During my recent DNS protocol experiments, I encountered an interesting behavior when attempting to query multiple record types simultaneously. While separate DNS packets for A and MX records work perfectly (returning both IP addresses and mail exchange data respectively), combining them in a single packet's question section yields only the A-record response.

The DNS protocol (RFC 1035) technically supports multiple questions in a single query packet. However, most DNS implementations handle this scenario differently:

// Sample DNS query structure with multiple questions
dns_query = {
    header: {
        id: 0x1234,
        flags: 0x0100,  // Standard query
        qdcount: 2,     // Two questions
        ancount: 0,
        nscount: 0,
        arcount: 0
    },
    questions: [
        { qname: "test.com", qtype: "A", qclass: "IN" },
        { qname: "test.com", qtype: "MX", qclass: "IN" }
    ]
}

Several factors contribute to this behavior:

• Server Configuration: Many DNS servers implement response filtering for performance and security reasons
• Protocol Interpretation: Some implementations treat multi-question packets as malformed
• Cache Optimization:
  Servers often prioritize A-records as the most commonly requested type

For reliable multi-record queries, consider these approaches:

# Using dig with multiple separate queries
dig test.com A +noall +answer
dig test.com MX +noall +answer

# Alternative: Use a DNS library with proper multi-query handling
const dns = require('dns');
dns.resolve('test.com', 'A', (err, addresses) => { ... });
dns.resolve('test.com', 'MX', (err, exchanges) => { ... });

Some specialized DNS servers (particularly authoritative servers) may properly handle multi-question packets. Test with:

# Using dig with multiple questions
dig test.com A MX +noall +answer

When working with raw DNS protocol implementation, I discovered an interesting behavior: A properly formatted DNS packet containing multiple record queries in its Question section often receives incomplete responses. For example, when querying both A and MX records for "example.com" in a single request, most DNS servers only respond with A records.

The RFC 1035 specification for DNS doesn't explicitly prohibit multiple questions in a single query, but section 4.1.2 states: "the question section contains QDCOUNT (usually 1) entries". This suggests that while technically possible, multi-question DNS packets aren't the expected standard implementation.

// Sample DNS query structure with multiple questions
typedef struct {
    uint16_t id;
    uint16_t flags;
    uint16_t qdcount;  // Typically set to 2 for dual queries
    uint16_t ancount;
    uint16_t nscount;
    uint16_t arcount;
    // Question 1: A record query
    unsigned char qname1[];
    uint16_t qtype1;
    uint16_t qclass1;
    // Question 2: MX record query
    unsigned char qname2[];
    uint16_t qtype2;
    uint16_t qclass2;
} dns_query_t;

Most DNS server implementations (BIND, PowerDNS, etc.) handle multi-question packets in one of three ways:

• Return only the first question's answer (common with A records)
• Return SERVFAIL (RFC 2136 compliant)
• Process all questions but with potential performance impacts

For reliable multi-record queries, consider these approaches:

// Using dig for parallel queries (Unix systems)
dig example.com A +noall +answer & dig example.com MX +noall +answer

// Python solution using dnspython
import dns.resolver
answers = dns.resolver.resolve('example.com', 'A')
mx_answers = dns.resolver.resolve('example.com', 'MX')

While combining queries seems efficient, DNS servers optimize for UDP packet size (typically 512 bytes). Multiple questions may:

• Trigger TCP fallback due to larger size
• Increase server CPU load for parallel processing
• Violate common client implementations

Modern extensions like DNS over HTTPS (DoH) and DNS over TLS (DoT) maintain similar behavior. The emerging EDNS(0) standard could potentially support proper multi-question handling, but current implementations remain conservative.