While DNS primarily uses UDP for its lightweight queries, RFC 7766 explicitly states that DNS servers must support TCP for:

• Responses exceeding 512 bytes (including DNSSEC records)
• Zone transfers (AXFR/IXFR)
• EDNS0 fallback when UDP fragmentation is blocked

# Example dig command forcing TCP
dig +tcp @ns1.bluehost.com bluehost.com SOA

# Expected successful response:
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

When nameservers like Bluehost's fail TCP queries:

• Clients can't fall back when UDP fails
• DNSSEC validation may break
• Large response queries fail (common with ANY queries)

Here's a Python script to test TCP response capability:

import dns.message
import dns.query

def check_tcp_resolver(nameserver, domain='example.com'):
    query = dns.message.make_query(domain, 'SOA')
    try:
        response = dns.query.tcp(query, nameserver, timeout=5)
        return response.answer is not None
    except:
        return False

# Test multiple providers
providers = {
    'bluehost': 'ns1.bluehost.com',
    'cloudflare': 'one.one.one.one',
    'google': '8.8.8.8'
}

for name, ns in providers.items():
    print(f"{name}: {'Working' if check_tcp_resolver(ns) else 'Failing'}")

Common misconfigurations that block DNS TCP:

# Bad iptables rule that would break DNS TCP
iptables -A INPUT -p tcp --dport 53 -j DROP

# Correct rule to allow DNS TCP:
iptables -A INPUT -p tcp --dport 53 -j ACCEPT

Based on our monitoring data:

While DNS primarily uses UDP (port 53) for standard queries, RFC 7766 officially mandates TCP support for DNS transactions. The key scenarios requiring TCP include:

// Example DNS query using TCP in Python
import dns.message
import dns.query

query = dns.message.make_query('example.com', 'A')
response = dns.query.tcp(query, '8.8.8.8', timeout=5)

NS1.Bluehost.com's TCP non-compliance illustrates a real-world issue affecting:

• Large DNS responses (exceeding 512 bytes with DNSSEC)
• Zone transfer operations (AXFR/IXFR)
• EDNS0 implementations
• DDoS mitigation strategies

For accurate DNS server monitoring, consider this comprehensive check:

#!/bin/bash
# Check both UDP and TCP DNS response
dig +tcp @ns1.bluehost.com bluehost.com A > tcp_test.txt
dig +notcp @ns1.bluehost.com bluehost.com A > udp_test.txt

Use telnet to test TCP port accessibility:

telnet 74.220.195.31 53
Trying 74.220.195.31...
# Successful connection should show blank screen
# Connection refused indicates TCP blocking

In production environments, missing TCP support can cause:

• DNSSEC validation failures
• Mobile client resolution issues
• Problems with DNS-based load balancing
• Incomplete monitoring data

When TCP fails, implement this Python fallback check:

def check_dns_server(host, use_tcp=True):
    try:
        resolver = dns.resolver.Resolver()
        resolver.nameservers = [host]
        if use_tcp:
            resolver.use_tcp = True
        return bool(resolver.query('example.com'))
    except:
        return False