TechnitiumSoftware/DnsServer · error · ArgumentException

Seconds elapsed must be 2 bytes.

Error message

Seconds elapsed must be 2 bytes.

What it means

Constructor argument guard in DhcpMessage: the seconds-elapsed (secs) field must be exactly 2 bytes. RFC 2131 defines secs as a 2-octet field recording seconds since the client began the address acquisition process; other lengths are invalid wire data.

Source

Thrown at DnsServerCore/Dhcp/DhcpMessage.cs:119

            if (yiaddr.AddressFamily != AddressFamily.InterNetwork)
                throw new ArgumentException("Address family not supported.", nameof(yiaddr));

            if (siaddr.AddressFamily != AddressFamily.InterNetwork)
                throw new ArgumentException("Address family not supported.", nameof(siaddr));

            if (giaddr.AddressFamily != AddressFamily.InterNetwork)
                throw new ArgumentException("Address family not supported.", nameof(giaddr));

            ArgumentNullException.ThrowIfNull(clientHardwareAddress);

            if (clientHardwareAddress.Length > 16)
                throw new ArgumentException("Client hardware address cannot exceed 16 bytes.", nameof(clientHardwareAddress));

            if (xid.Length != 4)
                throw new ArgumentException("Transaction ID must be 4 bytes.", nameof(xid));

            if (secs.Length != 2)
                throw new ArgumentException("Seconds elapsed must be 2 bytes.", nameof(secs));

            _op = op;
            _htype = hardwareAddressType;
            _hlen = Convert.ToByte(clientHardwareAddress.Length);
            _hops = 0;

            _xid = xid;

            _secs = secs;
            _flags = flags;

            _ciaddr = ciaddr;
            _yiaddr = yiaddr;
            _siaddr = siaddr;
            _giaddr = giaddr;

            _clientHardwareAddress = clientHardwareAddress;
            _chaddr = new byte[16];

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Build secs as exactly 2 bytes using BitConverter.GetBytes((ushort)seconds) in the correct endianness (network byte order).
2. When forwarding a parsed packet, reuse its original 2-byte secs slice.
3. Validate secs.Length == 2 before constructing.
4. Do not pass a full int/uint representation.

Example fix

// before
byte[] secs = BitConverter.GetBytes(0u); // 4 bytes
var msg = new DhcpMessage(op, htype, xid, secs, flags, ...); // throws [217]

// after: 2 bytes, network order
ushort s = 0;
byte[] secs = new byte[] { (byte)(s >> 8), (byte)(s & 0xff) };
var msg = new DhcpMessage(op, htype, xid, secs, flags, ...);

if (secs is null) throw new ArgumentNullException(nameof(secs));
if (secs.Length != 2) throw new ArgumentException("Seconds elapsed (secs) must be exactly 2 bytes.", nameof(secs));

var msg = new DhcpMessage(op, htype, xid, secs, flags, ciaddr, yiaddr, siaddr, giaddr, chaddr, sname, file, opts);

static bool IsValidSecs(byte[] secs) =>
    secs is not null && secs.Length == 2;

When it happens

Trigger: Calling the DhcpMessage constructor with a secs byte array whose Length is not 2.

Common situations: Passing a ushort/int directly instead of its 2-byte form; creating secs from the wrong buffer slice; a helper that emits 4 bytes (uint) instead of 2.

Related errors

• Client hardware address cannot exceed 16 bytes.
• Transaction ID must be 4 bytes.
• Server host name cannot exceed 63 bytes.
• Boot file name cannot exceed 127 bytes.
• Address family not supported.

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/80aae9c234928905. Report an issue: GitHub.