Add Ethernet.setHostname method and make DHCP hostname configurable.

examples/DhcpCustomHostname/DhcpCustomHostname.ino

@@ -0,0 +1,95 @@
+/*
+  DHCP with custom hostname
+
+  This sketch uses the DHCP extensions to the Ethernet library
+  to get an IP address via DHCP and print the address obtained.
+  using an Arduino WIZnet Ethernet shield.
+
+  It demonstrates how to set a custom hostname for the DHCP request.
+
+  Circuit:
+   Ethernet shield attached to pins 10, 11, 12, 13
+
+  created 14 December 2025
+  by Dmytro Lahoza
+ */
+
+#include <SPI.h>
+#include <Ethernet.h>
+
+// Enter a MAC address for your controller below.
+// Newer Ethernet shields have a MAC address printed on a sticker on the shield
+byte mac[] = {
+  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED
+};
+
+void setup() {
+  // You can use Ethernet.init(pin) to configure the CS pin
+  //Ethernet.init(10);  // Most Arduino shields
+  //Ethernet.init(5);   // MKR ETH Shield
+  //Ethernet.init(0);   // Teensy 2.0
+  //Ethernet.init(20);  // Teensy++ 2.0
+  //Ethernet.init(15);  // ESP8266 with Adafruit FeatherWing Ethernet
+  //Ethernet.init(33);  // ESP32 with Adafruit FeatherWing Ethernet
+
+  // Open serial communications and wait for port to open:
+  Serial.begin(9600);
+  while (!Serial) {
+    ; // wait for serial port to connect. Needed for native USB port only
+  }
+
+  // Set the hostname for the DHCP request
+  Ethernet.setHostname("arduino-ethernet");
+
+  // start the Ethernet connection:
+  Serial.println("Initialize Ethernet with DHCP:");
+  if (Ethernet.begin(mac) == 0) {
+    Serial.println("Failed to configure Ethernet using DHCP");
+    if (Ethernet.hardwareStatus() == EthernetNoHardware) {
+      Serial.println("Ethernet shield was not found.  Sorry, can't run without hardware. :(");
+    } else if (Ethernet.linkStatus() == LinkOFF) {
+      Serial.println("Ethernet cable is not connected.");
+    }
+    // no point in carrying on, so do nothing forevermore:
+    while (true) {
+      delay(1);
+    }
+  }
+  // print your local IP address:
+  Serial.print("My IP address: ");
+  Serial.println(Ethernet.localIP());
+}
+
+void loop() {
+  switch (Ethernet.maintain()) {
+    case 1:
+      //renewed fail
+      Serial.println("Error: renewed fail");
+      break;
+
+    case 2:
+      //renewed success
+      Serial.println("Renewed success");
+      //print your local IP address:
+      Serial.print("My IP address: ");
+      Serial.println(Ethernet.localIP());
+      break;
+
+    case 3:
+      //rebind fail
+      Serial.println("Error: rebind fail");
+      break;
+
+    case 4:
+      //rebind success
+      Serial.println("Rebind success");
+      //print your local IP address:
+      Serial.print("My IP address: ");
+      Serial.println(Ethernet.localIP());
+      break;
+
+    default:
+      //nothing happened
+      break;
+  }
+}

src/Dhcp.cpp

@@ -116,10 +116,15 @@ void DhcpClass::presend_DHCP()
 {
 }
 
+void DhcpClass::setCustomHostname(const char* name)
+{
+	_dhcpHostname = name;
+}
+
 void DhcpClass::send_DHCP_MESSAGE(uint8_t messageType, uint16_t secondsElapsed)
 {
-	uint8_t buffer[32];
-	memset(buffer, 0, 32);
+	uint8_t buffer[64];
+	memset(buffer, 0, 64);
 	IPAddress dest_addr(255, 255, 255, 255); // Broadcast address
 
 	if (_dhcpUdpSocket.beginPacket(dest_addr, DHCP_SERVER_PORT) == -1) {
@@ -153,14 +158,14 @@ void DhcpClass::send_DHCP_MESSAGE(uint8_t messageType, uint16_t secondsElapsed)
 	//put data in W5100 transmit buffer
 	_dhcpUdpSocket.write(buffer, 28);
 
-	memset(buffer, 0, 32); // clear local buffer
+	memset(buffer, 0, 64); // clear local buffer
 
 	memcpy(buffer, _dhcpMacAddr, 6); // chaddr
 
 	//put data in W5100 transmit buffer
 	_dhcpUdpSocket.write(buffer, 16);
 
-	memset(buffer, 0, 32); // clear local buffer
+	memset(buffer, 0, 64); // clear local buffer
 
 	// leave zeroed out for sname && file
 	// put in W5100 transmit buffer x 6 (192 bytes)
@@ -187,16 +192,18 @@ void DhcpClass::send_DHCP_MESSAGE(uint8_t messageType, uint16_t secondsElapsed)
 	memcpy(buffer + 10, _dhcpMacAddr, 6);
 
 	// OPT - host name
+	const char* hostname = _dhcpHostname ? _dhcpHostname : HOST_NAME;
+	size_t hostnameLen = strlen(hostname);
 	buffer[16] = hostName;
-	buffer[17] = strlen(HOST_NAME) + 6; // length of hostname + last 3 bytes of mac address
-	strcpy((char*)&(buffer[18]), HOST_NAME);
+	buffer[17] = hostnameLen + 6; // length of hostname + last 3 bytes of mac address
+	strcpy((char*)&(buffer[18]), hostname);
 
-	printByte((char*)&(buffer[24]), _dhcpMacAddr[3]);
-	printByte((char*)&(buffer[26]), _dhcpMacAddr[4]);
-	printByte((char*)&(buffer[28]), _dhcpMacAddr[5]);
+	printByte((char*)&(buffer[18 + hostnameLen]), _dhcpMacAddr[3]);
+	printByte((char*)&(buffer[18 + hostnameLen + 2]), _dhcpMacAddr[4]);
+	printByte((char*)&(buffer[18 + hostnameLen + 4]), _dhcpMacAddr[5]);
 
 	//put data in W5100 transmit buffer
-	_dhcpUdpSocket.write(buffer, 30);
+	_dhcpUdpSocket.write(buffer, 18 + hostnameLen + 6);
 
 	if (messageType == DHCP_REQUEST) {
 		buffer[0] = dhcpRequestedIPaddr;

src/Ethernet.cpp

@@ -25,11 +25,15 @@
 
 IPAddress EthernetClass::_dnsServerAddress;
 DhcpClass* EthernetClass::_dhcp = NULL;
+char EthernetClass::_hostname[32] = {0};
 
 int EthernetClass::begin(uint8_t *mac, unsigned long timeout, unsigned long responseTimeout)
 {
 	static DhcpClass s_dhcp;
 	_dhcp = &s_dhcp;
+	if (_hostname[0] != 0) {
+		_dhcp->setCustomHostname(_hostname);
+	}
 
 	// Initialise the basic info
 	if (W5100.init() == 0) return 0;
@@ -178,6 +182,16 @@ IPAddress EthernetClass::gatewayIP()
 	return ret;
 }
 
+void EthernetClass::setHostname(const char* hostname)
+{
+	if (strlen(hostname) < 32) {
+		strcpy(_hostname, hostname);
+		if (_dhcp) {
+			_dhcp->setCustomHostname(_hostname);
+		}
+	}
+}
+
 void EthernetClass::setMACAddress(const uint8_t *mac_address)
 {
 	SPI.beginTransaction(SPI_ETHERNET_SETTINGS);

src/Ethernet.h

@@ -75,6 +75,7 @@ class EthernetClass {
 private:
 	static IPAddress _dnsServerAddress;
 	static DhcpClass* _dhcp;
+	static char _hostname[32];
 public:
 	// Initialise the Ethernet shield to use the provided MAC address and
 	// gain the rest of the configuration through DHCP.
@@ -97,6 +98,7 @@ class EthernetClass {
 	static IPAddress gatewayIP();
 	static IPAddress dnsServerIP() { return _dnsServerAddress; }
 
+	void setHostname(const char* hostname);
 	void setMACAddress(const uint8_t *mac_address);
 	void setLocalIP(const IPAddress local_ip);
 	void setSubnetMask(const IPAddress subnet);
@@ -297,6 +299,7 @@ class DhcpClass {
 	unsigned long _lastCheckLeaseMillis;
 	uint8_t _dhcp_state;
 	EthernetUDP _dhcpUdpSocket;
+	const char *_dhcpHostname = NULL;
 
 	int request_DHCP_lease();
 	void reset_DHCP_lease();
@@ -311,6 +314,7 @@ class DhcpClass {
 	IPAddress getGatewayIp();
 	IPAddress getDhcpServerIp();
 	IPAddress getDnsServerIp();
+	void setCustomHostname(const char* name);
 
 	int beginWithDHCP(uint8_t *, unsigned long timeout = 60000, unsigned long responseTimeout = 4000);
 	int checkLease();