Inconsistent support of IPv6 literals

[candlerb]

Host operating system: output of uname -a

Linux prometheus 5.4.0-162-generic #179-Ubuntu SMP Mon Aug 14 08:51:31 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

snmp_exporter version: output of snmp_exporter -version

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

What did you do that produced an error?

Supplying a target IPv6 literal with square brackets but without a port.

• Works: curl -gvsS 'localhost:9116/snmp?target=2001:db8::1&module=if_mib&auth=public_v2'
• Works: curl -gvsS 'localhost:9116/snmp?target=[2001:db8::1]:161&module=if_mib&auth=public_v2'
• FAILS: curl -gvsS 'localhost:9116/snmp?target=[2001:db8::1]&module=if_mib&auth=public_v2'

The output in the latter case:

*   Trying 127.0.0.1:9116...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 9116 (#0)
> GET /snmp?target=[2001:db8::1]&module=if_mib&auth=public_v2 HTTP/1.1
> Host: localhost:9116
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain; charset=utf-8
< X-Content-Type-Options: nosniff
< Date: Fri, 15 Dec 2023 10:36:48 GMT
< Content-Length: 325
<
An error has occurred while serving metrics:

error collecting metric Desc{fqName: "snmp_error", help: "Error scraping target", constLabels: {module="if_mib"}, variableLabels: {}}: error connecting to target [2001:db8::1]: error establishing connection to host: dial udp: address [[2001:db8::1]]:161: missing port in address
* Connection #0 to host localhost left intact

Note the address [[2001:db8::1]]:161 - it looks like something has gone wrong when appending the default port.

Additional info

Discussion: https://groups.google.com/g/prometheus-users/c/xdaiJZqm8G4

blackbox_exporter seems to have the same problem with the dns_prober (at least), so there may be some code in a common library that needs fixing.

The string "missing port in address" doesn't exist in the snmp_exporter repository. It most likely comes from the go stdlib src/net/ipsock.go. Prometheus itself has code which tests for it:

./web/api/v1/api.go:	if err != nil && strings.HasSuffix(err.Error(), "missing port in address") {