snmpwalk/snmpget can't read global variable

thiagomedeiros · Sat Mar 09, 2024 5:02 pm

RouterOS ver 7.14

Global environment declared before:
test=123

Script code:

/system script add dont-require-permissions=no name=test policy=read,write,policy,test source=":global test; :put \$test;"

Listing name from SNMP:

snmpwalk -v3 -l authPriv -u mkuser -a SHA -A xxxxxxxx -x DES -X xxxxxxxxxxx 10.20.30.40  1.3.6.1.4.1.14988.1.1.8.1.1.2
Result:
iso.3.6.1.4.1.14988.1.1.8.1.1.2.11 = STRING: "test"

Trying to get value from snmpwalk:

snmpwalk -v3 -l authPriv -u mkuser -a SHA -A xxxxxxxx -x DES -X xxxxxxxxxxx 10.20.30.40  1.3.6.1.4.1.14988.1.1.18.1.1.2.11
Result:
iso.3.6.1.4.1.14988.1.1.18.1.1.2.11 = STRING: "
"

Trying to get value from snmpget:

snmpget -v3 -l authPriv -u mkuser -a SHA -A xxxxxxxx -x DES -X xxxxxxxxxxx 10.20.30.40  1.3.6.1.4.1.14988.1.1.18.1.1.2.11
Result:
iso.3.6.1.4.1.14988.1.1.18.1.1.2.11 = STRING: "
"

SNMP Scripts can't read global var?

patrikg · Sat Mar 09, 2024 6:54 pm

test=123

Should that just declare some int, and when you reading it with snmp, you just get int like char value of 123.
I think you should try to assign a string instead.

test="test"

I am not any expert of RouterOS scripts, but like another programming lang and script lang.
You can try to test my hypothesis with:

test=65

You should get "A"

optio · Sat Mar 09, 2024 8:07 pm

I think problem is that SNMP service is executing script with different user (some internal system) than ROS login user which created global variable. Users cannot read global variables created by another. Same issue is for eg. with netwatch tool and getting/setting global variables that are used with other scripts.
Try to set global variable in test script before :put it should work, but that global var. value will not be readable by scripts which are not executed by SNMP, it will not even be listed with /system/script/environment/print from CLI or Winbox.
You can try to set value to file and read from it instead as workaround.

@patrikg - lol

jaclaz · Sun Mar 10, 2024 2:15 am

In this thread Amm0 proposed a nice trick that could be useful in this context, making use of a value set as part of *something* unused in configuration:
viewtopic.php?t=205221

optio · Sun Mar 10, 2024 2:38 am

In this case personally I will rather use file in root path which is using tmpfs (assuming ROS is running on device with flash drive) than writing value into some config if presistance is not needed since such file will act as global variable, preserved only in RAM, no flash writes upon setting value and lost after reboot.

thiagomedeiros · Sun Mar 10, 2024 10:14 pm

@optio I think you are right.

Tryed this and worked:

:global test;
:set $test 123;
:put $test;

iso.3.6.1.4.1.14988.1.1.18.1.1.2.12 = STRING: "123"

thiagomedeiros · Sun Mar 10, 2024 11:17 pm

This is very weird:

Don't Work:
ping address=8.8.8.8 interval=200ms count=3 as-value

iso.3.6.1.4.1.14988.1.1.18.1.1.2.12 = ""

Works:
ping address=8.8.8.8 interval=200ms count=3

iso.3.6.1.4.1.14988.1.1.18.1.1.2.12 = STRING: "  SEQ HOST                                     SIZE TTL TIME       STATUS      
    0 8.8.8.8                                    56  58 36ms775us 
    1 8.8.8.8                                    56  58 34ms379us 
    2 8.8.8.8                                    56  58 34ms423us 
    sent=3 received=3 packet-loss=0% min-rtt=34ms379us avg-rtt=35ms192us 
   max-rtt=36ms775us 
"

Worked, need to write :put
:put [ping address=8.8.8.8 interval=200ms count=3 as-value]

Amm0 · Sun Mar 10, 2024 11:36 pm

RouterOS ver 7.14

[...]
Code: Select all
/system script add dont-require-permissions=no name=test policy=read,write,policy,test source=":global test; :put \$test;"
[/code]
[...]
SNMP Scripts can't read global var?

It's permissions I think. Perhaps dont-require-permissions=yes would allow it to be read via SNMP?

But @optio is right: passing around global variable is problematic.

thiagomedeiros · Sun Mar 10, 2024 11:47 pm

@Amm0

The SNMP run with a user called *sys, and this user can't read another vars from other users.

Amm0 · Sun Mar 10, 2024 11:58 pm

In this case personally I will rather use file in root path which is using tmpfs (assuming ROS is running on device with flash drive) than writing value into some config if presistance is not needed since such file will act as global variable, preserved only in RAM, no flash writes upon setting value and lost after reboot.

Since we're talking about 7.14, there is the JSON serialization – which make reading/writing RouterOS vars to disk easy. Using a tmpfs be a good idea if there is a lot of writes to the data. But if it's mainly read, flash should be fine.

Perhaps some script example help explain how to persist a variable. I wrapped the logic in two function $savestate and $getstate, but you can cut-and-paste/adapted as needed (including using a ramdisk via "/disk add type=tmpfs ...")

:global savestate do={
    :do {  /file set "state$[:jobname].json" contents=[:serialize to=json $1]  } on-error={
           /file add name="state$[:jobname].json" contents=[:serialize to=json $1]
    }
}

:global getstate do={
    :return [:deserialize from=json [/file get "state$[:jobname].json" contents]]
}

# examples:
# an array to persist
:global myvars {a="123";b="something"}

# call \$savestate which write the RouterOS array as JSON to a file name state[:jobname].json
$savestate $myvars
:put [$getstate]

# the state is override if saved state is called again.
$savestate "mydata"
:put [$getstate]

# note: script depends on [:jobname] so the persisted variables should be scoped to each /system/script plus CLI has one state.

Amm0 · Mon Mar 11, 2024 12:12 am

This is very weird:
[...]
Worked, need to write :put
:put [ping address=8.8.8.8 interval=200ms count=3 as-value]

SNMP is capturing stdout, as-value surpasses that. And /system/script don't "return" anything, like a function or [/cmd/get value] would – so :put is all ya got to SNMP.

Amm0 · Mon Mar 11, 2024 12:23 am

@Amm0

The SNMP run with a user called *sys, and this user can't read another vars from other users.

Oh I'm sure you're right. Basically since the 7.12 script permission changes, it's bad idea to relay passing around :globals. It's always been a little wonky.

Now there are quite few script helpers introduced in 7.12 too. That make actually using a file for storage easier to use. Until recently there was no JSON, :jobname, or /file/add. FWIW, you need the JSON... since you could NOT use ":tostr <array>" and ":toarray <str>" and get the same array back in all cases. JSON will deal with "marshaling" any RouterOS type, including arrays, date, etc.

As noted above, stashing values in config or comments is another way persist data.

thiagomedeiros · Mon Mar 11, 2024 12:55 am

Well, work is done. This use Zabbix to get a value from ping and loss inside Mikrotik.

Template for Zabbix 6.0

OBS: Only run with SNMP read/write access. Please use SNMP version 3!

You need to create the script into Mikrotik with a start name: Ping
Example: Ping - something else.

Into Mikrotik, create your script.

/system script add dont-require-permissions=no name="Ping 8.8.8.8" policy=read source=":put [ping address=8.8.8.8 interval=200ms count=5]"

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