For network administrators, understanding DNS packet fragmentation is crucial. When a DNS response packet is large and unable to fit within the MTU size, it’s divided into smaller fragments. MTU, or the maximum transmission unit, is the largest size of a packet or frame that can be sent across a data link. This process affects SPF in several ways, mainly because SPF relies heavily on DNS lookups. Let’s delve deeper into this important topic.
DNS Packet Fragmentation’s Working
DNS packet fragmentation is an inherent part of network communication when dealing with large data packets. Understanding this concept and managing the fragmentation process efficiently is non-negotiable if you want stable and reliable DNS communications.
Here’s how DNS packet fragmentation generally occurs-
Determining MTU and DNS Response Sizes
The DNS server sends a response and checks the size of the response packet. The packet is then compared with the network’s MTU size, as that’s the maximum size a packet can be before it must be fragmented.
Fragmentation
If the DNS response packet size is bigger than the MTU’s size, the packet is split into smaller fragments. Each fragment includes part of the original DNS response data along with additional headers to specify the position of the packet in the sequence and the total number of fragments.
The first fragment contains the original packet’s header and source or destination IP addresses. Subsequent fragments contain continuation headers.
Transmission
The smaller fragments are then sent individually over the network, and network routing decisions may direct them on different routes to reach their destination.
Reassembly
When these fragmented packets reach the destination, the receiver reassembles them to put together the original DNS response packet. The receiver keeps track of the fragments received and their sequence to reassemble them in the correct order.
Potential Issues
If any fragments are lost during transmission, the response may be incomplete, leading to DNS lookup failures. Delays may occur if the reassembly process is slowed down by waiting for missing fragments.
Mitigation and Alternatives
To steer clear of problems emerging from fragmentation, DNS servers use the Extension Mechanisms for DNS (shortened as EDNS) to evaluate what’s the maximum size of a response that a client can accept and then adjustments are made accordingly.
If the DNS response is too large for UDP, the server can fall back to TCP for transmission, as TCP does not face the same size limitations and does not fragment packets.
Effects of DNS Packet Fragmentation on Sender Policy Framework
Recipients’ mail servers perform DNS lookups to retrieve the SPF records corresponding to the email senders’ domains. So, if the SPF record is too large, the DNS response will exceed the maximum MTU size for a packet, leading to the following fragmentation issues-
Increased Latency
The entire process of email delivery slows down because fragmentation and reassembling take some time. These delays can be problematic for operations and customer support.
Packet Loss
Fragmented packets are likely to be lost or dropped during transmission, causing incomplete DNS responses.
Security Risks
Fragmented packets can be exploited for certain types of network attacks, such as fragmentation attacks.
Image sourced from isc.sans.edu
Best Practices for SPF Records To mitigate the Impact of DNS Packet Fragmentation on SPF
Use mechanisms like ‘include’ and ‘redirect’ to keep your SPF record concise, as they minimize the number of DNS lookups and nestle efficiently. These mechanisms shorten SPF records by delegating them to other domains; however, ensure you use them carefully to avoid excessive DNS lookups. Incorporating SPF flattening strategies can further optimize the efficiency of your SPF record.
Lastly, regularly check the size of your SPF record to ensure they remain within the acceptable limits, which is generally 512- bytes for DNS responses.
