Before you start
Learn what these tutorials can teach you and how you can get the most from them.
About this series
The Linux Professional Institute (LPI) certifies Linux system administrators at two levels: junior level (also called "certification level 1") and intermediate level (also called "certification level 2"). To attain certification level 1, you must pass exams 101 and 102; to attain certification level 2, you must pass exams 201 and 202.
developerWorks offers tutorials to help you prepare for each of the four exams. Each exam covers several topics, and each topic has a corresponding self-study tutorial on developerWorks.
- To prepare for certification level 1, see the developerWorks tutorials for LPI exams 101 and 102.
- To prepare for certification level 2, see the developerWorks tutorials for LPI exams 201 and 202.
For LPI exam 202, the seven topics and corresponding developerWorks tutorials are:
|LPI exam 202 topic||developerWorks tutorial||Tutorial summary|
|Topic 205||LPI exam 202 prep (topic 205):|
|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.|
|Topic 206||LPI exam 202 prep (topic 206):|
Mail and news
|Learn how to use Linux as a mail server and as a news server. Learn about mail transport, local mail filtering, mailing list maintenance software, and server software for the NNTP protocol.|
|Topic 207||LPI exam 202 prep (topic 207):|
|Learn how to use Linux as a DNS server, chiefly using BIND. Learn how to perform a basic BIND configuration, manage DNS zones, and secure a DNS server.|
|Topic 208||LPI exam 202 prep (topic 208):|
|Learn how to install and configure the Apache Web server, and learn how to implement the Squid proxy server.|
|Topic 210||LPI exam 202 prep (topic 210):|
Network client management
|Learn how to configure a DHCP server, an NIS client and server, an LDAP server, and PAM authentication support. See detailed objectives below.|
|Topic 212||LPI exam 202 prep (topic 212):|
|Learn how to configure a router, secure FTP servers, configure SSH, and perform various other security administration tasks.|
LPI exam 202 prep (topic 214):|
|(This tutorial) Review tools and commands that let you detect and solve networking problems. See detailed objectives below.|
The Linux Professional Institute does not endorse any third-party exam preparation material or techniques in particular. For details, please contact firstname.lastname@example.org.
About this tutorial
Welcome to "Network troubleshooting," the last of seven tutorials covering intermediate network administration on Linux. This tutorial re-examines the material covered in the first six tutorials on the Linux Professional Institute's 202 exam topics to give some general context for the entire series. Highlighted here are some of the tools you've previously covered --
traceroute, etc. -- with the focus on using those tools to fix problems.
As with the other tutorials in the developerWorks 201 and 202 series, this tutorial is intended to serve as a study guide and entry point for exam preparation, rather than complete documentation on the subject. Readers are encouraged to consult LPI's detailed objectives list and to supplement the information provided here with other material as needed.
This tutorial is organized according to the LPI objectives for this topic. Very roughly, expect more questions on the exam for objectives with higher weight.
|LPI exam objective||Objective weight||Objective summary|
Troubleshooting network issues
|Weight 1||Identify and correct common network setup issues. This objective includes knowledge of locations for basic configuration files and commands.|
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. This tutorial builds on material covered in the previous six tutorials in the LPI exam 202 series.
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. 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).
Network configuration tools
About network troubleshooting
To troubleshoot a network configuration, you need to know how to use several of the tools discussed in this tutorial series; you also need to be familiar with the configuration files that affect network status and behavior. This tutorial summarizes the main tools and configuration files you should be familiar with for effective troubleshooting.
For simplicity, this tutorial groups the tools according to whether a given tool applies more to configuration of a network in the first place or to diagnosis of network problems. Of course, in practice those elements are rarely separate.
LPI exam 202 prep (topic 205): Networking configuration discusses
in greater detail. This utility both reports on the current
status of network interfaces and lets you modify the
configuration of those interfaces. In most cases, if something is
wrong with a network -- like a particular machine does not appear to
access the network at all -- running
ifconfig with no options is
usually the first step you should take. If this fails to report
active interfaces, you can be pretty sure that the local machine
itself has a configuration problem. "Active" in this case means that it shows an IP address assigned; in most cases, you
should expect to see a number of packets in the RX and TX lines:
Listing 1. Using ifconfig
eth0 Link encap:Ethernet HWaddr 00:C0:9F:21:2F:25 inet addr:192.168.216.90 Bcast:188.8.131.52 Mask:255.255.254.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:6193735 errors:0 dropped:0 overruns:0 frame:0 TX packets:6982479 errors:0 dropped:0 overruns:0 carrier:0
Attempting to activate an interface with something like
ifconfig eth0 up ...
is a good first step to try to see if an interface can be activated
(in many cases, filling in additional options in the line).
LPI exam 202 prep (topic 205): Networking configuration discusses
in greater detail. This utility lets you both view and modify
the routing tables currently in effect for a local machine and a local
you may add and delete routes, set netmasks
and gateways, and perform various other tweaking tasks.
For the most part,
should be performed in initialization scripts, but in
attempting to diagnose and fix problems, experimenting with routing
options can help (you can copy successes to appropriate
initialization scripts for later use).
This utility also has aliases to employ different aspects of the utility:
You control these
capabilities with switches to
hostname is used to either set or display the current host, domain,
or node name of the system. These names are used by many of the
networking programs to identify the machine. The domain name is also
used by NIS/YP.
dmesg allows you to examine kernel log messages; it
works in cooperation with
syslogd. Any kernel process, including
those related to networking, are best accessed using the
utility, often filtered using other tools such as
grep, as well as
Manually setting ARP
You almost never need or want to mess with automatically discovered
ARP records. However, you may want to
manually configure the ARP cache in debugging situations. The utility
arp lets you do this.
The key flag options in the
arp utility are
-d for delete,
-s for set,
-f for set-from-file (default file is /etc/ethers).
For example, suppose that communication with a specific IP address on the local network is erratic or unreliable. One possible cause of this situation is if multiple machines are incorrectly configured to use the same IP address. When an ARP request is broadcast over the Ethernet network, it is not pre-determined which machine will respond first with an ARP reply. The end result might be that the data packets are delivered to one machine one time and to a different machine another time.
arp -n to debug the actual IP assignment is a first step. If
you can determine that the IP address at issue does not map to the
correct Ethernet device, that is a strong clue about what is going on.
But beyond that somewhat random detection, you can force the right ARP
mapping using the
arp -s (or
-f) option. Set an IP to map to the
actual Ethernet device it should map to; manually configured mapping will not
expire unless specifically set to do so using the
temp flag. If a
manual ARP mapping fixes the data loss problem, this is a strong sign
the problem is over-assigned IP addresses.
Network diagnostic tools
LPI exam 202 prep (topic 205): Networking configuration discusses
in greater detail. This utility displays a variety of information
on network connections, routing tables, interface statistics,
masquerade connections, and multicast memberships. Among other
netstat provides fairly detailed statistics on packets
that have been handled in various ways.
The manpage for
netstat provides information on the wide range of
switches and options available. This utility is a good general-purpose tool for digging into details of the status of networking on
the local machine.
A good starting point in finding out if you can connect to a given
host from the current machine (by either IP number or symbolic name)
is the utility
ping. As well as establishing that a route exists at
all -- including the resolution of names via DNS or other means if a
symbolic name is used --
ping gives you information on round-trip
times that may be indicative of network congestion or routing delays.
ping will indicate a percentage of dropped packets, but in
practical use you almost always see either 100 or 0 percent of packets lost
traceroute is a bit like a
ping on steroids. Rather
than simply report the fact that a route exists to a given host,
traceroute reports complete details on all the hops taken along
the way, including the timing of each router. Routes may change over
time, either because of dynamic changes in the Internet or because of
routing changes you have implemented locally. At a given moment
traceroute shows you an actual followed path.
Listing 2. traceroute shows the actual followed path
$ traceroute google.com traceroute: Warning: google.com has multiple addresses; using 184.108.40.206 traceroute to google.com (220.127.116.11), 30 hops max, 38 byte packets 1 ev1s-66-98-216-1.ev1servers.net (18.104.22.168) 0.466 ms 0.424 ms 0.323 ms 2 ivhou-207-218-245-3.ev1.net (22.214.171.124) 0.650 ms 0.452 ms 0.491 ms 3 ivhou-207-218-223-9.ev1.net (126.96.36.199) 0.497 ms 0.467 ms 0.490 ms 4 gateway.mfn.com (188.8.131.52) 36.487 ms 1.277 ms 1.156 ms 5 so-5-0-0.mpr1.atl6.us.above.net (184.108.40.206) 13.824 ms 14.073 ms 13.826 ms 6 220.127.116.11.google.com (18.104.22.168) 13.786 ms 13.940 ms 14.019 ms 7 22.214.171.124 (126.96.36.199) 14.783 ms 14.749 ms 14.476 ms 8 188.8.131.52 (184.108.40.206) 16.651 ms 16.421 ms 17.648 ms 9 220.127.116.11 (18.104.22.168) 14.816 ms 14.913 ms 14.775 ms
host, nslookup, and dig
All three utilities --
dig -- are used for
querying DNS entries; they largely overlap in their capabilities. Generally,
nslookup provided enhancement to
dig in turn enhanced
nslookup (though none of the three are exactly backward- or forward-compatible
with the others). All the tools rely on the same underlying kernel
facilities, so reported results should be consistent in all cases
(except where level of detail differs). For example, each of the three
is used to query google.com:
Listing 3. Using host, nslookup, and dig to query Google
$ host google.com google.com has address 22.214.171.124 google.com has address 126.96.36.199 google.com has address 188.8.131.52 $ nslookup google.com Server: 184.108.40.206 Address: 220.127.116.11#53 Non-authoritative answer: Name: google.com Address: 18.104.22.168 Name: google.com Address: 22.214.171.124 Name: google.com Address: 126.96.36.199 $ dig google.com ; <<>> DiG 9.2.4 <<>> google.com ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46137 ;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;google.com. IN A ;; ANSWER SECTION: google.com. 295 IN A 188.8.131.52 google.com. 295 IN A 184.108.40.206 google.com. 295 IN A 220.127.116.11 ;; Query time: 16 msec ;; SERVER: 18.104.22.168#53(22.214.171.124) ;; WHEN: Mon Apr 17 01:08:42 2006 ;; MSG SIZE rcvd: 76
Network configuration files
/etc/network/ and /etc/sysconfig/network-scripts/
The directory /etc/network/ contains a variety of data about
the current network on some Linux distributions, especially in the
file /etc/network/interfaces. Various utilities, especially
iwdown for wireless interfaces), are
contained in /etc/sysconfig/network-scripts/ on some distributions
(but the same scripts may live elsewhere instead on your
/var/log/syslog and /var/log/messages
Messages logged by the kernel or the
syslogd facility are stored in
the log files /var/log/syslog and /var/log/messages. LPI exam 201 prep (topic 211): System maintenance discusses system
logging in greater detail. The utility
dmesg is generally used to
LPI exam 202 prep (topic 207): Domain Name System discusses /etc/resolv.conf in greater detail. Generally, this file simply contains the information needed to find domain name servers. It may be configured either manually or via dynamic means such as RIP, DHCP, or NIS.
The file /etc/hosts is usually the first place a Linux system looks to attempt to resolve a symbolic hostname. You can add entries to either bypass DNS lookup (or sometimes YP or NIS facilities) or to name hosts that are not available on DNS, often because they are strictly names on the local network. See the examples in Listing 4.
Listing 4. /etc/hosts, the place to resolve symbolic hostnames
$ cat /etc/hosts # Set some local addresses 127.0.0.1 localhost 255.255.255.255 broadcasthost 192.168.2.1 artemis.gnosis.lan 192.168.2.2 bacchus.gnosis.lan # Set undesirable site patterns to loopback 127.0.0.1 *.doubleclick.com 127.0.0.1 *.advertising.com 127.0.0.1 *.valueclick.com
/etc/hostname and /etc/HOSTNAME
The file /etc/HOSTNAME (on some systems without the capitalization) is sometimes used for the symbolic name of the localhost as known on the network. However, use of this file varies between distributions; generally /etc/hosts is used exclusively on modern distributions.
/etc/hosts.allow and /etc/hosts.deny
LPI exam 201 prep (topic 209): File and service sharing and LPI exam 202 prep (topic 212): System security discusses the files /etc/hosts.allow and /etc/hosts.deny in greater detail. These configuration files are used for positive and negative access lists by a variety of network tools. Read the manpages on these configuration files for more information on the specification of wildcards, ranges, and specific permissions that may be granted or denied.
Beyond initial setup to enforce system security, you often want to examine the content of these when a connection fails that "just seems like" it should be working. Generally, examining access control issues comes after examining basic interface and routing information in a debugging effort. That is, if you cannot reach a particular host at all (or it cannot reach you), it does not matter whether the host has permissions to use the services you provide. But selective failures in connections and service utilization can often be because of access control issues.
A final word
Take advantage of every resource
For the subjects addressed in this tutorial, possibly the best resource for further information is the rest of this tutorial series. Nearly all the topics addressed here are detailed further in previous tutorials.
Quite a few people have written step-by-step guides to fixing a broken Linux network. One that looks good is "Simple Network Troubleshooting." Debian's similar quick guide is "How To Set Up A Linux Network." Since tutorials come and go and are updated on different schedules as distributions and commands change, you can always search the Internet to find currently available sources.
- Review the entire LPI exam prep tutorial series on developerWorks to learn Linux fundamentals and prepare for system administrator certification.
- At the LPIC Program, find task lists, sample questions, and detailed objectives for the three levels of the Linux Professional Institute's Linux system administration certification.
- TCP/IP Network Administration, Third Edition by Craig Hunt (O'Reilly, April 2002) is an excellent resource on Linux networking.
- For more in-depth information, the Linux Documentation Project has a variety of useful documents, especially its HOWTOs.
- In the developerWorks Linux zone, find more resources for Linux developers.
- Stay current with developerWorks technical events and Webcasts.
Get products and technologies
- With IBM trial software, available for download directly from developerWorks, build your next development project on Linux.
- This list of more than 700 Linux User Groups around the world can help you find local and distance study groups for LPI exams.
- Check out developerWorks blogs and get involved in the developerWorks community.
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