Server name resolution

Advanced Server Access implements a custom name resolution system, which is used to resolve user-supplied names to a server registered with Advanced Server Access.

For example, if you run sft ssh web0 the name web0 will be resolved in Advanced Server Access. Likewise, if the agent on web0 has a config file that specifies Bastion: bastion.example.com, then bastion.example.com will be resolved to establish a tunnel to web0.

If Advanced Server Access is unable to resolve a name to an enrolled server, Advanced Server Access will fall back to using locally supported name resolution and authentication methods to access the server. For example, if you run sft ssh web0.example.com and Advanced Server Access is unable to resolve web0.example.com to a server enrolled in Advanced Server Access, then the client behaves as though you ran ssh web0.example.com without using Advanced Server Access.

Active servers

Advanced Server Access only matches names against active servers. A server is considered active if all of the following conditions are true:

  1. The server was enrolled in Advanced Server Access
  2. The server was not subsequently deleted from Advanced Server Access
  3. The Advanced Server Access agent was recently running on the server, and able to contact the Advanced Server Access platform. Currently the threshold is 96 hours (4 days), but this is subject to change.

As a special case, if an Unmanaged Server is created in Advanced Server Access it will be considered active until it is deleted.

Matching

When resolving a name, Advanced Server Access will look for any server matching one of the following values.

Server ID

The server's id, a random UUID automatically assigned by the Advanced Server Access platform during enrollment.

Hostname

The server's hostname, as reported by the operating system on the server. If the OS hostname ends in ".local", then this will be ignored by the platform. For example, if the OS hostname is web0.local, you can access the server as web0, but not as web0.local.

If a CanonicalName value is specified in the Advanced Server Access agent configuration, then this value is used in place of the OS hostname.

AltNames

Any AltNames values specified in the Advanced Server Access agent configuration file on the server. AltNames can be used to add additional aliases to servers.

Default IP address

The default IP address of the server, as determined by Advanced Server Access.

Instance ID

If the server is an AWS or GCE cloud instance, then this is the provider-assigned ID of the instance.

Ambiguous names

If a name supplied to the Advanced Server Access client resolves to more than one server, the client will return an error to avoid inadvertently connecting to an unintended server.

One side effect of this is that if a server is replaced with a new server that has the same hostname, or if the agent is forced to re-enroll, a user with administrative privileges will need to manually delete the original server record before the new one can be addressed by the shared name. Alternatively, users can specify an unambiguous name such as the server ID, or wait for the original server record to become inactive.

If a Bastion name supplied in a Advanced Server Access agent configuration is ambiguous, the Advanced Server Access platform will attempt to choose the best fit by ranking matches in descending order of preference by matching:

  1. id or CanonicalName
  2. Cloud Instance ID
  3. Hostname
  4. AltNames
  5. Default IP Address

Ties are broken arbitrarily.

IP addresses

After a server has been resolved, Advanced Server Access attempts to determine the IP address that should be used to address the server.

If the server is being accessed directly by the Advanced Server Access client, the client will attempt to use the server's default IP address as described below.

If the server is being accessed via one or more bastions, and the immediately preceding bastion resides in the same AWS VPC or GCE Network as the server, Advanced Server Access will prefer the server's corresponding private IP address. Otherwise Advanced Server Access will use the default IP address.

Default IP address

Each server enrolled in Advanced Server Access is assigned a default IP address, which is the first available value from:

  1. An AccessAddress value specified in the Advanced Server Access agent configuration file.
  2. The public_ip_v4 address of an AWS instance, based on the instance's metadata.
  3. The external or internal IP address of a GCE or Azure instance, based on the instance's metadata.
  4. The numerically lowest IPv4 address in a public address range.
  5. The numerically lowest IPv4 address in a private range.