TechnitiumSoftware/DnsServer · error · ArgumentOutOfRangeException

Port 53 cannot be used for DNS-over-TLS service. Please use

Error message

Port 53 cannot be used for DNS-over-TLS service. Please use a different port.

What it means

Thrown by the DnsOverTlsPort setter when value == 53, after the range check (312) passes. Port 53 is reserved for plain DNS; DNS-over-TLS must use a different port (853 by convention). The server enforces this separation explicitly.

Source

Thrown at DnsServerCore/Dns/DnsServer.cs:7774

            set
            {
                if (string.IsNullOrWhiteSpace(value))
                    value = null;

                _dnsOverHttpsUnixSocket = value;
            }
        }

        public int DnsOverTlsPort
        {
            get { return _dnsOverTlsPort; }
            set
            {
                if ((value < ushort.MinValue) || (value > ushort.MaxValue))
                    throw new ArgumentOutOfRangeException(nameof(DnsOverTlsPort), "Port number valid range is from 0 to 65535.");

                if (value == 53)
                    throw new ArgumentOutOfRangeException(nameof(DnsOverTlsPort), "Port 53 cannot be used for DNS-over-TLS service. Please use a different port.");

                _dnsOverTlsPort = value;
            }
        }

        public int DnsOverHttpsPort
        {
            get { return _dnsOverHttpsPort; }
            set
            {
                if ((value < ushort.MinValue) || (value > ushort.MaxValue))
                    throw new ArgumentOutOfRangeException(nameof(DnsOverHttpsPort), "Port number valid range is from 0 to 65535.");

                if (value == 53)
                    throw new ArgumentOutOfRangeException(nameof(DnsOverHttpsPort), "Port 53 cannot be used for DNS-over-HTTPS service. Please use a different port.");

                if (value == 853)
                    throw new ArgumentOutOfRangeException(nameof(DnsOverHttpsPort), "Port 853 is reserved for DNS-over-TLS service. Please use a different port for DNS-over-HTTPS service.");

View on GitHub (pinned to d0484b6c1e)

Solutions

Use the conventional DoT port 853 (or another non-53 port).
Keep plain DNS on 53 and run DoT on a dedicated port.
Pre-validate the DoT port is not 53 before assigning.
Re-submit a corrected value via the settings API.

Example fix

// before
_dnsServer.DnsOverTlsPort = 53;  // throws: 53 reserved for plain DNS

// after
_dnsServer.DnsOverTlsPort = 853;  // conventional DoT port

Defensive patterns

Strategy: validation

Validate before calling

if (parsed == 53) parsed = 853; // 53 reserved for plain DNS
_dnsServer.DnsOverTlsPort = parsed;

Type guard

static bool IsAcceptableDoTPort(int value) => value != 53 && value >= 0 && value <= 65535;

Try / catch

try { _dnsServer.DnsOverTlsPort = parsed; }
catch (ArgumentOutOfRangeException ex) when (ex.ParamName == nameof(DnsServer.DnsOverTlsPort) && parsed == 53)
{ _dnsServer.DnsOverTlsPort = 853; }

Prevention

Never use 53 for DoT.
Default migrated 53 values to 853.
Reserve 53 for the plain DNS listener.

When it happens

Trigger: Assigning DnsServer.DnsOverTlsPort = 53 via the settings API or config restore. The range guard passes; this dedicated guard then fires.

Common situations: Assuming TLS DNS can run on 53 alongside plain DNS. Config migration that left the DoT port at 53. UI mix-up between the standard DNS port and the DoT port.

Understand the failure class

SSL/TLS and certificate errors — how TLS handshakes and certificate validation fail.

Related errors

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/fcc3138e5e5a0921. Report an issue: GitHub.