Perl script to check if possible to ssh into remote server - perl

Question

I have a perl script in which i want to first check if it possible to ssh into the server before i actually do ssh. So if it possible to ssh, then go ahead and ssh into that server, otherwise handle this exception appropriately.
So i have this script that goes through a textfile with a list of databases, and tries to ssh into each server to perform a bash command that checks if the server is performing MySQL SST or not:
#!/usr/bin/perl -w
use strict;
use warnings;
use 5.010;
use DBI;
use Encode;
use IO::File;
use JSON;
use utf8;
BEGIN {
binmode STDERR, ':utf8';
binmode STDIN, ':utf8';
binmode STDOUT, ':utf8';
}
my $time = localtime();
my $file = '/opt/db-servers/db.txt';
open my $info, $file or die "Could not open $file: $!";
while( my $hostname = <$info>) {
chomp( $hostname );
my $xtrabk_check = `ssh $hostname ps -ef |grep mysql | grep wsrep_sst_xtrabackup-v2`;
my $role_check = `ssh $hostname ps -ef |grep mysql | grep donor`;
my $error_log = `ssh $hostname ps -ef |grep mysql`;
if ( $xtrabk_check ne "" ){
if ( $role_check ne "" ){
my $cmd ="curl -vs -o /dev/null -X POST --data-urlencode 'payload={\"channel\": \"#db-wsrep-status-log\", \"username\": \"db-wsrep-status-log\", \"text\": \"$time: $hostname: --role Donor \n```$error_log```\", \"icon_emoji\": \":scorpion:\"}' 2>&1 /dev/null https://hooks.slack.com/services/GVadasdd/B6LSMF5GV/BIApnzoIldfdsrw343wf";
system($cmd);
}
else {
my $cmd ="curl -vs -o /dev/null -X POST --data-urlencode 'payload={\"channel\": \"#db-wsrep-status-log\", \"username\": \"db-wsrep-status-log\", \"text\": \"$time: $hostname: State transfer in progress, setting sleep higher mysqld \n```$error_log```\", \"icon_emoji\": \":scorpion:\"}' 2>&1 https://hooks.slack.com/services/GVadasdd/B6LSMF5GV/BIApnzoIldfdsrw343wf";
system($cmd);
}
}
}
close $info;
So probably before performing the three lines below, i want to be able to know if that server is down (hence cannot ssh into it) or not (then ssh into it with the commands below):
my $xtrabk_check = `ssh $hostname ps -ef |grep mysql | grep wsrep_sst_xtrabackup-v2`;
my $role_check = `ssh $hostname ps -ef |grep mysql | grep donor`;
my $error_log = `ssh $hostname ps -ef |grep mysql`;
How can i go about achieving this? By the way, the slack URL have been altered so security worries there.

Answer 1

By far the most reliable way to determine whether you can ssh into a server or not is to actually attempt to ssh into it. Just attempt to connect and, if it fails, report the failure and abort any further attempts:
use Try::Tiny;
# Declaring them here so they don't disappear at the end of the try block
my ($xtrabk_check, $role_check, $error_log);
try {
$xtrabk_check = `ssh $hostname ps -ef |grep mysql | grep wsrep_sst_xtrabackup-v2`;
$role_check = `ssh $hostname ps -ef |grep mysql | grep donor`;
$error_log = `ssh $hostname ps -ef |grep mysql`;
} catch {
die "Failed to connect to ssh: $_";
}

Answer 2

Instead of running ssh directly, use any of the Perl modules supporting SSH (for instance, Net::OpenSSH). That would allow you to connect to the remote host and run as many commands as you need over that single connection and it will detect any connection error and report it appropriately.
You can also parallelize the operation using Net::OpenSSH::Parallel.