I did an adhoc scan for 6 servers and got the result as No Host Alive on all. When i talk with the support team they mentioned that these servers are behind the firewall. Please let me know how to scan the servers which are behind firewall
In order to scan a particular target, we first do some basic connectivity tests to insure the target is alive and on the network. This significantly speeds the vulnerability assessment up, as we'd otherwise spend time scanning hosts that aren't really there. Our basica host-alive process includes: ICMP Echo requests, TCP/SYN probes to ports 21-23, 25, 53, 80, 88, 110-111, 135, 139, 443, 445, and UDP probes to ports 53, 111, 135, 137, 161, 500. These ports can be modified in the Scan Option profile settings under Additional-->Host Discovery; there is also an option to "scan dead hosts".
If there are no responses from the target host, then the service cannot connect to the host. From the service's point of view the host system is considered "not alive".
This link might provide additional insight.
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