DNS resolver for captive portal
This commit is contained in:
Jeroen Domburg
+207
@@ -0,0 +1,207 @@
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#include <esp8266.h>
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typedef struct __attribute__ ((packed)) {
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uint16_t id;
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uint8_t flags;
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uint8_t rcode;
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uint16_t qdcount;
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uint16_t ancount;
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uint16_t nscount;
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uint16_t arcount;
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} DnsHeader;
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typedef struct __attribute__ ((packed)) {
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uint8_t len;
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uint8_t data;
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} DnsLabel;
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typedef struct __attribute__ ((packed)) {
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//before: label
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uint16_t type;
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uint16_t class;
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} DnsQuestionFooter;
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typedef struct __attribute__ ((packed)) {
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//before: label
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uint16_t type;
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uint16_t class;
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uint32_t ttl;
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uint16_t rdlength;
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//after: rdata
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} DnsResourceFooter;
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#define FLAG_QR (1<<7)
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#define FLAG_AA (1<<2)
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#define FLAG_TC (1<<1)
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#define FLAG_RD (1<<0)
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#define QTYPE_A 1
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#define QTYPE_NS 2
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#define QTYPE_CNAME 5
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#define QTYPE_SOA 6
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#define QTYPE_WKS 11
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#define QTYPE_PTR 12
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#define QTYPE_HINFO 13
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#define QTYPE_MINFO 14
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#define QTYPE_MX 15
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#define QTYPE_TXT 16
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#define QCLASS_IN 1
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#define QCLASS_ANY 255
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//Function to put unaligned 16-bit network values
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static void ICACHE_FLASH_ATTR setn16(void *pp, int16_t n) {
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char *p=pp;
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*p++=(n>>8);
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*p++=(n&0xff);
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}
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//Function to put unaligned 32-bit network values
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static void ICACHE_FLASH_ATTR setn32(void *pp, int32_t n) {
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char *p=pp;
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*p++=(n>>24)&0xff;
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*p++=(n>>16)&0xff;
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*p++=(n>>8)&0xff;
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*p++=(n&0xff);
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}
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static uint16_t ntohs(uint16_t *in) {
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char *p=(char*)in;
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return ((p[0]<<8)&0xff00)|(p[1]&0xff);
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}
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//Parses a label.
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//Returns pointer to start of next fields in packet
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static char* ICACHE_FLASH_ATTR labelToStr(char *packet, char *labelPtr, int packetSz, char *res, int resMaxLen) {
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int i, j, k;
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char *endPtr=NULL;
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i=0;
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do {
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if ((*labelPtr&0xC0)==0) {
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j=*labelPtr++; //skip past length
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//Add separator period if there already is data in res
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if (i<resMaxLen && i!=0) res[i++]='.';
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//Copy label to res
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for (k=0; k<j; k++) {
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if ((labelPtr-packet)>packetSz) return NULL;
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if (i<resMaxLen) res[i++]=*labelPtr++;
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}
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} else if ((*labelPtr&0xC0)==0xC0) {
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//Compressed label pointer
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endPtr=labelPtr+2;
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int offset=ntohs(((uint16_t *)labelPtr))&0x3FFF;
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//Check if offset points to somewhere outside of the packet
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if (offset>packetSz) return NULL;
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labelPtr=&packet[offset];
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}
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//check for out-of-bound-ness
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if ((labelPtr-packet)>packetSz) return NULL;
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} while (*labelPtr!=0);
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res[i]=0; //zero-terminate
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if (endPtr==NULL) endPtr=labelPtr+1;
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return endPtr;
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}
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char *strToLabel(char *str, char *label, int maxLen) {
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char *len=label; //ptr to len byte
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char *p=label+1; //ptr to next label byte to be written
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while (1) {
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if (*str=='.' || *str==0) {
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*len=((p-len)-1); //write len of label bit
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len=p; //pos of len for next part
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p++; //data ptr is one past len
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if (*str==0) break; //done
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str++;
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} else {
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*p++=*str++; //copy byte
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// if ((p-label)>maxLen) return NULL; //check out of bounds
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}
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}
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*len=0;
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return p; //ptr to first free byte in resp
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}
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static void ICACHE_FLASH_ATTR captdnsRecv(void* arg, char *pusrdata, unsigned short length) {
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struct espconn *conn=(struct espconn *)arg;
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char buff[512];
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char reply[512];
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int i;
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char *rend=&reply[length];
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char *p=pusrdata;
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DnsHeader *hdr=(DnsHeader*)p;
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DnsHeader *rhdr=(DnsHeader*)&reply[0];
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p+=sizeof(DnsHeader);
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os_printf("DNS packet: id 0x%X flags 0x%X rcode 0x%X qcnt %d ancnt %d nscount %d arcount %d len %d\n",
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ntohs(&hdr->id), hdr->flags, hdr->rcode, ntohs(&hdr->qdcount), ntohs(&hdr->ancount), ntohs(&hdr->nscount), ntohs(&hdr->arcount), length);
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//Some sanity checks:
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if (length>512) return; //Packet is longer than DNS implementation allows
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if (length<sizeof(DnsHeader)) return; //Packet is too short
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if (hdr->ancount || hdr->nscount || hdr->arcount) return; //this is a reply, don't know what to do with it
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if (hdr->flags&FLAG_TC) return; //truncated, can't use this
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//Reply is basically the request plus the needed data
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os_memcpy(reply, pusrdata, length);
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rhdr->flags|=FLAG_QR;
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for (i=0; i<ntohs(&hdr->qdcount); i++) {
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//Grab the labels in the q string
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p=labelToStr(pusrdata, p, length, buff, sizeof(buff));
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if (p==NULL) return;
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DnsQuestionFooter *qf=(DnsQuestionFooter*)p;
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p+=sizeof(DnsQuestionFooter);
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os_printf("Q %d (type 0x%X class 0x%X) for %s\n", i, ntohs(&qf->type), ntohs(&qf->class), buff);
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if (ntohs(&qf->type)==QTYPE_A) {
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//They want to know the IPv4 address of something.
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//Build the response.
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rend=strToLabel(buff, rend, sizeof(reply)-(rend-reply)); //Add the label
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if (rend==NULL) return;
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DnsResourceFooter *rf=(DnsResourceFooter *)rend;
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rend+=sizeof(DnsResourceFooter);
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setn16(&rf->type, QTYPE_A);
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setn16(&rf->class, QCLASS_IN);
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setn32(&rf->ttl, 1);
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setn16(&rf->rdlength, 4); //IPv4 addr is 4 bytes;
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*rend++=192; //hardcoded :X
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*rend++=168;
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*rend++=4;
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*rend++=1;
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setn16(&rhdr->ancount, ntohs(&rhdr->ancount)+1);
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os_printf("Added A rec to resp. Resp len is %d\n", (rend-reply));
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} else if (ntohs(&qf->type)==QTYPE_NS) {
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//Give ns server. Basically is whatever.
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rend=strToLabel(buff, rend, sizeof(reply)-(rend-reply)); //Add the label
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DnsResourceFooter *rf=(DnsResourceFooter *)rend;
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rend+=sizeof(DnsResourceFooter);
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setn16(&rf->type, QTYPE_NS);
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setn16(&rf->class, QCLASS_IN);
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setn16(&rf->ttl, 1);
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setn16(&rf->rdlength, 4);
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*rend++=2;
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*rend++='n';
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*rend++='s';
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*rend++=0;
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setn16(&rhdr->ancount, ntohs(&rhdr->ancount)+1);
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os_printf("Added NS rec to resp. Resp len is %d\n", (rend-reply));
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}
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}
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espconn_sent(conn, (uint8*)reply, rend-reply);
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}
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void ICACHE_FLASH_ATTR captdnsInit(void) {
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static struct espconn conn;
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static esp_udp udpconn;
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conn.type=ESPCONN_UDP;
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conn.proto.udp=&udpconn;
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conn.proto.udp->local_port = 53;
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espconn_regist_recvcb(&conn, captdnsRecv);
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espconn_create(&conn);
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}
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@@ -0,0 +1,5 @@
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#ifndef CAPTDNS_H
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#define CAPTDNS_H
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void ICACHE_FLASH_ATTR captdnsInit(void);
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#endif
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@@ -20,6 +20,7 @@
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#include "stdout.h"
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#include "auth.h"
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#include "espfs.h"
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#include "captdns.h"
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//#define SHOW_HEAP_USE
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@@ -89,6 +90,8 @@ static void ICACHE_FLASH_ATTR prHeapTimerCb(void *arg) {
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void user_init(void) {
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stdoutInit();
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ioInit();
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captdnsInit();
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// 0x40200000 is the base address for spi flash memory mapping, ESPFS_POS is the position
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// where image is written in flash that is defined in Makefile.
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