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The MultiNet Intrusion Prevention System

Jeremy Begg, Managing Director, VSM Software Services Pty Ltd.

MultiNet® is a TCP/IP network stack for OpenVMS which runs on all OpenVMS hardware platforms.  It was initially created by TGV Inc as a VAX/VMS port of the BSD Unix TCP/IP stack, and has for the past 12 years been owned and enhanced by Process Software LLC[1].

The most recent release (V5.3, February 2009) is based on BSD 4.4 and includes an interesting new security feature, the MultiNet Intrusion Prevention System, or “IPS”.  IPS enables the system manager to configure MultiNet so that it detects intrusion attempts and then blocks further access from the remote system that is the source of the attack.  Several of the commonly-targeted MultiNet services such as SSH, FTP and TELNET come equipped to use IPS and mechanisms are provided to allow any network application to do likewise. 

This article describes the author’s early experiences with IPS in a production environment.

[1] MultiNet is a registered trademark and Process Software and the Process Software logo are trademarks of Process Software.

Why Use IPS?

VSM Software Services Pty Ltd runs an AlphaServer DS20E to provide services to customers on the Internet including DNS, webserving and email.  We also allow incoming FTP and SSH access for server management and customer website maintenance.  In addition to the DS20E we also have a number of VAX, Alpha and Integrity systems for product support & development.

There is no doubt that the Internet has become a hostile place, even for servers running OpenVMS.  In operating our services we regularly see VMS breakin events logged by MultiNet’s SSH and FTP servers and by PMDF’s POP3, IMAP and SMTP servers. We also see suspicious DNS update attempts but (fortunately for us!) not much in the way of Denial-of-Service attacks (i.e. attempts to flood the server with so much traffic that it can’t maintain acceptable responsiveness).

To date we have used a variety of methods to control this activity, all of them relying on manual observation of an attack and manual updating of a configuration file.  For example,

  • PORT_ACCESS mapping rules in PMDF[2] to block access from persistently annoying systems

  • Configuring DNS to refuse recursive lookups from outside the local network
  • Manually updating MULTINET:FILTER-SE0.DAT to block all IP traffic from specified hosts.

MultiNet IPS uses information provided by network services to detect suspicious activity and then dynamically updates the MultiNet packet filter to disrupt that activity.  (In this context the term “network service” means a server process on the local system which provides a TCP/IP application service such as SSH, POP3, HTTP, etc.)

We already had a lot of experience with MultiNet’s packet filter mechanism so the ability to have it automatically updated by IPS was very appealing.

[2] PMDF is a standards-based, robust, electronic mail platform and gateway for OpenVMS, Solaris, Tru64 Unix, Windows and Linux.  PMDF has been supported and developed by Process Software since October 2000.

Configuring IPS

IPS is documented in Chapter 32 of the MultiNet V5.3 Installation & Administration Guide.  It is installed as part of the standard MultiNet installation but until its configuration files are set up it doesn’t do anything. These files are described in detail in the MultiNet documentation but briefly the procedure is as follows:


  2. Edit MULTINET:FILTER_SERVER_CONFIG.TXT and set desired configuration options.  In particular, use INCLUDE statements to specify the service-specific configuration files which will be loaded when IPS starts.

  3. Set up each of the service-specific configuration files specified in step 2.  For example, copy MULTINET:SSH_FILTER_CONFIG.TEMPLATE to MULTINET:SSH_FILTER_CONFIG.TXT.

  4.  Edit each of the service-specific configuration files to specify the criteria for each service, such as the template packet filter rule and the trigger for activating the packet filter.

  5. When all configuration files have been prepared, issue the command
    $ MULTINET SET/IPS/RELOAD to activate them.

Services Protected by IPS

The MultiNet installation kit comes with templates for protecting the  most common MultiNet services including FTP, IMAP, POP3, REXEC, RLOGIN, RSHELL, SMTP, SNMP, SSH and TELNET.

OpenVMS systems which are running MultiNet V5.3 and PMDF V6.4 can use IPS to protect the PMDF IMAP, POP3 and SMTP servers.  The configuration files for these are shipped in the PMDF_TABLE: directory as part of the standard PMDF V6.4 installation.

Service-specific Configuration File

There is a configuration file for each service.  Reasonable defaults are provided in the .TEMPLATE files but some changes are required:

destination_address     This must match the IP address (in CIDR format) of the interface to be monitored for intrusion activity.  For example, 


This specifies that the interface with IP address is to be monitored.  The subnet portion of the address (/27 in this case) is required but ignored.

exclude_address           This specifies one or more remote IP addresses which are to be ignored, i.e. the packet filter will never block those addresses.  The template files come with this configured to block a commonly-used local address ( but sites may wish to remove or change it.

Each template file also specifies a prototype packet filter entry which looks a little odd at first glance:

 proto_filter "deny tcp log"

The two IP address ranges in CIDR format will be automatically replaced by the actual source and destination addresses when the rule is activated by MultiNet IPS.  This rule only needs to be changed if you wish to change the action, e.g. from “deny” to “drop”.

Monitoring IPS

The emulator makes extensive use of OOP, particularly of the features offered by the C++ language.  While C and C++ are reviled by some for their perceived cryptic nature (although there is no rule that says C or C++ code has to be cryptic), they are commonly considered to be the languages of choice for low-level, portable programming found in operating systems, device drivers, and emulators.  C and C++ give programmers a level of control over the bits and bytes of their code few other high-level languages offer, and C and C++ compilers that produce blazingly fast code are available for virtually any platform.

The Intrusion Prevention System generates a wealth of evidence for its effectiveness including assorted MultiNet log files, OPCOM, SNMP and the OpenVMS Security Audit logs.

There are several log files created by MultiNet IPS:

  • MULTINET:FILTER_SERVER.OUT is the primary log file for the filter server process.
  • MULTINET:FILTER_SERVER_HOURLY_LOG.yyyymmdd is a “day file” containing a  summary of filter actions each hour during the day. A new file is created every day at 1am.

Here is an extract from the FILTER_SERVER.OUT file on one of our systems:


20-MAR-2009 12:14:07.28 - Using configuration file MULTINET_ROOT:[MULTINET]FILTER_SERVER_CONFIG.TXT;
20-MAR-2009 12:14:07.30 - Processing include file "multinet:ssh_filter_config.txt"
20-MAR-2009 12:14:07.30 - Using configuration file MULTINET_ROOT:[MULTINET]SSH_FILTER_CONFIG.TXT;
20-MAR-2009 20:37:03.46 - Event message received
20-MAR-2009 20:37:03.46 -     Component: SSH
20-MAR-2009 20:37:03.46 -     Rule     : SSH_BOGUS_ID
20-MAR-2009 20:37:03.46 -     Time     : 20-MAR-2009 20:37:03.46
20-MAR-2009 20:37:03.47 -     Src Port : 54232
20-MAR-2009 20:37:03.47 -     Src Addr :
20-MAR-2009 20:37:03.47 -     Dst Addr :
20-MAR-2009 20:37:03.47 -     Process  : SSHD Master
20-MAR-2009 20:37:03.47 -     PID      : 208000AD
20-MAR-2009 21:12:01.31 - Event message received
20-MAR-2009 21:12:01.31 -     Component: SSH
20-MAR-2009 21:12:01.31 -     Rule     : SSH_INVALIDUSER
20-MAR-2009 21:12:01.31 -     Time     : 20-MAR-2009 21:12:01.31
20-MAR-2009 21:12:01.32 -     Src Port : 55839
20-MAR-2009 21:12:01.32 -     Src Addr :
20-MAR-2009 21:12:01.32 -     Dst Addr :
20-MAR-2009 21:12:01.32 -     Process  : SSHD 0000
20-MAR-2009 21:12:01.32 -     PID      : 20800B83
20-MAR-2009 21:12:06.49 - Event message received
20-MAR-2009 21:12:06.49 -     Component: SSH
20-MAR-2009 21:12:06.49 -     Rule     : SSH_INVALIDUSER
20-MAR-2009 21:12:06.49 -     Time     : 20-MAR-2009 21:12:06.49
20-MAR-2009 21:12:06.50 -     Src Port : 55991
20-MAR-2009 21:12:06.50 -     Src Addr :
20-MAR-2009 21:12:06.50 -     Dst Addr :
20-MAR-2009 21:12:06.50 -     Process  : SSHD 0001
20-MAR-2009 21:12:06.50 -     PID      : 20800C04

... Event messages removed for brevity ...

20-MAR-2009 21:12:47.13 - Event message received
20-MAR-2009 21:12:47.13 -     Component: SSH
20-MAR-2009 21:12:47.13 -     Rule     : SSH_INVALIDUSER
20-MAR-2009 21:12:47.13 -     Time     : 20-MAR-2009 21:12:47.12
20-MAR-2009 21:12:47.13 -     Src Port : 50039
20-MAR-2009 21:12:47.13 -     Src Addr :
20-MAR-2009 21:12:47.14 -     Dst Addr :
20-MAR-2009 21:12:47.14 -     Process  : SSHD 0009
20-MAR-2009 21:12:47.14 -     PID      : 20800C20
20-MAR-2009 21:12:47.14 - Creating a filter for component ssh rule
20-MAR-2009 21:12:47.14 -                   src address   =
20-MAR-2009 21:12:47.14 -                   dst address   =
20-MAR-2009 21:12:47.14 -                   interface     =  se0
20-MAR-2009 21:12:47.14 -                   filter expires   20-MAR-2009
21-MAR-2009 00:00:00.50 - Performing daily maintenance

The extract above shows that the filter server started at 20-MAR-2009 12:14:07.28 and loaded a single service-specific configuration file (for SSH).  At 20:37:03.46 the SSH server reported suspicious activity but this was not followed by any other such activity within the specified timeout (5 minutes by default) and so was ignored.  Then at 21:12:01 the SSH server reported more suspicious activity and this time the remote system ( persisted in its breakin attempts.  After ten such reports in the space of under a minute the IPS created a packet filter to block the remote system. 

The FILTER_SERVER_HOURLY_LOG files contain hourly snapshots of IPS activity.  For example the time period corresponding to the extract above looks like this:

Filter server hourly snapshot for hour 21 of 03/20/2009

Component  ssh
    Rule ssh_bogus_id

        number of hits:      0
        destination address:

            number of still-queued events:   0
            number of all events:            0
            number of filters created:       0
            Address entry to be deleted:     21-MAR-2009 00:42:03.46

     Rule ssh_authfailed
        number of hits:      0
        destination address:

     Rule ssh_userauth
        number of hits:      0
        destination address:

     Rule ssh_invaliduser
        number of hits:      10
        destination address:

            number of still-queued events:   0
            number of all events:            10
            number of filters created:       1
            Address entry to be deleted:     21-MAR-2009 01:12:47.19

In addition to the log files, the regular MultiNet interface commands can be used to see what packet filters are in place at any given moment:

$ mu show/int se0/filter
            VMS Device = EWA0
            IP Address =
            No common links defined

MultiNet Packet Filter List for se0:

Logging is disabled
                     Source Address / Port
Action  Proto   Hits  Destination Address / Port
------  -----  -----  ---------------------------------------------------drop      tcp     29


Note: The above output was generated some days after the events shown in the log file extracts.

Closing Remarks

We’re still in the early days of using IPS here at VSM but it’s already proven to be effective at limiting intrusion activity.  In general the default settings are very good but some adjustments might give better results, and here are a few suggestions.

  • In setting up the SSH filters for IPS we chose to change the default proto_filter rule from “deny” to “drop”.  In our experience with SSH-based attacks the remote systems keep sending packets to the server even though they’re getting “administratively denied” responses.  Changing the rule to “drop” causes nothing whatsoever to be sent back to the remote systems, and they seem to stop trying much sooner.

  • The default trigger for many events is 10 occurrences inside 5 minutes.  At our site where most SSH users are what you might call “sophisticated” we could probably tighten this to (say) three login failures inside 5 minutes.  On the other hand if we had a large number of users the probability that any given user would enter the wrong password would be somewhat higher, and “3 in 5” might be too restrictive.

  • Once we get PMDF upgraded on our primary server we will look at implementing IPS for PMDF’s IMAP, POP3 and SMTP servers.  All of them are popular targets for password-hunting netbots.

If you run MultiNet at your site and you suffer from intrusion attempts, have a look at IPS - it might be just what you need!

For more information

The author will be happy to field enquiries of a technical nature about MultiNet IPS or indeed any aspect of MultiNet or PMDF; please send your enquiry to jeremy@vsm.com.au   (For technical support about a specific problem please use your usual support channel.)

For general enquiries about MultiNet, PMDF or any other Process Software product please visit the Process Software web site, http://www.process.com.

The MultiNet documentation can be viewed on-line at: