Mini SNMP daemon

February 23, 2020 - 5 mins read

This is an SNMP server for small and embedded systems, currently Linux and FreeBSD are supported. It is easily portable to other UNIX systems since it’s written in C. The daemon is very small (~40 kiB) and does not have nowhere near the feature set of Net-SNMP, therefore it has a very low impact on system resources.

See my mini HowTo: Playing with SNMP for a quick introduction to setting up the SNMP tools and MIBs to avoid having to use numerical OIDs.

Features:

Supports SNMP version 1 and 2c
Supports SNMP get, getnext and getbulk
Supports both IPv4 and IPv6
Supports communication over UDP and TCP sockets
Supports the most important performance data (uptime, CPU load, memory usage)
Supports the most important network data (bytes/packets in/out, error counts)
Supports the most important disk data (disk space/inodes available/used/free)
Tested with net-snmp, cacti, and MRTG

Example

First start the daemon:

$ sudo ./mini-snmpd -i eth0,eth1,wlan0

Then do an SNMP walk:

$ snmpwalk -v2c -c public 127.0.0.1 .
SNMPv2-MIB::sysDescr.0 = STRING: 
SNMPv2-MIB::sysObjectID.0 = OID: SNMPv2-SMI::enterprises
SNMPv2-MIB::sysUpTime.0 = Timeticks: (5954) 0:00:59.54
SNMPv2-MIB::sysContact.0 = STRING: 
SNMPv2-MIB::sysName.0 = STRING: luthien
SNMPv2-MIB::sysLocation.0 = STRING: 
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: eth0
IF-MIB::ifDescr.2 = STRING: eth1
IF-MIB::ifDescr.3 = STRING: wlan0
IF-MIB::ifOperStatus.1 = INTEGER: up(1)
IF-MIB::ifOperStatus.2 = INTEGER: lowerLayerDown(7)
IF-MIB::ifOperStatus.3 = INTEGER: up(1)
IF-MIB::ifInOctets.1 = Counter32: 1791842
IF-MIB::ifInOctets.2 = Counter32: 0
IF-MIB::ifInOctets.3 = Counter32: 1909293573
IF-MIB::ifInUcastPkts.1 = Counter32: 18071
IF-MIB::ifInUcastPkts.2 = Counter32: 0
IF-MIB::ifInUcastPkts.3 = Counter32: 2010670
IF-MIB::ifInDiscards.1 = Counter32: 12
IF-MIB::ifInDiscards.2 = Counter32: 0
IF-MIB::ifInDiscards.3 = Counter32: 0
IF-MIB::ifInErrors.1 = Counter32: 0
IF-MIB::ifInErrors.2 = Counter32: 0
IF-MIB::ifInErrors.3 = Counter32: 0
IF-MIB::ifOutOctets.1 = Counter32: 1199703
IF-MIB::ifOutOctets.2 = Counter32: 0
IF-MIB::ifOutOctets.3 = Counter32: 179994204
IF-MIB::ifOutUcastPkts.1 = Counter32: 6437
IF-MIB::ifOutUcastPkts.2 = Counter32: 0
IF-MIB::ifOutUcastPkts.3 = Counter32: 1269885
IF-MIB::ifOutDiscards.1 = Counter32: 0
IF-MIB::ifOutDiscards.2 = Counter32: 0
IF-MIB::ifOutDiscards.3 = Counter32: 0
IF-MIB::ifOutErrors.1 = Counter32: 0
IF-MIB::ifOutErrors.2 = Counter32: 0
IF-MIB::ifOutErrors.3 = Counter32: 0
HOST-RESOURCES-MIB::hrSystemUptime.0 = Timeticks: (4290528121) 496 days, 14:08:01.21
UCD-SNMP-MIB::memTotalReal.0 = INTEGER: 8055412 kB
UCD-SNMP-MIB::memAvailReal.0 = INTEGER: 961136 kB
UCD-SNMP-MIB::memShared.0 = INTEGER: 0 kB
UCD-SNMP-MIB::memBuffer.0 = INTEGER: 474372 kB
UCD-SNMP-MIB::memCached.0 = INTEGER: 2473876 kB
UCD-SNMP-MIB::dskIndex.1 = INTEGER: 1
UCD-SNMP-MIB::dskPath.1 = STRING: /
UCD-SNMP-MIB::dskTotal.1 = INTEGER: 222230176
UCD-SNMP-MIB::dskAvail.1 = INTEGER: 20289308
UCD-SNMP-MIB::dskUsed.1 = INTEGER: 201940864
UCD-SNMP-MIB::dskPercent.1 = INTEGER: 91
UCD-SNMP-MIB::dskPercentNode.1 = INTEGER: 12
UCD-SNMP-MIB::laIndex.1 = INTEGER: 1
UCD-SNMP-MIB::laIndex.2 = INTEGER: 2
UCD-SNMP-MIB::laIndex.3 = INTEGER: 3
UCD-SNMP-MIB::laNames.1 = STRING: Load-1
UCD-SNMP-MIB::laNames.2 = STRING: Load-5
UCD-SNMP-MIB::laNames.3 = STRING: Load-15
UCD-SNMP-MIB::laLoad.1 = STRING: 0.06
UCD-SNMP-MIB::laLoad.2 = STRING: 0.14
UCD-SNMP-MIB::laLoad.3 = STRING: 0.28
UCD-SNMP-MIB::laConfig.1 = STRING: 1
UCD-SNMP-MIB::laConfig.2 = STRING: 5
UCD-SNMP-MIB::laConfig.3 = STRING: 15
UCD-SNMP-MIB::laLoadInt.1 = INTEGER: 6
UCD-SNMP-MIB::laLoadInt.2 = INTEGER: 14
UCD-SNMP-MIB::laLoadInt.3 = INTEGER: 28
UCD-SNMP-MIB::ssCpuRawUser.0 = Counter32: 2263123
UCD-SNMP-MIB::ssCpuRawNice.0 = Counter32: 8820
UCD-SNMP-MIB::ssCpuRawSystem.0 = Counter32: 353194
UCD-SNMP-MIB::ssCpuRawIdle.0 = Counter32: 46493489
UCD-SNMP-MIB::ssRawInterrupts.0 = Counter32: 50186444
UCD-SNMP-MIB::ssRawContexts.0 = Counter32: 243678129
UCD-SNMP-MIB::ssRawContexts.0 = No more variables left in this MIB View (It is past the end of the MIB tree)

Building Really Small Binaries

By simply calling ./configure && make you don’t really get a small mini-snmpd binary. Sure, most people know about setting CFLAGS=-Os before calling the configure script – that’s how you reach the ~40 kiB mentioned above.

CFLAGS="-Os" ./configure && make clean all && strip mini-snmpd && ll mini-snmpd && size mini-snmpd
-rwxrwxr-x 1 jocke jocke 39520 nov  8 21:35 mini-snmpd*
   text	   data	    bss	    dec	    hex	 filename
  32766	   1028	  16032	  49826	   c2a2	 mini-snmpd

To get really crazy with things you can try this, it works for me but YMMV as usual:

CFLAGS="-W -Wall -Os -U_FORTIFY_SOURCE -fno-stack-protector -fomit-frame-pointer -ffunction-sections -fdata-sections -Wl,--gc-sections -fno-asynchronous-unwind-tables -fmerge-all-constants -fno-ident -Wl,-z,norelro -Wl,--hash-style=gnu -Wl,--build-id=none " ./configure --disable-ipv6
make clean all && strip -S --strip-unneeded --remove-section=.note.gnu.gold-version --remove-section=.comment --remove-section=.note --remove-section=.note.gnu.build-id --remove-section=.note.ABI-tag mini-snmpd && ll mini-snmpd && size mini-snmpd
-rwxrwxr-x 1 jocke jocke 30696 nov  8 21:37 mini-snmpd*
   text	   data	    bss	    dec	    hex	 filename
  27305	    964	  15968	  44237	   accd	 mini-snmpd

This insane amount of arguments to GCC saves you ~9kiB. Which begs the question, is there anything else you can do, how low can we go?! Let me introduce you to upx:

upx --ultra-brute mini-snmpd && ll mini-snmpd && size mini-snmpd
                       Ultimate Packer for eXecutables
                          Copyright (C) 1996 - 2013
UPX 3.91        Markus Oberhumer, Laszlo Molnar & John Reiser   Sep 30th 2013

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
     30696 ->     15604   50.83%  linux/ElfAMD   mini-snmpd

Packed 1 file.
-rwxrwxr-x 1 jocke jocke 15604 nov  8 21:37 mini-snmpd*
   text	   data	    bss	    dec	    hex	 filename
      0	      0	      0	      0	      0	 mini-snmpd

Yeah, running size on the binary afterwards is kinda useless, but WOW! Using upx actually cut the size down to almost half of what we got with the insane arguments above – saved ~15kiB on a 30kiB binary!

On embedded targets some of these tricks may just about save you if you’re running out of flash. However, there are often nasty compiler bugs to be found just by changing optimization level to -Os. So when it comes to embedded I always recommend playing it safe and going with -O2 and no further optimizations, unless you want to spend a lot of time looking for weird bugs!

Project Info

This is the continuation of the hard work on mini-snmpd by Robert Ernst. Unfortunately his mini-snmpd homepage has gone offline, so that and the lack of updates over the last couple of years is what prompted my setting up a GitHub project to act as a focal point for future development. I’ve also taken the liberty of setting up an FTP mirror of any previous releases I could find. If you happen to come by any older version(s), send me an email! :)

mini-snmpd is licensed under the GNU GPL v2.