Dns

Dns Server

The DnsServer trait and DnsAction is the interface between the cross compiled part and the logic.

pub enum DnsAction {
    SendPacket {
        payload: [u8; 576],
        len: usize,
    },
    Ignore,
}

pub trait DnsServer {
    fn handle_message(
        &mut self,
        buffer: &[u8],
    ) -> DhcpAction;
}

Message Format

DNS messages are binary-encoded over UDP. The wire format consists of a fixed 12-byte header, followed by a variable number of question and answer records. All multi-byte integers are big-endian.

Everything that is gray in the following tables is not implemented and only encoded with a default value.

Header

Header Flags

Question

Each question record contains an encoded name, a 16-bit type field, and a 16-bit class field (always 1 for IN / Internet in this implementation).

Supported question types:

Value	Type	Description
1	A	IPv4 address
5	CNAME	Canonical name alias
15	MX	Mail exchange
16	TXT	Arbitrary text
28	AAAA	IPv6 address

Answer

Answer records extend the question format with a 32-bit TTL (time-to-live in seconds) and a 16-bit len of the data, followed by the record data.

Name

Domain names are encoded as a sequence of length-prefixed labels, terminated by a zero byte. For example, captive.apple.com encodes as [7]captive[5]apple[3]com[0]. DNS compression pointers (top two bits 0xC0) are also supported for parsing — they redirect the reader to an earlier offset in the packet.

Record

Record payloads are type-specific:

This is just a example for the IPv4 type. For other types view the code inside logic/src/dns.rs.

Poisoned Dns Server

The PoisonedDnsServer is a simple DNS server designed to support a captive portal. Rather than forwarding or resolving queries, it responds to every DNS query — regardless of the requested domain - with an A record pointing to SERVER_IP. This forces all DNS-dependent traffic on the network to resolve to the device running the portal.

Behaviour:

• Parses the incoming DNS query.
• Takes the name from the first question record.
• Responds with a single A record mapping that name to SERVER_IP with a TTL of 0.
• Echoes back the original identification, opcode, and RD flag, and sets AA and RA to true.
• Returns DnsAction::Ignore if the packet cannot be parsed or contains no questions.

The TTL of 0 prevents clients from caching the poisoned response, which ensures they re-query on each connection attempt - useful if the portal IP changes or the device restarts.