Inconsistency in handling IPv6 literals

[Brian Candler]

I'd like to raise this as a general issue for discussion.  I am working with these:

blackbox_exporter, version 0.24.0 (branch: HEAD, revision: 0b0467473916fd9e8526e2635c2a0b1c56011dff)
  build user:       root@e5bbfcc8184e
  build date:       20230516-11:07:25
  go version:       go1.20.4
  platform:         linux/amd64
  tags:             netgo

snmp_exporter, version 0.25.0 (branch: HEAD, revision: 9c42d6c874d479314e612bca69558c81f8e26287)
  build user:       root@880115266f70
  build date:       20231210-10:05:18
  go version:       go1.21.5
  platform:         linux/amd64
  tags:             netgo

In blackbox_exporter, it's inconsistent whether IPv6 literals need to be enclosed in square brackets or not.

- With the icmp prober, IPv6 literals *must not* be enclosed in brackets AFAICS

- With the dns prober, IPv6 literals *must not* be enclosed in brackets, unless you also provide a port. e.g. [::1] is invalid, but [::1]:53 is OK, as far as I can tell

- With the tcp prober, I don't think the port can be omitted anyway; hence IPv6 literals *must* be enclosed in brackets and followed by :port

This makes it difficult to write generic code to map a device to a target plus module, because it depends on what underlying prober the module is using.

Looking at the wider Prometheus ecosystem:

- Prometheus service discovery requires square brackets, because __address__ is an address:port combination

- snmp_exporter does *not* allow the square brackets in the target address if it doesn't have a port. Conversely, you can add the square brackets, but if you do you *must* add a port; it doesn't add :161 automatically.  (This is like the dns prober in blackbox_exporter)

# curl -vsS 'localhost:9116/snmp?target=[2001:db8::1]&module=mikrotik&auth=workshop_v3'
error collecting metric Desc{fqName: "snmp_error", help: "Error scraping target", constLabels: {module="mikrotik"}, variableLabels: {}}: error connecting to target [2001:db8::1]: error establishing connection to host: dial udp: address [[2001:db8::1]]:161: missing port in address

That looks like a straightforward bug to me: it seems to be attempting to add the :161 port, but is mangling it with an extra set of brackets.

If that can be fixed (and maybe it's the same issue in dns prober), then it will be possible to use brackets around IPv6 addresses without ports.

That leaves the icmp prober in blackbox exporter. I think it should allow square brackets, if not actually require them (there's no useful concept of "port" here, it will always send an echo-request). It might be more consistent to require square brackets

P.S. I would expect the blackbox_exporter http prober to require square brackets around IPv6 literals in URLs, but I can't them to work: e.g.

curl -g 'localhost:9115/probe?target=http:%2f%2f%5b2001:db8::1%5d&module=http_2xx_example'

doesn't make an outbound connection to 2001:db8::1. But quite possibly I'm doing something wrong.

[Ben Kochie]

Yea, that's some inconsistency we should fix. I agree, we should allow square brackets everywhere, but only require them if a port is required.

Consistently using https://pkg.go.dev/net#SplitHostPort should help.

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[Ben Kochie]

Oh, I forgot to add, it would be helpful to file issues on the affected repos.

[Brian Candler]

Done:

https://github.com/prometheus/snmp_exporter/issues/1076

https://github.com/prometheus/blackbox_exporter/issues/1171

BTW, there's no problem with http prober and URLs with IPv6 literals. It turns out the http prober module I was testing with initially had

      preferred_ip_protocol: "ip4"
      ip_protocol_fallback: false

[Brian Candler]

net.SplitHostPort only works if you provide both address and port, and it only accepts IPv6 literals if they are in square brackets.

https://go.dev/play/p/ezUD89xLd6z

Therefore I think the logic has to be more sophisticated:

* Contains no colons: just a hostname

* Contains exactly one colon: parse with SplitHostPort

* Contains two or more colons:

    * does not start with "[": just a hostname

    * starts with "[" and ends with "]": just a hostname inside the brackets

    * otherwise: parse with SplitHostPort