Discovering assets

A discovery scan finds, identifies, and builds an inventory of all the connected devices and assets on your internal network. Running a discovery scan routinely will help you keep track of and know exactly what is on your network.

Discovery scans are configured by site, explorer, and scope. In order to run a scan against a specific site, an explorer must be activated and either assigned to that site or configured for all sites.

When creating a new scan, you have multiple parameters you can set, ranging from scheduling a date to more advanced options. To launch a discovery scan, browse to the Inventory page and click the New Scan button in the upper right.

Rumble inventory menu buttons

Site

Rumble organizes information into organizations and sites. Organizations are distinct entities that are useful for keeping data separate and contain a collection of sites. Sites are used to model segmented networks, particularly independent networks which use the same private IP address ranges.

For example, you might have multiple physical locations with their own local networks, all using the 10.0.0.0/8 private IP range. By defining them as sites, you can set up an explorer for each, and the networks and assets will be treated as completely independent even if similar systems are seen at the same IP addresses in each.

Since scan analysis occurs at the site level, the boundaries you define for a site set the default scope for scans for that site.

Explorer

Select the explorer to run the scan from, chosen from the set of registered explorers for the site. The explorer you choose must be able to directly communicate with the networks and addresses you define for the discovery scope.

The chosen explorer should ideally be able to reach all addresses in the scope directly, without a firewall in the way. Stateful firewalls and VPN gateways may interfere with the discovery process.

Discovery scope

The discover scope defines what IP ranges will be scanned. The scope uses the site settings when specified as “defaults”, but may be changed on a per scan basis as well. The scope should include at least one IP address or hostname. IP address ranges can be specified in most standard formats:

  • 10.0.0.1
  • 10.0.0.0/24
  • 10.0.0.0/255.255.255.0
  • 10.0.0.1-10.0.0.255

Hostnames specified in the scope will be resolved at runtime by the assigned explorer. If the hostname returns multiple IP addresses, all addresses in the response will be scanned. Hostnames can also have masks applied, indicating that the mask should expand to each resolved address of the hostname. For example, if example.com resolves to both 1.2.3.4 and 5.6.7.8, the input of example.com/24 would become 1.2.3.0/24 and 5.6.7.0/24.

Scan name

You can assign a name to your Scan task to make it easier to keep track of.

Scan speed

Specify the maximum packet rate for the overall discovery process, in network packets per second. 500 is conservative, 3000 works for most LANs including WiFi, 10000 or more may be helpful for large sites with fast connectivity.

Schedule

You can set a date and frequency for your scan task. Dates and times take into account your browser’s advertised timezone.

Scans scheduled to start in the past will be launched immediately and then repeated at the specified time based at the frequency selected.

Scheduling grace period

Specify the number of hours to wait for an available explorer before giving up on this scan. A zero or negative value will result in the scan retrying indefinitely until an explorer becomes available.

Advanced scan options

The Advanced tab can be used to display and modify additional scan settings, such as network exclusions, scan speed, the ports covered by the TCP scan, and which probes are enabled. The default settings should work for most organizations but may need to be tweaked for slow networks or unreliable links.

Maximum host rate

As well as setting an overall scan rate in packets per second, you can also control the maximum rate at which packets are sent to any single host IP address. This is useful when you have devices which are easily overloaded by network traffic. The default should be safe for most systems.

Max group size

When Rumble scans your network, it spreads the scan load across many IP addresses at once. The max group size determines how many IP addresses can be actively being scanned at once – allowing for the fact that hosts may take some time to respond to probes. The max group size needs to be at least as large as the overall scan speed, or else it would limit the speed of the scan to below the set value. If you provide a value that’s lower than the overall scan speed, it will be increased automatically at scan time.

The max group size is mostly useful when dealing with stateful network devices that can only track a limited number of connections at once, as a way to restrict how many active TCP sessions will result from a Rumble scan.

Max TTL

The IPv4 standards define the Time To Live or TTL as a time in seconds. However, the standards also specify that every device processing a packet must decrease the TTL by at least one, no matter how quickly it is processed. The TTL value therefore serves two purposes: it determines the maximum time a connection attempt can take to be processed, and also the maximum number of network hops it can pass through. The TTL value can be used to limit a Rumble scan from traversing distant networks.

TCP ports

The Included TCP ports and Excluded TCP ports fields can be used to override the default scan ports. The string “defaults” will lookup the current default port list at scan time. The current port list is:

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Prescan modes for large IP spaces

Sometimes, the scope of your IP space is unknown, subnet usage is unknown, and the total number of assets is unknown. These unknowns can make it challenging to optimize your discovery scans for efficiency and speed. And when your IP space is large, like a /16 space with a few thousand IPs in use, a full discovery scan can take more time to complete, since it looks at more than 500 TCP ports and 15 UDP ports on every address. In these types of cases, you may want to tune your scan settings to prefilter ranges and IP addresses before a full scan.

Rumble has two prescan modes that you can use to run a faster scan: subnet sampling and host ping.

Subnet sampling

Professional Enterprise

To speed up scans of large subnets you can use the “Only scan subnets with active hosts” advanced scan option. If this option is on, a prescan runs against the target space to identify the subnets with an active host. This mode leverages heuristics Rumble has collected to identify addresses that are more likely to be responsive across subnets. This process allows Rumble to quickly scan larger spaces by identifying the subnets that are in use, before starting full probes. All subnets that are identified as having active hosts are then fully scanned – unless you enable host pings.

There are two tweakable parameters for subnet sampling. The sample rate determines what percentage of addresses in each subnet are prescanned to determine if the subnet should be scanned. The subnet size determines how many IP addresses are in each subnet. By default, the subnet size is 256 addresses, corresponding to a /24 subnet, and 3% of the addresses in each subnet are prescanned.

Host ping

After you have some insights on the subnets that are in use, you may want to limit the full scan to only addresses that respond to the most common ping methods, such as ICMP and some TCP and UDP ports. If you choose the “Limit scans to pingable hosts” advanced scan option, only hosts that respond to a ping request will be fully scanned.

Note that it is relatively common for enterprise firewalls to be set up to block ping, or for hosts to be set up not to respond to ping requests. Limiting scans to pingable hosts can therefore result in assets being missed entirely, even if their IP addresses are probed. If your goal is to speed up scan times, subnet sampling is usually the better option.

It’s possible to use both subnet sampling and limiting scans to pingable hosts at the same time, but this is not recommended except as a last resort for reducing scan times.