TechnitiumSoftware/DnsServer · error · ArgumentException

Server host name cannot exceed 63 bytes.

Error message

Server host name cannot exceed 63 bytes.

What it means

Thrown by the ServerHostName setter when a non-null value has Length >= 64. DHCP server host names are bounded by the 64-byte sname field in the DHCP message, so the library rejects anything that would not fit (max 63 usable characters).

Source

Thrown at DnsServerCore/Dhcp/Scope.cs:2001

        }

        public IPAddress ServerAddress
        {
            get { return _serverAddress; }
            set
            {
                ValidateIpv4(value, nameof(ServerAddress));
                _serverAddress = value;
            }
        }

        public string ServerHostName
        {
            get { return _serverHostName; }
            set
            {
                if ((value != null) && (value.Length >= 64))
                    throw new ArgumentException("Server host name cannot exceed 63 bytes.");

                _serverHostName = value;
            }
        }

        public string BootFileName
        {
            get { return _bootFileName; }
            set
            {
                if ((value != null) && (value.Length >= 128))
                    throw new ArgumentException("Boot file name cannot exceed 127 bytes.");

                _bootFileName = value;
            }
        }

        public IPAddress RouterAddress

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Truncate or choose a host name of at most 63 characters before assignment.
2. Validate input length in your config/UI layer.
3. If the FQDN is too long, use a short hostname instead.
4. Note: the check is on character Length; for non-ASCII ensure byte length also stays under 64.

Example fix

// before
scope.ServerHostName = fqdn; // may exceed 63

// after
scope.ServerHostName = (fqdn == null || fqdn.Length <= 63) ? fqdn : fqdn.Substring(0, 63);

if (hostName != null && Encoding.UTF8.GetByteCount(hostName) >= 64)
    throw new ConfigurationException("ServerHostName must be <= 63 bytes");
scope.ServerHostName = hostName;

static bool IsValidServerHostName(string s)
    => s == null || Encoding.UTF8.GetByteCount(s) <= 63;

try { scope.ServerHostName = value; }
catch (ArgumentException ex) when (ex.Message.Contains("Server host name"))
{ /* truncate and retry, or prompt user */ }

When it happens

Trigger: Assigning scope.ServerHostName = value where value != null && value.Length >= 64. Occurs when the host name is an FQDN with many labels, a stale long value, or unvalidated user input.

Common situations: Config importing the machine FQDN verbatim, templated names that exceed 63 chars, or forgetting the 63-byte DHCP limit.

Related errors

• Boot file name cannot exceed 127 bytes.
• Ending address must be greater than starting address.
• Lease time in days must be between 0 to 999.
• Lease time in hours must be between 0 to 23.
• Lease time in minutes must be between 0 to 59.

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/db853a86e1a4646a. Report an issue: GitHub.