1.200. It will lease an IP
989
address for 600 seconds if the client doesn't ask for a specific time frame.
990
Otherwise the maximum (allowed) lease will be 7200 seconds. The server will also
991
"advise" the client that it should use 255.255.255.0 as its subnet mask,
992
192.168.1.255 as its broadcast address, 192.168.1.254 as the router/gateway and
993
192.168.1.1 and 192.168.1.2 as its DNS servers.
996
If you need to specify a WINS server for your Windows clients, you will need to
997
include the netbios-name-servers option, e.g.
1000
option netbios-name-servers 192.168.1.1;
1003
Dhcpd configuration settings are taken from the DHCP mini-HOWTO, which can be found
1004
<ulink url="http://www.tldp.org/HOWTO/DHCP/index.html">here</ulink>.
1007
<sect2 id="dhcp-references" status="review">
1008
<title>References</title>
1013
The <ulink url="https://help.ubuntu.com/community/dhcp3-server">dhcp3-server Ubuntu Wiki</ulink> page has more information.
1018
For more <filename>/etc/dhcp3/dhcpd.conf</filename> options see the
1019
<ulink url="http://manpages.ubuntu.com/manpages/&distro-short-codename;/en/man5/dhcpd.conf.5.html">dhcpd.conf man page</ulink>.
1024
Also see the <ulink url="http://www.dhcp-handbook.com/dhcp_faq.html">DHCP FAQ</ulink>
1032
<sect1 id="NTP" status="review">
1033
<title>Time Synchronisation with NTP</title>
1035
This page describes methods for keeping your computer's time accurate. This is useful for servers, but is not necessary (or desirable) for desktop machines.
1038
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.
1041
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 (often via GPS), 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!
1044
Ubuntu has two ways of automatically setting your time: ntpdate and ntpd.
1047
<sect2 id="ntpdate" status="review">
1048
<title>ntpdate</title>
1050
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. However, a server's clock is likely to drift considerably between reboots, so it makes sense to correct the time occasionally. The easiest way to do this is to get cron to run ntpdate every day. With your favorite editor, as root, create a file
1051
<code>/etc/cron.daily/ntpdate</code>
1055
ntpdate -s ntp.ubuntu.com
1058
The file <code>/etc/cron.daily/ntpdate</code> must also be executable.
1060
<screen>sudo chmod 755 /etc/cron.daily/ntpdate
1063
<sect2 id="ntpd" status="review">
1066
ntpdate is a bit of a blunt instrument - it can only adjust the time once a day, in one big correction. The ntp daemon ntpd is far
1067
more subtle. It calculates the drift of your system clock and continuously adjusts it, so there are no large corrections that could
1068
lead to inconsistent logs for instance. The cost is a little processing power and memory, but for a modern server this is negligible.
1072
To install ntpd, from a terminal prompt enter:
1076
<command>sudo apt-get install ntp</command>
1080
<sect2 id="timeservers" status="review">
1081
<title>Changing Time Servers</title>
1083
In both cases above, your system will use Ubuntu's NTP server at <emphasis>ntp.ubuntu.com</emphasis> by default. This is OK, but you might
1084
want to use several servers to increase accuracy and resilience, and you may want to use time servers that are geographically closer
1085
to you. to do this for ntpdate, change the contents of <filename>/etc/cron.daily/ntpdate</filename> to:
1089
ntpdate -s ntp.ubuntu.com pool.ntp.org
1093
And for ntpd edit <filename>/etc/ntp.conf</filename> to include additional server lines:
1097
server ntp.ubuntu.com
1102
You may notice <emphasis>pool.ntp.org</emphasis> in the examples above. This is a really good idea which uses round-robin DNS to
1103
return an NTP server from a pool, spreading the load between several different servers. Even better, they have pools for different
1104
regions - for instance, if you are in New Zealand, so you could use <emphasis>nz.pool.ntp.org</emphasis> instead of
1105
<emphasis>pool.ntp.org</emphasis>. Look at <ulink url="http://www.pool.ntp.org/">http://www.pool.ntp.org/</ulink> for more details.
1109
You can also Google for NTP servers in your region, and add these to your configuration. To test that a server works, just type:
1113
<command>sudo ntpdate ntp.server.name</command>
1117
<sect2 id="ntp-references" status="review">
1118
<title>References</title>
1123
See the <ulink url="https://help.ubuntu.com/community/UbuntuTime">Ubuntu Time</ulink> wiki page for more information.
1128
<ulink url="http://support.ntp.org/bin/view/Support/WebHome">NTP Support</ulink>
1133
<ulink url="http://www.ntp.org/ntpfaq/NTP-a-faq.htm">The NTP FAQ and HOWTO</ulink>
added
removed
serverguide/C/network-config.xml
