You can monitor your system’s Web performance quite easily with graphical Linux tools. You’ll learn how to use several in this chapter, including MRTG, which is based on SNMP and monitors raw network traffic, and Webalizer, which tracks Web site hits.


Most servers, routers and firewalls keep their operational statistics in object identifiers (OIDs) that you can remotely retrieve via the Simple Network Management Protocol (SNMP). For ease of use, equipment vendors provide Management Information Base (MIB) files for their devices that define the functions of the OIDs they contain. That’s a lot of new terms to digest in two sentences, so take a moment to look more closely.


OIDs are arranged in a structure of management information (SMI) tree defined by the SNMP standard. The tree starts from a root node, which then descends through branches and leaves that each add their own reference value to the path separated by a period.. Figure 22-1 shows an OID structure in which the path to the enterprises OID branch passes through the org, dod, internet, and private branches first. The OID path for enterprises is, therefore,

Figure 22-1 SNMP OID Structure


Management Information Bases (MIBs) are text definitions of each of the OID branches. Table 22-1 shows how some commonly used OIDs map to their MIB definitions. For example, the SMI org MIB defines all the topmost OIDs found at the next layer, which is named dod; the internet MIB under dod defines the function of the topmost OIDs in the directory, mgmt, experimental, and private branches. This MIB information is very useful for SNMP management programs, enabling you to click on an OID and see its value, type, and description.

Table 22-1 OIDs And Their Equivalent MIBs


You can refer to an OID by substituting the values in a branch with one of these more readable MIB aliases. For example, you can reference the OID as enterprises. by substituting the branch name (enterprises) for its OID numbers (

Remember, only the OID value at the very tip of a branch, the leaf, actually has a readable value. Think of OIDs like the directory structure on a hard disk. Each branch is equivalent to a subdirectory, and the very last value at the tip (the leaf) correlates to a file containing data.

The Linux snmpget command outputs the value of a single leaf, and the snmpwalk command provides the values of all leaves under a branch. I’ll discuss these commands later; for now, all you need to know is that the command output frequently doesn’t list the entire OID, just the MIB file in which it was found and the alias within the MIB. For example


Here the OID value was found in the SNMPv2-MIB file and occupies position zero in the sysUpTime alias.

Equipment manufacturers are usually assigned their own dedicated OID branch under the enterprises MIB, and they must also provide information in universally accepted OIDs for ease of manageability. For example, NIC interface data throughput values must always be placed in a predefined location in the general tree, but a memory use value on a customized processor card may be defined in a MIB under the manufacturer’s own OID branch.

SNMP Community Strings

As a security measure, you need to know the SNMP password, or community string, to query OIDs. There are a number of types of community strings, the most commonly used ones are the Read Only or “get” community string that only provides access for viewing statistics and system parameters. In many cases the Read Only community string or password is set to the word “public;” you should change it from this easy-to-guess value whenever possible. The Read/Write or “set” community string is for not only viewing statistics and system parameters but also for updating the parameters.

SNMP Versions

There are currently three versions of SNMP.

  1. SNMP Version 1: The first version of SNMP to be implemented, version 1 was designed to be a protocol to provide device statistics and error reporting without consuming a lot of system resources. Security was limited to community strings and access controls based on the IP address of the querying server. Data communication wasn’t encrypted.
  2. SNMP Version 2: The second version of SNMP, often referred to as v2c, expanded the number of supported error codes, increased the size of counters used to track data, and had the ability to do bulk queries that more efficiently loaded response packets with data. SNMP v2c is backward compatible with version 1.
  3. SNMP Version 3: This version provides greater security and remote configuration capabilities than its predecessors. Access isn’t limited to a single community string for read-only and read/write access, as usernames and passwords have been introduced. Views of OIDs in a MIB can also be limited on a per-user basis. Support for encrypted SNMP data transfer and transfer error detection is also provided.

Remember their differences, because you will need to specify the version number when doing SNMP queries.

Doing SNMP Queries

Configuring SNMP on a server isn’t hard, but it does require a number of detailed steps.

Installing SNMP Utilities on a Linux Server

If you intend to use your Linux box to query your network devices, other servers or even itself using MRTG or any other tool, you need to have the SNMP utility tools package net-snmp-utils installed. This package may also require pre-requisite packages, so it is best to use an automated package updater such yum or apt to do this.

When searching for these packages the filenames will start with the package name followed by a version number, as in net-snmp-utils-5.1.1-2.i386.rpm. .

SNMP Utilities Command Syntax

The SNMP utility tools package installs a number of new commands on your system for doing SNMP queries, most notably snmpget for individual OIDs and snmpwalk for obtaining the contents of an entire MIB. Both commands require you to specify the community string with a -c operator. They also require you to specify the version of the SNMP query to be used with a -v 1, -v 2c, or -v 3 operator for versions 1, 2, and 3, respectively. The first argument is the name or IP address of the target device and all other arguments list the MIBs to be queried.

This example gets all the values in the interface MIB of the local server using SNMP version 1 and the community string of craz33guy.

[root@bigboy tmp]# snmpwalk -v 1 -c craz33guy localhost interface
IF-MIB::ifDescr.1 = STRING: lo
IF-MIB::ifDescr.2 = STRING: eth0
IF-MIB::ifDescr.3 = STRING: eth1
IF-MIB::ifPhysAddress.1 = STRING:
IF-MIB::ifPhysAddress.2 = STRING: 0:9:5b:2f:9e:d5
IF-MIB::ifPhysAddress.3 = STRING: 0:b0:d0:46:32:71
[root@bigboy tmp]#

Upon inspecting the output of the snmpwalk command, you can see that the second interface seems to have the name eth0 and the MAC address 0:9:5b:2f:9e:d5. You can now retrieve the individual MAC address using the snmpget command.

[root@bigboy tmp]# snmpget -v 1 -c const1payted localhost ifPhysAddress.2
IF-MIB::ifPhysAddress.2 = STRING: 0:9:5b:2f:9e:d5
[root@bigboy tmp]#

You can confirm this information using the ifconfig command for interface eth0; the very first line shows a matching MAC address.

[root@bigboy tmp]# ifconfig -a eth0
eth0      Link encap:Ethernet  HWaddr 00:09:5B:2F:9E:D5
          inet addr:  Bcast:   Mask:
[root@bigboy tmp]#

You’ll now see how you can configure SNMP on your Linux server to achieve these results.

Configuring Simple SNMP on a Linux Server

By default Fedora, installs the net-snmp package as its SNMP server product. This package uses a configuration file named /etc/snmp/snmpd.conf in which the community strings and other parameters may be set. The version of the configuration file that comes with net-snmp is quite complicated. I suggest archiving it and using a much simpler version with only a single line containing the keyword rocommunity followed by the community string. Here is an example.

1) Save the old configuration file

[root@bigboy tmp]# cd /etc/snmp/
[root@bigboy snmp]# mv snmpd.conf snmpd.conf.old
[root@bigboy snmp]# vi snmpd.conf

2) Enter the following line in the new configuration file to set the Read Only community string to craz33guy.

rocommunity craz33guy

3) Configure Linux to start SNMP services on each reboot with the chkconfig command:

[root@bigboy root]# chkconfig snmpd on
[root@bigboy root]#

4) Start SNMP to load the current configuration file.

[root@bigboy root]# service snmpd start
Starting snmpd: [ OK ]
[root@bigboy root]#

5) Test whether SNMP can read the system and interface MIBs using the snmpwalk command.

[root@bigboy snmp]# snmpwalk -v 1 -c craz33guy localhost system
SNMPv2-MIB::sysDescr.0 = STRING: Linux bigboy 2.4.18-14 #1 Wed Sep 4 11:57:57 EDT 2002 i586
SNMPv2-MIB::sysObjectID.0 = OID: NET-SNMP-MIB::netSnmpAgentOIDs.10
SNMPv2-MIB::sysUpTime.0 = Timeticks: (425) 0:00:04.25
SNMPv2-MIB::sysContact.0 = STRING: root@localhost
SNMPv2-MIB::sysName.0 = STRING: bigboy
[root@bigboy snmp]# snmpwalk -v 1 -c craz33guy localhost interface
IF-MIB::ifNumber.0 = INTEGER: 3
IF-MIB::ifIndex.1 = INTEGER: 1
IF-MIB::ifIndex.2 = INTEGER: 2
IF-MIB::ifIndex.3 = INTEGER: 3
IF-MIB::ifDescr.1 = STRING: lo
IF-MIB::ifDescr.2 = STRING: wlan0
IF-MIB::ifDescr.3 = STRING: eth0
[root@bigboy snmp]#

Now that you know SNMP is working correctly on your Linux server, you can configure SNMP statistics gathering software, such as MRTG, to create online graphs of your traffic flows.

SNMP On Other Devices

In the example, you were polling localhost. You can poll any SNMP-aware network device that has SNMP enabled. All you need is the IP address and SNMP Read Only string and you’ll be able to get similar results. Here is an example of a query of a device with an IP address of

[root@bigboy snmp]# snmpwalk -v 1 -c chir1qui interface

Note: When issuing snmpwalk and snmpget commands, remember to use the proper version switch (-v 1, -v 2c, or -v 3) for the version of SNMP you’re using.

Basic SNMP Security

The security precautions that need to be taken with SNMP vary depending on the version you are using. This section outlines the basic steps for protecting your MIB data.

SNMP Versions 1 and 2

The most commonly supported versions of SNMP don’t encrypt your community string password so you shouldn’t do queries over insecure networks, such as the Internet. You should also make sure that you use all reasonable security measures to allow queries only from trusted IP addresses either via a firewall or the SNMP security features available in the snmp.conf file. You can also configure your server to use the TCP wrappers feature outlined in Appendix I, “Miscellaneous Linux Topics,” to limit access to specific servers without the need of a firewall.

In case you need it, the snmpd.conf file can support limiting MIB access to trusted hosts and networks.

The snmpd.conf file has two security sections; a section with very restrictive access sits at the top of the file and is immediately followed by a less restrictive section. The example that follows is a modification of the less restrictive section. You will have to comment out the more restrictive statements at the top of the file for it to work correctly.

##     source           community
##       ========     ======           =========
com2sec  local       localhost        craz33guy
com2sec  network_1    craz33guy
com2sec  network_2   craz33guy

##   sec.model
##       =================  =========         ========
group    MyROGroup_1        v1                local
group    MyROGroup_1        v1                network_1
group    MyROGroup_2        v2c               network_2

##     incl/excl  MIB.subtree  mask
##   ==============   =========  ===========  ====
view all-mibs         included   .1           80

##      MIB
##   context sec.model sec.level prefix read     write  notif
##      ==========   ======= ========= ========= ====== ====     =====  =====
access  MyROGroup_1  ""       v1       noauth    exact  all-mibs none   none
access  MyROGroup_2  ""       v2c      noauth    exact  all-mibs none   none

In our example:

  • Only three networks (localhost,, and are allowed to access the server with the craz33guy community string.
  • Each network is matched to a either a group called MyROGroup_1 using SNMP version 1, or group called MyROGroup_2 using SNMP version 2.
  • All the MIBs on the server are defined by the view named all-mibs.
  • An access statement ensures that only the defined networks have read only access to all the MIBs. MyROGroup_1 only has version 1 access with MyROGroup_2 only having version 2 access.
  • Modification of the MIBs via SNMP is denied because the word “none” is in the write section of the access statement.

These precautions are probably unnecessary in a home environment where access is generally limited to devices on the home network by a NAT firewall.

SNMP Version 3

SNMP Version 3 SNMPv3 is a much more secure alternative to earlier versions as it encrypts all its data and uses a username / password combination for client authentication. The username should be located in the /etc/snmp/snmpd.conf file with a corresponding automatically generated password located in the /var/net-snmp/snmpd.conf file. Here is how it’s done.

1. Install the net-snmp-devel package as it contains the utility that will allow you to generate the password.

2. Stop the snmpd process.

[root@bigboy tmp]# service snmpd stop
Stopping snmpd: [  OK  ]
[root@bigboy tmp]#

3. Automatically create the /etc/snmp/snmpd.conf and /var/net-snmp/snmpd.conf username and password entries using the net-snmp-config command. In this example the authentication password of “rootsrockreggae” for the read only (-ro) username “username4snmpv3” is encrypted using the MD5 algorithm. The data received will not be encrypted.

[root@bigboy tmp]# net-snmp-config --create-snmpv3-user -ro \
-a MD5 -A rootsrockreggae username4snmpv3

adding the following line to /var/net-snmp/snmpd.conf:
   createUser username4snmpv3 MD5 "rootsrockreggae" DES
adding the following line to /etc/snmp/snmpd.conf:
   rouser username4snmpv3
[root@bigboy tmp]# service snmpd start
Starting snmpd: [  OK  ]
[root@bigboy tmp]#

4. To encrypt the data received we use the net-snmp-config command again to create a username just for this purpose. A new read only username “securev3user” and authentication password “mandeville” are used, but this time the data will be encrypted using the DES algorithm with the privacy password of “savlamar”.

[root@bigboy tmp]# net-snmp-config --create-snmpv3-user -ro \
-a MD5 -A mandeville -x DES -X savlamar securev3user

adding the following line to /var/net-snmp/snmpd.conf:
   createUser securev3user MD5 "mandeville" DES savlamar
adding the following line to /etc/snmp/snmpd.conf:
   rouser securev3user
[root@bigboy tmp]#

5. Start the snmpd process.

[root@bigboy tmp]# service snmpd start
Starting snmpd: [  OK  ]
[root@bigboy tmp]#

6. First we’ll do a query from remote host smallfry. We specify the authentication password and authentication encryption method, and we also use the -l flag to indicate that authentication will be used, but that data privacy will be disabled (the authNoPriv option).

[root@smallfry ~]# snmpget -v 3 -u username4snmpv3 -l authNoPriv \
-a MD5 -A rootsrockreggae SNMPv2-MIB::sysORDescr.8

SNMPv2-MIB::sysORDescr.8 = STRING: The management information definitions for the SNMP User-based Security Model.
[root@smallfry ~]#

The query returns an easy to read string, “The management information definitions for the SNMP User-based Security Model”. This unencrypted string can also be seen in the tshark packet capture of the server’s interface.

[root@bigboy tmp]# tshark -n -i eth1 -x port 161
Capturing on eth1

  0.005889 -> SNMP get-response

0000  00 c0 4f 46 0c 2e 00 b0 d0 46 32 71 08 00 45 00   ..OF.....F2q..E.
0010  00 f0 00 00 40 00 40 11 b3 b2 c0 a8 02 c8 c0 a8   ....@.@.........
0020  02 32 00 a1 80 0a 00 dc 87 38 30 81 d1 02 01 03   .2.......80.....
0030  30 11 02 04 45 a2 23 54 02 03 00 ff e3 04 01 01   0...E.#T........
0040  02 01 03 04 38 30 36 04 0d 80 00 1f 88 80 71 11   ....806.......q.
0050  68 72 0e b1 e7 45 02 01 12 02 01 39 04 0f 75 73
0060  65 72 6e 61 6d 65 34 73 6e 6d 70 76 33 04 0c 46   ername4snmpv3..F
0070  6c 74 26 51 4d aa 65 61 59 06 1a 04 00 30 7f 04   lt&QM.eaY....0..
0080  0d 80 00 1f 88 80 71 11 68 72 0e b1 e7 45 04 00
0090  a2 6c 02 04 43 4e da d7 02 01 00 02 01 00 30 5e   .l..CN........0^
00a0  30 5c 06 0a 2b 06 01 02 01 01 09 01 03 08 04 4e   0\..+..........N
00b0  54 68 65 20 6d 61 6e 61 67 65 6d 65 6e 74 20 69   The management i
00c0  6e 66 6f 72 6d 61 74 69 6f 6e 20 64 65 66 69 6e   nformation defin
00d0  69 74 69 6f 6e 73 20 66 6f 72 20 74 68 65 20 53   itions for the S
00e0  4e 4d 50 20 55 73 65 72 2d 62 61 73 65 64 20 53   NMP User-based S
00f0  65 63 75 72 69 74 79 20 4d 6f 64 65 6c 2e         ecurity Model.

4 packets captured
[root@bigboy tmp]#

7. Next we’ll do a query that will return a response over an encrypted data channel while crossing the network (the authPriv option).

[root@smallfry ~]# snmpget -v 3 -u securev3user -l authPriv \
-a MD5 -A mandeville  -x DES -X savlamar SNMPv2-MIB::sysORDescr.8
SNMPv2-MIB::sysORDescr.8 = STRING: The management information definitions for the SNMP User-based Security Model.
[root@smallfry ~]#

The query returns the same string, but the tshark packet capture only sees encrypted data, with only the username being visible.

[root@bigboy tmp] # tshark -n -i eth1 -x port 161
Capturing on eth1
  0.003675 -> SNMP Source port: 161  Destination port: 32778 [UDP CHECKSUM INCORRECT]

0000  00 c0 4f 46 0c 2e 00 b0 d0 46 32 71 08 00 45 00   ..OF.....F2q..E.
0010  01 00 00 00 40 00 40 11 b3 a2 c0 a8 02 c8 c0 a8   ....@.@.........
0020  02 32 00 a1 80 0a 00 ec 87 48 30 81 e1 02 01 03   .2.......H0.....
0030  30 11 02 04 17 52 82 96 02 03 00 ff e3 04 01 03   0....R..........
0040  02 01 03 04 3e 30 3c 04 0d 80 00 1f 88 80 71 11   ....>0<.......q.
0050  68 72 0e b1 e7 45 02 01 11 02 02 00 8e 04 0c 73   hr...E.........s
0060  65 63 75 72 65 76 33 75 73 65 72 04 0c 01 b2 00   ecurev3user.....
0070  6e 23 07 83 dc a2 b6 d6 3d 04 08 00 00 00 11 4e   n#......=......N
0080  df 19 a3 04 81 88 36 dd e0 ce e0 52 19 ff 58 7e   ......6....R..X~
0090  be fa d1 96 20 2b 28 65 59 30 e8 d4 cb 18 9f 8f   .... +(eY0......
00a0  1e 5b a3 d6 ae f7 4a 86 bd ed 2a 4b a8 df 52 fb   .[....J...*K..R.
00b0  00 b4 a8 37 3d 74 9e 6d 1d 56 9a ba f2 13 fa 72   ...7=t.m.V.....r
00c0  4d 47 fb 88 7b d3 54 e1 9d b3 66 f0 29 ab 8a 55   MG..{.T...f.)..U
00d0  6f 77 65 40 87 ab 0c 51 d9 0e bf 33 7f 9a cb ea   owe@...Q...3....
00e0  37 50 3c 8e 65 dd 8f 3c 49 71 96 59 f9 d3 a8 23   7P<.e..<Iq.Y...#
00f0  81 c6 1b b2 c2 d0 57 9b 98 1b 89 1e ca 77 3d 84   ......W......w=.
0100  6f af b6 9b 86 3a 2f 66 44 1a 41 51 03 bc         o....:/fD.AQ..

4 packets captured
[root@bigboy tmp] #

8. Your password and privilege information are stored in /var/net-snmp/snmpd.conf using the format displayed when you used the net-snmp-config command. here is an example.

# File: /var/net-snmp/snmpd.conf before SNMP starts

createUser securev3user MD5 "mandeville" DES savlamar
createUser username4snmpv3 MD5 "rootsrockreggae" DES

The snmpd daemon will completely encrypt the SNMP password data in this file when it restarts which helps to further increase security. We can see an example of this configuration here.

# File: /var/net-snmp/snmpd.conf after SNMP starts

usmUser 1 3 0x80001f88780711168720eb1e745
NULL . 0xd951
ac1d95033f4afgf31243eb6907df .
0xf1f4bb00452211d27b50c273c09031ac 0x00
usmUser 1 3 0x80001f8880711168720eb1e745
1.1.2 0x5e35c9f5352519aa4f53eded09bbdddd
. 0x5e35c9f5122519aa4f53eded09bbdddd ""
setserialno 1464593474

Practice using the net-snmp-config command so that you can become familiar with the syntax it uses to edit the SNMP configuration files. When in doubt, you can get a full syntax listing of the command if you use it without any arguments like this:

[root@bigboy tmp]# net-snmp-config
 SNMP Setup commands:

   --create-snmpv3-user [-ro] [-A authpass] [-X privpass]
                        [-a MD5|SHA] [-x DES|AES] [username]
[root@bigboy tmp]#

With experience, you should become confident enough to edit the configuration files by yourself.

As you can see, SNMPv3 is more secure than previous versions and should be your first SNMP choice whenever possible.

Simple SNMP Troubleshooting

If your SNMP queries fail, then verify that:

  • You restarted your snmp.conf file so the configuration settings become active. Remember, the snmpd.conf file is only read by the snmpd daemon when it starts up.
  • You are using the correct community string.
  • Firewalls aren’t preventing SNMP queries from the SNMP client to the SNMP target.
  • Your SNMP security policy allows the query from your network.
  • Any TCP wrappers configuration on your SNMP target machine allows SNMP queries from your SNMP client. Generally speaking in a home environment protected by NAT your TCP wrappers files (/etc/hosts.allow) and (/etc/hosts.deny) should be blank.
  • Network routing between the client and target devices is correct. A simple ping or traceroute test should be sufficient.
  • The snmpd daemon is running on the SNMP client.
  • You are querying using the correct SNMP version.
  • Your /var/log/messages file does not contain errors that may have occurred while starting snmpd.

Troubleshooting to get functioning SNMP queries is important as many other supporting applications, such as MRTG which I’ll discuss next, rely on them in order to work correctly.


MRTG (Multi-Router Traffic Grapher) is a public domain package for producing graphs of various router statistics via a Web page. You can easily create graphs of traffic flow statistics through your home network’s firewall/router or even your Linux box’s NIC cards using MRTG. The product is available from the MRTG Web site ( and also on your distribution CDs. Figure 22-2 shows a sample MRTG graph.

Figure 22-2 A Typical MRTG Web Page


MRTG Download and Installation

You need to install MRTG before proceeding. Most RedHat and Fedora Linux software products are available in the RPM format. When searching for the file, remember that the MRTG RPM’s filename usually starts with mrtg and a version number, as in mrtg-2.10.5-3.i386.rpm.

In addition to MRTG, you need to install the SNMP utility tools as explained earlier and you need to have a Web server package installed for MRTG to work. RedHat Linux usually comes with the Apache Web server software preinstalled. The easiest way to tell if Apache is installed is to run the rpm -q httpd command. If you don’t get a positive response, . By default Apache expects the HTML files for your Web site to be located in /var/www/html. MRTG places its HTML files in /var/www/mrtg.

Configuring MRTG

By default, MRTG maps the inbound and outbound data throughput rates on the device it is polling. Methods for specifying other OIDs, such as CPU and memory usage, are discussed in Chapter 23, “Advanced MRTG for Linux“. For now, I’ll stick with the default configuration.

When the MRTG RPM is installed, it creates a directory called /etc/mrtg in which all future configuration files are stored. To create a replacement default /etc/mrtg/mrtg.cfg configuration file for the server, follow these steps.

1) Use MRTG’s cfgmaker command to create a configuration file named mrtg.cfg for the server (bigboy) using a Read Only community string of craz33guy. Place all data files in the directory /var/www/mrtg.

[root@bigboy tmp]# cfgmaker --output=/etc/mrtg/mrtg.cfg \
--global "workdir: /var/www/mrtg" -ifref=ip \
--global 'options[_]: growright,bits' \

--base: Get Device Info on craz33guy@localhost:
--base: Vendor Id:
--base: Populating confcache
--snpo: confcache craz33guy@localhost: Descr lo --> 1
--snpo: confcache craz33guy@localhost: Descr wlan0 --> 2
 --base: Walking ifAdminStatus
--base: Walking ifOperStatus
--base: Writing /etc/mrtg/mrtg.cfg
[root@bigboy tmp]#

SNMPv2: As explained in the SNMP section, there are different versions of SNMP. If your query doesn’t work, check to make sure you are using the required version and then check other SNMP configuration parameters on the target device. You can specify MRTG’s SNMP query version with the –snmp-options cfgmaker option. Here is an example of cfgmaker using an SNMP version 2 query of a router with an IP address of The –snmp-options option’s five colons before the 2 are important.

[root@bigboy tmp]# cfgmaker --output=/etc/mrtg/ \
-ifref=ip --global "workdir: /var/www/mrtg" \
--snmp-options=:::::2 craz33guy@

SNMPv3: The cfgmaker command can also be used to poll SNMPv3 enabled devices, but you have to first install the Net::SNMP PERL module.

[root@bigboy tmp]# yum -y install perl-Net-SNMP


[root@bigboy tmp]# perl -MCPAN -e "install Net::SNMP"

If you fail to install the module, you will get an error looking like this:

Undefined subroutine &main::snmpmapOID called at ./cfgmaker line 1480.

Next you need to know the SNMPv3 ID of the host you intend to poll. In fedora, this is value is located in the /var/net-snmp/snmpd.conf file. You can use the grep command to obtain it.

[root@bigboy tmp]# grep oldEngineID /var/net-snmp/snmpd.conf
oldEngineID 0x80001f8880711168720eb1e745
[root@bigboy tmp]#

You can then repeat the cfgmaker command with options specific to the privacy and authentication schemes configured on the SNMP target device. Using the configurations of our previous SNMPv3 example, our configuration for encrypted authentication only would look like this:

[root@bigboy tmp]# cfgmaker --global 'WorkDir: /var/www/mrtg' \
--global 'Options[_]: growright, bits' \
--output=/etc/mrtg/ \
--enablesnmpv3 --username=username4snmpv3 \
--authpassword=rootsrockreggae --authproto=md5 \
--snmp-options=:::::3 \
--contextengineid=0x80001f8880711168720eb1e745 \

Our configuration for encrypted authentication and data privacy only would look like this:

[root@bigboy tmp]# cfgmaker --global 'WorkDir: /var/www/mrtg' \
--global 'Options[_]: growright, bits' \
--output=/etc/mrtg/ \
--enablesnmpv3 --username=securev3user --authpassword=mandeville \
--authproto=md5 --privpassword=savlamar --privprotocol=des \
--snmp-options=:::::3 \
--contextengineid=0x80001f8880711168720eb1e745 \

Note: The MRTG cfgmaker command reliably supports SNMPv3 as of MRTG version 2.15. Prior to this version you would commonly see this error when attempting to do DNMPv3 queries.

SNMP V3 requires a --username parameter as part of the User Security Model for router securev3user@ at ./cfgmaker line 121.

2) Edit /etc/mrtg/mrtg.cfg, and remove the sections related to interfaces you don’t need to monitor. A certain candidate would be the virtual loopback interface Lo: (with the IP address of, which doesn’t pass any network traffic at all.

3) Run MRTG using /etc/mrtg/mrtg.cfg as your argument three times. You’ll get an error the two times as MRTG tries to move old data files, and naturally, the first time it is run, MRTG has no data files to move.

[root@bigboy tmp]# env LANG=C /usr/bin/mrtg /etc/mrtg/mrtg.cfg
Rateup WARNING: /usr/bin/rateup could not read the primary log file for localhost_192.168.1.100
Rateup WARNING: /usr/bin/rateup The backup log file for localhost_192.168.1.100 was invalid as well
Rateup WARNING: /usr/bin/rateup Can't remove localhost_192.168.1.100.old updating log file
Rateup WARNING: /usr/bin/rateup Can't rename localhost_192.168.1.100.log to localhost_192.168.1.100.old updating log file
[root@bigboy tmp]# env LANG=C /usr/bin/mrtg /etc/mrtg/mrtg.cfg
Rateup WARNING: /usr/bin/rateup Can't remove localhost_192.168.1.100.old updating log file
[root@bigboy tmp]# env LANG=C /usr/bin/mrtg /etc/mrtg/mrtg.cfg
[root@bigboy tmp]#

4) Use MRTG’s indexmaker command to create a Web index page using your new mrtg.cfg file as a guide. The MRTG Web GUI expects to find the index file in the default MRTG Web directory of /var/www/mrtg/, so the format of the command would be.

[root@bigboy tmp]# indexmaker --output=/var/www/mrtg/index.html \

5) MRTG is run every five minutes by default, and the file that governs this is /etc/cron.d/mrtg. For MRTG to work correctly, edit this file, replacing all occurrences of /usr/bin/mrtg with env LANG=C /usr/bin/mrtg. The explanation for changing the language character set for MRTG is given in the “Troubleshooting MRTG” section.

This isn’t all, you need to view the graphs too. This will be covered later, but first I’ll show you how to poll multiple devices.

Getting MRTG To Poll Multiple Devices

The Fedora Core MRTG installation process creates a cron file named /etc/cron.d/mrtg. This file tells the cron daemon to run MRTG using the /etc/mrtg/mrtg.cfg file every five minutes to poll your network devices. You can configure MRTG to poll multiple devices, each with a separate configuration file. Here’s how:

1) Create a new configuration file using the steps from the previous section; choose a filename that is not mrtg.cfg.

2) Add a new MRTG line in /etc/cron.d/mrtg for each new configuration file you create.

0-59/5 * * * * root env LANG=C /usr/bin/mrtg /etc/mrtg/mrtg.cfg
0-59/5 * * * * root env LANG=C /usr/bin/mrtg /etc/mrtg/device1.cfg
0-59/5 * * * * root env LANG=C /usr/bin/mrtg /etc/mrtg/device2.cfg

3) Run the indexmaker command, and include all of your /etc/mrtg configuration files, to regenerate your Web index page.

[root@bigboy tmp]# indexmaker --output=/var/www/mrtg/index.html \
/etc/mrtg/mrtg.cfg /etc/mrtg/device1.cfg /etc/mrtg/device2.cfg

4) Other versions of Linux keep their MRTG cron entries inside the /etc/crontab file. Edit this file using the same syntax as the Fedora /etc/cron.d/mrtg file, and then restart the cron daemon to re-read the configuration:

[root@bigboy tmp]# service crond restart

You could also create a script with the /usr/bin/mrtg /etc/mrtg/device.cfg entries in it and make cron run it every five minutes. This way you can just edit the script each time you add a device without having to restart cron.

Configuring Apache To Work With MRTG

MRTG is useful because it can provide a graphical representation of your server’s performance statistics via a Web browser.

With Fedora Core, MRTG creates an add-on configuration file named /etc/httpd/conf.d/mrtg.conf that includes all the necessary Apache commands for MRTG to work.

Some configuration may need to be done, because by default MRTG accepts Web requests from the Linux console only. You can add your home network to the file by inserting the network on the Allow from line, or you can allow universal access by commenting out that line along with the Deny from line. This example adds access from the network.

<Location /mrtg>
    Order deny,allow
    Deny from all
    Allow from localhost

If you want to access MRTG from the Internet, then you’ll have to comment out the Deny statement and allow from all IP addresses:

<Location /mrtg>
    Order deny,allow
    Allow from all

Remember to restart Apache once you have made these modifications in order for these changes to take effect.

Note: With newer versions of Fedora, Apache automatically reads the add-on files in the /etc/httpd/conf.d/ directory. With Fedora Core 1, you have to specifically configure the Apache configuration file /etc/httpd/conf/httpd.conf to find it. You can do this yourself by inserting this line at the very bottom of the main Apache configuration file before restarting Apache for the change to take effect.

include "/etc/httpd/conf.d/mrtg.conf"

Basic Security

If you are accessing MRTG graphs from the Internet, you may want to add password protection to the directory by using a .htaccess

How To View The MRTG Graphs In Your Web Browser

You can now access your MRTG graphs by pointing your browser to the URL:


Using MRTG To Monitor Other Subsystems

MRTG will generate HTML pages with daily, weekly, monthly, and yearly statistics for your interfaces. By default, MRTG provides only network interface statistics. Chapter 23, “Advanced MRTG for Linux“, has detailed examples and explanations of how to monitor Linux disk, CPU, memory, and Web connection data. The MRTG Web site,, also has links to other sites that show you how to monitor many other subsystems on a variety of devices and operating systems.

Troubleshooting MRTG

There are many simple steps you can use to troubleshoot MRTG. Take a look at some of the most common ones.

Basic Steps

MRTG won’t work if SNMP queries don’t work. Make sure you follow the SNMP troubleshooting steps if you have any difficulties.

Setting The Correct Character Set

MRTG usually works only if your system uses an ASCII-based (Western European) character set. If it isn’t set, then you’ll get errors such as this every time you run MRTG from the command line or as part of a cron job:

[root@bigboy tmp]# mrtg /etc/mrtg/mrtg.cfg
ERROR: Mrtg will most likely not work propperly when the environment
       variable LANG is set to UTF-8. Please run mrtg in an envir..
       where this is not the case:

       env LANG=C /usr/bin/mrtg ...
[root@bigboy tmp]#

Your system’s character set is defined in /etc/sysconfig/i18n, and the current Fedora default of en_US.UTF-8 won’t work, but en_US will after a system reboot. This is not necessarily a good idea, especially if the native language Linux uses on your system is not ASCII based, other things may fail to work.

A better solution is to always run MRTG using this command instead of using just plain /usr/bin/mrtg.

env LANG=C /usr/bin/mrtg

This will modify the character set used by MRTG alone and shouldn’t affect anything else.

Fedora Core 1 MRTG Errors With Net-SNMP

A bug appears in the MRTG implementation for some Fedora Core 1 MRTG versions when polling another Fedora Core 1 server.

When using a -ifref=ip statement with the cfgmaker command, every line in the configuration file that is generated becomes commented out. When it works, this statement is very convenient, because it makes MRTG provide graphs sorted by the IP addresses of the interfaces instead of the default, which is the much harder to recognize interface MAC address. Upgrading to the latest Core 1 version of MRTG will fix the problem.

### Interface 6 >> Descr:  | Name:  | Ip: ''
### The following interface is commented out because:
### * has a speed of which makes no sense
### * got 'Received SNMP response with error code
###       error status: noSuchName
###       index 1 (OID:
###     SNMPv1_Session (remote host: "localhost" [].161)
###                       community: "craz33guy"
###                      request ID: 824482716
###                     PDU bufsize: 8000 bytes
###                         timeout: 2s
###                         retries: 5
# Target[localhost_192.168.1.100]: /
# SetEnv[localhost_192.168.1.100]: MRTG_INT_IP="" MRTG_INT_DES
# MaxBytes[localhost_192.168.1.100]: 0
# Title[localhost_192.168.1.100]: Traffic Analysis for
# PageTop[localhost_192.168.1.100]: Traffic Analysis for

As all the lines in the configuration file are commented out with a # character, indexmaker fails to create an index.html file and gives errors.

[root@bigboy tmp]# indexmaker --output=/var/www/mrtg/stats/index.html /etc/mrtg/mrtg.cfg
Use of uninitialized value in hash element at /usr/bin/indexmaker line 307.
[root@bigboy tmp]#


Webalizer is a Web server log file analysis tool that comes installed by default on RedHat/Fedora Linux. Each night, Webalizer reads your Apache log files and creates a set of Web pages that enable you to view Web surfer statistics for your site. The information provided includes a list of your Web site’s most popular pages sorted by hits along with traffic graphs showing the times of day when your site is most popular.

How To View Your Webalizer Statistics

Fedora creates an add-on configuration file named /etc/httpd/conf.d/Webalizer.conf that includes all the necessary Apache commands for Webalizer to work. As in the case of the MRTG add-on file mentioned above, you have to edit it to allow access to the Webalizer pages from locations other than the Linux console. You also have to restart Apache to make the changes take effect.

By default, Webalizer places its index page in the directory /var/www/html/usage and allows you to view your data by visiting the URL http://server-ip-address/usage.

The Webalizer Configuration File

Webalizer stores its configuration in the file /etc/Webalizer.conf. The default settings should be sufficient for your Web server, but you may want to adjust the directory in which Webalizer places your graph statistics. This can be adjusted with the OutputDir directive in the file. After adjustments, Webalizer functions with few annoyances; however, be aware that running in quiet mode could hide deeper problems that could occur in future.

The top Command

You can monitor the amount of memory and CPU resources your system is using the top command.

[root@bigboy tmp]# top

  3:04pm  up 25 days, 23:23,  2 users,  load average: 0.00, 0.02, 0.00
78 processes: 76 sleeping, 2 running, 0 zombie, 0 stopped
CPU states:  0.9% user,  0.5% system,  0.0% nice,  0.8% idle
Mem:   384716K av,  327180K used,   57536K free,       0K shrd,  101544K buff
Swap:  779112K av,       0K used,  779112K free                  130776K cached

27191 root      15   0  1012 1012   780 R     5.6  0.2   0:00 top
 4545 root      16   0  5892 5888  4956 S     0.9  1.5 169:26 magicdev
    1 root      15   0   476   476   432 S     0.0  0.1   0:05 init
    2 root      15   0     0     0     0 SW    0.0  0.0   0:00 keventd
    5 root      15   0     0     0     0 SW    0.0  0.0   0:41 kswapd
    6 root      25   0     0     0     0 SW    0.0  0.0   0:00 bdflush

[root@bigboy tmp]#

Here the CPU usage is under 1.0% and 14% of memory (57536K) is free. The amount of free memory may appear low, but in this case, the server doesn’t seem to be swapping idle processes from memory to the swap disk partition as it isn’t being used at all. Excessive swapping can cause your system to slow down dramatically, the simplest ways to avoid this is to add more RAM or reduce the number of processes or users that are active on your system.

If your system seems slow but the CPU and memory usage is low, then start looking at networking problems, such as poor duplex negotiation, bad cables, and network congestion due to excessive traffic.

The vmstat Command

You can also determine memory and swap usage with the vmstat command, which provides a summary of what top produces. In the example, memory is still 14% free (57,452MB used from a total of 130,780) and swap isn’t being used at all.

[root@bigboy tmp]# vmstat
   procs                       memory    swap          io     system         cpu
  r  b  w   swpd   free   buff   cache  si  so    bi    bo   in    cs  us  sy  id
  0  0  0      0  57452 101584 130780   0   0     0     4   18     1   3   1   1
[root@bigboy tmp]#

As your memory fills up, your system will temporarily store programs and data on your hard disk’s “swap” partition. Excess swapping of programs and data between disk and memory can cause your system to slow down significantly and memory usage should be monitored to allow you to plan ways to either increase RAM or tune the way your system operates. System tuning is beyond the scope of this book, but there are many reference guides which can show you how to do this.

The free Utility

The free utility can determine the amount of free RAM on your system. The output is easier to understand than vmstat’s. Here’s a sample.

[root@bigboy tmp]# free
             total       used       free     shared     buffers     cached
Mem:        126060     119096       6964          0       58972      40028
-/+ buffers/cache:      20096     105964
Swap:       522072       15496     506576
[root@bigboy tmp]#

You should generally try to make your system run with at least 20% free memory on average, which should allow it to handle moderate spikes in usage caused by running memory-intensive cron batch jobs or tape backups. If you cannot achieve this, consider running more efficient versions of programs, offloading some applications to servers with less load, and, of course, upgrading the capacity of your RAM.


In many cases using MRTG in a basic configuration to monitor the volume of network traffic to your server isn’t enough. You may also want to see graphs of CPU, disk, and memory usage. This chapter explains how to find the values you want to monitor in the SNMP MIB files and then how to use this information to configure MRTG.

All the chapter’s examples assume that the SNMP Read Only string is craz33guy and that the net-snmp-utils RPM package is installed

Locating And Viewing The Contents Of Linux MIBs

Residing in memory, MIBs are data structures that are constantly updated via the SNMP daemon. The MIB configuration text files are located on your hard disk and loaded into memory each time SNMP restarts.

You can easily find your Fedora Linux MIBs by using the locate command and filtering the output to include only values with the word “snmp” in them. As you can see in this case, the MIBs are located in the /usr/share/snmp/mibs directory:

[root@bigboy tmp]# locate mib | grep snmp
[root@bigboy tmp]#

As the MIB configurations are text files you can search for keywords in them using the grep command. This examples searches for the MIBs that keep track of TCP connections and returns the RFC1213 and TCP MIBs as the result.

[root@silent mibs]# grep -i tcp /usr/share/snmp/mibs/*.txt | grep connections
RFC1213-MIB.txt: "The limit on the total number of TCP connections
RFC1213-MIB.txt: "The number of times TCP connections have made a
TCP-MIB.txt:     "The number of times TCP connections have made a
[root@silent mibs]#

You can use the vi editor to look at the MIBs. Don’t change them, because doing so could cause SNMP to fail. MIBs are very complicated, but fortunately the key sections are commented.

Each value tracked in a MIB is called an object and is often referred to by its object ID or OID. In this snippet of the RFC1213-MIB.txt file, you can see that querying the tcpActiveOpens object returns the number of active open TCP connections to the server. The SYNTAX field shows that this is a counter value.

MIBs usually track two types of values. Counter values are used for items that continuously increase as time passes, such as the amount of packets passing through a NIC or amount of time CPU been busy since boot time. Integer values change instant by instant and are useful for tracking such statistics as the amount of memory currently being used.

tcpActiveOpens OBJECT-TYPE
    SYNTAX  Counter
    ACCESS  read-only
    STATUS  mandatory
            "The number of times TCP connections have made a
            direct transition to the SYN-SENT state from the
            CLOSED state."
    ::= { tcp 5 }

You’ll explore the differences between SNMP and MRTG terminologies in more detail later. Understanding them will be important in understanding how to use MRTG to track MIB values.

Testing Your MIB Value

Once you have identified an interesting MIB value for your Linux system you can then use the snmpwalk command to poll it. Many times the text aliases in a MIB only reference the OID branch and not the OID the data located in a leaf ending in an additional number like a “.0” or “.1”. The snmpget command doesn’t work with branches giving an error stating that the MIB variable couldn’t be found.

In the example below, the ssCpuRawUser OID alias was found to be interesting, but the snmpget command fails to get a value. Follow up with the snmpwalk command shows that the value is located in ssCpuRawUser.0 instead. The snmpget is then successful in retrieving the “counter32” type data with a current value of 396271.

[root@bigboy tmp]# snmpget -v1 -c craz33guy localhost ssCpuRawUser
Error in packet
Reason: (noSuchName) There is no such variable name in this MIB.
Failed object: UCD-SNMP-MIB::ssCpuRawUser
[root@bigboy tmp]#

[root@bigboy tmp]# snmpwalk -v1 -c craz33guy localhost ssCpuRawUser
UCD-SNMP-MIB::ssCpuRawUser.0 = Counter32: 396241
[root@bigboy tmp]# snmpget -v1 -c craz33guy localhost ssCpuRawUser.0
UCD-SNMP-MIB::ssCpuRawUser.0 = Counter32: 396271
[root@bigboy tmp]#

The MIB values that work successfully with snmpget are the ones you should use with MRTG.

Differences In MIB And MRTG Terminology

Always keep in mind that MRTG refers to MIB counter values as counter values. It refers to MIB integer and gauge values as gauge. By default, MRTG considers all values to be counters.

MRTG doesn’t plot counter values as a constantly increasing graph, it plots only how much the value has changed since the last polling cycle. CPU usage is typically tracked by MIBs as a counter value; fortunately, you can edit your MRTG configuration file to make it graph this information in a percentage use format (more on this later).

The syntax type, the MIB object name, and the description of what it does are the most important things you need to know when configuring MRTG; I’ll come back to these later.

The CPU And Memory Monitoring MIB

The UCD-SNMP-MIB MIB keeps track of a number of key performance MIB objects, including the commonly used ones in Table 23-1.

Table 23-1 Important Objects In The UCD-SNMP-MIB MIB

UCD-SNMP-MIB Object Variable MIB Type MRTG Type Description

The TCP/IP Monitoring MIB

The TCP-MIB MIB keeps track of data connection information and contains the very useful tcpActiveOpens and tcpCurrEstab objects. Table 23.2 details the most important objects in TCP-MIB.

Table 23-2 Important Objects In The TCP-MIB MIB

UCD-SNMP-MIB Object Variable MIB Type MRTG Type Description

Manually Configuring Your MRTG File

The MRTG cfgmaker program creates configuration files for network interfaces only, simultaneously tracking two OIDs: the NIC’s input and output data statistics. The mrtg program then uses these configuration files to determine the type of data to record in its data directory. The indexmaker program also uses this information to create the overview, or Summary View Web page for the MIB OIDs you’re monitoring.

This Summary View page shows daily statistics only. You have to click on the Summary View graphs to get the Detailed View page behind it with the daily, weekly, monthly, and annual graphs. Some of the parameters in the configuration file refer to the Detailed View, others refer to the Summary View.

If you want to monitor any other pairs of OIDs, you have to manually create the configuration files, because cfgmaker isn’t aware of any OIDs other than those related to a NIC. The mrtg and indexmaker program can be fed individual OIDs from a customized configuration file and will function as expected if you edit the file correctly.

Parameter Formats

MRTG configuration parameters are always followed by a graph name surrounded by square brackets and a colon. The format looks like this:

Parameter[graph name]: value

For ease of editing, the parameters for a particular graph are usually grouped together. Each graph can track two OIDS listed in the Target parameter, which is usually placed at the very top of the graph name list. The two OID values are separated by an & symbol; the first one can be is the input OID, and the second one is the output OID.

Legend Parameters

On the Detailed View Web page, each graph has a legend that shows the max, average, and current values of the graph’s OID statistics. You can use the legendI parameter for the description of the input graph (first graph OID) and the legendO for the output graph (second graph OID).

The space available under each graph’s legend is tiny so MRTG also has legend1 and legend2 parameters that are placed at the very bottom of the page to provide more details. Parameter legend1 is the expansion of legendI, and legend2 is the expansion of legendO.

The Ylegend is the legend for the Y axis, the value you are trying to compare. In the case of a default MRTG configuration this would be the data flow through the interface in bits or bytes per second. Here is an example of the legends of a default MRTG configuration:

YLegend[graph1]: Bits per second
Legend1[graph1]: Incoming Traffic in Bits per Second
Legend2[graph1]: Outgoing Traffic in Bits per Second
LegendI[graph1]: In
LegendO[graph1]: Out

You can prevent MRTG from printing the legend at the bottom of the graph by leaving the value of the legend blank like this:


Later you’ll learn how to match the legends to the OIDs for a variety of situations.

Options Parameters

Options parameters provide MRTG with graph formatting information. The growright option makes sure the data at the right of the screen is for the most current graph values. This usually makes the graphs more intuitively easy to read. MRTG defaults to growing from the left.

The nopercent option prevents MRTG from printing percentage style statistics in the legends at the bottom of the graph. The gauge option alerts MRTG to the fact that the graphed values are of the gauge type. If the value you are monitoring is in bytes, then you can convert the output to bits using the bits option. Likewise, you can convert per second values to per minute graphs using the perminute option. Here are some examples for two different graphs:

options[graph1]: growright,nopercent,perminute

options[graph2]: gauge,bits

If you place this parameter at the top with a label of [_] it gets applied to all the graphs defined in the file. Here’s an example.

options[_]: growright

Title Parameters

The title on the Summary Page is provided by the Title parameter, the PageTop parameter tells the title for the Detailed View page. The PageTop string must start with < H1 > and end with < H1 >.

Title[graph1]: Interface eth0

PageTop[graph1]: < H1 >Detailed Statistics For Interface eth0 < H1 >

Scaling Parameters

The MaxBytes parameter is the maximum amount of data MRTG will plot on a graph. Anything more than this seems to disappear over the edge of the graph.

MRTG also tries to adjust its graphs so that the largest value plotted on the graph is always close to the top. This is so even if you set the MaxBytes parameter.

When you are plotting a value that has a known maximum and you always want to have this value at the top of the vertical legend, you may want to turn off MRTG’s auto scaling. If you are plotting percentage CPU usage, and the server reaches a maximum of 60%, with scaling, MRTG will have a vertical plot of 0% to 60%, so that the vertical peak is near the top of the graph image.

When scaling is off, and MaxBytes is set to 100, then the peak will be only 60% of the way up as the graph plots from 0% to 100%. The example removes scaling from the yearly, monthly, weekly, and daily views on the Detailed View page and gives them a maximum value of 100.

Unscaled[graph1]: ymwd
MaxBytes[graph1]: 100

Defining The MIB Target Parameters

As stated before, MRTG always tries to compare two MIB OID values that are defined by the Target parameter. You have to specify the two MIB OID objects, the SNMP password and the IP address of the device you are querying in this parameter, and separate them with an & character:

Target[graph1]: mib-object-1.0&mib-object-2.0:<SNMP-password>@<IP-address>

The numeric value, in this case .0, at the end of the MIB is required. The next example uses the SNMP command to return the user mode CPU utilization of a Linux server. Notice how the .0 is tagged onto the end of the output.

[root@silent mibs]# snmpwalk -v 1 -c craz33guy localhost ssCpuRawUser
UCD-SNMP-MIB::ssCpuRawUser.0 = Counter32: 926739
[root@silent mibs]#

The MRTG legends map to the MIBs listed in the target as shown in Table 23-3.

Table 23-3 Mapping MIBs To The Graph Legends

Legend Maps To Target MIB

So in the example below, legend1 and legendI describe mib-object-1.0 and legend2 and legendO describe mib-object-2.0.

Target[graph1]: mib-object-1.0&mib-object-2.0:<SNMP-password>@<IP-address>

Plotting Only One MIB Value

If you want to plot only one MIB value, you can just repeat the target MIB in the definition as in the next example, which plots only mib-object-1. The resulting MRTG graph actually superimposes the input and output graphs one on top of the other.

Target[graph1]: mib-object-1.0&mib-object-1.0:<SNMP-password>@<IP-address>

Adding MIB Values Together For a Graph

You can use the plus sign between the pairs of MIB object values to add them together. The next example adds mib-object-1.0 and mib-object-3.0 for one graph and adds mib-object-2.0 and mib-object-4.0 for the other.

Target[graph1]: mib-object-1.0&mib-object-2.0:<SNMP-password>@<IP-address> + mib-object-3.0&mib-object-4.0:<SNMP-password>@<IP-address>

You can use other mathematical operators, such as subtract (-), multiply (*), and divide (%). Left and right parentheses are also valid. There must be white spaces before and after all these operators for MRTG to work correctly. If not, you’ll get oddly shaded graphs.

Sample Target: Total CPU Usage

Linux CPU usage is occupied by system processes, user mode processes, and a few processes running in nice mode. This example adds them all together in a single plot.

Target[graph1]:ssCpuRawUser.0&ssCpuRawUser.0:<SNMP-password>@<IP-address> + ssCpuRawSystem.0&ssCpuRawSystem.0:<SNMP-password>@<IP-address> + ssCpuRawNice.0&ssCpuRawNice.0:<SNMP-password>@<IP-address>

Be sure to place this command on a single line

Sample Target: Memory Usage

Here is an example for the plotting the amount of free memory versus the total RAM installed in the server. Notice that this is a gauge type variable.

Target[graph1]: memAvailReal.0&memTotalReal.0:<SNMP-password>@<IP-address>
options[graph1]: nopercent,growright,gauge

Next, plot the percentage of available memory. Notice how the mandatory white spaces separate the mathematical operators from the next target element.

Target[graph1]: ( memAvailReal.0& memAvailReal.0:<SNMP-password>@<IP-Address> ) * 100 / ( memTotalReal.0&memTotalReal.0:<SNMP-password>@<IP-Address> )
options[graph1]: nopercent,growright,gauge

Sample Target: Newly Created Connections

HTTP traffic caused by Web browsing usually consists of many very short lived connections. The tcpPassiveOpens MIB object tracks newly created connections and is suited for this type of data transfer. The tcpActiveOpens MIB object monitors new connections originating from the server. On smaller Web sites you may want to use the perminute option to make the graphs more meaningful.

Target[graph1]: tcpPassiveOpens.0& tcpPassiveOpens.0:<SNMP-password>@<IP-address>
MaxBytes[graph1]: 1000000
Options[graph1]: perminute

Sample Target: Total TCP Established Connections

Other protocols such as FTP and SSH create longer established connections while people download large files or stay logged into the server. The tcpCurrEstab MIB object measures the total number of connections in the established state and is a gauge value.

Target[graph1]: tcpCurrEstab.0&tcpCurrEstab.0:<SNMP-password>@<IP-address>
MaxBytes[graph1]: 1000000
Options[graph1]: gauge

Sample Target: Disk Partition Usage

In this example, you’ll monitor the /var and /home disk partitions on the system.

1) First use the df -k command to get a list of the partitions in use.

[root@bigboy tmp]# df -k
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda8               505605    128199    351302  27% /
/dev/hda1               101089     19178     76692  21% /boot
/dev/hda5              1035660    122864    860188  13% /home
/dev/hda6               505605      8229    471272   2% /tmp
/dev/hda3              3921436    890092   2832140  24% /usr
/dev/hda2              1510060    171832   1261520  73% /var
[root@bigboy tmp]#

2) Add two entries to your snmpd.conf file.

disk  /home
disk  /var

3) Restart the SNMP daemon to reload the values.

[root@bigboy tmp]# service snmpd restart

4) Use the snmpwalk command to query the the dskPercent MIB. Object dskPercent.1 refers to the first disk entry in snmpd.conf (/home), and dskPercent.2 refers to the second (/var).

[root@bigboy tmp]# snmpwalk -v 1 -c craz33guy localhost dskPercent.1
UCD-SNMP-MIB::dskPercent.1 = INTEGER: 13
[root@bigboy tmp]# snmpwalk -v 1 -c craz33guy localhost dskPercent.2
UCD-SNMP-MIB::dskPercent.2 = INTEGER: 73
[root@bigboy tmp]#

Your MRTG target for these gauge MIB objects should look like this:

Target[graph1]: dskPercent.1& dskPercent.1:<SNMP-password>@<IP-address>
options[graph1]: growright,gauge

Defining Global Variables

You have to make sure MRTG knows where the MIBs you’re using are located. The default location MRTG uses may not be valid. Specify their locations with the global LoadMIBs parameter. You must also define where the HTML files will be located; the example specifies the default Fedora MRTG HTML directory.

LoadMIBs: /usr/share/snmp/mibs/UCD-SNMP-MIB.txt, /usr/share/snmp/mibs/TCP-MIB.txt
workdir: /var/www/mrtg/

Implementing Advanced Server Monitoring

You now can combine all you have learned to create a configuration file that monitors all these variables, and then you can integrate it into the existing MRTG configuration.

A Complete Sample Configuration

Here is a sample configuration file that is used to query server localhost for CPU, memory, disk, and TCP connection information.

# File: /etc/mrtg/server-info.cfg
# Configuration file for non bandwidth server statistics

# Define global options

LoadMIBs: /usr/share/snmp/mibs/UCD-SNMP-MIB.txt,/usr/share/snmp/mibs/TCP-MIB.txt
workdir: /var/www/mrtg/

# CPU Monitoring
# (Scaled so that the sum of all three values doesn't exceed 100)

Target[server.cpu]:ssCpuRawUser.0&ssCpuRawUser.0:craz33guy@localhost + ssCpuRawSystem.0&ssCpuRawSystem.0:craz33guy@localhost + ssCpuRawNice.0&ssCpuRawNice.0:craz33guy@localhost
Title[server.cpu]: Server CPU Load
PageTop[server.cpu]: < H1 >CPU Load - System, User and Nice Processes< /H1 >
MaxBytes[server.cpu]: 100
ShortLegend[server.cpu]: %
YLegend[server.cpu]: CPU Utilization
Legend1[server.cpu]: Current CPU percentage load
LegendI[server.cpu]: Used
Options[server.cpu]: growright,nopercent
Unscaled[server.cpu]: ymwd

# Memory Monitoring (Total Versus Available Memory)

Target[server.memory]: memAvailReal.0&memTotalReal.0:craz33guy@localhost
Title[server.memory]: Free Memory
PageTop[server.memory]: < H1 >Free Memory< /H1 >
MaxBytes[server.memory]: 100000000000
ShortLegend[server.memory]: B
YLegend[server.memory]: Bytes
LegendI[server.memory]: Free
LegendO[server.memory]: Total
Legend1[server.memory]: Free memory, not including swap, in bytes
Legend2[server.memory]: Total memory
Options[server.memory]: gauge,growright,nopercent
kMG[server.memory]: k,M,G,T,P,X

# Memory Monitoring (Percentage usage)
Title[server.mempercent]: Percentage Free Memory
PageTop[server.mempercent]: < H1 >Percentage Free Memory< /H1 >
Target[server.mempercent]: ( memAvailReal.0&memAvailReal.0:craz33guy@localhost ) * 100 / ( memTotalReal.0&memTotalReal.0:craz33guy@localhost )
options[server.mempercent]: growright,gauge,transparent,nopercent
Unscaled[server.mempercent]: ymwd
MaxBytes[server.mempercent]: 100
YLegend[server.mempercent]: Memory %
ShortLegend[server.mempercent]: Percent
LegendI[server.mempercent]: Free
LegendO[server.mempercent]: Free
Legend1[server.mempercent]: Percentage Free Memory
Legend2[server.mempercent]: Percentage Free Memory

# New TCP Connection Monitoring (per minute)

Target[server.newconns]: tcpPassiveOpens.0&tcpActiveOpens.0:craz33guy@localhost
Title[server.newconns]: Newly Created TCP Connections
PageTop[server.newconns]: < H1 >New TCP Connections< /H1 >
MaxBytes[server.newconns]: 10000000000
ShortLegend[server.newconns]: c/s
YLegend[server.newconns]: Conns / Min
LegendI[server.newconns]: In
LegendO[server.newconns]: Out
Legend1[server.newconns]: New inbound connections
Legend2[server.newconns]: New outbound connections
Options[server.newconns]: growright,nopercent,perminute

# Established TCP Connections

Target[server.estabcons]: tcpCurrEstab.0&tcpCurrEstab.0:craz33guy@localhost
Title[server.estabcons]: Currently Established TCP Connections
PageTop[server.estabcons]: < H1 >Established TCP Connections< /H1 >
MaxBytes[server.estabcons]: 10000000000
YLegend[server.estabcons]: Connections
LegendI[server.estabcons]: In
Legend1[server.estabcons]: Established connections
Options[server.estabcons]: growright,nopercent,gauge

# Disk Usage Monitoring

Target[server.disk]: dskPercent.1&dskPercent.2:craz33guy@localhost
Title[server.disk]: Disk Partition Usage
PageTop[server.disk]: < H1 >Disk Partition Usage /home and /var< /H1 >
MaxBytes[server.disk]: 100
ShortLegend[server.disk]: %
YLegend[server.disk]: Utilization
LegendI[server.disk]: /home
LegendO[server.disk]: /var
Options[server.disk]: gauge,growright,nopercent
Unscaled[server.disk]: ymwd

Testing The Configuration

The next step is to test that MRTG can load the configuration file correctly.

Restart SNMP to make sure the disk monitoring commands in the snmpd.conf file are activated. Run the /usr/bin/mrtg command followed by the name of the configuration file three times. If all goes well, MRTG will complain only about the fact that certain database files don’t exist. MRTG then creates the files. By the third run, all the files are created and MRTG should operate smoothly.

[root@bigboy tmp]# service snmpd restart
[root@bigboy tmp]# env LANG=C /usr/bin/mrtg /etc/mrtg/server-stats.cfg

Creating A New MRTG Index Page To Include This File

Use the indexmaker command and include your original MRTG configuration file from Chapter 22, “ Monitoring Server Performance“, (/etc/mrtg/mrtg.cfg) plus the new one you created (/etc/mrtg/server-stats.cfg).

[root@bigboy tmp]# indexmaker --output=/var/www/mrtg/index.html \
/etc/mrtg/mrtg.cfg /etc/mrtg/server-stats.cfg

Configuring cron To Use The New MRTG File

The final step is to make sure that MRTG is configured to poll your server every five minutes using this new configuration file. To do so, add this line to your /etc/cron.d/mrtg file.

0-59/5 * * * * root env LANG=C /usr/bin/mrtg /etc/mrtg/server-stats.cfg

Some versions of Linux require you to edit your /etc/crontab file instead. See Chapter 22, “ Monitoring Server Performance“, for more details. You will also have to restart cron with the service crond restart for it to read its new configuration file that tells it to additionally run MRTG every five minutes using the new MRTG configuration file.

[root@bigboy tmp]# service crond restart

Monitoring Non Linux MIB Values

All the MIBs mentioned so far are for Linux systems; other types of systems will need additional MIBs whose correct installation may be unclear in user guides or just not available. In such cases, you’ll need to know the exact value of the OID.


Imagine that your small company has purchased a second-hand Cisco switch to connect its Web site servers to the Internet. The basic MRTG configuration shown in Chapter 22, “ Monitoring Server Performance“, provides the data bandwidth statistics, but you want to measure the CPU load the traffic is having on the device, as well. Downloading MIBs from Cisco and using them with the snmpget command was not a success. You do not know what to do next. Find The OIDs

When MIB values fail, it is best to try to find the exact OID value. Like most network equipment manufacturers, Cisco has an FTP site from which you can download both MIBs and OIDs. The SNMP files for Cisco’s devices can be found at in the /pub/mibs directory; OIDs are in the oid directory beneath that.

After looking at all the OID files, you decide that the file CISCO-PROCESS-MIB.oid will contain the necessary values and find these entries inside it.

"cpmCPUTotalPhysicalIndex"  ""
"cpmCPUTotal5sec"           ""
"cpmCPUTotal1min"           ""
"cpmCPUTotal5min"           ""
"cpmCPUTotal5secRev"        ""
"cpmCPUTotal1minRev"        ""
"cpmCPUTotal5minRev"        ""

Testing The OIDs

As you can see, all the OIDs are a part of the same tree starting with The OIDs provided may be incomplete, so it is best to use the snmpwalk command to try to get all the values below this root first.

[root@bigboy tmp]# snmpwalk -v1 -c craz33guy cisco-switch
SNMPv2-SMI::enterprises. = INTEGER: 0
SNMPv2-SMI::enterprises. = Gauge32: 32
SNMPv2-SMI::enterprises. = Gauge32: 32
SNMPv2-SMI::enterprises. = Gauge32: 32
[root@bigboy tmp]#

Although listed in the OID file,,, and are not supported. Notice also how SNMP has determined that the first part of the OID value ( in the original OID file maps to the word “enterprise”.

Next, you can use one the snmpget command to set only one of the OID values returned by snmpwalk.

[root@bigboy tmp]# snmpget -v1 -c craz33guy cisco-switch \
SNMPv2-SMI::enterprises. = Gauge32: 33
[root@bigboy tmp]#

Success! Now you can use this OID value, enterprises., for your MRTG queries.

Speeding up MRTG with RRDtool

MRTG is a very useful program but it has a limitation. All the graphs and web pages are recreated each time a device is polled. This can potentially overload your MRTG server especially if you have a large number of monitored devices and the graphs take more than five minutes to generate. RRDtool is an application written by the creator of MRTG that can store general purpose data, but generates graphs on demand. Integrating MRTG with RRDtool can have very noticeable performance benefits. The example that follows will show you how to quickly implement a general purpose solution.


The use of RRDtool is needed to reduce the load on a monitoring server that has been experiencing very sluggish performance due to the amount of MRTG graphs it has to regenerate every polling cycle.

  • Due to space constraints, the RRD database needs to be located in the /var partition.
  • The server has a default Apache configuration with the CGI files needed for dynamically generated content being located in the /var/www/cgi-bin directory.
  • A CGI script is required that will read the new MRTG data in RRDtool format.
  • The MRTG configuration file is /etc/mrtg/mrtg.cfg.

Here’s how to proceed.

Installing RRDtool

The RRDtool and RRDtool PERL module file can be downloaded from its website at, but installation can be tricky as the installation program may look for certain supporting libraries in the wrong directories.

Fortunately the prerequisite rrdtool and rrdtool-perl packages now come as part of most Linux distributions. For more details on installing packages, see Chapter 6, “Installing Linux Software“).

Storing the MRTG Data in RRDtool Format

This phase of the integration process can be done in a few minutes, but the steps can be tricky:

  • The first step is to add some new options to your cfgmaker command. The first indicates that MRTG should only store rrdtool formatted data, and the second defines the /var/mrtg directory in which it should be stored. For added security, the directory should be external to your web server’s document root.
--global 'LogFormat: rrdtool' --global "workdir: /var/mrtg"  --global 'IconDir: /mrtg'
Finally, you should also specify an icon directory which specifies the location of all miscellaneous MRTG web page icons. The RRD web interface script we’ll install later uses an incorrect location. The icon directory /mrtg is actually a partial URL location. In this Fedora scenario we are using the default Apache configuration which locates the MRTG icon files in the /var/www/mrtg directory. If you are using a non default Apache MRTG configuration or are using other Linux distributions or versions you may have to copy the icons to the custom directory in which the MRTG PNG format icon files are located.
The cfgmaker program is simple to use and is covered in in Chapter 22, “Monitoring Server Performance“.
  • The next step is to create the data repository directory /var/mrtg and make it be owned by the apache user and process that runs the default Linux web server application.
[root@bigboy tmp]# mkdir /var/mrtg
[root@bigboy tmp]# chown apache /var/mrtg
[root@bigboy tmp]#
Note: If you are using SELinux you’ll have to change the context of this directory to match that of the /var/www/html directory so that the apache process will be able to read the database files when your CGI script needs them. These commands compare the contexts of the both directories and apply the correct set to /var/mrtg.
Please refer to Chapter 20, “ The Apache Web Server” for more details on file contexts with Apache.
[root@bigboy tmp]# ls -alZ /var/www | grep html
drwxr-xr-x  root     root     system_u:object_r:httpd_sys_content_t html
[root@bigboy tmp]# ls -alZ /var | grep mrtg
drwxr-xr-x  apache   root     root:object_r:var_t              mrtg
[root@bigboy tmp]# chcon -R -u system_u -r object_r -t httpd_sys_content_t /var/mrtg
[root@bigboy tmp]#
  • We now need to test that the RRD files are being created correctly. Run MRTG using the /etc/mrtg/mrtg.cfg file as the source configuration file then test to see if the contents of the /var/mrtg directory have changed. Success!
[root@bigboy tmp]# ls /var/mrtg/
[root@bigboy tmp]#

The files are being created properly. Now we need to find a script to read the new data format and present it in a web format. This will be discussed next.

The MRTG / RRDtool Integration Script

The MRTG website recommends the script located on the mrtg-rrd website ( as being a good one to use. Let’s go ahead and install it.

  • Download the script using wget. The site lists several versions; make sure you get the latest one.
[root@bigboy tmp]# wget
           => `mrtg-rrd-0.7.tar.gz'
Connecting to||:21... connected.
Logging in as anonymous ... Logged in!
15:24:50 (53.53 KB/s) - `mrtg-rrd-0.7.tar.gz' saved [20863]
[root@bigboy tmp]# ls
[root@bigboy tmp]#
  • Extract the contents of the tar file.
[root@bigboy tmp]# tar -xzvf mrtg-rrd-0.7.tar.gz
[root@bigboy tmp]#
  • Create the /var/www/cgi-bin/mrtg directory and copy the mrtg-rrd.cgi file to it.
[root@bigboy tmp]# mkdir -p /var/www/cgi-bin/mrtg
[root@bigboy tmp]# cp mrtg-rrd-0.7/mrtg-rrd.cgi /var/www/cgi-bin/mrtg/
[root@bigboy tmp]#
  • Edit the mrtg-rrd.cgi file and make it refer to the /etc/mrtg/mrtg.cfg file for its configuration details, or you can specify all the .cfg files in your /etc/mrtg directory.
# File: mrtg-rrd.cgi (Single File)

# EDIT THIS to reflect all your MRTG config files
BEGIN { @config_files = qw(/etc/mrtg/mrtg.cfg); }
# File: mrtg-rrd.cgi (multipl .cfg files)

# EDIT THIS to reflect all your MRTG config files
BEGIN { @config_files = </etc/mrtg/*.cfg>; }
  • You should now be able to access your MRTG RRD graphs by visiting this URL:

Once installed, RRDtool operates transparently with MRTG. You’ll have to remember to add the RRD statements to any new MRTG configurations and also add the configuration file to the CGI script. Our monitoring server can now breathe a little easier.


The troubleshooting techniques for advanced MRTG are similar to those mentioned in Chapter 22, “ Monitoring Server Performance“, but because you have done some customizations you’ll have to go the extra mile.

  • Verify the IP address and community string of the target device you intend to poll.
  • Make sure you can do an SNMP walk of the target device. If not, revise your access controls on the target device and any firewall rules that may impede SNMP traffic.
  • Ensure you can do an SNMP get of the specific OID value listed in your MRTG configuration file.
  • Check your MRTG parameters to make sure they are correct. Gauge values defined as counter and vice versa will cause your graphs to have continuous zero values. Graph results that are eight times what you expect may have the bits parameter set.
  • There are a few errors common to initial RRDtool integration.
Web messages like this where the reference to the MRTG configuration file in the CGI script was incorrect
Error: Cannot open config file: No such file or directory
“Permission Denied” web messages are usually caused by incorrect file permissions and / or SELinux contexts
Error: RRDs::graph failed, opening '/var/mrtg/localhost_192.168.1.100.rrd': Permission denied
Errors in the /var/log/httpd/errorlog file referring to files or directories that don’t exist can be caused by an incorrect IconDir statement in the MRTG configuration file.
[Wed Jan 04 15:42:13 2006] [error] [client] File does not exist: /var/www/html/var,
referer: http://bigboy/cgi-bin/mrtg/mrtg-rrd.cgi/ 

[Wed Jan 04 15:45:46 2006] [error] [client] script not found or unable to stat:
 /var/www/cgi-bin/mrtg/mrtg-l.png, referer: http://bigboy/cgi-bin/mrtg/mrtg-rrd.cgi/

These quick steps should be sufficient in most cases and will reward you with a more manageable network.

Monitoring tool

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