This section provides an overview of the DNS Snooping feature.
In the 12.2 release, the DNS Snooping feature is supported only on the GGSN and P-GW.
ECS, using L7 rules, can be configured to filter subscriber traffic based on domain name. While this works fine for HTTP-based traffic, a subscriber's initial HTTP request may result in additional flows being established that use protocols other than HTTP and/or may be encrypted. Also, a domain may be served by multiple servers, each with its own IP address. This means that using an IP rule instead of an HTTP rule will result in multiple IP rules, one for each server "behind" the domain. This necessitates service providers to maintain a list of IP addresses for domain-based filters.
The DNS Snooping feature enables a set of IP rules to be installed based on the response from a DNS query. The rule in this case contains a fully qualified domain name (for example, m.google.com) or its segment (for example, google) and a switch that causes the domain to be resolved to a set of IP addresses. The rules installed are thus IP rules. Any actions specified in the domain rule are inherited by the resulting IP rules.
When configured, DNS snooping is done on live traffic for every subscriber.
The DNS Snooping feature enables operators to create ruledefs specifying domain names or their segments. On defining the ruledefs, the gateway will monitor all the DNS responses sent towards the UE, and snoop only the DNS response that has q-name or a-name as specified in the rules, and identify all the IP addresses resulting from the DNS response. A table of these IP addresses is maintained per destination context per rulebase per instance and shared across subscribers of the same destination context same rulebase per instance. In case DNS queries made by different subscribers produce different results, all the IP entries in the table are stored based on their Time to Live (TTL) and the configurable timer. The TTL or the timer whichever is greater is used for aging out the IP entry. Dynamic IP rules are created for these IP entries within the same rule having the domain name, applying the same charging action to these dynamic rules. This solution will have the exact IP entries as obtained live from snooping DNS responses. They will be geographically and TTL correct.
DNS Snooping is a licensed Cisco feature. A separate feature license may be required. Contact your Cisco account representative for detailed information on specific licensing requirements. For information on installing and verifying licenses, refer to the Managing License Keys section of the Software Management Operations chapter in the System Administration Guide.
Limitations and Dependencies
This section identifies limitations and dependencies for the DNS Snooping feature.
On a SessMgr kill or card switchover, the dynamic IP rules created based on domain name resolution will be lost. Until a new DNS query is made, the dynamic IP based rules will not be applied. These rules will be recreated on new DNS traffic. So, SessMgr recovery is not supported for these dynamic IP rules.
The ip server-domain-name ruledef can be used as a predefined dynamic rule, static rule, or as a part of group of ruledefs. However, it cannot be used as a dynamic-only rule, as dynamic-only rules apply up to L4 and this is an L7 rule.
Operators must define valid domain-name servers, the DNS responses from which will be considered correct and snooped and included in the list of dynamic-learnt IP addresses. If the list of valid domain-name servers is not provided, then the DNS responses from all DNS servers will be considered valid and included in the list of learnt IP addresses. Also, in case subscribers make DNS queries to their self-created DNS servers and hack the response being sent, it can result in inclusion of invalid IP addresses in the list. In this case, the IP addresses will be learnt and the traffic may be free-rated or blocked incorrectly depending on the action set. Therefore the above is suggested to avoid attacks on DNS traffic.
There is a limit on the total number of learnt IP addresses per server-domain-name ruledef for memory and performance considerations. Any more IP addresses across this limit will not be learnt and hence the charging-action will not be applied to these IP addresses. Similarly, there is a limit on the total number of server-domain-name ruledefs that can be configured.
If same IP address is returned in DNS responses for different DNS q-names (same IP hosting multiple URLs), than while rule matching, the higher priority rule having this learnt-IP address will be matched. This can have undesired rule-matching as explained next.
For example, if DNS queries for both www.facebook.com and www.cnn.com returned the IP address 220.127.116.11. Here we have allow action for domain www.facebook.com and block or no action for www.cnn.com which is at a lower priority than allow rule. In this if the actual request for www.cnn.com comes than as the server IP is same, it will match the higher priority allow rule for domain www.facebook.com (considering there are no other rule lines or all lines match) and thus, free rated incorrectly. However, this will happen only of same IP address is returned for different q-names, which is rare and cannot be handled.
In the 12.2 release, the lookup for IPv6 learnt IP addresses will not be optimized. Hash based lookup (optimization) is done for IPv4 address lookup. In a later release Longest Prefixed Match (LPM) based optimization will be considered for both IPv4 and IPv6 learnt IP address matching.