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snmp-lab

Experiments extending the Net-SNMP agent

Introduction

This project is to share my experience in extending the Net-SNMP agent by creating a custom MIB and then populating the generated stubs from mib2c(1).

I am writing this in November, 2024 and the current source version of Net-SNMP is 5.9.4, my development box is a ThinkPad on Ubuntu 22 LTS (Jammy Jellyfish).

Building Net-SNMP from scratch

  1. Obtain and unpack the source tar ball.

  2. ./configure (there will be interactive questions)

    1. I use SNMPv2c for development and use SNMPv3 for production.
    2. My configuration summary:
    ---------------------------------------------------------
            Net-SNMP configuration summary:
---------------------------------------------------------

  SNMP Versions Supported:    1 2c 3
  Building for:               linux
  Net-SNMP Version:           5.9.4.pre2
  Network transport support:  Callback Unix Alias TCP UDP TCPIPv6 UDPIPv6 IPv4Base SocketBase TCPBase UDPIPv4Base UDPBase IPBase IPv6Base
  SNMPv3 Security Modules:     usm
  Agent MIB code:            default_modules =>  snmpv3mibs mibII ucd_snmp notification notification-log-mib target agent_mibs agentx disman/event disman/schedule utilities host
  MYSQL Trap Logging:         unavailable
  Embedded Perl support:      enabled
  SNMP Perl modules:          building -- embeddable
  SNMP Python modules:        disabled
  Crypto support from:        use_pkg_config_for_openssl
  Authentication support:     MD5
  Encryption support:         
  Local DNSSEC validation:    disabled

---------------------------------------------------------

  3. make;sudo make install

    1. binaries installed in /usr/local/bin and /usr/local/sbin
    2. headers installed in /usr/local/include/net-snmp
    3. libraries installed in /usr/local/lib
    4. mib installed in /usr/local/share/snmp/mibs

  4. configuration files for snmp utilities and snmpd

    1. configuration files reside in /usr/local/share/snmp
    2. examples: snmp.conf (utilities) snmpd.conf (snmpd)
    3. copy these or use snmpconf(1) to generate your own

  5. almost there!

    1. snmpd(8) does not install as a systemd service
    2. logs to /var/log/snmpd.log

Test run vanilla agent

  1. (optional) I like to run tcpdump(8) to see the agent traps.

    1. Ensure snmpd.conf has the correct IP address for your "manager" (i.e. development machine)
    2. tcpdump -v port 162

  2. Run snmpd

    1. Must run as root for port 161
    2. I prefer to run snmpd(8) in the foreground while developing
    3. Start the agent like ./snmpd -f -V
    4. Should have observed the start trap (via tcpdump)

  3. Tail the log

    1. tail -f /var/log/snmpd.log
    2. Should a version message waiting for you

  4. Exercise the agent

    1. snmpwalk -v 2c -c public localhost system

    2. You should see something like this:

      gsc@waifu:138>snmpwalk -v 2c -c public localhost system
MODULE-IDENTITY MACRO (lines 55..79 parsed and ignored).
OBJECT-IDENTITY MACRO (lines 81..103 parsed and ignored).
OBJECT-TYPE MACRO (lines 212..298 parsed and ignored).
NOTIFICATION-TYPE MACRO (lines 302..334 parsed and ignored).
TEXTUAL-CONVENTION MACRO (lines 8..48 parsed and ignored).
SNMPv2-MIB::sysDescr.0 = STRING: Linux waifu 6.8.0-49-generic #49~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Nov  6 17:42:15 UTC 2 x86_64
SNMPv2-MIB::sysObjectID.0 = OID: NET-SNMP-MIB::netSnmpAgentOIDs.10
DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (14624) 0:02:26.24
SNMPv2-MIB::sysContact.0 = STRING: mongohax
SNMPv2-MIB::sysName.0 = STRING: waifu
SNMPv2-MIB::sysLocation.0 = STRING: margaritaville
SNMPv2-MIB::sysServices.0 = INTEGER: 12
SNMPv2-MIB::sysORLastChange.0 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORID.1 = OID: SNMP-FRAMEWORK-MIB::snmpFrameworkMIBCompliance
SNMPv2-MIB::sysORID.2 = OID: SNMP-MPD-MIB::snmpMPDCompliance
SNMPv2-MIB::sysORID.3 = OID: SNMP-USER-BASED-SM-MIB::usmMIBCompliance
SNMPv2-MIB::sysORID.4 = OID: SNMPv2-MIB::snmpMIB
SNMPv2-MIB::sysORID.5 = OID: SNMP-VIEW-BASED-ACM-MIB::vacmBasicGroup
SNMPv2-MIB::sysORID.6 = OID: TCP-MIB::tcpMIB
SNMPv2-MIB::sysORID.7 = OID: UDP-MIB::udpMIB
SNMPv2-MIB::sysORID.8 = OID: IP-MIB::ip
SNMPv2-MIB::sysORID.9 = OID: SNMP-NOTIFICATION-MIB::snmpNotifyFullCompliance
SNMPv2-MIB::sysORID.10 = OID: NOTIFICATION-LOG-MIB::notificationLogMIB
SNMPv2-MIB::sysORDescr.1 = STRING: The SNMP Management Architecture MIB.
SNMPv2-MIB::sysORDescr.2 = STRING: The MIB for Message Processing and Dispatching.
SNMPv2-MIB::sysORDescr.3 = STRING: The management information definitions for the SNMP User-based Security Model.
SNMPv2-MIB::sysORDescr.4 = STRING: The MIB module for SNMPv2 entities
SNMPv2-MIB::sysORDescr.5 = STRING: View-based Access Control Model for SNMP.
SNMPv2-MIB::sysORDescr.6 = STRING: The MIB module for managing TCP implementations
SNMPv2-MIB::sysORDescr.7 = STRING: The MIB module for managing UDP implementations
SNMPv2-MIB::sysORDescr.8 = STRING: The MIB module for managing IP and ICMP implementations
SNMPv2-MIB::sysORDescr.9 = STRING: The MIB modules for managing SNMP Notification, plus filtering.
SNMPv2-MIB::sysORDescr.10 = STRING: The MIB module for logging SNMP Notifications.
SNMPv2-MIB::sysORUpTime.1 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.2 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.3 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.4 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.5 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.6 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.7 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.8 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.9 = Timeticks: (0) 0:00:00.00
SNMPv2-MIB::sysORUpTime.10 = Timeticks: (0) 0:00:00.00

    3. If all this works out, congratulations you have built a working snmpd (agent).

Extend the agent to support a custom MIB

  1. In this section, I use a simple custom MIB to generate C stubs via mib2c(1) and then add these to snmpd, followed by verification via snmpwalk(1).

Demo MIB

  1. Start off simple with two scalars in GUY-COLE-SCALAR-MIB.
  2. Copy GUY-COLE-SCALAR-MIB.txt to /usr/local/share/snmp/mibs
  3. Seed your environment: export MIBS="+GUY-COLE-SIMPLE-MIB"
  4. Validate: snmptranslate -IR guyColeSimple (should run without error).
  5. Run mib2c(1): mib2c guyCole
    1. The OID 1.3.6.1.4.1.5088 is based on my IANA enterprise assignment.
    2. Pick "Net-SNMP style code"
    3. Pick "writing code for generic some scalars"
    4. Generates two files: "guyCole.h" and guyCole.c"
  6. Configure and build snmpd(8) for new MIB
    1. Copy the generated files to the Net-SNMP directory, i.e. "net-snmp-5.9.4/agent/mibgroup"
    2. Run configure again: ./configure --with-mib-modules="guyCole"
    3. My configuration summary:
    ---------------------------------------------------------
            Net-SNMP configuration summary:
---------------------------------------------------------

  SNMP Versions Supported:    1 2c 3
  Building for:               linux
  Net-SNMP Version:           5.9.4.pre2
  Network transport support:  Callback Unix Alias TCP UDP TCPIPv6 UDPIPv6 IPv4Base SocketBase TCPBase UDPIPv4Base UDPBase IPBase IPv6Base
  SNMPv3 Security Modules:     usm
  Agent MIB code:            default_modules guyCole =>  snmpv3mibs mibII ucd_snmp notification notification-log-mib target agent_mibs agentx disman/event disman/schedule utilities host
  MYSQL Trap Logging:         unavailable
  Embedded Perl support:      enabled
  SNMP Perl modules:          building -- embeddable
  SNMP Python modules:        disabled
  Crypto support from:        use_pkg_config_for_openssl
  Authentication support:     MD5
  Encryption support:         
  Local DNSSEC validation:    disabled

---------------------------------------------------------
    1. make;sudo make install
  7. Exercise the agent
    1. Restart the agent with logging enabled
      1. ./snmpd -DguyCole -f -V
    2. Invoke snmpwalk(1) 
      gsc@waifu:355>snmpwalk -v 2c -c public localhost 1.3.6.1.4.1.5088.1.1
SNMPv2-SMI::enterprises.5088.1.1.0 = INTEGER: 0
    3. Set fresh integer value using snmpset(1) 
      snmpset -v 2c -c private localhost 1.3.6.1.4.1.5088.1.1.0 i 5
SNMPv2-SMI::enterprises.5088.1.1.0 = INTEGER: 5
    4. Verify update 
      snmpwalk -v 2c -c public localhost 1.3.6.1.4.1.5088.1.1
SNMPv2-SMI::enterprises.5088.1.1.0 = INTEGER: 5

476 snmpset -v 2c -c private localhost 1.3.6.1.4.1.5088.1.2.0 s "aaaaa" 477 snmpwalk -v 2c -c public localhost 1.3.6.1.4.1.5088.1

Net-SNMP Coding Documentation http://www.net-snmp.org/wiki/index.php/Agent_Architecture https://openhpi.sourceforge.net/subagent-manual/index.html

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Experiments extending the Net-SNMP agent

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c snmp snmp-agent snmpd net-snmp net-snmp-extension

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