Checking for the Availability of the Loopback Interface

You should not normally have to manually create a loopback interface because Ubuntu creates one automatically for you during installation. To check that one is set up, you can use the ip command with a couple parameters to list all networking interfaces available, including the lo interface if it exists. This example shows only the information for lo:

Click here to view code image

matthew@seymour:~$ ip address show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever

What you see in this example is evidence that the loopback interface is present and active. The inet listed is the IP number assigned to the localhost, typically 127.0.0.1, along with the broadcast mask 255.0.0.0. You can also see the IPv6 address that is assigned to lo, which is ::1/128, referred to as the inet6.

matthew@seymour:~$ ifconfig
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:270 errors:0 dropped:0 overruns:0 frame:0
          TX packets:270 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:20748 (20.7 KB)  TX bytes:20748 (20.7 KB)

Configuring the Loopback Interface Manually

The localhost interface’s IP address is specified in a text configuration file that is used by Ubuntu to keep records of various network-wide IP addresses. The file is called /etc/hosts and usually exists on a system, even if it is empty. The Linux kernel and other networking tools use this file to access local IP addresses and hostnames. If you have not configured any other networking interfaces, you might find that the file looks something like this:

Click here to view code image

127.0.0.1    localhost
127.0.1.1    seymour

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

The first line defines the special localhost interface and assigns it IP address 127.0.0.1. You might hear or read about terms such as localhost, loopback, and dummy interface; all these terms refer to the use of the IP address 127.0.0.1. The term loopback interface is used because, to Linux networking drivers, it looks as though the machine is talking to a network that consists of only one machine; the kernel sends network traffic to and from itself on the same computer. This is sometimes referred to as a dummy interface because the interface doesn’t really exist; it is not a real address as far as the outside world is concerned; it exists only for the local machine, to trick the kernel into thinking that it and any network-aware programs running that require a network interface to operate have one available without them actually being aware that the connection is a connection to the same machine. It is a dummy not in the sense of stupid or silent, but in the sense that it is a mockup or substitute for something real.

Each networked Ubuntu machine on a LAN uses this same IP address for its localhost. If for some reason you discover that an Ubuntu computer does not have this interface, perhaps because some well-meaning person deleted it without understanding it was needed, you can use sudo and edit the /etc/hosts file to add the localhost entry as you saw previously and then use the ifconfig and route commands using your sudo permissions to create the interface, like this:

Click here to view code image

matthew@seymour:~$ sudo ip addr add 127.0.0.1/24 dev lo
matthew@seymour:~$ sudo ip route add 127.0.0.1/24 dev lo

These commands create the localhost interface in memory (all interfaces, such as eth0 or ppp0, are created in memory when using Linux) and then add the IP address 127.0.0.1 to an internal (in-memory) table so that the Linux kernel’s networking code can keep track of routes to different addresses.

Use the ip command as shown previously to test the interface.

TCP/IP Addressing

To understand networking with Linux, you need to know the basics of TCP/IP addressing. Internet IP addresses (also known as public IP addresses) are different from those used internally on a local area network (LAN). Internet IP addresses are assigned (for the United States and some other hosts) by the American Registry for Internet Numbers (ARIN; see www.arin.net). Entities that need Internet addresses apply to this agency to be assigned addresses. ARIN assigns Internet service providers (ISPs) one or more blocks of IP addresses, which the ISPs can then assign to their subscribers.

You will quickly recognize the current form of TCP/IP addressing, known as IP version 4 (IPv4). In this method, a TCP/IP address is expressed as a series of four decimal numbers: a 32-bit value expressed in a format known as dotted-decimal format, such as 192.168.0.1. Each set of numbers is known as an octet (eight 1s and 0s, such as 10000000 to represent 128) and ranges from 0 to 255.

The first octet usually determines what class the network belongs to. There are three classes of networks:

▸ Class A—Consists of networks with the first octet ranging from 1 to 126. There are only 126 Class A networks, each composed of up to 16,777,214 hosts. (If you are doing the math, there are potentially 16,777,216 addresses, but no host portion of an address can be all 0s or 255s.) The 10. network is reserved for local network use, and the 127. network is reserved for the loopback address, 127.0.0.1. TCP/IP uses loopback addressing to enable Linux network-related client and server programs to communicate on the same host. This address does not appear and is not accessible on your LAN.

▸ Class B—Consists of networks defined by the first two octets, with the first ranging from 128 to 191. The 128. network is also reserved for local network use. There are 16,382 Class B networks, each with 65,534 possible hosts.

▸ Class C—Consists of a network defined by the first three octets with the first ranging from 192 to 223. The 192. network is another that is reserved for local network use. There are a possible 2,097,150 Class C networks of up to 254 hosts each.

No host portion of an IP address can be all 0s or 255s. These addresses are reserved for broadcast addresses. IP addresses with all 0s in the host portion are reserved for network-to-network broadcast addresses. IP addresses with all 255s in the host portion are reserved for local network broadcasts. Broadcast messages are not typically seen by users.

These classes are the standard, but a netmask also determines what class your network is in. The netmask determines what part of an IP address represents the network and what part represents the host. Common netmasks for the different classes are as follows:

▸ Class A—255.0.0.0

▸ Class B—255.255.0.0

▸ Class C—255.255.255.0

Because of the allocation of IP addresses for Internet hosts, getting a Class A network is now impossible. Getting a Class B network is also nearly impossible (all the addresses have been given out, but some companies are said to be willing to sell theirs), and Class C network availability is dropping rapidly, with the continued growth of Internet use worldwide.

Using IP Masquerading in Ubuntu

Three blocks of IP addresses are reserved for use on internal networks and hosts not directly connected to the Internet. The address ranges are from 10.0.0.0 to 10.255.255.255, or 1 Class A network; from 172.16.0.0 to 172.31.255.255, or 16 Class B networks; and from 192.168.0.0 to 192.168.255.255, or 256 Class C networks. Use these IP addresses when building a LAN for your business or home. Which class you choose can depend on the number of hosts on your network.

Internet access for your internal network can be provided by another PC or a router. The host or device is connected to the Internet and is used as an Internet gateway to forward information to and from your LAN. The host should also be used as a firewall to protect your network from malicious data and users while functioning as an Internet gateway.

A PC used in this fashion typically has at least two network interfaces. One is connected to the Internet and the other is connected to the computers on the LAN (via a hub or switch). Some broadband devices also incorporate four or more switching network interfaces. Data is then passed between the LAN and the Internet via NAT, sometimes known in networking circles as IP masquerading.

Ports

Most servers on your network perform more than one task. For example, web servers often have to serve both standard and secure pages. You might also be running an FTP server on the same host. For this reason, applications are provided ports to use to make “direct” connections for specific software services. These ports help TCP/IP distinguish services so that data can get to the correct application. If you check the file /etc/services, you see the common ports and their usage. For example, for FTP, HTTP, and POP3 (email retrieval server), you see the following:

Click here to view code image

ftp      21/tcp
http     80/tcp      http     # WorldWideWeb HTTP
pop3     110/tcp     pop-3    # POP version 3

The ports defined in /etc/services in this example are 21 for FTP, 80 for HTTP, and 110 for POP3. Some other common port assignments are 25 for Simple Mail Transfer Protocol (SMTP) and 22 for Secure Shell (SSH) remote login. Note that these ports are not set in stone, and you can set up your server to respond to different ports. For example, although port 22 is listed in /etc/services as a common default for SSH, you can configure the sshd server to listen on a different port by editing its configuration file, /etc/ssh/sshd_config. The default setting (commented out with a pound sign, #) looks like this:

#Port 22

Edit the entry to use a different port, making sure to select an unused port number, as follows:

Port 2224

Save your changes and then restart the sshd server with sudo service ssh restart. Remote users must now access the host through port 2224, which can be done using ssh’s -p (port) option, like this:

Click here to view code image

matthew@seymour:~$ ssh -p 2224 remote_host_name_or_IP

Subnetting

Within Class A and B networks, there can be separate networks called subnets. Subnets are considered part of the host portion of an address for network class definitions. For example, in the 128. Class B network, you can have one computer with address 128.10.10.10 and another with address 128.10.200.20; these computers are on the same network (128.10.), but they have different subnets (128.10.10. and 128.10.200.). Because of this, communication between the two computers requires either a router or a switch. Subnets can be helpful for separating workgroups within a company.

Often subnets can be used to separate workgroups that have no real need to interact with or to shield from other groups’ information passing among members of a specific workgroup. For example, if your company is large enough to have its own HR department and payroll section, you could put those departments’ hosts on their own subnet and use your router configuration to limit the hosts that can connect to this subnet. This configuration prevents networked workers who are not members of the designated departments from being able to view some of the confidential information the HR and payroll personnel work with.

Subnet use also enables your network to grow beyond 254 hosts and share IP addresses. With proper routing configuration, users might not even know they are on a different subnet from their co-workers. Another common use for subnetting is with networks that cover a wide geographic area. It is not practical for a company with offices in Chicago and London to have both offices on the same subnet, so using a separate subnet for each office is the best solution.

Subnet Masks

TCP/IP uses subnet masks to show which part of an IP address is the network portion and which part is the host. Subnet masks are usually referred to as netmasks. For a pure Class A network, the netmask is 255.0.0.0; for a Class B network, the netmask is 255.255.0.0; and for a Class C network, the netmask is 255.255.255.0. You can also use netmasks to deviate from the standard classes.

By using customized netmasks, you can subnet your network to fit your needs. For example, say that your network has a single Class C address. You have a need to subnet your network. Although this is not possible with a normal Class C subnet mask, you can change the mask to break your network into subnets. By changing the last octet to a number greater than zero, you can break the network into as many subnets as you need.

For more information on how to create customized subnet masks, see Day 6, “The Art of Subnet Masking,” in Sams Teach Yourself TCP/IP Network Administration in 21 Days. That chapter goes into great detail on how to create custom netmasks and explains how to create an addressing cheat sheet for hosts on each subnet. The Linux Network Administrators Guide, at www.tldp.org/LDP/nag2/index.html, also has good information about how to create subnets.

Broadcast, Unicast, and Multicast Addressing

Information can get to systems through three types of addresses: unicast, multicast, and broadcast. Each type of address is used according to the purpose of the information being sent, as explained here:

▸ Unicast—Sends information to one specific host. Unicast addresses are used for Telnet, FTP, SSH, or any other information that needs to be shared in a one-to-one exchange of information. Although it is possible that any host on the subnet/network can see the information being passed, only one host is the intended recipient and will take action on the information being received.

▸ Multicasting—Broadcasts information to groups of computers sharing an application, such as a video conferencing client or an online gaming application. All the machines participating in the conference or game require the same information at precisely the same time to be effective.

▸ Broadcasting—Transmits information to all the hosts on a network or subnet. Dynamic Host Configuration Protocol (DHCP) uses broadcast messages when the DHCP client looks for a DHCP server to get its network settings, and Reverse Address Resolution Protocol (RARP) uses broadcast messages for hardware address-to-IP address resolution. Broadcast messages use .255 in all the host octets of the network IP address. (10.2.255.255 broadcasts to every host in your Class B network.)

Network Interface Cards

A computer must have a network interface card (NIC) to connect to a network. Currently, there are several topologies (ways of connecting computers) for network connections. These topologies range from the old and mostly outdated 10BASE-2 to the much newer and popular wireless Wi-Fi, or 802.11, networking.

Each NIC has a unique address (the hardware address, known as Media Access Control [MAC] address), which identifies that NIC. This address is six pairs of hexadecimal bits separated by colons (:). A MAC address looks similar to this: 00:60:08:8F:5A:D9. The hardware address is used by DHCP (see the section “Dynamic Host Configuration Protocol,” later in this chapter) to identify a specific host. In addition, Address Resolution Protocol (ARP) and Reverse Address Resolution Protocol (RARP) use MAC addresses to map hosts to IP addresses.

This section covers some of the different types of NICs used to connect to a network, including several that have long been obsolete but that you may still find in use in older systems.

Network Cable

Currently, three types of network cable are available: coaxial, UTP, and fiber. Coaxial cable looks a lot like the coaxial cable used to connect your television to the cable jack or antenna. UTP looks a lot like the cable that runs from your phone to the wall jack (though the jacks are a bit wider). Fiber cable looks sort of like the RCA cables used on a stereo or like the cable used on electrical appliances in a home (with two separate segments connected together). The following sections discuss UTP and fiber network cable in more detail.

Hubs and Switches

Hubs and switches are used to connect several hosts together in a star architecture network. They can have any number of connections; the common sizes are 4, 8, 16, 24, and 48 connections (ports), and each port has a light that comes on when a network connection is made (link light). Hubs and switches enable you to expand your network easily; you can just add new hubs or switches when you need to add new connections. Each unit can connect to the other hubs or switches on the network, typically through a port on the hub or switch called an uplink port. This means two hubs or switches, connected by their uplink ports, can act as one hub or switch. Having a central location where all the hosts on your network can connect allows for easier troubleshooting of problems. If one host goes down, none of the other hosts are affected (depending on the purpose of the downed host). Because hubs and switches are not directly involved with the Linux operating system, compatibility is not an issue.

If you are constructing a small to midsize network, it is important to consider whether you intend to use either hubs or switches. Hubs and switches are visually the same in that they have rows of network ports. However, under the hood, the difference is quite important. Data is sent as packets of information across the network; with a hub, the data is transmitted simultaneously to all the network ports, regardless of which port the destination computer is attached to.

Switches, however, are more intelligent because they can direct packets of information to the correct network port that leads to the destination computer. They do this by “learning” the MAC addresses of each computer that is attached to them. In short, using switches minimizes excess packets being sent across the network, thus increasing network bandwidth available. In a small network with a handful of computers, the use of hubs might be perfectly acceptable, and you will find that hubs are generally cheaper than switches. However, for larger networks of 15 computers or more, you should consider implementing a switched network.

Routers and Bridges

Routers and bridges are used to connect different networks to your network and to connect different subnets within your network. Routers and bridges both serve the same purpose of connecting networks and subnets, but they do so using different techniques. The information in the following sections helps you choose the connection method that best suits your needs.

Initializing New Network Hardware

All the initial network configuration and hardware initialization for Ubuntu is normally done during installation. At times, however, you may have to reconfigure networking on your system, such as when a host needs to be moved to a different subnet or a different network, or if you replace any of your computer’s networking hardware.

Linux creates network interfaces in memory when the kernel recognizes that a NIC or another network device is attached to the system. These interfaces are unlike other Linux interfaces, such as serial communications ports, and they do not have a corresponding device file in the /dev directory. Unless support for a particular NIC is built in to your kernel, Linux must be told to load a specific kernel module to support your NIC. More than 100 such modules are located in the /lib/modules/5.5.XX-XX/kernel/net directory (where XX-XX is your version of the kernel).

You can initialize a NIC in several ways when using Linux. When you first install Ubuntu, automatic hardware probing detects and configures your system to use any installed NICs. If you remove the original NIC and replace it with a different make and model, your system will not automatically detect and initialize the device unless you configure Ubuntu to use automatic hardware detection when booting. Ubuntu should detect the absence of the old NIC and the presence of the new NIC at boot time.

If you do not use automatic hardware detection and configuration, you can initialize network hardware by doing the following:

▸ Manually editing the /etc/modprobe.conf file to prompt the system to recognize and support the new hardware upon reboot

▸ Manually loading or unloading the new device’s kernel module with the modprobe command

The following sections explain these methods in greater detail.

alias eth0 8139too

alias eth0 eepro100

matthew@seymour:~$ sudo modprobe 8139too

matthew@seymour:~$ dmesg
...
eth0: RealTek RTL8139 Fast Ethernet at 0xce8ee000, 00:30:1b:0b:07:0d, IRQ 11
eth0: Identified 8139 chip type ôRTL-8139C'
eth0: Setting half-duplex based on auto-negotiated partner ability 0000.
...

matthew@seymour:~$ dmesg
...
eepro100.c:v1.09j-t 9/29/99 Donald Becker http://cesdis.gsfc.nasa.gov/linux/drive
rs/eepro100.html
eepro100.c: $Revision: 1.36 $ 2000/11/17 Modified by Andrey V. Savochkin
&#x194;<[email protected]> and others
PCI: Found IRQ 10 for device 00:0d.0
eth0: Intel Corporation 82557 [Ethernet Pro 100], 00:90:27:91:92:B5, IRQ 10.
 Board assembly 721383-007, Physical connectors present: RJ45
 Primary interface chip i82555 PHY #1.
 General self-test: passed.
 Serial sub-system self-test: passed.
 Internal registers self-test: passed.
 ROM checksum self-test: passed (0x04f4518b).
...

Command-Line Network Interface Configuration

You can configure a network interface from the command line by using the basic Linux networking utilities. You configure your network client hosts either with commands to change your current settings or by editing a number of system files. Traditionally, two commands, ifconfig (which has generally been abandoned for ip, as mentioned earlier in this chapter) and ip route, are used for network configuration. The netstat command displays information about the network connections.

ifconfig [network device] options

matthew@seymour:~$ ifconfig
eth0      Link encap:Ethernet  HWaddr 00:90:f5:8e:52:b5
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
          Interrupt:30 Base address:0xc000
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:314 errors:0 dropped:0 overruns:0 frame:0
          TX packets:314 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:25204 (25.2 KB)  TX bytes:25204 (25.2 KB)
wlan0     Link encap:Ethernet  HWaddr 00:16:ea:d4:58:88
          inet addr:192.168.1.106  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::216:eaff:fed4:5888/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:325832 errors:0 dropped:0 overruns:0 frame:0
          TX packets:302754 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:207381807 (207.3 MB)  TX bytes:40442735 (40.4 MB)

matthew@seymour:~$ sudo ifconfig eth0 down

matthew@seymour:~$ sudo ifconfig eth0 192.168.2.9 netmask 255.255.255.0 up

matthew@seymour:~$ sudo ifconfig eth0 catcat.fakeurl.com up

matthew@seymour:~$ sudo ip addr show

matthew@seymour:~$ sudo ip addr add 192.168.2.9 dev eth1

matthew@seymour:~$ sudo ip addr del 192.168.2.9 dev eth1

matthew@seymour:~$ sudo ip link set eth1 up

matthew@seymour:~$ sudo ip link set eth1 down

matthew@seymour:~$ sudo ip route show

matthew@seymour:~$ sudo ip route add 10.10.30.0/24 via 192.168.50.100 dev eth0

matthew@seymour:~$ sudo ip route del 10.10.30.0/24

matthew@seymour:~$ sudo ip route add default via 192.168.36.100

matthew@seymour:~$ ip route
default via 192.168.1.1 dev enp1s0 proto static metric 100
169.254.0.0/16 dev enp1s0 scope link metric 1000
192.168.1.0/24 dev enp1s0 proto kernel scope link src 192.168.1.148 metric 100

Network Configuration Files

As previously stated, five network configuration files can be modified to make changes to basic network interaction of your system:

▸ /etc/hosts—A listing of addresses, hostnames, and aliases

▸ /etc/services—Network service and port connections

▸ /etc/nsswitch.conf—Linux network information service configuration

▸ /etc/resolv.conf—Domain Name System (DNS) domain (search) settings

▸ /etc/host.conf—Network information search order (by default, /etc/hosts and then DNS)

After these files are modified, the changes are active. With most configuration files, you can add comments with a hash mark (#) preceding a comment. All these files have man pages where you can find more information.

127.0.0.1       localhost
127.0.1.1       optimus
# 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
ff02::3 ip6-allhosts

# Each line describes one service, and is of the form:
#
# service-name port/protocol [aliases ...]  [# comment]

tcpmux     1/tcp              # TCP port service multiplexer
tcpmux     1/udp              # TCP port service multiplexer
rje        5/tcp              # Remote Job Entry
rje        5/udp              # Remote Job Entry
echo       7/tcp
echo       7/udp
discard    9/tcp      sink null
discard    9/udp      sink null
systat     11/tcp     users

passwd:         compat
group:          compat
shadow:         compat

hosts:          files dns mdns
networks:       files
protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis

nameserver 192.172.3.8
nameserver 192.172.3.9
search mydomain.com

# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager

matthew@seymour:~$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp2s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN group default qlen 1000
    link/ether d8:d3:85:94:5d:3f brd ff:ff:ff:ff:ff:ff
3: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether d8:d3:85:94:5d:3e brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.149/24 brd 192.168.1.255 scope global dynamic noprefixroute enp1s0

network:
    Version: 2
    Renderer: NetworkManager/ networkd
    ethernets:
       DEVICE_NAME:
          Dhcp4: yes/no
          Addresses: [IP_ADDRESS/NETMASK]
          Gateway: GATEWAY
          Nameservers:
             Addresses: [NAMESERVER_1, NAMESERVER_2]
       DEVICE_NAME:
          Dhcp4: yes/no
          Addresses: [IP_ADDRESS/NETMASK]
          Gateway: GATEWAY
          Nameservers:
             Addresses: [NAMESERVER_1, NAMESERVER_2]

matthew@seymour:~$ sudo netplan try
[sudo] password for matthew:
Do you want to keep these settings?

Press ENTER before the timeout to accept the new configuration

Changes will revert in 112 seconds
Configuration accepted.

matthew@seymour:~$ sudo netplan apply

matthew@seymour:~$ sudo netplan -d apply

matthew@seymour:~$ sudo systemctl restart network-manager

matthew@seymour:~$ sudo systemctl restart system-networkd

order hosts, bind

Using Graphical Configuration Tools

Ubuntu provides options for desktop users to configure networking using graphical configuration tools. In most cases, all you need to know is contained in Chapter 1, “Installing Ubuntu and Post-Installation Configuration,” in the section “Configuring Wireless Networks.” Power users and unique setups generally eschew the GUI and use command-line tools.

How DHCP Works

DHCP provides persistent storage of network parameters by holding identifying information for each network client that might connect to the network. The three most common pairs of identifying information are as follows:

▸ Network subnet/host address—Enables hosts to connect to the network at will

▸ Subnet/hostname—Enables the specified host to connect to the subnet

▸ Subnet/hardware address—Enables a specific client to connect to the network after getting the hostname from DHCP

DHCP also allocates to the client’s temporary or permanent network (IP) addresses. When a temporary assignment, known as a lease, elapses, the client can request to have the lease extended, or, if the address is no longer needed, the client can relinquish the address. For hosts that will be permanently connected to a network with adequate addresses available, DHCP allocates infinite leases.

DHCP offers your network some advantages. First, it shifts responsibility for assigning IP addresses from the network administrator (who can accidentally assign duplicate IP addresses) to the DHCP server. Second, DHCP makes better use of limited IP addresses. If a user is away from the office for whatever reason, the user’s host can release its IP address for use by other hosts.

Like most other things in life, DHCP is not perfect. Servers cannot be configured through DHCP alone because DNS does not know what addresses DHCP assigns to a host. This means that DNS lookups are not possible on machines configured through DHCP alone; therefore, services cannot be provided. However, DHCP can make assignments based on DNS entries when using subnet/hostname or subnet/hardware address identifiers.

Activating DHCP at Installation and Boot Time

Ubuntu automatically defaults your network interfaces to using DHCP because it is the simplest way of setting up a network interface. With dynamic, or DHCP-assigned IP addressing schemes for your NIC, the broadcast address is set at 255.255.255.255 because dhclient, the DHCP client used for IP configuration, is initially unaware of where the DHCP server is located, so the request must travel every network until a server replies.

You can find the instruction to use DHCP for your NIC in /etc/netplan/*.yaml, in a line that says dhcp.

Other settings specific to obtaining DHCP settings are saved in the file /etc/dhcp/dhclient.conf and are documented in the dhclient.conf man page. More than 100 options are also documented in the dhcpoptions man page.

However, using DHCP is not very complicated. If you want to use DHCP and know that there is a server on your network, you can quickly configure your NIC by using the dhclient, as follows:

Click here to view code image

matthew@seymour:~$ sudo dhclient
Internet Systems Consortium DHCP Client V3.1.3
Copyright 2004-2009 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/eth0/00:90:f5:8e:52:b5
Sending on   LPF/eth0/00:90:f5:8e:52:b5
Listening on LPF/virbr0/ee:1a:62:7e:e2:a2
Sending on   LPF/virbr0/ee:1a:62:7e:e2:a2
Listening on LPF/wlan0/00:16:ea:d4:58:88
Sending on   LPF/wlan0/00:16:ea:d4:58:88
Sending on   Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 7
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 3
DHCPOFFER of 192.168.1.106 from 192.168.1.1
DHCPREQUEST of 192.168.1.106 on wlan0 to 255.255.255.255 port 67
DHCPACK of 192.168.1.106 from 192.168.1.1
bound to 192.168.1.106 -renewal in 35959 seconds.

In this example, the first Ethernet device, eth0, has been assigned IP address 192.168.1.106 from a DHCP server at 192.168.1.1. The renewal will take place in 35959 seconds, or about 10 hours. (Cool tip: Google converts this for you if you search for “35959 seconds in hours.”)

DHCP Software Installation and Configuration

Installation of the DHCP client and server is fairly straightforward, mainly because Ubuntu already includes dhclient in a default installation but also because installing software is easy using synaptic or apt-get.

matthew@seymour:~$ sudo cp /etc/dhcp/dhclient.conf/etc/dhcp/dhclient.conf.backup

Using DHCP to Configure Network Hosts

Configuring your network with DHCP can look difficult but is actually easy if your needs are simple. The server configuration can take a bit of work depending on the complexity of your network and how much you want DHCP to do.

Configuring the server takes some thought and a little bit of work. Luckily, the work involves editing only a single configuration file, /etc/dhcp/dhcpd.conf. To start the server at boot time, use the systemd enable command.

The /etc/dhcp3/dhcpd.conf file contains all the information needed to run dhcpd. Ubuntu includes a sample dhcpd.conf file in /usr/share/doc/dhcp*/dhcpd.conf.sample. The DHCP server source files also contain a sample dhcpd.conf file.

You can think of the /etc/dhcp/dhcpd.conf file at as a three-part file. The first part contains the following configurations for DHCP itself:

▸ Setting the domain name—option domain-name "example.org"

▸ Setting DNS servers—option domain-name-servers ns1.example.org and ns2.example.org (IP addresses can be substituted.)

▸ Setting the default and maximum lease times—default-lease-time 3600 and max-lease-time 14400

Other settings in the first part include whether the server is the primary (authoritative) server and what type of logging DHCP should use. These settings are considered defaults, and you can override them with the subnet and host portion of the configuration in more complex situations.

The next part of the dhcpd.conf file deals with the different subnets that your DHCP server serves; this section is quite straightforward. Each subnet is defined separately and can look like this:

Click here to view code image

subnet 10.5.5.0 netmask 255.255.255.224 {
 range 10.5.5.26 10.5.5.30;
 option domain-name-servers ns1.internal.example.org;
 option domain-name "internal.example.org";
 option routers 10.5.5.1;
 option broadcast-address 10.5.5.31;
 default-lease-time 600;
 max-lease-time 7200;
}

This defines the IP addressing for the 10.5.5.0 subnet. It defines the IP address range from 10.5.5.26 through 10.5.5.30 to be dynamically assigned to hosts that reside on that subnet. This example shows that you can set any TCP/IP option from the subnet portion of the configuration file. It shows which DNS server the subnet will connect to, which can be good for DNS server load balancing, or which can be used to limit the hosts that can be reached through DNS. It defines the domain name, so you can have more than one domain on your network. It can also change the default and maximum lease times.

If you want your server to ignore a specific subnet, you can do so as follows:

Click here to view code image

subnet 10.152.187.0 netmask 255.255.255.0 {
}

This defines no options for the 10.152.187.0 subnet; therefore, the DHCP server ignores it.

The last part of the dhcpd.conf file is for defining hosts. This can be good if you want a computer on your network to have a specific IP address or other information specific to that host. The key to completing the host section is to know the hardware address of the host. As you learned in the “Hardware Devices for Networking” section, earlier in this chapter, the hardware address is used to differentiate the host for configuration. You can obtain your hardware address by using the ip command, as described previously:

Click here to view code image

host hopper {
  hardware ethernet 08:00:07:26:c0:a5;
  fixed-address hopper.matthewhelmke.com;
}

This example takes the host with the hardware address 08:00:07:26:c0:a5 and does a DNS lookup to assign the IP address for hopper.matthewhelmke.com to the host.

DHCP can also define and configure booting for diskless clients, like this:

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host bumblebee {
  hardware ethernet 0:0:c0:5d:bd:95;
  filename "vmunix.bumblebee";
  server-name "kernigan.matthewhelmke.com";
}

The diskless host bumblebee gets its boot information from server kernigan.matthewhelmke.com and uses vmunix.bumblebee kernel. All other TCP/IP configuration can also be included.

Other Uses for DHCP

A whole host of options can be used in dhcpd.conf: Entire books are dedicated to DHCP. The most comprehensive book is The DHCP Handbook, by Ralph Droms and Ted Lemon. You can define NIS domains, configure NetBIOS, set subnet masks, and define time servers or many other types of servers (to name a few of the DHCP options you can use). The preceding example gets your DHCP server and client up and running.

The DHCP server distribution contains an example of the dhcpd.conf file that you can use as a template for your network. The file shows a basic configuration that can get you started with explanations for the options used.

Support for Wireless Networking in Ubuntu

The Linux kernel that ships with Ubuntu provides extensive support for wireless networking. Related wireless tools for configuring, managing, or displaying information about a wireless connection include the following:

▸ iwconfig—Sets the network name, encryption, transmission rate, and other features of a wireless network interface

▸ iwlist—Displays information about a wireless interface, such as rate, power level, or frequency used

▸ iwpriv—Sets optional features of a wireless network interface, such as roaming

▸ iwspy—Shows wireless statistics of a number of nodes

Support varies for wireless devices, but most modern (that is, post-2005) wireless devices should work with Ubuntu. In general, Linux wireless device software (usually in the form of a kernel module) supports the creation of an Ethernet device that can be managed using traditional interface tools such as ifconfig—with wireless features of the device managed by the various wireless software tools.

For example, when a wireless networking device is first recognized and initialized for use, the driver most likely reports a new device, like so:

Click here to view code image

zd1211rw 5-4:1.0: firmware version 4725

zd1211rw 5-4:1.0: zd1211b chip 050d:705c v4810 
high 00-17-3f AL2230_RF pa0 G—ns

zd1211rw 5-4:1.0: eth2

usbcore: registered new interface driver zd1211rw

This output (from the dmesg command) shows that the eth2 device has been reported. If DHCP is in use, the device should automatically join the nearest wireless subnet and be automatically assigned an IP address. If not, the next step is to use a wireless tool such as iwconfig to set various parameters of the wireless device. The iwconfig command, along with the device name (eth2 in this example), shows the status:

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matthew@seymour:~$ iwconfig eth2
eth2      IEEE 802.11b/g  ESSID:"SKY35120"  Nickname:"zd1211"
          Mode:Managed  Frequency:2.462 GHz  
Access Point: 00:18:4D:06:8E:2A
          Bit Rate=24 Mb/s
          Encryption key:0EFD-C1AF-5C8D-B2C6-7A89-3790-07A7-AC64-0AB5
-C36E-D1E9-A230-1DB9-D227-2EB6-D6C8   Security mode:open
          Link Quality=100/100  Signal level=82/100
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

This example shows a 24Mbps connection to a network named SKY35120. To change a parameter, such as the transmission rate, use a command-line option with the iwconfig command like this:

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matthew@seymour:~$ sudo iwconfig eth2 rate 11M

Other options supported by the iwconfig command include essid, to set the NIC to connect to a specific network by name; mode, to enable the NIC to automatically retrieve settings from an access point or connect to another wireless host; and freq, to set a frequency to use for communication. Additional options include channel, frag, enc (for encryption), power, and txpower. Details and examples of these options are in the iwconfig man page.

You can then use the ifconfig command or perhaps a graphical Ubuntu tool to set the device networking parameters, and the interface will work as on a hardwired LAN. One handy output of the iwconfig command is the link quality output, which you can use in shell scripts or other graphical utilities for signal-monitoring purposes.

Choosing from Among Available Wireless Protocols

The Institute of Electrical and Electronics Engineers (IEEE) started to look seriously at wireless networking in 1990. This is when the 802.11 standard was first introduced by the Wireless Local Area Networks Standards Working Group. The group based the standard roughly around the architecture used in cellular phone networks. A wireless network is controlled by a base station, which can be just a transmitter attached to the network or, more commonly these days, a router.

A larger network may use more than one base station. Networks with more than one base station are usually referred to as distribution systems. You can use a distribution system to increase coverage area and support roaming of wireless hosts. You can also use external omnidirectional antennas to increase the coverage area or, if required, you can use point-to-point or directional antennas to connect distant computers or networks.

The 802.11 standard specifies that wireless devices use a frequency range of 2400MHz to 2483.5MHz. This is the standard used in North America and Europe. In Japan, however, wireless networks are limited to a frequency range of 2471MHz to 2479MHz. Within these ranges, each network is given up to 79 nonoverlapping frequency channels to use. This reduces the chance of two closely located wireless networks using the same channel at the same time. It also allows for channel hopping, which can be used for security.

Configuring Digital Subscriber Line Access

Ubuntu also supports the use of a digital subscriber line (DSL) service. Ubuntu refers to the different types of DSL available as xDSL (which includes ADSL, IDSL, SDSL, and other flavors of DSL service), and you can configure all of them by using the Internet Connection Wizard. DSL service generally provides 256Kbps to 24Mbps transfer speeds and transmits data over copper telephone lines from a central office to individual subscriber sites (such as your home). Many DSL services (technically, cable rather than DSL) provide asymmetric speeds with download speeds greater than upload speeds.

A DSL connection requires that you have an Ethernet NIC (sometimes a USB interface that is not easily supported in Linux) in your computer or notebook. Many users also configure a gateway, firewall, or other computer with at least two NICs to share a connection with a LAN. We looked at the hardware and protocol issues earlier in this chapter. Advanced configuration of a firewall or router, other than what was addressed during your initial installation of Ubuntu, is beyond the scope of this book.

Understanding PPP over Ethernet

Establishing a DSL connection with an ISP providing a static IP address is easy. Unfortunately, many DSL providers use a type of PPP called Point-to-Point Protocol over Ethernet (PPPoE) that provides dynamic IP address assignment and authentication by encapsulating PPP information inside Ethernet frames. Roaring Penguin’s rp-pppoe clients are available from the Roaring Penguin site (www.roaringpenguin.com/files/download/rp-pppoe-3.11.tar.gz), and these clients make the difficult-to-configure PPPoE connection much easier to deal with. You can download and install newer versions.

Configuring a PPPoE Connection Manually

You should need to use the steps in this section only if you are using a modem supplied by your ISP and not a router. The basic steps involved in manually setting up a DSL connection using Ubuntu involve connecting the proper hardware and then running a simple configuration script if you use rp-pppoe from Roaring Penguin.

First, connect your DSL modem to your telephone line and then plug in your Ethernet cable from the modem to your computer’s NIC. If you plan to share your DSL connection with the rest of your LAN, you need at least two network cards, designated eth0 (for your LAN) and eth1 (for the DSL connection).

The following example assumes that you have more than one computer and will share your DSL connection on a LAN.

First, log in as root and ensure that your first eth0 device is enabled and up (perhaps using the ifconfig command). Next, bring up the other interface but assign a null IP address like this:

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matthew@seymour:~$ sudo ifconfig eth1 0.0.0.0 up

Now use the adsl-setup command to set up your system, as follows:

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matthew@seymour:~$ sudo /sbin/adsl-setup

You are presented with a text script and asked to enter your username and the Ethernet interface used for the connection (such as eth1). You are then asked to use “on-demand” service or have the connection stay up all the time (until brought down by the root operator). You can also set a timeout, in seconds, if desired. You are then asked to enter the IP addresses of your ISP’s DNS servers if you haven’t configured the DNS information on the network interface.

After that, you are prompted to enter your password two times and must choose the type of firewall and IP masquerading to use. (You learned about IP masquerading in the “Using IP Masquerading in Ubuntu” section, earlier in this chapter.) The actual configuration is done automatically. Using a firewall is essential, so choose this option unless you intend to craft your own set of firewall rules (a discussion of which is beyond the scope of this book). After you have chosen your firewall and IP masquerading setup, you are asked to confirm, save, and implement your settings. You are also given a choice to allow users to manage the connection, which is a handy option for home users.

Changes are made to your system’s /etc/netplan/*.yaml, /etc/ppp/pap-secrets, and /etc/ppp/chap-secrets files.

After configuration has finished, use the adsl-start command to start a connection and DSL session, like this:

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matthew@seymour:~$ sudo /sbin/adsl-start

The DSL connection should be nearly instantaneous, but if problems occur, check to make sure that your DSL modem is communicating with the phone company’s central office by examining the status LEDs on the modem. This varies from modem to modem, so consult your modem user’s manual.

Make sure all cables are properly attached, that your interfaces are properly configured, and that you have entered the correct information to the setup script.

If IP masquerading is enabled, other computers on your LAN on the same subnet address (such as 192.168.0.XXX) can use the Internet but must have the same name server entries and a routing entry with the DSL-connected computer as a gateway. For example, if the host computer with the DSL connection has IP address 192.168.0.1, and other computers on your LAN use addresses in the 192.168.0.XXX range, use the route command on each computer like this:

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matthew@seymour:~$ sudo route add default gw 192.168.0.1

Note that you can also use a hostname instead if each computer has an /etc/hosts file with hostname and IP address entries for your LAN. To stop your connection, use the adsl-stop command:

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matthew@seymour:~$ sudo /sbin/adsl-stop

Configuring Dial-up Internet Access

Most ISPs provide dial-up connections that support PPP because it is a fast and efficient protocol for using TCP/IP over serial lines. PPP is designed for two-way networking, and TCP/IP provides the transport protocol for data. One hurdle faced by new Ubuntu users is how to set up PPP and connect to the Internet. Understanding the details of the PPP protocol is not necessary to use it, and setting up a PPP connection is easy. You can configure PPP connections manually by using the command line or graphically during an X session using Ubuntu’s Network Configuration tool. These approaches produce the same results.

PPP uses several components on your system. The first is a daemon called pppd, which controls the use of PPP. The second is a driver called the high-level data link control (HDLC), which controls the flow of information between two machines. A third component of PPP is a routine called chat that dials the other end of the connection for you when you want it to. Although PPP has many “tunable” parameters, the default settings work well for most people.

Ubuntu includes some useful utilities to get your dial-up connection up and running. In this section, we look at two options that will have you on the Internet in no time.

The first way is to configure a connection using pppconfig, a command-line utility to help you configure specific dial-up connection settings.

Enter the following command:

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matthew@seymour:~$ sudo pppconfig

Before you connect for the first time, you need to add yourself to both the dip and dialout groups by using these commands:

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matthew@seymour:~$ sudo adduser YOURNAMEHERE dip
matthew@seymour:~$ sudo adduser YOURNAMEHERE dialout

After you have done this, it is just a simple matter of issuing the pon command to connect and the poff command to disconnect. You can create as many different profiles as you need and can launch specific ones by using the command pon profilename, again using the poff command to disconnect.

Troubleshooting Connection Problems

The Linux Documentation Project (www.tldp.org) offers many in-depth resources for configuring and troubleshooting connection problems. Google is also an invaluable tool for dealing with specific questions about connections. For many other useful references, see the “References” section at the end of this chapter.

Here are a few troubleshooting tips culled from many years of experience:

▸ If your modem connects and then hangs up, you are probably using the wrong password or dialing the wrong number. If the password and phone number are correct, it is likely an authentication protocol problem.

▸ If you get connected but cannot reach websites, it is likely a domain name resolver problem, meaning that DNS is not working. If it worked yesterday and you haven’t “adjusted” the associated files, it is probably a problem on the ISP’s end. Call and ask.

▸ Always make certain that everything is plugged in. Check again (and again).

▸ If the modem works in Windows but not in Linux no matter what you do, it is probably a software modem no matter what it said on the box.

▸ If everything just stops working (and you do not see smoke), it is probably a glitch at the ISP or the telephone company. Take a break and give them some time to fix it.

▸ Never configure a network connection when you have had too little sleep or too much caffeine; you will just have to redo it tomorrow.