XNETMON -- an X windows based SNMP network management
station from SNMP Research.
alarm, control, manager, map, routing, security,
status; DECnet, ethernet, IP, OSI, ring, star; NMS,
SNMP, X; DOS, UNIX, VMS; sourcelib.
The XNETMON application implements a powerful network
management station based on the X window system. It
provides network managers tools for fault management,
configuration management, performance management, and
security management. It can be successfully used with
many types of networks including those based on various
LAN media, and wide area networks. XNETMON has been
used with multiprotocol devices including those which
support TCP/IP, DECnet, and OSI protocols. The fault
management tool displays the map of the network confi-
guration with node and link state indicated in one of
several colors to indicate current status. Alarms may
be enabled to alert the operator of events occurring in
the network. Events are logged to disk. The confi-
guration management tool may be used to edit the net-
work management information base stored in the network
management station to reflect changes occurring in the
network. Other features include graphs and tabular
tools for use in fault and performance management and
mechanisms by which additional variables, such as
vendor-specific variables, may be added. The XNETMON
application comes complete with source code including a
powerful set of portable libraries for generating and
parsing SNMP messages. Output data from XNETMON may be
transferred via flat files for additional report gen-
eration by a variety of statistical packages.
The XNETMON application is based on the Simple Network
Management Protocol (SNMP). Polling is performed via
the powerful SNMP get-next operator and the SNMP get
operator. Trap directed polling is used to regulate
the focus and intensity of the polling.
The monitored and managed nodes must implement the SNMP
over UDP per RFC 1098 or must be reachable via a proxy
X windows workstation with UDP socket library. Mono-
chrome is acceptable but color is far superior.
X windows version 11 release 3 or later.
This is a commercial product available under license
P.O. Box 8593
Knoxville, TN 37996-4800
(615) 573-1434 (Voice)
(615) 573-9197 (FAX)
Attn: Dr. Jeff Case
xnetperfmon -- a graphical network performance and
fault management tool from SNMP Research.
manager, status; DECnet, ethernet, IP, OSI, ring, star;
NMS, SNMP, X; DOS, UNIX, VMS; sourcelib.
Xnetperfmon may be used to plot SNMP variables as a
graphical display. These graphs are often useful for
fault and performance management. Variables may be
plotted as gauges versus time. Alternatively, counters
may be plotted as delta count/delta time (rates). The
user may easily customize the variables to be plotted,
labels, step size, update interval, and the like. The
scales automatically adjust whenever a point to be
plotted would go off scale.
The xnetperfmon application communicates with remote
agents or proxy agents via the Simple Network Manage-
ment Protocol (SNMP).
All plots for a single invocation of xnetperfmon must
be for variables provided by a single network manage-
ment agent. However, multiple invocations of xnetperf-
mon may be active on a single display simultaneously or
proxy agents may be used to summarize information at a
Systems supporting X windows.
X Version 11 release 2 or later.
status; ping, X; HP.
Xup uses the X-Windows to display the status of an
"interesting" set of hosts.
Xup uses ping to determine host status.
Polling for status increases network load.
Runs only on HP series 300 and 800 workstations.
Version 10 of X-Windows.
A standard command for the HP 300 & 800 Workstations.
Appendix: Network Management Tutorial
This tutorial is an overview of the practice of network
management. Reading this section is no substitute for know-
ing your system, and knowing how it is used. Do not wait
until things break to learn what they ought to do or how
they usually work: a crisis is not the time for determining
how "normal" packet traces should look. Furthermore, it
takes little imagination to realize that you do not want to
be digging through manuals while your boss is screaming for
network service to be restored.
We assume an acquaintance with the TCP/IP protocol suite and
the Internet architecture. There are many available refer-
ences on these topics, several of which are listed below in
Since many of the details of network management are system-
specific, this tutorial is a bit superficial. There is,
however, a more fundamental problem in prescribing network
management practices: network management is not a well-
understood endeavor. At present, the cutting edge of net-
work management is the use of distributed systems to collect
and exchange status information, and then to display the
data as histograms or trend lines. It is not clear that we
know what data should be collected, how to analyze it when
we get it, or how to structure our collection systems. For
now, automated, real-time control of internets is an aspira-
tion, rather than a reality. The communications systems
that we field are apparently more complex than we can
comprehend, which no doubt accounts in part for their fre-
quently surprising behavior.
The first section of this tutorial lists the overall goals
and functions of network management. It presents several
aspects of network management, including system monitoring,
fault detection and isolation, performance testing, confi-
guration management, and security. These discussions are
followed by a bibliographic section. The tutorial closes
with some final advice for network managers.
1. Network Management Goals and Functions
An organization's view of network management goals is shaped
by two factors:
1. people in the organization depend on the system
2. LANs, routers, lines, and other communications
resources have costs.
From the organizational vantage point, the ultimate goal of
network management is to provide a consistent, predictable,
acceptable level of service from the available data communi-
cations resources. To achieve this, a network manager must
first be able to perform fault detection, isolation, and
correction. He must also be able to effect configuration
changes with a minimum of disruption, and measure the utili-
zation of system components.
People actually managing networks have a different focus.
Network managers are usually evaluated by the availability
and performance of their communications systems, even though
many factors of net performance are beyond their control.
To them, the most important requirement of a network manage-
ment tool is that it allows the detection and diagnosis of
faults before users can call to complain: users (and bosses)
can often be placated just by knowing that a network problem
has been diagnosed. Another vital network management func-
tion is the ability to collect data that justify current or
future expenditures for the data communications plant and
Following a section on system monitoring, this tutorial
addresses fault, performance, configuration, and security
management. By fault management, we mean the detection,
diagnosis, and correction of network malfunctions. Under
the subject of performance management, we include support
for predictable, efficient service, as well as capacity
planning and capacity testing. Configuration management
includes support for orderly configuration changes (usually,
system growth), and local administration of component names
and addresses. Security management includes both protecting
system components from damage and protecting sensitive
information from unintentional or malicious disclosure or
Readers familiar with the ISO management standards and
drafts will note both that we have borrowed heavily from the
"OSI Management Framework," except that we have omitted the
"account management" function. Account management seems a
bit out of place with the other network management
functions. The logging required by account management is
likely to be done by specialized, dedicated subsystems that
are distinct from other network management components.
Hence, this tutorial does not cover account management.
Rest assured, however, that account management, if required,
will be adequately supported and staffed.
For those with a DoD background, security may also seem out
of place as a subtopic of network management. Without
doubt, communications security is an important issue that
should be considered in its own right. Because of the
requirements of trust for security mechanisms, security com-
ponents will probably not be integrated subcomponents of a
larger network management system. Nevertheless, because a
network manager has a responsibility to protect his system
from undue security risks, this tutorial includes a discus-
sion on internet security.
2. System Monitoring
System monitoring is a fundamental aspect of network manage-
ment. One can divide system monitoring into two rough
categories: error detection and baseline monitoring.
System errors, such as misformatted frames or dropped pack-
ets, are not in themselves cause for concern. Spikes in
error rates, however, should be investigated. It is sound
practice to log error rates over time, so that increases can
be recognized. Furthermore, logging error rates as a func-
tion of traffic rates can be used to detect congestion.
Investigate unusual error rates and other anomalies as they
are detected, and keep a notebook to record your
Day-to-day traffic should be monitored, so that the opera-
tional baselines of a system and its components can be
determined. As well as being essential for performance
management, baseline determination and traffic monitoring
are the keys to early fault detection.
A preliminary step to developing baseline measurements is
construction of a system map: a graphical representation of
the system components and their interfaces. Then, measure-
ments of utilization (i.e., use divided by capacity) are
needed. Problems are most likely to arise, and system tun-
ing efforts are most likely to be beneficial, at highly
It is worthwhile to develop a source/destination traffic
matrix, including a breakdown of traffic between the local
system and other internet sites. Both volume and type of
traffic should be logged, along with its evolution over
time. Of particular interest for systems with diskless
workstations is memory swapping and other disk server
access. For all systems, broadcast traffic and routing
traffic should be monitored. Sudden increases in the vari-
ance of delay or the volume of routing traffic may indicate
thrashing or other soft failures.
In monitoring a system, long-term averages are of little
use. Hourly averages are a better indicator of system use.
Variance in utilization and delay should also be tracked.
Sudden spikes in variance are tell-tale signs that a problem
is looming or exists. So, too, are trends of increased
packet or line errors, broadcasts, routing traffic, or
3. Fault Detection and Isolation
When a system fails, caution is in order. A net manager
should make an attempt to diagnose the cause of a system
crash before rebooting. In many cases, however, a quick
diagnosis will not be possible. For some high priority
applications, restoring at least some level of service will
have priority over fault repair or even complete fault diag-
nosis. This necessitates prior planning. A net manager
must know the vital applications at his site. If applica-
tions require it, he must also have a fall-back plan for
bringing them online. Meanwhile, repeated crashes or
hardware failures are unambiguous signs of a problem that
must be corrected.
A network manager should prepare for fault diagnosis by
becoming familiar with how diagnostic tools respond to net-
work failure. In times of relative peace, a net manager
should occasionally unplug the network connection from an
unused workstation and then "debug" the problem.
When diagnosing a fault or anomaly, it is vital to proceed
in an orderly manner, especially since network faults will
usually generate spurious as well as accurate error mes-
sages. Remember to keep in mind that the network itself is
failing. Do not place too much trust in anything obtained
remotely. Furthermore, it is unlikely to be significant
that remote information such as DNS names or NFS files can-
not be obtained.
Even spurious messages can be revealing, because they pro-
vide clues to the problem. From the data at hand, develop
working hypotheses about probable causes of the problems you
detect. Direct your further data gathering efforts so that
the information you get will either refute or support your
An orderly approach to debugging is facilitated if it is
guided by a model of network behavior. The following por-
tions of this section present such a model, along with a
procedure for checking network connectivity. The section
concludes with some hints for diagnosing a particularly
tricky class of connectivity problem.
3.1 A Network Model as a Diagnostic Framework
The point of having a model of how things work is to have a
basis for developing educated guesses about how things go
wrong. The problem of cascading faults -- faults generating
other faults -- makes use of a conceptual model a virtual
In general, only problems in a component's hardware or
operating system will generate simultaneous faults in multi-
ple protocol layers. Otherwise, faults will propagate vert-
ically (up the protocol stack) or horizontally (between
peer-level communications components). Applying a concep-
tual model that includes the architectural relations of net-
work components can help to order an otherwise senseless
barrage of error messages and symptoms.
The model does not have to be formal or complex to bring
structure to debugging efforts. A useful start is something
as simple as the following:
1. Applications programs use transport services:
TCP/UDP. Before using service, applications that
accept host names as parameters must translate the
names into IP addresses. Translation may be based
on a static table lookup (/etc/hosts file in UNIX
hosts), the DNS, or yellow pages. Nslookup and
DiG are tools for monitoring the activities of the
2. Transport protocol implementations use IP ser-
vices. The local IP module makes the initial
decision on forwarding. An IP datagram is for-
warded directly to the destination host if the
destination is on the same network as the source.
Otherwise, the datagram is forwarded to a gateway
attached to the network. On BSD hosts, the con-
tents of a host's routing table are visible by use
of the "netstat" command.*
3. IP implementations translate the IP address of a
datagram's next hop (either the destination host
or a gateway) to a local network address. For
ethernets, the Address Resolution Protocol (ARP)
is commonly used for this translation. On BSD
systems, an interface's IP address and other con-
figuration options can be viewed by use of the
"ifconfig" command, while the contents of a host's
ARP cache may be viewed by use of "arp" command.
4. IP implementations in hosts and gateways route
datagrams based on subnet and net identifiers.
Subnetting is a means of allocating and preserving
IP address space, and of insulating users from the
topological details of a multi-network campus.
Sites that use subnetting reserve portions of the
IP address's host identifier to indicate particu-
lar networks at their campus. Subnetting is
highly system-dependent. The details are a criti-
cal, though local, issue. As for routing between
separate networks, a variety of gateway-to-gateway
protocols are used. Traceroute is a useful tool
for investigating routing problems. The tool,
"query," can be used to examine RIP routing
A neophyte network manager should expand the above descrip-
tion so that it accurately describes his particular system,
* Initial forwarding may actually be complex and
vulnerable to multiple points of failure. For example,
when sending an IP datagram, 4.3BSD hosts first look
for a route to the particular host. If none has been
specified for the destination, then a search is made
for a route to the network of the destination. If this
search also fails, then as a last resort, a search is
made for a route to a "default" gateway. Routes to
hosts, networks, and the "default" gateway may be stat-
ic, loaded at boot time and perhaps updated by operator
commands. Alternatively, they may be dynamic, loaded
from redirects and routing protocol updates.
and learn the tools and techniques for monitoring the opera-
tions at each of the above stages.
3.2 A Simple Procedure for Connectivity Check
In this section, we describe a procedure for isolating a
TCP/IP connectivity problem.** In this procedure, a series
of tests methodically examine connectivity from a host,
starting with nearby resources and working outward. The
steps in our connectivity-testing procedure are:
1. As an initial sanity check, ping your own IP address
and the loopback address.
2. Next, try to ping other IP hosts on the local subnet.
Use numeric addresses when starting off, since this
eliminates the name resolvers and host tables as poten-
tial sources of problems. The lack of an answer may
indicate either that the destination host did not
respond to ARP (if it is used on your LAN), or that a
datagram was forwarded (and hence, the destination IP
address was resolved to a local media address) but that
no ICMP Echo Reply was received. This could indicate a
length-related problem, or misconfigured IP Security.
3. If an IP router (gateway) is in the system, ping both
its near and far-side addresses.
4. Make sure that your local host recognizes the gateway
as a relay. (For BSD hosts, use netstat.)
Still using numeric IP addresses, try to ping hosts
beyond the gateway. If you get no response, run hop-
check or traceroute, if available. Note whether your
packets even go to the gateway on their way to the des-
tination. If not, examine the methods used to instruct
your host to use this gateway to reach the specified
destination net (e.g., is the default route in place?
Alternatively, are you successfully wire-tapping the
IGP messages broadcast on the net you are attached to?)
** Thanks to James VanBokkelen, president of FTP
Software, for sharing with us a portion of a PC/TCP
support document, the basis for the above connectivity
If traceroute is not available, ping, netstat, arp, and
a knowledge of the IP addresses of all the gateway's
interfaces can be used to isolate the cause of the
problem. Use netstat to determine your next hop to the
destination. Ping that IP address to ensure the router
is up. Next, ping the router interface on the far sub-
net. If the router returns "network unreachable" or
other errors, investigate the router's routing tables
and interface status. If the pings succeed, ping the
close interface of the succeeding next hop gateway, and
so on. Remember the routing along the outbound and
return paths may be different.
6. Once ping is working with numeric addresses, use ping
to try to reach a few remote hosts by name. If ping
fails when host names are used, check the operation of
the local name-mapping system (i.e., with nslookup or
DiG). If you want to use "shorthand" forms ("myhost"
instead of "myhost.mydomain.com"), be sure that the
alias tables are correctly configured.
7. Once basic reachability has been established with ping,
try some TCP-based applications: FTP and TELNET are
supported on almost all IP hosts, but FINGER is a
simpler protocol. The Berkeley-specific protocols
(RSH, RCP, REXEC and LPR) require extra configuration
on the server host before they can work, and so are
poor choices for connectivity testing.
If problems arise in steps 2-7 above, rerunning the tests
while executing a line monitor (e.g., etherfind, netwatch,
or tcpdump) can help to pinpoint the problem.
The above procedure is sound and useful, especially if lit-
tle is known about the cause of the connectivity problem.
It is not, however, guaranteed to be the shortest path to
diagnosis. In some cases, a binary search on the problem
might be more effective (i.e., try a test "in the middle,"
in a spot where the failure modes are well defined). In
other cases, available information might so strongly suggest
a particular failure that immediately testing for it is in
order. This last "approach," which might be called "hunting
and pecking," should be used with caution: chasing one will
o' the wisp after another can waste much time and effort.
Note that line problems are still among the most common
causes of connectivity loss. Problems in transmission
across local media are outside the scope of this tutorial.
But, if a host or workstation loses or cannot establish con-
nectivity, check its physical connection.
3.3 Limited Connectivity
An interesting class of problems can result in a particu-
larly mysterious failure: TELNET or other low-volume TCP
connections work, but large file transfers fail. FTP
transfers may start, but then hang. There are several pos-
sible culprits in this problem. The most likely suspects
are IP implementations that cannot fragment or reassemble
datagrams, and TCP implementations that do not perform
dynamic window sizing (a.k.a. Van Jacobson's "Slow Start"
algorithm). Another possibility is mixing incompatible
frame formats on an ethernet.
Even today, some IP implementations in the Internet cannot
correctly handle fragmentation or reassembly. They will
work fine for small packets, but drop all large packets.
The problem can also be caused by buffer exhaustion at gate-
ways that connect interfaces of widely differing bandwidth.
Datagrams from a TCP connection that traverses a bottleneck
will experience queue delays, and will be dropped if buffer
resources are depleted. The congestion can be made worse if
the TCP implementation at the traffic source does not use
the recommended algorithms for computing retransmission
times, since spuriously retransmitted datagrams will only
add to the congestion.* Fragmentation, even if correctly
implemented, will compound this problem, since processing
delays and congestion will be increased at the bottleneck.
Serial Line Internet Protocol (SLIP) links are especially
vulnerable to this and other congestion problems. SLIP
lines are typically an order of magnitude slower than other
gateway interfaces. Also, the SLIP lines are at times con-
figured with MTUs (Maximum Transfer Unit, the maximum length
of an IP datagram for a particular subnet) as small as 256
* To avoid this problem, TCP implementations on the In-
ternet must use "exponential backoff" between succes-
sive retransmissions, Karn's algorithm for filtering
samples used to estimate round-trip delay between TCP
peers, and Jacobson's algorithm for incorporating vari-
ance into the "retransmission time-out" computation for
TCP segments. See Section 22.214.171.124 of RFC 1122, "Re-
quirements for Internet Hosts -- Communication Layers."
bytes, which virtually guarantees fragmentation.
To alleviate this problem, TCP implementations behind slow
lines should advertise small windows. Also, if possible,
SLIP lines should be configured with an MTU no less than 576
bytes. The tradeoff to weigh is whether interactive traffic
will be penalized too severly by transmission delays of
lengthy datagrams from concurrent file transfers.
Misuse of ethernet trailers can also cause the problem of
hanging file transfers. "Trailers" refers to an ethernet
frame format optionally employed by BSD systems to minimize
buffer copying by system software. BSD systems with ether-
net interfaces can be configured to send large frames so
that their address and control data are at the end of a
frame (hence, a "trailer" instead of a "header"). After a
memory page is allocated and loaded with a received ethernet
frame, the ethernet data will begin at the start of the
memory page boundary. Hence, the ethernet control informa-
tion can be logically stripped from the end merely by
adjusting the page's length field. By manipulating virtual
memory mapping, this same page (sans ethernet control infor-
mation), can then be passed to the local IP module without
additional allocation and loading of memory. The disadvan-
tage in using trailers is that it is non-standard. Many
implementations cannot parse trailers.
The hanging FTP problem will appear if a gateway is not con-
figured to recognize trailers, but a host or gateway immedi-
ately "upstream" on an ethernet uses them. Short datagrams
will not be formatted with trailers, and so will be pro-
cessed correctly. When the bulk data transfer starts, how-
ever, full-sized frames will be sent, and will use the
trailer format. To the gateway that receives them, they
appear simply as misformatted frames, and are quietly
dropped. The solution, obviously, is to insure that all
hosts and gateways on an ethernet are consistent in their
use of trailers. Note that RFC 1122, "Internet Host
Requirements," places very strict restrictions on the use of
4. Performance Testing
Performance management encompasses two rather different
activities. One is passive system monitoring to detect
problems and determine operational baselines. The goal is
to measure system and component utilization and so locate
bottlenecks, since bottlenecks should receive the focus of
performance tuning efforts. Also, performance data is usu-
ally required by upper level management to justify the costs
of communications systems. This is essentially identical to
system monitoring, and is addressed at greater length in
Section 2, above.
Another aspect of performance management is active perfor-
mance testing and capacity planning. Some work in this area
can be based on analysis. For example, a rough estimate of
gateway capacity can be deduced from a simple model given by
Charles Hedrick in his "Introduction to Administration of an
Internet-based Local Network," which is
per-packet processing time =
switching time +
(packet size) * (transmission bps).
Another guideline for capacity planning is that in order to
avoid excessive queuing delays, a system should be sized at
about double its expected load. In other words, system
capacity should be so high that utilization is no greater
Although there are more sophisticated analytic models of
communications systems than those above, their added com-
plexity does not usually gain a corresponding accuracy.
Most analytic models of communications nets require assump-
tions about traffic load distributions and service rates
that are not merely problematic, but are patently false.
These errors tend to result in underestimating queuing
delays. Hence, it is often necessary to actually load and
measure the performance of a real communications system if
one is to get accurate performance predictions. Obviously,
this type of testing is performed on isolated systems or
during off hours. The results can be used to evaluate
parameter settings or predict performance during normal
Simulations can be used to supplement the testing of real
systems. To be believable, however, simulations require
validation, which, in turn, requires measurements from a
real system. Whether testing or simulating a system's per-
formance, actual traffic traces should be incorporated as
input to traffic generators. The performance of a communi-
cations system will be greatly influenced by its load
characteristics (burstiness, volume, etc.), which are them-
selves highly dependent on the applications that are run.
When tuning a net, in addition to the usual configuration
parameters, consider the impact of the location of gateways
and print and file servers. A few rules of thumb can guide
the location of shared system resources. First, there is
the principle of locality: a system will perform better if
most traffic is between nearby destinations. The second
rule is to avoid creating bottlenecks. For example, multi-
ple diskservers may be called for to support a large number
of workstations. Furthermore, to avoid LAN and diskserver
congestion, workstations should be configured with enough
memory to avoid frequent swapping.
As a final note on performance management, proceed cau-
tiously if your ethernet interface allows you to customize
its collision recovery algorithm. This is almost always a
bad idea. The best that it can accomplish is to give a few
favored hosts a disproportionate share of the ethernet
bandwidth, perhaps at the cost of a reduction in total sys-
tem throughput. Worse, it is possible that differing colli-
sion recovery algorithms may exhibit a self-synchronizing
behavior, so that excess collisions are generated.
5. Configuration Management
Configuration management is the setting, collecting, and
storing of the state and parameters of network resources.
It overlaps all other network management functions. Hence,
some aspects of configuration management have already been
addressed (e.g., tuning for performance). In this section,
we will focus on configuration management activities needed
to "hook up" a net or campus to a larger internet. We will
not, of course, include specific details on installing or
maintaining internetted communications systems. We will,
however, skim over some of the TCP/IP configuration
Configuration management includes "name management" -- the
control and allocation of system names and addresses, and
the translation between names and addresses. Name-to-
address translation is performed by "name servers." We con-
clude this section with a few strictures on the simultaneous
use of two automated name-servers, the Domain Name System
(DNS), and Yellow Pages (YP).
5.1 Required Host Configuration Data for TCP/IP internets
In a TCP/IP internet, each host needs several items of
information for internet communications. Some will be
host-specific, while other information will be common for
all hosts on a subnet. In a soon to be published RFC docu-
ment,* R. Droms identifies the following configuration data
required by internet hosts:
+ An IP address, a host specific value that can be
hard-coded or obtained via BOOTP, the Reverse
Address Resolution Protocol (RARP) or Dynamic RARP
+ Subnet properties, such as the subnet mask and the
Maximum Transmission Unit (MTU); obviously, these
values are not host-specific.
+ Addresses of "entry" gateways to the internet;
addresses of default gateways are usually hard-
coded; though the ICMP "redirect" message can be
used to refine a host's routing tables, there is
currently no dynamic TCP/IP mechanism or protocol
for a host to locate a gateway; an IETF working
group is busy on this problem.
+ For hosts in internets using the Domain Name Sys-
tem (DNS) for name-to-address translation, the
location of a local DNS server is needed; this
information is not host-specific, and usually
+ Host name (domain name, for hosts using DNS);
obviously host-specific; either hard-coded or
obtained in a boot procedure.
+ For diskless hosts, various boot services. BOOTP
is the standard Internet protocol for downloading
boot configuration information. The Trivial File
Transfer Protocol (TFTP) is typically used for
downloading boot images. Sun computers use the
"bootparams" RPC mechanism for downloading initial
configuration data to a host.
There are ongoing developments, most notably the work of the
Dynamic Host Configuration Working Group of the IETF, to
support dynamic, automatic gathering of the above data. In
the meantime, most systems will rely on hand-crafted confi-
* Draft "Dynamic Configuration of Internet Hosts."
5.3 Connecting to THE Internet
The original TCP/IP Internet (spelled with an upper-case
"I") is still active, and still growing. An interesting
aspect of the Internet is that it spans many independently
Connection to the Internet requires: a registered network
number, for use in IP addresses; a registered autonomous
system number (ASN), for use in internet routing; and, a
registered domain name. Fielding a primary and backup DNS
server is a condition for registering a domain name.
The Defense Data Network (DDN) Network Information Center
(NIC) is responsible for registering network numbers, auto-
nomous system numbers, and domain names. Regional nets will
have their own policies and requirements for Internet con-
nections, but all use the NIC for this registration service.
Contact the NIC for further information, at:
DDN Network Information Center
SRI International, Room EJ291
333 Ravenswood Avenue
Menlo Park, CA 94025
1-800-235-3155 (toll-free hotline)
5.4 YP and DNS: Dueling name servers.
The Domain Name System (DNS) provides name service: it
translates host names into IP addresses (this mapping is
also called "resolution"). Two widespread DNS implementa-
tions are "bind" and "named." The Sun Yellow Pages (YP)
system can be configured to provide an identical service, by
providing remote, keyed access to the "hosts.byname" map.
Unfortunately, if both DNS and the YP hosts.byname map are
installed, they can interact in disruptive ways.
The problem has been noted in systems in which DNS is used
as a fallback, to resolve hostnames that YP cannot. If DNS
is slow in responding, the timeout in program ypserv may
expire, which triggers a repeated request. This can result
in disaster if DNS was initially slow because of congestion:
the slower things get, the more requests are generated,
which slows things even more. A symptom of this problem is
that failures by the DNS server or network will trigger
numerous requests to DNS.
Reportedly, the bug in YP that results in the avalanche of
DNS requests has been repaired in SunOS 4.1. The problem,
however, is more fundamental than an implementation error.
The YP map hosts.byname and the DNS contain the same class
of information. One can get an answer to the same query
from each system. These answers may well be different:
there is not a mechanism to maintain consistency between the
systems. More critical, however, is the lack of a mechanism
or procedure to establish which system is authoritative.
Hence, running the DNS and YP name services in parallel is
pointless. If the systems stay consistent, then only one is
needed. If they differ, there is no way to choose which is
The YP hosts.byname service and DNS are comparable, but
incompatible. If possible, a site should not run both ser-
vices. Because of Internet policy, sites with Internet con-
nections MUST use the DNS. If YP is also used, then it
should be configured with YP-to-DNS disabled.
Hacking a system so that it uses DNS rather than the YP
hosts.byname map is not trivial, and should not be attempted
by novices. The approach is to rebuild the shared C link-
library, so that system calls to gethostbyname() and
gethostbyaddr() will use DNS rather than YP. To complete
the change, programs that do not dynamically link the shared
C library (rcp, arp, etc.) must also be rebuilt.
Modified shared C libraries for Sun 3s and Sun 4s are avail-
able via anonymous FTP from host uunet.uu.net, in the sun-
fixes directory. Note that use of DNS routines rather than
YP for general name resolution is not a supported SunOS
feature at this time.
6. Internet Security
The guidelines and advice in this section pertain to enhanc-
ing the protection of data that are merely "sensitive." By
themselves, these measures are insufficient for protecting
"classified" data. Implementing the policies required to
protect classified data is subject to stringent, formal
review procedures, and is regulated by agencies such as the
Defense Investigative Service (DIS) and the National Secu-
rity Agency (NSA).
A network manager must realize that he is responsible for
protecting his system and its users. Furthermore, though
the Internet may appear to be a grand example of a coopera-
tive joint enterprise, recent incidents have made it clear
that not all Internet denizens are benign.
A network manager should be aware that the network services
he runs have a large impact on the security risks to which
his system is exposed. The prudent network manager will be
very careful as to what services his site provides to the
rest of the Internet, and what access restrictions are
enforced. For example, the protocol "finger" may provide
more information about a user than should be given to the
world at large. Worse, most implementations of the protocol
TFTP give access to all world-readable files.
This section highlights several basic security considera-
tions for Internet sites. It then lists several sources of
information and advice on improving the security of systems
connected to the Internet.
6.1 Basic Internet Security
Two major Internet security threats are denial of service
and unauthorized access.
Denial of service threats often take the form of protocol
spoofers or other malicious traffic generators. These prob-
lems can be detected through system monitoring logs. If an
attack is suspected, immediately contact your regional net
office (e.g., SURANET, MILNET). In addition, DDN users
should contact SCC, while other Internet users should con-
tact CERT (see below). A cogent description of your
system's symptoms will be needed.
At your own site, be prepared to isolate the problems (e.g.,
by limiting disk space available to the message queue of a
mail system under attack). As a last resort, coping with an
attack may require taking down an Internet connection. It
is better, however, not to be too quick to quarantine your
site, since information for coping with the attack may come
via the Internet.
Unauthorized access is a potentially more ominous security
threat. The main avenues are attacks against passwords and
attacks against privileged system processes.
An appallingly common means of gaining entry to systems is
by use of the initial passwords to root, sysdiag, and other
management accounts that systems are shipped with. Only
slightly less vulnerable are common or trivial passwords,
since these are readily subverted by dictionary attacks.*
Obvious steps can reduce the risk of password attacks: pass-
words should be short-lived, at least eight characters long,
with a mix of upper and lower case, and preferably random.
The distasteful aspect of memorizing a random string can be
alleviated if the password is pronounceable.
Improving passwords does not remove all risks. Passwords
transmitted over an ethernet are visible to all attached
systems. Furthermore, gateways have the potential to inter-
cept passwords used by any FTP or TELNET connections that
traverse them. It is a bad idea for the root account to be
accessed by FTP or TELNET if the connections must cross
Attacks against system processes are another avenue of unau-
thorized access. The principle is that by subverting a sys-
tem process, the attacker can then gain its access
One approach to reducing this risk is to make system pro-
grams harder to subvert. For example, the widespread attack
in November 1988 by a self-replicating computer program
("worm," analogous to a tapeworm) subverted the "fingerd"
process, by loading an intrusive bootstrap program (known
variously as a "grappling hook" or "vector" program), and
then corrupting the stack space so that a subroutine's
return address was overwritten with the address of the
bootstrap program.** The security hole in fingerd consisted
of an input routine that did not have a length check. Secu-
rity fixes to "fingerd" include the use of a revised input
A more general protection is to apply the principle of
"least privilege." Where possible, system routines should
run under separate user IDs, and should have no more
privilege than is necessary for them to function.
* Exotic fantasy creatures and women's names are well
represented in most password dictionaries.
** An early account of the Internet Worm incident of
November 1988 is given by Eugene Spafford in the Janu-
ary 89 issue of "Computer Communications Review."
Several other articles on the worm incident are in the
June 89 issue of the "Communications of the ACM."
To further protect against attacks on system processes, sys-
tem managers should regularly check their system programs to
ensure that they have not been tampered with or modified in
any way. Checksums should be used for this purpose. Using
the operating system to check a file's last date of modifi-
cation is insufficient, since the date itself can be
Finally, to avoid the unauthorized replacement of system
code, care should be exercised in assigning protection to
its directory paths.
Some system programs actually have "trap doors" that facili-
tate subversion. A trap door is the epitome of an undocu-
mented feature: it is a hidden capability of a system pro-
gram that allows a knowledgeable person to gain access to a
system. The Internet Worm exploited what was essentially a
trap door in the BSD sendmail program.
Ensuring against trap doors in software as complex as send-
mail may be infeasible. In an ideal world, the BSD sendmail
program would be replaced by an entire mail subsystem (i.e.,
perhaps including mail user agents, mail transfer agents,
and text preparation and filing programs). Any site using
sendmail should at least obtain the less vulnerable,
toughened distribution from ucbarpa.berkeley.edu, in file
~ftp/4.3/sendmail.tar.Z. Sites running SunOS should note
that the 4.0.3 release closed the security holes exploited
by the Internet Worm. Fixes for a more obscure security
hole in SunOS are available from host uunet.uu.net in
~ftp/sun-fixes; these improvements have been incorporated in
Sendmail has problems other than size and complexity. Its
use of root privileges, its approach to alias expansion, and
several other design characteristics present potential ave-
nues of attack. For UNIX sites, an alternative mail server
to consider is MMDF, which is now at version 2. MMDF is
distributed as part of the SCO UNIX distribution, and is
also available in the user contributed portion of 4.3BSD.
Though free, MMDF is licensed, and resale is restricted.
Sites running MMDF should be on the mmdf email list;
requests to join this list should be sent to:
Programs that masquerade as legitimate system code but which
contain trap doors or other aides to unauthorized access are
known as trojan horses. Computer "viruses," intrusive
software that infects seemingly innocent programs and pro-
pagates when the infected programs are executed or copied,
are a special case of trojan horse programs.*
To guard against trojan horse attacks, be wary of programs
downloaded from remote sources. At minimum, do not download
executables from any but the most trusted sources. Also, as
noted above, to avoid proliferation of "infected" software,
checksums should be computed, recorded, and periodically
6.2 Security Information Clearing-Houses
The Internet community can get security assistance from the
Computer Emergency Response Team (CERT), established by
DARPA in November 1988. The Coordination Center for the
CERT (CERT/CC) is located at the Software Engineering Insti-
tute at Carnegie Mellon University. The CERT is intended to
respond to computer security threats such as the November
'88 worm attack that invaded many defense and research com-
puters. Consult RFC 1135 (Reynolds, J., "The Helminthiasis
of the Internet", USC/ISI, December 1989), for further
CERT assists Internet sites in response to security attacks
or other emergency situations. It can immediately tap
experts to diagnose and solve the problems, as well as
establish and maintain communications with the affected com-
puter users and with government authorities as appropriate.
Specific responses will be taken in accordance with the
nature of the problem and the magnitude of the threat.
CERT is also an information clearing-house for the identifi-
cation and repair of security vulnerabilities, informal
assessments of existing systems in the research community,
improvement to emergency response capability, and both ven-
dor and user security awareness. This security information
is distributed by periodic bulletins, and is posted to the
USENET news group comp.security.announce. In addition, the
security advisories issued by CERT, as well as other useful
security-related information, are available via anonymous
FTP from cert.sei.cmu.edu.
For immediate response to attacks or incidents, CERT mans a
* Virus attacks have been seen against PCs, but as yet
have rarely been directed agains UNIX systems.
24-hour hotline at (412) 268-7090. To subscribe to CERT's
security announcement bulletin, or for further information,
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890
For DDN users, the Security Coordination Center (SCC) serves
a function similar to CERT. The SCC is the DDN's clearing-
house for host/user security problems and fixes, and works
with the DDN Network Security Officer. The SCC also distri-
butes the DDN Security Bulletin, which communicates informa-
tion on network and host security exposures, fixes, and con-
cerns to security and management personnel at DDN facili-
ties. It is available online, via kermit or anonymous FTP,
from nic.ddn.mil, in SCC:DDN-SECURITY-yy-nn.TXT (where "yy"
is the year and "nn" is the bulletin number). The SCC pro-
vides immediate assistance with DDN-related host security
problems; call (800) 235-3155 (6:00 a.m. to 5:00 p.m.
Pacific Time) or send e-Mail to SCC@NIC.DDN.MIL. For 24
hour coverage, call the MILNET Trouble Desk (800) 451-7413
or AUTOVON 231-1713.
The CERT/CC and the SCC communicate on a regular basis and
support each other when problems occur. These two organiza-
tions are examples of the incident response centers that are
forming; each serving their own constituency or focusing on
a particular area of technology.
Other network groups that discuss security issues are:
comp.protocols.tcp-ip, comp.virus (mostly PC-related, but
occasionally covers Internet topics), misc.security, and the
BITNET Listserv list called VIRUS-L.
7. Internet Information
There are many available references on the TCP/IP protocol
suite, the internet architecture, and the DDN Internet. A
soon to be published FYI RFC document, "Where to Start: A
Bibliography of General Internetworking Information." pro-
vides a bibliography of online and hard copy documents,
reference materials, and multimedia training tools that
address general networking information and "how to use the
Internet." It presents a representative collection of
materials that will help the reader become familiar with the
concepts of internetworking. Inquires on the current status
of this document can be sent to firstname.lastname@example.org or by
postal mail to:
Corporation for National Research Initiatives
1895 Preston White, Suite 100
Reston, VA 22091
Attn: IAB Secretariat.
Two texts on networking are especially noteworthy. Inter-
networking With TCP/IP, by Douglas Comer, is an informative
description of the TCP/IP protocol suite and its underlying
architecture. The UNIX System Administration Handbook, by
Nemeth, Snyder, and Seebass, is a "must have" for system
administrators who are responsible for UNIX hosts. In addi-
tion to covering UNIX, it provides a wealth of tutorial
material on networking, the Internet, and network manage-
A great deal of information on the Internet is available
online. An automated, online reference service is available
from CSNET. To obtain a bibliography of their online offer-
ings, send the email message
The DDN NIC also offers automated access to many NIC docu-
ments, online files, and WHOIS information via electronic
mail. To use the service, send an email message with your
request specified in the SUBJECT field of the message. For
a sampling of the type of offerings available through this
service, send the following message
The DDN Protocol Implementations and Vendors Guide, pub-
lished by the DDN Network Information Center (DDN NIC),* is
* Products mentioned in the guide are not specifically
an online reference to products and implementations associ-
ated with the DoD Defense Data Network (DDN) group of com-
munication protocols, with emphasis on TCP/IP and OSI proto-
cols. It contains information on protocol policy and
evaluation procedures, a discussion of software and hardware
implementations, and analysis tools with a focus on protocol
and network analyzers. To obtain the guide, invoke FTP at
your local host and connect to host NIC.DDN.MIL (internet
address 126.96.36.199 or 10.0.0.51). Log in using username
'anonymous' with password 'guest' and get the file
The DDN Protocol Guide is also available in hardcopy form.
To obtain a hardcopy version of the guide, contact the DDN
Network Information Center:
By U.S. mail:
DDN Network Information Center
333 Ravenswood Avenue, Room EJ291
Menlo Park, CA 94025
1-800-235-3155 (toll-free hotline)
For further information about the guide, or for information
on how to list a product in a subsequent edition of the
guide, contact the DDN NIC.
There are many additional online sources on Internet Manage-
ment. RFC 1118, "A Hitchhiker's Guide to the Internet," by
Ed Krol, is a useful introduction to the Internet routing
algorithms. For more of the nitty-gritty on laying out and
configuring a campus net, see Charles Hedrick's "Introduc-
tion to Administration of an Internet-based Local Network,"
available via anonymous FTP from cs.rutgers.edu (sometimes
listed in host tables as aramis.rutgers.edu), in subdirec-
tory runet, file tcp-ip-admin. Finally, anyone responsible
for systems connected to the Internet must be thoroughly
versed in the Host Requirements RFCs (RFC 1122 and RFC 1123)
endorsed or recommended by the Defense Communications