I am using scheduler/API to scan target machines for Vulnerabilities(Using Nessus). But before the scan starts, I want to check if the servers are available or not for scan. I understand from this question that, "ping sends icmp, that can be filtered by the firewall".
So other than ICMP ping , what can I use to verify if the servers are available or not ?
The good way to test if the server is online and delivering some service... is to test the given service.
So, if they are used as a web server, do an HTTP/HTTPS query and see if you get a reply. You may even want to check the content of the reply to make sure that it comes from the server you thing about, and not some proxy.
If they are mail hosts, do an SMTP or IMAP connection (based on their services), and verify that you get a valid reply.
If they allow remote SSH connection, try to establish an SSH connection.
Etc.
