feat(icmp) support DoNotFragment + Size

[antoinekh]

Summary

It's really a network-related need.
We use Gatus for temporary tests and we need to do MTU validation.

This pull request adds 2 capabilities to ICMP:

• Ability to set the DoNotFragement bit
• Ability to configure the size

This will my issue #506

Example of configuration with these options:

  - name: 10.35.206.132_df
    group: static
    url: "icmp://10.35.206.132"
    interval: 3s
    conditions:
      - "[CONNECTED] == true"
    client:
      timeout: 1s
      df: true
      size: 3500

Checklist

• [X ] Tested and/or added tests to validate that the changes work as intended, if applicable.
• [X ] Updated documentation in README.md, if applicable.

[TwiN]

Suggested change

- [Table of Contents](#table-of-contents)

[TwiN]

Could you revert this & other similar changes?

[TwiN]

What does Df mean? DoFragment?
Could you clarify that in the comment?

[antoinekh]

Ah that's my markdown auto format in my vscode, I'll revert.
And add info on the Df bit
I’ll take explanation for this good https://stackoverflow.com/questions/45953716/when-to-set-dont-fragment-flag-in-ip-header

[antoinekh]

Should be better now.
Let me know if you need more informations.

I'm not sure if this is really a feature that should be added to Gatus, but it doesn't cost much to add it.

[TwiN]

looks like this file has been added by accident?

[antoinekh]

My bad, fixed !

[TwiN]

I'In hindsight, maybe this should be under icmp, i.e.

client:
  icmp:
    df: true
    size: 25

What do you think?

[antoinekh]

It's done, as you said, it makes more sense that way

[TwiN]

Suggested change

	DF bool `yaml:"df"` // Don't Fragment flag used for the ICMP client
	DF bool `yaml:"df"` // Don't Fragment flag used for the ICMP client

[TwiN]

Based on what I just looked at, the current default value is 25 (on my machine, at least 🤣)

Side note, I also see that there's a comment on L112 that may no longer be valid - it says // limit for pro-ping, below 25 it's not working.

[antoinekh]

I've set this configuration because it's the only way I've found to make it work.
But maybe I'm missing something in Go.

I did the tests by validating with network captures:

• A classic linux ping can have a data of 0.
• If I don't specify a Size or defaultSize by default the Size is 24.
  So you are right I can remove defaultSize = 24

But then, If I don't specifiy anything in icmp in my yaml file, the ping is not working.

That's why I put this code with the Size in it.

        // If Icmp is nil, let's initialize it with default values
        c.ICMPConfig = &ICMPConfig{
            DF:   false,
            Size: 24,
        }

But maybe this is not the good way to initialise it.

If I test with a Size <24 it doesn't work either.
I notice that this limitation exists in other tools that use pro-bing
For example
https://github.com/SuperQ/smokeping_prober
Packetdata size in bytes. Default 56 (Range: 24 - 65535)

That's why I put

    if c.ICMPConfig != nil {
        // limit for pro-ping, below 24 it's not working
        if c.ICMPConfig.Size < 24 {
            c.ICMPConfig.Size = 24
        }

And my mistake: the limit is 24, not 25.