V-285226

Discussion

Limiting the number of concurrent sessions reduces the risk of denial-of-service (DoS) to the DNS implementation. Infoblox DNS servers configured in a Grid do not use zone transfers. Data is replicated using an encrypted management connection. However, when a zone contains both Infoblox Grid DNS servers and non-Grid DNS servers, a DNS protocol-compliant zone transfer is performed. Name servers do not have direct user connections but accept client connections for queries. Original restriction on client connections should be high enough to prevent a self-imposed denial of service, after which the connections are monitored and fine-tuned to best meet the organization's specific requirements. Primary name servers also make outbound connections to secondary name servers to provide zone transfers and accept inbound connection requests from clients wishing to provide a dynamic update. Primary name servers should explicitly limit zone transfers to be made only to designated secondary name servers. Because zone transfers involve the transfer of entire zones and use TCP connections, they place substantial demands on network resources relative to normal DNS queries. Errant or malicious frequent zone transfer requests on the name servers of the enterprise can overload the primary zone server and result in DoS to legitimate users. Primary name servers should be configured to limit the hosts from which they will accept dynamic updates. Additionally, the number of concurrent clients, especially TCP clients, needs to be kept to a level that does not risk placing the system in a DoS state.

Check Content

Verify inbound and outbound zone transfer limits are configured. These values control the amount of concurrent zone transfers to non-Grid DNS service members. 

For DNS Member settings:
1. From the Infoblox GUI, navigate to Data Management >> DNS >> Members tab. 
2. Review each server with the DNS service enabled, select each server, then click "Edit". 
3. Click "Toggle Advanced Mode", and then select General >> Advanced tab. 
4. Verify "Maximum inbound and outbound concurrent zone transfer limitations" and "Maximum concurrent inbound zone transfers per remote name server" are configured.
5. When complete, click "Cancel" to exit the "Properties" screen. 

For Global DNS settings:
1. From the Infoblox GUI, navigate to Infoblox Grid >> Grid Manager >> DNS >> Services >> Grid DNS Properties (Toolbar menu).
2. Toggle Advanced Mode, and select General >> Advanced tab.
3. Verify Maximum inbound and outbound concurrent zone transfer limitations and "Maximum concurrent inbound zone transfers per remote name server" are configured.
4. When complete, click "Cancel" to exit the "Properties" screen.

If maximum inbound and outbound concurrent zone transfer limits are not configured for non-Infoblox Grid DNS service members, this is a finding.

Note: Default values are conservative to protect the DNS service. Adjustment to higher values should be reviewed prior to change, to account for potential implications. This only affects non-Grid secondary DNS servers.