DNS resolver for captive portal

user/captdns.c

@@ -0,0 +1,207 @@
+#include <esp8266.h>
+
+typedef struct __attribute__ ((packed)) {
+	uint16_t id;
+	uint8_t flags;
+	uint8_t rcode;
+	uint16_t qdcount;
+	uint16_t ancount;
+	uint16_t nscount;
+	uint16_t arcount;
+} DnsHeader;
+
+
+typedef struct __attribute__ ((packed)) {
+	uint8_t len;
+	uint8_t data;
+} DnsLabel;
+
+
+typedef struct __attribute__ ((packed)) {
+	//before: label
+	uint16_t type;
+	uint16_t class;
+} DnsQuestionFooter;
+
+
+typedef struct __attribute__ ((packed)) {
+	//before: label
+	uint16_t type;
+	uint16_t class;
+	uint32_t ttl;
+	uint16_t rdlength;
+	//after: rdata
+} DnsResourceFooter;
+
+
+
+
+#define FLAG_QR (1<<7)
+#define FLAG_AA (1<<2)
+#define FLAG_TC (1<<1)
+#define FLAG_RD (1<<0)
+
+#define QTYPE_A  1
+#define QTYPE_NS 2
+#define QTYPE_CNAME 5
+#define QTYPE_SOA 6
+#define QTYPE_WKS 11
+#define QTYPE_PTR 12
+#define QTYPE_HINFO 13
+#define QTYPE_MINFO 14
+#define QTYPE_MX 15
+#define QTYPE_TXT 16
+
+#define QCLASS_IN 1
+#define QCLASS_ANY 255
+
+
+//Function to put unaligned 16-bit network values
+static void ICACHE_FLASH_ATTR setn16(void *pp, int16_t n) {
+	char *p=pp;
+	*p++=(n>>8);
+	*p++=(n&0xff);
+}
+
+//Function to put unaligned 32-bit network values
+static void ICACHE_FLASH_ATTR setn32(void *pp, int32_t n) {
+	char *p=pp;
+	*p++=(n>>24)&0xff;
+	*p++=(n>>16)&0xff;
+	*p++=(n>>8)&0xff;
+	*p++=(n&0xff);
+}
+
+static uint16_t ntohs(uint16_t *in) {
+	char *p=(char*)in;
+	return ((p[0]<<8)&0xff00)|(p[1]&0xff);
+}
+
+
+//Parses a label.
+//Returns pointer to start of next fields in packet
+static char* ICACHE_FLASH_ATTR labelToStr(char *packet, char *labelPtr, int packetSz, char *res, int resMaxLen) {
+	int i, j, k;
+	char *endPtr=NULL;
+	i=0;
+	do {
+		if ((*labelPtr&0xC0)==0) {
+			j=*labelPtr++; //skip past length
+			//Add separator period if there already is data in res
+			if (i<resMaxLen && i!=0) res[i++]='.';
+			//Copy label to res
+			for (k=0; k<j; k++) {
+				if ((labelPtr-packet)>packetSz) return NULL;
+				if (i<resMaxLen) res[i++]=*labelPtr++;
+			}
+		} else if ((*labelPtr&0xC0)==0xC0) {
+			//Compressed label pointer
+			endPtr=labelPtr+2;
+			int offset=ntohs(((uint16_t *)labelPtr))&0x3FFF;
+			//Check if offset points to somewhere outside of the packet
+			if (offset>packetSz) return NULL;
+			labelPtr=&packet[offset];
+		}
+		//check for out-of-bound-ness
+		if ((labelPtr-packet)>packetSz) return NULL;
+	} while (*labelPtr!=0);
+	res[i]=0; //zero-terminate
+	if (endPtr==NULL) endPtr=labelPtr+1;
+	return endPtr;
+}
+
+
+char *strToLabel(char *str, char *label, int maxLen) {
+	char *len=label; //ptr to len byte
+	char *p=label+1; //ptr to next label byte to be written
+	while (1) {
+		if (*str=='.' || *str==0) {
+			*len=((p-len)-1);	//write len of label bit
+			len=p;				//pos of len for next part
+			p++;				//data ptr is one past len
+			if (*str==0) break;	//done
+			str++;
+		} else {
+			*p++=*str++;	//copy byte
+//			if ((p-label)>maxLen) return NULL;	//check out of bounds
+		}
+	}
+	*len=0;
+	return p; //ptr to first free byte in resp
+}
+
+
+
+static void ICACHE_FLASH_ATTR captdnsRecv(void* arg, char *pusrdata, unsigned short length) {
+	struct espconn *conn=(struct espconn *)arg;
+	char buff[512];
+	char reply[512];
+	int i;
+	char *rend=&reply[length];
+	char *p=pusrdata;
+	DnsHeader *hdr=(DnsHeader*)p;
+	DnsHeader *rhdr=(DnsHeader*)&reply[0];
+	p+=sizeof(DnsHeader);
+	os_printf("DNS packet: id 0x%X flags 0x%X rcode 0x%X qcnt %d ancnt %d nscount %d arcount %d len %d\n", 
+		ntohs(&hdr->id), hdr->flags, hdr->rcode, ntohs(&hdr->qdcount), ntohs(&hdr->ancount), ntohs(&hdr->nscount), ntohs(&hdr->arcount), length);
+	//Some sanity checks:
+	if (length>512) return; 									//Packet is longer than DNS implementation allows
+	if (length<sizeof(DnsHeader)) return; 						//Packet is too short
+	if (hdr->ancount || hdr->nscount || hdr->arcount) return;	//this is a reply, don't know what to do with it
+	if (hdr->flags&FLAG_TC) return;								//truncated, can't use this
+	//Reply is basically the request plus the needed data
+	os_memcpy(reply, pusrdata, length);
+	rhdr->flags|=FLAG_QR;
+	for (i=0; i<ntohs(&hdr->qdcount); i++) {
+		//Grab the labels in the q string
+		p=labelToStr(pusrdata, p, length, buff, sizeof(buff));
+		if (p==NULL) return;
+		DnsQuestionFooter *qf=(DnsQuestionFooter*)p;
+		p+=sizeof(DnsQuestionFooter);
+		os_printf("Q %d (type 0x%X class 0x%X) for %s\n", i, ntohs(&qf->type), ntohs(&qf->class), buff);
+		if (ntohs(&qf->type)==QTYPE_A) {
+			//They want to know the IPv4 address of something.
+			//Build the response.
+			rend=strToLabel(buff, rend, sizeof(reply)-(rend-reply)); //Add the label
+			if (rend==NULL) return;
+			DnsResourceFooter *rf=(DnsResourceFooter *)rend;
+			rend+=sizeof(DnsResourceFooter);
+			setn16(&rf->type, QTYPE_A);
+			setn16(&rf->class, QCLASS_IN);
+			setn32(&rf->ttl, 1);
+			setn16(&rf->rdlength, 4); //IPv4 addr is 4 bytes;
+			*rend++=192; //hardcoded :X
+			*rend++=168;
+			*rend++=4;
+			*rend++=1;
+			setn16(&rhdr->ancount, ntohs(&rhdr->ancount)+1);
+			os_printf("Added A rec to resp. Resp len is %d\n", (rend-reply));
+		} else if (ntohs(&qf->type)==QTYPE_NS) {
+			//Give ns server. Basically is whatever.
+			rend=strToLabel(buff, rend, sizeof(reply)-(rend-reply)); //Add the label
+			DnsResourceFooter *rf=(DnsResourceFooter *)rend;
+			rend+=sizeof(DnsResourceFooter);
+			setn16(&rf->type, QTYPE_NS);
+			setn16(&rf->class, QCLASS_IN);
+			setn16(&rf->ttl, 1);
+			setn16(&rf->rdlength, 4);
+			*rend++=2;
+			*rend++='n';
+			*rend++='s';
+			*rend++=0;
+			setn16(&rhdr->ancount, ntohs(&rhdr->ancount)+1);
+			os_printf("Added NS rec to resp. Resp len is %d\n", (rend-reply));
+		}
+	}
+	espconn_sent(conn, (uint8*)reply, rend-reply);
+}
+
+void ICACHE_FLASH_ATTR captdnsInit(void) {
+	static struct espconn conn;
+	static esp_udp udpconn;
+	conn.type=ESPCONN_UDP;
+	conn.proto.udp=&udpconn;
+	conn.proto.udp->local_port = 53;
+	espconn_regist_recvcb(&conn, captdnsRecv);
+	espconn_create(&conn);
+}

user/captdns.h

@@ -0,0 +1,5 @@
+#ifndef CAPTDNS_H
+#define CAPTDNS_H
+void ICACHE_FLASH_ATTR captdnsInit(void);
+
+#endif

user/user_main.c

@@ -20,6 +20,7 @@
 #include "stdout.h"
 #include "auth.h"
 #include "espfs.h"
+#include "captdns.h"
 
 //#define SHOW_HEAP_USE
 
@@ -89,6 +90,8 @@ static void ICACHE_FLASH_ATTR prHeapTimerCb(void *arg) {
 void user_init(void) {
 	stdoutInit();
 	ioInit();
+	captdnsInit();
+
 
 	// 0x40200000 is the base address for spi flash memory mapping, ESPFS_POS is the position
 	// where image is written in flash that is defined in Makefile.