srv-plz - SRV record extractor

Service Please!

srv-plz looks up a DNS SRV record from the specified DNS server and outputs the result.

$ SRV_DNS=127.0.0.1:8600 srv-plz example.service.consul

srv-plz includes support for injecting SRV records into invoked commands. This is useful for enhancing tools like curl which do not support SRV records (it is on curl's TODO list). Here we recursively resolve a Consul query and invoke curl in a URL with the result:

$ SRV_DNS=127.0.0.1:8600 srv-plz example.service.consul -r -c -- curl https://%SRV%/health

Usage

$ srv-plz --help
usage:  srv-plz <options> [service1 [service2 [...]]] [-- command]

srv-plz resolves DNS SRV records and outputs the result.

The resolver is specified with "--dns <IP:port>" argument or by setting
the SRV_DNS environment variable.  If only an IP address is set, port 53 is used.

If no DNS resolver is specified, the system resolver is used.

The default output is "host:port".  This may be customized with the --template
argument.  Possible fields are Target, Port, Priority, and Weight.
The default template is "{{.Target}}:{{.Port}}\n" for SRV records and "{{.Target}}\n" for A/AAAA records.

If "--command" is flagged, each SRV record will be injected into the command
specified after "--", using "%SRV%" or the "--match" argument as a matcher.  Example:

    srv-plz webserver.service.consul -r -c -- curl https://%SRV%/health

Arguments:
  -a, --a                 Check A records, not SRV records
  -6, --aaaa              Check AAAA records, not SRV records
  -c, --command           for each record, invoke exec.Command on the args after '--', replacing %SRV% with its template
  -d, --dns string        DNS resolver to use. Must be in form IP (using port 53) or IP:port
  -h, --help              show help
  -l, --limit uint32      only return N records (default 1)
  -m, --match string      specify forward args after '--' to shell with <srv> replaced by the lookup (default "%SRV%")
  -r, --recurse           recurse with the same resolver
  -t, --template string   output using template (default "{{.Target}}:{{.Port}}")

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Installing

Binaries for multiple platforms are released on GitHub through GitHub Actions.

You can also install for various platforms with Homebrew from neomantra/homebrew-tap:

brew tap neomantra/homebrew-tap
brew install srv-plz

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Example Usage

Lookup with system resolver, without or with recursion:

$ srv-plz _http._tcp.mxtoolbox.com
mxtoolbox.com.:80

$ srv-plz -r _http._tcp.mxtoolbox.com
13.225.202.38:80

Lookup with custom resolver, either via CLI or SRV_DNS environment variable:

$ srv-plz -d 10.4.20.69:8600 -r webserver.service.consul 
10.4.20.69:55420

$ SRV_DNS=10.4.20.69:8600 srv-plz -r webserver.service.consul 
10.4.20.69:55420

Lookup with a custom output template. Note the shell expression $'\n' is used to include a newline at the end.

$ srv-plz -d 10.4.20.69:8600 -r webserver.service.consul -t $'t:{{.Target}} p:{{.Port}} w:{{.Weight}} p:{{.Priority}}\n'
t:10.0.10.25 p:26877 w:1 p:1

Check a URL with multiple records of multiple services using curl:

$ srv-plz -d consul.example.com:8600 -r -n 10 -c webserver.service.consul other.service.consul -- curl -s http://%SRV%/health

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Building

Building is performed with task:

$ task
task: [build] go build -o srv-plz cmd/srv-plz/main.go

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Credits and License

Thanks to github.com/miekg/dns for the heavy lifting.

Copyright (c) 2022-2023 Neomantra BV. Authored by Evan Wies.

Released under the MIT License, see LICENSE.txt.