net-wait-go utility / library

What is it?

Both utility and GO package to wait for port to open (TCP, UDP).

Why do we need it?

In dockerized applications usually we deploy several containers with the main program container.
We need to know if containers are started, so we can continue to execute our program or fail it after some deadline.

There are a lot of examples through the internet that advise us to use several bash commands like nc, timeout, curl. But what if we have GO program minimal docker image FROM scratch that does not have bash? We could use this package as library in our program — just put couple lines of code to check if some ports are available.

But also this package can be donwloaded as utility and used from command line.

Library usage

Simple

import "github.com/antelman107/net-wait-go/wait"

if !wait.New().Do([]string{"postgres:5432"}) {
    logger.Error("db is not available")
    return
}

All optional settings definition

import "github.com/antelman107/net-wait-go/wait"

if !wait.New(
      wait.WithProto("tcp"),
      wait.WithWait(200*time.Millisecond),
      wait.WithBreak(50*time.Millisecond),
      wait.WithDeadline(15*time.Second),
      wait.WithDebug(true),
).Do([]string{"postgres:5432"}) {
    logger.Error("db is not available")
    return
}

Utility usage

net-wait-go

  -addrs string
        address:port(,address:port,address:port,...)
  -deadline uint
        deadline in milliseconds (default 10000)
  -debug
        debug messages toggler
  -delay uint
        break between requests in milliseconds (default 50)
  -packet string
        UDP packet to be sent
  -proto string
        tcp (default "tcp")
  -wait uint
		delay of single request in milliseconds (default 100)

1 service check

net-wait-go -addrs ya.ru:443 -debug true
2020/06/30 18:07:38 ya.ru:443 is OK

return code is 0

2 services check

net-wait-go -addrs ya.ru:443,yandex.ru:443 -debug true
2020/06/30 18:09:24 yandex.ru:443 is OK
2020/06/30 18:09:24 ya.ru:443 is OK

return code is 0

2 services check (fail)

net-wait-go -addrs ya.ru:445,yandex.ru:445 -debug true
2020/06/30 18:09:24 yandex.ru:445 is FAILED
2020/06/30 18:09:24 ya.ru:445 is is FAILED
...
return code is 1

UDP support

Since UDP as protocol does not provide connection between a server and clients, it is not supported in the most of popular utilities:

wait-for-it (issue — https://github.com/vishnubob/wait-for-it/issues/29)

netcat (nc) has following note in its manual page:

CAVEATS
	  UDP port scans will always succeed (i.e. report the port as open)

net-wait-go provides UDP support, working following way:

sends a meaningful packet to the server
waits for a message back from the server (1 byte at least)

UDP packet example

Counter Strike game server is accessible via UDP. Let's check random Counter Strike server by sending A2S_INFO packet (https://developer.valvesoftware.com/wiki/Server_queries#A2S_INFO)

net-wait-go -proto udp -addrs 46.174.53.245:27015,185.158.113.136:27015 -packet '/////1RTb3VyY2UgRW5naW5lIFF1ZXJ5AA==' -debug true
 
2020/07/12 15:13:25 udp 185.158.113.136:27015 is OK
2020/07/12 15:13:25 udp 46.174.53.245:27015 is OK

# return code is 0

-packet value here is the base64-encoded A2S_INFO packet, which is documented here — https://github.com/wriley/steamserverinfo/blob/master/steamserverinfo.go#L133.

The sources

Available here — github.com/antelman107/net-wait-go.