Tutorial For Lpi Exam 202: Part 1

Topic 205: Network Configuration

David Mertz, Ph.D.
Professional Neophyte
September, 2005

Welcome to "Network Configuration", the first of seven tutorials covering intermediate network administration on Linux. In this tutorial, you will learn how to configure a basic TCP/IP network, from the hardware layer (usually ethernet, modem, ISDN, or 802.11), through the routing of network addresses. Higher level servers that may operate on these configured networks are covered in later tutorials.

Before You Start

About this series

The Linux Professional Institute (LPI) certifies Linux system administrators at junior and intermediate levels. There are two exams at each certification level. This series of seven tutorials helps you prepare for the second of the two LPI intermediate level system administrator exams--LPI exam 202. A companion series of tutorials is available for the other intermediate level exam--LPI exam 201. Both exam 201 and exam 202 are required for intermediate level certification. Intermediate level certification is also known as certification level 2.

Each exam covers several or topics and each topic has a weight. The weight indicate the relative importance of each topic. Very roughly, expect more questions on the exam for topics with higher weight. The topics and their weights for LPI exam 202 are:

Topic 205: Network Configuration (8) Topic 206: Mail and News (9) Topic 207: Domain Name System (DNS) (8) Topic 208: Web Services (6) Topic 210: Network Client Management (6) Topic 212: System Security (10) * Topic 214: Network Troubleshooting (1)

About this tutorial

Welcome to "Network Configuration", the first of seven tutorials covering intermediate network administration on Linux. In this tutorial, you will learn how to configure a basic TCP/IP network, from the hardware layer (usually ethernet, modem, ISDN, or 802.11), through the routing of network addresses. Higher level servers that may operate on these configured networks are covered in later tutorials.


To get the most from this tutorial, you should already have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this tutorial.

About network configuration

Network layers

It is useful in thinking about Linux networking, and this network configuration tutorial, to keep in mind the OSI seven layer Reference Model. What we call "network configuration" essentially lives on the second and third layers: Data Link Layer and Network Layer, and in the interfaces between them. In practice, this amounts to either ethernet or serial interfaces like modems the Data Link Layer, and Internet Protocol (IP) for the Network Layer. Later tutorials in this series deal with higher level layers, though most server applications discussed do not cleanly separate all seven layers (or even the top four where they operate)

The first network layer is the Physical Layer: actual wires (or wireless channels) and circuits. A practical network administrator needs to be ready to inspect cabling and install new network peripherals, from time-to-time; but those issues are mostly not in scope of these tutorials. Clearly, however, a loose wire, fried ethernet card, or broken plug is just as capable of creating network problems as is misconfigured software.

Layer 4 is the Transport Layer, which concretely means either TCP or UDP in IP networks. TCP and UDP are utilized at higher levels via the Berkeley Sockets Interface, which is a well-tested library on all modern computer systems. For background on how applications (such as those discussed in later tutorials of this series) utilize TCP or UDP read:

Programming Linux sockets, Part 1 http://www-128.ibm.com/developerworks/edu/l-dw-linux-sock-i.html
Programming Linux sockets, Part 2 http://www-128.ibm.com/developerworks/edu/l-dw-linux-sock2-i.html

Other resources

As with most Linux tools, it is always useful to examine the manpages for any utilities discussed. Versions and switches might change between utility or kernel version, or with different Linux distributions. For more in depth information, the Linux Documentation Project has a variety of useful documents, especially its HOWTOs. See http://www.tldp.org/. A variety of books on Linux networking have been published; I have found O'Reilly's TCP/IP Network Administration, by Craig Hunt to be quite helpful (find whatever edition is most current when you read this).

Basic Networking Configuration

Address Resolution Protocol

The first thing to understand about ethernet devices, either 802.11a/b/g wireless or more traditional CAT5/CAT6 wired networks is that every ethernet device has a unique six-byte ID in it. Those IDs are assigned in blocks to manufacturers, and you can look up the assignments at <http://www.iana.org/assignments/ethernet-numbers>. Ethernet generally "just works" at the hardware level, but a system needs to map an ethernet ID to the IP address it will use to enable IP networking.

The Address Resolution Protocol (ARP) lets machines discover each others' IP address within a local ethernet network. ARP, as a protocol, is generally implemented within network device drivers (kernel modules); the tool arp lets you examine the status of the ARP system, and tweak it in some ways. At this point, we assume that each machine has been configured to know its own IP address, either by static assignment or dynamically with DHCP.

When a Linux system (or any device with ethernet) wishes to address an IP address, the ARP request message, "who is X.X.X.X tell Y.Y.Y.Y" is sent using the Ethernet broadcast address. The target system forms an ARP response "X.X.X.X is hh:hh:hh:hh:hh:hh", and sends it to the requesting host. An ARP response is cached for a short time in /proc/net/arp to avoid the need to continually reestablish the mapping between hardware ethernet addresses and IP addresses.

A nice description of ARP can be found at <http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/arp.html>.

The arp utility

The Linux utility arp lets you examine and modify the status of ARP mappings. A simple status report might look like:

ARP status report

$ arp -n
Address          HWtype  HWaddress           Flags Mask    Iface      ether   00:03:2F:09:61:C7   C             eth0

This tells you the specific hardware device assigned to IP address on this network (the number used is suggestive of a router/gateway, which indeed it is in this case). The fact that only this single mapping is listed does not necessarily mean that no other devices exist on the local network, but simply that the ARP records for other devices have expired. ARP expires records after a short time--on the order of minutes, rather than seconds or hours--to allow networks to reconfigure themselves if hosts are added or removed, or if settings are changed on machines. By caching for an ARP record for a short time, a new request should not be necessary during most client/server application sessions.

Any sort of IP request on a host that may be on the local network causes the kernel to send out an ARP request, and if an ARP reply is received, add the host to the ARP cache. For example:

Interacting with additional IP addresses

$ ping -c 1 > /dev/null
$ ping -c 1 > /dev/null
$ ping -c 1 > /dev/null
$ ping -c 1 > /dev/null
$ ping -c 1 > /dev/null
$ arp -n
Address          HWtype  HWaddress           Flags Mask    Iface      ether   00:03:2F:09:61:C7   C             eth0    ether   00:30:65:2C:01:11   C             eth0    ether   00:11:24:9D:1E:4B   C             eth0    ether   00:48:54:83:82:AD   C             eth0

In this case, the first four addresses really exist on the local ethernet network, but does not, and so no ARP reply is received. Notice also that addresses you may succeed in connecting to via non-local routing will also not cause anything to be added to the ARP cache. For example:

$ ping -c 1 google.com
PING google.com ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=235 time=109 ms
--- google.com ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 109.123/109.123/109.123/0.000 ms
$ arp -n
Address          HWtype  HWaddress           Flags Mask    Iface      ether   00:03:2F:09:61:C7   C             eth0

Google is reachable (because routing is already configured), but is non-local and is not added to ARP.

Also see Topic 214: Network Troubleshooting/Manually setting ARP.


Point-to-Point Protocol (PPP) is used to establish internet links over dial-up modems, direct serial connections, DSL, and other types of point-to-point links, including sometimes as PPPoE (as a "pseudo-layer over ethernet). The pppd daemon works together with the kernel PPP driver to establish and maintain a PPP link with another system (called the peer) and to negotiate Internet Protocol (IP) addresses for each end of the link.

PPP, specifically pppd authenticate its peer and/or supply authentication information to the peer. Such authentication is performed using either the simple password system Password Authentication Protocol (PAP) or the per-session Challenge Handshake Authentication Protocol (CHAP). Of the two, CHAP is more secure if both ends support it.

Options for PPP in general are stored in /etc/ppp/options. Configuration of PAP is via the "PAP secrets" file /etc/ppp/pap-secrets, and for "CHAP secrets," /etc/ppp/pap-secrets.

The PAP/CHAP secrets file

The /etc/ppp/pap-secrets file contains white space separated field for client, server, secret, and "acceptable local IP address". The last may be blank (and generally is for dynamic IP allocation). The PAP secrets file should be configured correspondingly for each peer. Even though PPP is a peer protocol, for connection purposes we call the requesting machine the client, and the waiting machine the server for configuration purposes. So, for example, the machine bacchus on my network might have a configuration like:

/etc/ppp/pap-secrets on bacchus

# Every regular user can use PPP and uses passwords from /etc/passwd
# INBOUND connections
# client   server  secret            acceptable local IP addresses
*          bacchus ""                *
chaos      bacchus chaos-password
# OUTBOUND connections
bacchus      *     bacchus-password

Machine bacchus will accept connections claiming to be any regular user, or also claiming to be machine chaos (and demanding the password "chaos-password" in the latter case). To other machines, bacchus will simply use its own name, and offer the password "bacchus-password" to every peer.

Correspondingly, the machine chaos on my network might have:

/etc/ppp/pap-secrets on chaos

# client   server  secret            acceptable local IP addresses
chaos      bacchus chaos-password
bacchus    chaos   bacchus-password

Machine chaos is more conservative in whom it will connect to. It is only willing to exchange credentials with bacchus. You may configure each /etc/ppp/options file to decide if credentials are demanded though.

Using CHAP secrets requires that you allow for both peer machines to authenticate each other. As long as bidirection authentication is configued in PAP secrets, a CHAP secrets file may look just the same as the above examples.

Connecting with mgetty

The PAP secrets file can be used with the AUTO_PPP function of mgetty. mgetty 0.99+ is preconfigured to startup pppd with the "login" option. This tells pppd to consult /etc/passwd (and /etc/shadow in turn) after a user has passed this file.

In general, a getty program may be configured to allow connections from serial devices, including modems and direct serial ports. For example, for a hard-wired line or a console tty, you might run:

/sbin/getty 9600 ttyS1

in your inittab. For a old style dial-in line with a 9600/2400/1200 baud modem:

/sbin/getty -mt60 ttyS1 9600,2400,1200

Configuring routing

In the discussion of Address Resolution Protocol, we saw how IP addresses are assigned within a local network. However, to communicate with machines outside of a local network, it is necessary to have a gateway/router. Basically, a gateway is simply a machine that connects to more than one network, and can therefore take packets transmitted within one network, and re-transmit them on other networks it is connected to. This is where the name "internet" comes with: it is a "network of networks", in which every gateway can eventually reach every other network said to be "on the internet."

The tutorial in this series, Topic 210: Network Client Management, discusses DHCP, which will assign both client IP addresses and gateway address. However, with a fixed IP address on a client, or in debugging situations, the Linux command route allows you to view and modify routing tables. The newer command ip also lets you modify routing tables, using a somewhat more powerful syntax. A routing table simply lets you determine which gateway or host to send a packet to, given a specific pattern in the address. And address pattern is specified by combining an address with a subnet mask. A subnet mask is a bit pattern, usually represented in dotted quad notation, that tell the kernel which bits of a destination to treat as the network address, and which remaining bits to treat as a subnet. The command ip can accept the simpler /NN format for bitmasks. In general, in a mask and address, zero bits are "wildcards".

For example, a simple network with a single external gateway is likely to have a routing table similar to:

Typical simple routing table

$ route -n
Kernel IP routing table
Destination  Gateway      Genmask        Flags Metric Ref Use Iface  U     0      0     0 eth0        UG    0      0     0 eth0

What this means is simply that any IP address that matches "192.168.2.*" is on the local network, and will be delivered directly to the proper host (resolved with ARP). Any other address will be sent to the gateway "", which will be required to forward a packet appropriately. The machine must be connected to one or more external networks.

However, for a more complex case you may route specific patterns differently. In an invented example, let us say that you wish to route specific /16 addresses through other gateways. You might use:

Changing routing on /16 networks

$ route add -net netmask gw
$ route add -net netmask gw
$ route -n
Kernel IP routing table
Destination  Gateway      Genmask        Flags Metric Ref Use Iface  U     0      0     0 eth0    UG    0      0     0 eth0    UG    0      0     0 eth0        UG    0      0     0 eth0

Addresses of the form "216.109.*" and "216.239.* will now be routed through the gateways and, respectively (both on the local network themselves). Addresses that are local or outside the pattern spaces given, will be routed as before. The command route delete may be used correspondingly to remove routes.

Advanced Networking Configuration And Troubleshooting

About network utilities

Linux comes with a number of standard utilities you will want to be familiar with in customizing and troubleshooting a network configuration. Although much of Linux networking code lives in the kernel itself, almost everything about the behavior of networks is configurable using command-line utilities. Many distributions also come with higher level and/or graphical high-level configuration tools, but those do not contain anything that can not be peformed and scripted using the command-line tools.

The ping utility

The most basic way of checking whether a Linux host has access to an IP address (or once DNS and/or /etc/hosts is configured, to a named host) is with the utility ping. ping operates at the basic IP layer, it does not rely on data link layer like TCP or IP, but instead uses the Internet Control Message Protocol (ICMP). If you cannot reach a host with ping, you can assume you will not reach it with any other tool, so ping is always the first step in establishing whether a connection to a host is available. See man ping for details on options. By default ping sends a message every one second until cancelled, but you may change timings, limit message count, and change output details. When run, ping returns some details on round trip time and dropped packages, but for the most part either you can ping a host or you cannot. Some examples:

Local and non-local ping examples

$ ping -c 2 -i 2 google.com
PING google.com ( 56 data bytes
64 bytes from icmp_seq=0 ttl=237 time=43.861 ms
64 bytes from icmp_seq=1 ttl=237 time=36.956 ms

--- google.com ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max = 36.956/40.408/43.861 ms

$ ping
PING ( 56 data bytes
64 bytes from icmp_seq=0 ttl=255 time=4.64 ms
64 bytes from icmp_seq=1 ttl=255 time=2.176 ms
--- ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max = 2.176/3.408/4.64 ms

The ifconfig utility

Network interfaces are configured with the tool ifconfig. Usually this is run as part of the initialization process, but interfaces may be modified and tuned later, in some cases (especially for debugging). If run with no switches, ifconfig displays the current status. You may use the forms ifconfig <interface> up and ifconfig <interface> down to start and stop network interfaces. Some other switches change the display or limit the display to specific interfaces. See man ifconfig for more details.

An informational display might look something like:

Using ifconfig to examine network interfaces

$ ifconfig
eth0  Link encap:Ethernet  HWaddr 00:12:F0:21:4C:F8
      inet addr:  Bcast:  Mask:
      inet6 addr: fe80::212:f0ff:fe21:4cf8/64 Scope:Link
      RX packets:540 errors:0 dropped:0 overruns:0 frame:0
      TX packets:233 errors:0 dropped:0 overruns:0 carrier:1
      collisions:0 txqueuelen:1000
      RX bytes:49600 (48.4 KiB)  TX bytes:42067 (41.0 KiB)
      Interrupt:21 Base address:0xc000 Memory:ffcfe000-ffcfefff

ppp0  Link encap:Point-Point Protocol
      inet addr:  P-t-P: Mask:
      RX packets:0 errors:0 dropped:0 overruns:0
      TX packets:0 errors:0 dropped:0 overruns:0

lo    Link encap:Local Loopback
      inet addr:  Mask:
      inet6 addr: ::1/128 Scope:Host
      UP LOOPBACK RUNNING  MTU:16436  Metric:1
      RX packets:4043 errors:0 dropped:0 overruns:0 frame:0
      TX packets:4043 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0
      RX bytes:368044 (359.4 KiB)  TX bytes:368044 (359.4 KiB)

In this display, two networks are configured, one on ethernet, one on PPP (plus the local loopback). In other case, you might have multiple ethernet interfaces configured, or other interface types. If so, the system is called multi-homed.

The netstat utility

With Linux utilities, there tends to be a fair overlap in functionality. The tool netstat displays information that may also be provided by several utilities, such as ifconfig and route. You may also find extensive general statistics on network activity. For example:

Network statistics report

$ netstat -s
    12317 total packets received
    0 forwarded
    0 incoming packets discarded
    12255 incoming packets delivered
    11978 requests sent out
    1 ICMP messages received
    0 input ICMP message failed.
    ICMP input histogram:
        echo replies: 1
    0 ICMP messages sent
    0 ICMP messages failed
    ICMP output histogram:
    7 active connections openings
    5 passive connection openings
    0 failed connection attempts
    0 connection resets received
    3 connections established
    11987 segments received
    11885 segments send out
    0 segments retransmited
    0 bad segments received.
    3 resets sent
    101 packets received
    0 packets to unknown port received.
    0 packet receive errors
    92 packets sent
    1 TCP sockets finished time wait in fast timer
    1490 delayed acks sent
    Quick ack mode was activated 5 times
    3632 packets directly queued to recvmsg prequeue.
    126114 of bytes directly received from backlog
    161977 of bytes directly received from prequeue
    1751 packet headers predicted
    3469 packets header predicted and directly queued to user
    17 acknowledgments not containing data received
    4696 predicted acknowledgments
    0 TCP data loss events

Other utilties: tcpdump, lsof, nc (netcat)

There are several other utilities you should be aware of for network configuration. As usual, their respective manpages contain full usage information. Detailed discussion of these is contained later in this tutorial series, in Topic 214: Network Troubleshooting.

tcpdump lets you monitor all the packets that pass through network interfaces, optionally limited to particular interfaces or filtered on various criteria. Often saving this packet summary information, then filtering or summarizing it with text processing tools, is useful for debugging network problem. For example, you can examing packets communicated with a particular remote host.

lsof lists open files on a running Linux system. But in particular, you may use the lsof -i option to examine only the pseudo-files for a particular IP connection, or for network connections in general. E.g.:

$ lsof -i
vino-serv 7812  dqm   33u  IPv4  12824
        TCP *:5900 (LISTEN)
gnome-cup 7832  dqm   18u  IPv4  12865
        TCP localhost.localdomain:32771->localhost.localdomain:ipp (ESTABLISHED)
telnet    8909  dqm    3u  IPv4  15771

nc and netcat are aliases. netcat is a simple unix utility which reads and writes data across network connections, using TCP or UDP protocol. It is a "back-end" tool that can be used directly or driven by other programs and scripts. In many respects netcat is similar to telnet, but it is more versatile in allowing UDP interaction, and sending unfiltered binary data.