TechnitiumSoftware/DnsServer · error · ArgumentOutOfRangeException

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

Error message

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

What it means

Thrown by the DnsOverQuicPort setter when value == 53, after the range check (317) passes. Port 53 is reserved for plain DNS; DNS-over-QUIC must use a different port (853 by convention).

Source

Thrown at DnsServerCore/Dns/DnsServer.cs:7807

                    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.");

                _dnsOverHttpsPort = value;
            }
        }

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

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

                _dnsOverQuicPort = value;
            }
        }

        public IReadOnlyCollection<NetworkAccessControl> DnsReverseProxyNetworkACL
        {
            get { return _dnsReverseProxyNetworkACL; }
            set
            {
                if ((value is null) || (value.Count == 0))
                    _dnsReverseProxyNetworkACL = null;
                else if (value.Count > byte.MaxValue)
                    throw new ArgumentOutOfRangeException(nameof(DnsReverseProxyNetworkACL), "Network Access Control List cannot have more than 255 entries.");
                else
                    _dnsReverseProxyNetworkACL = value;
            }
        }

View on GitHub (pinned to d0484b6c1e)

Solutions

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

Example fix

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

// after
_dnsServer.DnsOverQuicPort = 853;  // conventional DoQ port

Defensive patterns

Strategy: validation

Validate before calling

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

Type guard

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

Try / catch

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

Prevention

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

When it happens

Trigger: Assigning DnsServer.DnsOverQuicPort = 53 via the settings API or config restore. Range guard passes; this dedicated guard fires.

Common situations: Assuming QUIC-based DNS can share port 53 with plain DNS. Config migration leaving DoQ at 53. UI confusion between the plain DNS port and the DoQ port.

Related errors

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/fced586fee759ac2. Report an issue: GitHub.