text: /*
   * tinysvcmdns - a tiny MDNS implementation for publishing services
   * Copyright (C) 2011 Darell Tan
   * All rights reserved.
   * 
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in the
   *    documentation and/or other materials provided with the distribution.
   * 3. The name of the author may not be used to endorse or promote products
   *    derived from this software without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
   * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
   * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
   * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  
  #include "mdns.h"
  #include <stdint.h>
  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <assert.h>
  
  #ifdef _WIN32
  #include <winsock.h>
  #include <in6addr.h>
  #else
  #include <netinet/in.h>
  #endif
  
  
  struct name_comp {
  	uint8_t *label;	// label
  	size_t pos;		// position in msg
  
  	struct name_comp *next;
  };
  
  // ----- label functions -----
  
  // duplicates a name
  //inline uint8_t *dup_nlabel(const uint8_t *n) {
  uint8_t *dup_nlabel(const uint8_t *n) {
  	assert(n[0] <= 63);	// prevent mis-use
  	return (uint8_t *) strdup((char *) n);
  }
  
  // duplicates a label
  uint8_t *dup_label(const uint8_t *label) {
  	int len = *label + 1;
  	uint8_t *newlabel;
  	if (len > 63)
  		return NULL;
  	newlabel = malloc(len + 1);
  	strncpy((char *) newlabel, (char *) label, len);
  	newlabel[len] = '\0';
  	return newlabel;
  }
  
  uint8_t *join_nlabel(const uint8_t *n1, const uint8_t *n2) {
  	int len1, len2;
  	uint8_t *s;
  
  	assert(n1[0] <= 63 && n2[0] <= 63);	// detect misuse
  
  	len1 = strlen((char *) n1);
  	len2 = strlen((char *) n2);
  
  	s = malloc(len1 + len2 + 1);
  	strncpy((char *) s, (char *) n1, len1);
  	strncpy((char *) s+len1, (char *) n2, len2);
  	s[len1 + len2] = '\0';
  	return s;
  }
  
  // returns a human-readable name label in dotted form
  char *nlabel_to_str(const uint8_t *name) {
  	char *label, *labelp;
  	const uint8_t *p;
  
  	assert(name != NULL);
  
  	label = labelp = malloc(256);
  
  	for (p = name; *p; p++) {
  		strncpy(labelp, (char *) p + 1, *p);
  		labelp += *p;
  		*labelp = '.';
  		labelp++;
  
  		p += *p;
  	}
  
  	*labelp = '\0';
  
  	return label;
  }
  
  // returns the length of a label field
  // does NOT uncompress the field, so it could be as small as 2 bytes
  // or 1 for the root
  static size_t label_len(uint8_t *pkt_buf, size_t pkt_len, size_t off) {
  	uint8_t *p;
  	uint8_t *e = pkt_buf + pkt_len;
  	size_t len = 0;
  
  	for (p = pkt_buf + off; p < e; p++) {
  		if (*p == 0) {
  			return len + 1;
  		} else if ((*p & 0xC0) == 0xC0) {
  			return len + 2;
  		} else {
  			len += *p + 1;
  			p += *p;
  		}
  	}
  
  	return len;
  }
  
  // creates a label
  // free() after use
  uint8_t *create_label(const char *txt) {
  	int len;
  	uint8_t *s;
  
  	assert(txt != NULL);
  	len = strlen(txt);
  	if (len > 63)
  		return NULL;
  
  	s = malloc(len + 2);
  	s[0] = len;
  	strncpy((char *) s + 1, txt, len);
  	s[len + 1] = '\0';
  
  	return s;
  }
  
  // creates a uncompressed name label given a DNS name like "apple.b.com"
  // free() after use
  uint8_t *create_nlabel(const char *name) {
  	char *label;
  	char *p, *e, *lenpos;
  	int len = 0;
  
  	assert(name != NULL);
  
  	len = strlen(name);
  	label = malloc(len + 1 + 1);
  	if (label == NULL)
  		return NULL;
  
  	strncpy((char *) label + 1, name, len);
  	label[len + 1] = '\0';
  
  	p = label;
  	e = p + len;
  	lenpos = p;
  
  	while (p < e) {
  		char *dot = memchr(p + 1, '.', e - p - 1);
  		*lenpos = 0;
  
  		if (dot == NULL)
  			dot = e + 1;
  		*lenpos = dot - p - 1;
  
  		p = dot;
  		lenpos = dot;
  	}
  
  	return (uint8_t *) label;
  }
  
  // copies a label from the buffer into a newly-allocated string
  // free() after use
  static uint8_t *copy_label(uint8_t *pkt_buf, size_t pkt_len, size_t off) {
  	int len;
  
  	if (off > pkt_len)
  		return NULL;
  	
  	len = pkt_buf[off] + 1;
  	if (off + len > pkt_len) {
  		DEBUG_PRINTF("label length exceeds packet buffer\n");
  		return NULL;
  	}
  
  	return dup_label(pkt_buf + off);
  }
  
  // uncompresses a name
  // free() after use
  static uint8_t *uncompress_nlabel(uint8_t *pkt_buf, size_t pkt_len, size_t off) {
  	uint8_t *p;
  	uint8_t *e = pkt_buf + pkt_len;
  	size_t len = 0;
  	char *str, *sp;
  	if (off >= pkt_len)
  		return NULL;
  
  	// calculate length of uncompressed label
  	for (p = pkt_buf + off; *p && p < e; p++) {
  		size_t llen = 0;
  		if ((*p & 0xC0) == 0xC0) {
  			uint8_t *p2 = pkt_buf + (((p[0] & ~0xC0) << 8) | p[1]);
  			llen = *p2 + 1;
  			p = p2 + llen - 1;
  		} else {
  			llen = *p + 1;
  			p += llen - 1;
  		}
  		len += llen;
  	}
  
  	str = sp = malloc(len + 1);
  	if (str == NULL)
  		return NULL;
  
  	// FIXME: must merge this with above code
  	for (p = pkt_buf + off; *p && p < e; p++) {
  		size_t llen = 0;
  		if ((*p & 0xC0) == 0xC0) {
  			uint8_t *p2 = pkt_buf + (((p[0] & ~0xC0) << 8) | p[1]);
  			llen = *p2 + 1;
  			strncpy(sp, (char *) p2, llen);
  			p = p2 + llen - 1;
  		} else {
  			llen = *p + 1;
  			strncpy(sp, (char *) p, llen);
  			p += llen - 1;
  		}
  		sp += llen;
  	}
  	*sp = '\0';
  
  	return (uint8_t *) str;
  }
  
  // ----- RR list & group functions -----
  
  const char *rr_get_type_name(enum rr_type type) {
  	switch (type) {
  		case RR_A:		return "A";
  		case RR_PTR:	return "PTR";
  		case RR_TXT:	return "TXT";
  		case RR_AAAA:	return "AAAA";
  		case RR_SRV:	return "SRV";
  		case RR_NSEC:	return "NSEC";
  		case RR_ANY:	return "ANY";
  	}
  	return NULL;
  }
  
  void rr_entry_destroy(struct rr_entry *rr) {
  	struct rr_data_txt *txt_rec;
  	assert(rr);
  
  	// check rr_type and free data elements
  	switch (rr->type) {
  		case RR_PTR:
  			if (rr->data.PTR.name)
  				free(rr->data.PTR.name);
  			// don't free entry
  			break;
  
  		case RR_TXT:
  			txt_rec = &rr->data.TXT;
  			while (txt_rec) {
  				struct rr_data_txt *next = txt_rec->next;
  				if (txt_rec->txt)
  					free(txt_rec->txt);
  
  				// only free() if it wasn't part of the struct
  				if (txt_rec != &rr->data.TXT)
  					free(txt_rec);
  
  				txt_rec = next;
  			}
  			break;
  
  		case RR_SRV:
  			if (rr->data.SRV.target)
  				free(rr->data.SRV.target);
  			break;
  
  		case RR_AAAA:
  			if (rr->data.AAAA.addr)
  				free(rr->data.AAAA.addr);
          	break;
  
  		default:
  			// nothing to free
  			break;
  	}
  
  	free(rr->name);
  	free(rr);
  }
  
  // destroys an RR list (and optionally, items)
  void rr_list_destroy(struct rr_list *rr, char destroy_items) {
  	struct rr_list *rr_next;
  
  	for (; rr; rr = rr_next) {
  		rr_next = rr->next;
  		if (destroy_items)
  			rr_entry_destroy(rr->e);
  		free(rr);
  	}
  }
  
  int rr_list_count(struct rr_list *rr) {
  	int i = 0;
  	for (; rr; i++, rr = rr->next);
  	return i;
  }
  
  struct rr_entry *rr_entry_remove(struct rr_group *group, struct rr_entry *entry, enum rr_type type) {
  	struct rr_group *g;
  
  	for (g = group; g; g = g->next) {
  		struct rr_list *lrr = g->rr, *prr= NULL;
  		for (; lrr; lrr = lrr->next) {
  			if (lrr->e->type == type) {
  				switch (type) {
  				case RR_PTR:
  					if (lrr->e->data.PTR.entry == entry) {
  						struct rr_entry *e = lrr->e;
  						if (prr == NULL) {
  							g->rr = lrr->next;
  						} else {
  							prr->next = lrr->next;
  						}
  						free(lrr);
  						return e;
  					}
  					break;
  				default:
  					break;
  				}
  			}
  			prr = lrr;
  		}
  	}
  
  	return NULL;
  }
  
  struct rr_entry *rr_list_remove(struct rr_list **rr_head, struct rr_entry *rr) {
  	struct rr_list *le = *rr_head, *pe = NULL;
  	for (; le; le = le->next) {
  		if (le->e == rr) {
  			if (pe == NULL) {
  				*rr_head = le->next;
  				free(le);
  				return rr;
  			} else {
  				pe->next = le->next;
  				free(le);
  				return rr;
  			}
  		}
  		pe = le;
  	}
  	return NULL;
  }
  
  void rr_group_clean(struct rr_group **head) {
  	struct rr_group *le = *head, *pe = NULL;
  
  	while (le) {
  		if (le->rr == NULL) {
  			free(le->name);
  			if (pe == NULL) {
  				*head = le->next;
  				 free(le);
  				 le = *head;
  			 } else {
  				pe->next = le->next;
  				free(le);
  				le = pe->next;
  			 }
  		} else {
  			pe = le;
  			le = le->next;
          }
  	}
  }
  
  // appends an rr_entry to an RR list
  // if the RR is already in the list, it will not be added
  // RRs are compared by memory location - not its contents
  // return value of 0 means item not added
  int rr_list_append(struct rr_list **rr_head, struct rr_entry *rr) {
  	struct rr_list *node = malloc(sizeof(struct rr_list));
  	node->e = rr;
  	node->next = NULL;
  
  	if (*rr_head == NULL) {
  		*rr_head = node;
  	} else {
  		struct rr_list *e = *rr_head, *taile;
  		for (; e; e = e->next) {
  			// already in list - don't add
  			if (e->e == rr) {
  				free(node);
  				return 0;
  			}
  			if (e->next == NULL)
  				taile = e;
  		}
  		taile->next = node;
  	}
  	return 1;
  }
  
  #define FILL_RR_ENTRY(rr, _name, _type)	\
  	rr->name = _name;			\
  	rr->type = _type;			\
  	rr->ttl  = DEFAULT_TTL;		\
  	rr->cache_flush = 1;		\
  	rr->rr_class  = 1;
  
  struct rr_entry *rr_create_a(uint8_t *name, struct in_addr addr) {
  	DECL_MALLOC_ZERO_STRUCT(rr, rr_entry);
  	FILL_RR_ENTRY(rr, name, RR_A);
  	rr->data.A.addr = addr.s_addr;
  	rr->ttl = DEFAULT_TTL_FOR_RECORD_WITH_HOSTNAME; // 120 seconds -- see RFC 6762 Section 10
  	return rr;
  }
  
  struct rr_entry *rr_create_aaaa(uint8_t *name, struct in6_addr *addr) {
  	DECL_MALLOC_ZERO_STRUCT(rr, rr_entry);
  	FILL_RR_ENTRY(rr, name, RR_AAAA);
  	rr->data.AAAA.addr = addr;
  	rr->ttl = DEFAULT_TTL_FOR_RECORD_WITH_HOSTNAME; // 120 seconds -- see RFC 6762 Section 10
  	return rr;
  }
  
  struct rr_entry *rr_create_srv(uint8_t *name, uint16_t port, uint8_t *target) {
  	DECL_MALLOC_ZERO_STRUCT(rr, rr_entry);
  	FILL_RR_ENTRY(rr, name, RR_SRV);
  	rr->data.SRV.port = port;
  	rr->data.SRV.target = target;
  	rr->ttl = DEFAULT_TTL_FOR_RECORD_WITH_HOSTNAME; // 120 seconds -- see RFC 6762 Section 10
  	return rr;
  }
  
  struct rr_entry *rr_create_ptr(uint8_t *name, struct rr_entry *d_rr) {
  	DECL_MALLOC_ZERO_STRUCT(rr, rr_entry);
  	FILL_RR_ENTRY(rr, name, RR_PTR);
  	rr->cache_flush = 0;	// PTRs shouldn't have their cache flush bit set
  	rr->data.PTR.entry = d_rr;
  	rr->ttl = DEFAULT_TTL_FOR_RECORD_WITH_HOSTNAME; // 120 seconds -- see RFC 6762 Section 10
  	return rr;
  }
  
  struct rr_entry *rr_create(uint8_t *name, enum rr_type type) {
  	DECL_MALLOC_ZERO_STRUCT(rr, rr_entry);
  	FILL_RR_ENTRY(rr, name, type);
  	return rr;
  }
  
  void rr_set_nsec(struct rr_entry *rr_nsec, enum rr_type type) {
  	assert(rr_nsec->type == RR_NSEC);
  	assert((type / 8) < sizeof(rr_nsec->data.NSEC.bitmap));
  
  	rr_nsec->data.NSEC.bitmap[ type / 8 ] = 1 << (7 - (type % 8));
  }
  
  void rr_add_txt(struct rr_entry *rr_txt, const char *txt) {
  	struct rr_data_txt *txt_rec;
  	assert(rr_txt->type == RR_TXT);
  
  	txt_rec = &rr_txt->data.TXT;
  
  	// is current data filled?
  	if (txt_rec->txt == NULL) {
  		txt_rec->txt = create_label(txt);
  		return;
  	}
  
  	// find the last node
  	for (; txt_rec->next; txt_rec = txt_rec->next);
  
  	// create a new empty node
  	txt_rec->next = malloc(sizeof(struct rr_data_txt));
  
  	txt_rec = txt_rec->next;
  	txt_rec->txt = create_label(txt);
  	txt_rec->next = NULL;
  }
  
  // adds a record to an rr_group
  void rr_group_add(struct rr_group **group, struct rr_entry *rr) {
  	struct rr_group *g;
  
  	assert(rr != NULL);
  
  	if (*group) {
  		g = rr_group_find(*group, rr->name);
  		if (g) {
  			rr_list_append(&g->rr, rr);
  			return;
  		}
  	}
  
  	MALLOC_ZERO_STRUCT(g, rr_group);
  	g->name = dup_nlabel(rr->name);
  	rr_list_append(&g->rr, rr);
  
  	// prepend to list
  	g->next = *group;
  	*group = g;
  }
  
  // finds a rr_group matching the given name
  struct rr_group *rr_group_find(struct rr_group* g, uint8_t *name) {
  	for (; g; g = g->next) {
  		if (cmp_nlabel(g->name, name) == 0)
  			return g;
  	}
  	return NULL;
  }
  
  struct rr_entry *rr_entry_find(struct rr_list *rr_list, uint8_t *name, uint16_t type) {
  	struct rr_list *rr = rr_list;
  	for (; rr; rr = rr->next) {
  		if (rr->e->type == type && cmp_nlabel(rr->e->name, name) == 0) 
  			return rr->e;
  	}
  	return NULL;
  }
  
  // looks for a matching entry in rr_list
  // if entry is a PTR, we need to check if the PTR target also matches
  struct rr_entry *rr_entry_match(struct rr_list *rr_list, struct rr_entry *entry) {
  	struct rr_list *rr = rr_list;
  	for (; rr; rr = rr->next) {
  		if (rr->e->type == entry->type && cmp_nlabel(rr->e->name, entry->name) == 0) {
  			if (entry->type != RR_PTR) {
  				return rr->e;
  			} else if (cmp_nlabel(MDNS_RR_GET_PTR_NAME(entry), MDNS_RR_GET_PTR_NAME(rr->e)) == 0) {
  				// if it's a PTR, we need to make sure PTR target also matches
  				return rr->e;
  			}
  		}
  	}
  	return NULL;
  }
  
  void rr_group_destroy(struct rr_group *group) {
  	struct rr_group *g = group;
  
  	while (g) {
  		struct rr_group *nextg = g->next;
  		free(g->name);
  		rr_list_destroy(g->rr, 1);
  		free(g);
  		g = nextg;
  	}
  }
  
  uint8_t *mdns_write_u16(uint8_t *ptr, const uint16_t v) {
  	*ptr++ = (uint8_t) (v >> 8) & 0xFF;
  	*ptr++ = (uint8_t) (v >> 0) & 0xFF;
  	return ptr;
  }
  
  uint8_t *mdns_write_u32(uint8_t *ptr, const uint32_t v) {
  	*ptr++ = (uint8_t) (v >> 24) & 0xFF;
  	*ptr++ = (uint8_t) (v >> 16) & 0xFF;
  	*ptr++ = (uint8_t) (v >>  8) & 0xFF;
  	*ptr++ = (uint8_t) (v >>  0) & 0xFF;
  	return ptr;
  }
  
  uint16_t mdns_read_u16(const uint8_t *ptr) {
  	return  ((ptr[0] & 0xFF) << 8) | 
  			((ptr[1] & 0xFF) << 0);
  }
  
  uint32_t mdns_read_u32(const uint8_t *ptr) {
  	return  ((ptr[0] & 0xFF) << 24) | 
  			((ptr[1] & 0xFF) << 16) | 
  			((ptr[2] & 0xFF) <<  8) | 
  			((ptr[3] & 0xFF) <<  0);
  }
  
  // initialize the packet for reply
  // clears the packet of list structures but not its list items
  void mdns_init_reply(struct mdns_pkt *pkt, uint16_t id) {
  	// copy transaction ID
  	pkt->id = id;
  
  	// response flags
  	pkt->flags = MDNS_FLAG_RESP | MDNS_FLAG_AA;
  
  	rr_list_destroy(pkt->rr_qn,   0);
  	rr_list_destroy(pkt->rr_ans,  0);
  	rr_list_destroy(pkt->rr_auth, 0);
  	rr_list_destroy(pkt->rr_add,  0);
  
  	pkt->rr_qn    = NULL;
  	pkt->rr_ans   = NULL;
  	pkt->rr_auth  = NULL;
  	pkt->rr_add   = NULL;
  
  	pkt->num_qn = 0;
  	pkt->num_ans_rr = 0;
  	pkt->num_auth_rr = 0;
  	pkt->num_add_rr = 0;
  }
  
  // destroys an mdns_pkt struct, including its contents
  void mdns_pkt_destroy(struct mdns_pkt *p) {
  	rr_list_destroy(p->rr_qn, 1);
  	rr_list_destroy(p->rr_ans, 1);
  	rr_list_destroy(p->rr_auth, 1);
  	rr_list_destroy(p->rr_add, 1);
  
  	free(p);
  }
  
  
  // parse the MDNS questions section
  // stores the parsed data in the given mdns_pkt struct
  static size_t mdns_parse_qn(uint8_t *pkt_buf, size_t pkt_len, size_t off, 
  		struct mdns_pkt *pkt) {
  	const uint8_t *p = pkt_buf + off;
  	struct rr_entry *rr;
  	uint8_t *name;
     
  	assert(pkt != NULL);
  
  	rr = malloc(sizeof(struct rr_entry)); 
  	memset(rr, 0, sizeof(struct rr_entry));
  
  	name = uncompress_nlabel(pkt_buf, pkt_len, off);
  	p += label_len(pkt_buf, pkt_len, off);
  	rr->name = name;
  
  	rr->type = mdns_read_u16(p);
  	p += sizeof(uint16_t);
  
  	rr->unicast_query = (*p & 0x80) == 0x80;
  	rr->rr_class = mdns_read_u16(p) & ~0x80;
  	p += sizeof(uint16_t);
  
  	rr_list_append(&pkt->rr_qn, rr);
  	
  	return p - (pkt_buf + off);
  }
  
  // parse the MDNS RR section
  // stores the parsed data in the given mdns_pkt struct
  static size_t mdns_parse_rr(uint8_t *pkt_buf, size_t pkt_len, size_t off, 
  		struct mdns_pkt *pkt) {
  	const uint8_t *p = pkt_buf + off;
  	const uint8_t *e = pkt_buf + pkt_len;
  	struct rr_entry *rr;
  	uint8_t *name;
  	size_t rr_data_len = 0;
  	struct rr_data_txt *txt_rec;
  	int parse_error = 0;
  
  	assert(pkt != NULL);
  
  	if (off > pkt_len)
  		return 0;
  
  	rr = malloc(sizeof(struct rr_entry)); 
  	memset(rr, 0, sizeof(struct rr_entry));
  
  	name = uncompress_nlabel(pkt_buf, pkt_len, off);
  	p += label_len(pkt_buf, pkt_len, off);
  	rr->name = name;
  
  	rr->type = mdns_read_u16(p);
  	p += sizeof(uint16_t);
  
  	rr->cache_flush = (*p & 0x80) == 0x80;
  	rr->rr_class = mdns_read_u16(p) & ~0x80;
  	p += sizeof(uint16_t);
  
  	rr->ttl = mdns_read_u32(p);
  	p += sizeof(uint32_t);
  
  	// RR data
  	rr_data_len = mdns_read_u16(p);
  	p += sizeof(uint16_t);
  
  	if (p + rr_data_len > e) {
  		DEBUG_PRINTF("rr_data_len goes beyond packet buffer: %zu > %zu\n", rr_data_len, e - p);
  		rr_entry_destroy(rr);
  		return 0;
  	}
  
  	e = p + rr_data_len;
  
  	// see if we can parse the RR data
  	switch (rr->type) {
  		case RR_A:
  			if (rr_data_len < sizeof(uint32_t)) {
  				DEBUG_PRINTF("invalid rr_data_len=%zu for A record\n", rr_data_len);
  				parse_error = 1;
  				break;
  			}
  			rr->data.A.addr = ntohl(mdns_read_u32(p)); /* addr already in net order */
  			p += sizeof(uint32_t);
  			break;
  
  		case RR_AAAA: {
  			int i;
  			if (rr_data_len < sizeof(struct in6_addr)) {
  				DEBUG_PRINTF("invalid rr_data_len=%zu for AAAA record\n", rr_data_len);
  				parse_error = 1;
  				break;
  			}
  			rr->data.AAAA.addr = malloc(sizeof(struct in6_addr));
  			for (i = 0; i < sizeof(struct in6_addr); i++)
  				rr->data.AAAA.addr->s6_addr[i] = p[i];
  			p += sizeof(struct in6_addr);
  			break;
  			}
  
  
  		case RR_PTR:
  			rr->data.PTR.name = uncompress_nlabel(pkt_buf, pkt_len, p - pkt_buf);
  			if (rr->data.PTR.name == NULL) {
  				DEBUG_PRINTF("unable to parse/uncompress label for PTR name\n");
  				parse_error = 1;
  				break;
  			}
  			p += rr_data_len;
  			break;
  
  		case RR_TXT:
  			txt_rec = &rr->data.TXT;
  
  			// not supposed to happen, but we should handle it
  			if (rr_data_len == 0) {
  				DEBUG_PRINTF("WARN: rr_data_len for TXT is 0\n");
  				txt_rec->txt = create_label("");
  				break;
  			}
  
  			while (1) {
  				txt_rec->txt = copy_label(pkt_buf, pkt_len, p - pkt_buf);
  				if (txt_rec->txt == NULL) {
  					DEBUG_PRINTF("unable to copy label for TXT record\n");
  					parse_error = 1;
  					break;
  				}
  				p += txt_rec->txt[0] + 1;
  
  				if (p >= e)
  					break;
  
  				// allocate another record
  				txt_rec->next = malloc(sizeof(struct rr_data_txt));
  				txt_rec = txt_rec->next;
  				txt_rec->next = NULL;
  			}
  			break;
  
  		default:
  			// skip to end of RR data
  			p = e;
  	}
  
  	// if there was a parse error, destroy partial rr_entry
  	if (parse_error) {
  		rr_entry_destroy(rr);
  		return 0;
  	}
  
  	rr_list_append(&pkt->rr_ans, rr);
  	
  	return p - (pkt_buf + off);
  }
  
  // parse a MDNS packet into an mdns_pkt struct
  struct mdns_pkt *mdns_parse_pkt(uint8_t *pkt_buf, size_t pkt_len) {
  	uint8_t *p = pkt_buf;
  	size_t off;
  	struct mdns_pkt *pkt;
  	int i;
  
  	if (pkt_len < 12) 
  		return NULL;
  
  	MALLOC_ZERO_STRUCT(pkt, mdns_pkt);
  
  	// parse header
  	pkt->id 			= mdns_read_u16(p); p += sizeof(uint16_t);
  	pkt->flags 			= mdns_read_u16(p); p += sizeof(uint16_t);
  	pkt->num_qn 		= mdns_read_u16(p); p += sizeof(uint16_t);
  	pkt->num_ans_rr 	= mdns_read_u16(p); p += sizeof(uint16_t);
  	pkt->num_auth_rr 	= mdns_read_u16(p); p += sizeof(uint16_t);
  	pkt->num_add_rr 	= mdns_read_u16(p); p += sizeof(uint16_t);
  
  	off = p - pkt_buf;
  
  	// parse questions
  	for (i = 0; i < pkt->num_qn; i++) {
  		size_t l = mdns_parse_qn(pkt_buf, pkt_len, off, pkt);
  		if (! l) {
  			DEBUG_PRINTF("error parsing question #%d\n", i);
  			mdns_pkt_destroy(pkt);
  			return NULL;
  		}
  
  		off += l;
  	}
  
  	// parse answer RRs
  	for (i = 0; i < pkt->num_ans_rr; i++) {
  		size_t l = mdns_parse_rr(pkt_buf, pkt_len, off, pkt);
  		if (! l) {
  			DEBUG_PRINTF("error parsing answer #%d\n", i);
  			mdns_pkt_destroy(pkt);
  			return NULL;
  		}
  
  		off += l;
  	}
  
  	// TODO: parse the authority and additional RR sections
  
  	return pkt;
  }
  
  // encodes a name (label) into a packet using the name compression scheme
  // encoded names will be added to the compression list for subsequent use
  static size_t mdns_encode_name(uint8_t *pkt_buf, size_t pkt_len, size_t off,
  		const uint8_t *name, struct name_comp *comp) {
  	struct name_comp *c, *c_tail = NULL;
  	uint8_t *p = pkt_buf + off;
  	size_t len = 0;
  
  	if (name) {
  		while (*name) {
  			int segment_len;
  			// cache the name for subsequent compression
  			DECL_STRUCT(new_c, name_comp);
  
  			// find match for compression
  			for (c = comp; c; c = c->next) {
  				if (cmp_nlabel(name, c->label) == 0) {
  					mdns_write_u16(p, 0xC000 | (c->pos & ~0xC000));
  					return len + sizeof(uint16_t);
  				}
  
  				if (c->next == NULL)
  					c_tail = c;
  			}
  
  			// copy this segment
  			segment_len = *name + 1;
  			strncpy((char *) p, (char *) name, segment_len);
  			MALLOC_ZERO_STRUCT(new_c, name_comp);
  
  			new_c->label = (uint8_t *) name;
  			new_c->pos = p - pkt_buf;
  			c_tail->next = new_c;
  
  			// advance to next name segment
  			p += segment_len;
  			len += segment_len;
  			name += segment_len;
  		}
  	}
  
  	*p = '\0';	// root "label"
  	len += 1;
  
  	return len;
  }
  
  // encodes an RR entry at the given offset
  // returns the size of the entire RR entry
  static size_t mdns_encode_rr(uint8_t *pkt_buf, size_t pkt_len, size_t off, 
  		struct rr_entry *rr, struct name_comp *comp) {
  	uint8_t *p = pkt_buf + off, *p_data;
  	size_t l;
  	struct rr_data_txt *txt_rec;
  	uint8_t *label;
  	int i;
  
  	assert(off < pkt_len);
  
  	// name
  	l = mdns_encode_name(pkt_buf, pkt_len, off, rr->name, comp);
  	assert(l != 0);
  	p += l;
  
  	// type
  	p = mdns_write_u16(p, rr->type);
  
  	// class & cache flush
  	p = mdns_write_u16(p, (rr->rr_class & ~0x8000) | (rr->cache_flush << 15));
  
  	// TTL
  	p = mdns_write_u32(p, rr->ttl);
  	
  	// data length (filled in later)
  	p += sizeof(uint16_t);
  
  	// start of data marker
  	p_data = p;
  
  	switch (rr->type) {
  		case RR_A:
  			/* htonl() needed coz addr already in net order */
  			p = mdns_write_u32(p, htonl(rr->data.A.addr));
  			break;
  
  		case RR_AAAA:
  			for (i = 0; i < sizeof(struct in6_addr); i++)
  				*p++ = rr->data.AAAA.addr->s6_addr[i];
  			break;
  
  		case RR_PTR:
  			label = rr->data.PTR.name ? 
  					rr->data.PTR.name : 
  					rr->data.PTR.entry->name;
  			p += mdns_encode_name(pkt_buf, pkt_len, p - pkt_buf, label, comp);
  			break;
  
  		case RR_TXT:
  			txt_rec = &rr->data.TXT;
  			for (; txt_rec; txt_rec = txt_rec->next) {
  				int len = txt_rec->txt[0] + 1;
  				strncpy((char *) p, (char *) txt_rec->txt, len);
  				p += len;
  			}
  			break;
  
  		case RR_SRV:
  			p = mdns_write_u16(p, rr->data.SRV.priority);
  			
  			p = mdns_write_u16(p, rr->data.SRV.weight);
  
  			p = mdns_write_u16(p, rr->data.SRV.port);
  
  			p += mdns_encode_name(pkt_buf, pkt_len, p - pkt_buf, 
  					rr->data.SRV.target, comp);
  			break;
  
  		case RR_NSEC:
  			p += mdns_encode_name(pkt_buf, pkt_len, p - pkt_buf, 
  					rr->name, comp);
  
  			*p++ = 0;	// bitmap window/block number
  
  			*p++ = sizeof(rr->data.NSEC.bitmap);	// bitmap length
  
  			for (i = 0; i < sizeof(rr->data.NSEC.bitmap); i++) 
  				*p++ = rr->data.NSEC.bitmap[i];
  
  			break;
  
  		default:
  			DEBUG_PRINTF("unhandled rr type 0x%02x\n", rr->type);
  	}
  
  	// calculate data length based on p
  	l = p - p_data;
  
  	// fill in the length
  	mdns_write_u16(p - l - sizeof(uint16_t), l);
  
  	return p - pkt_buf - off;
  }
  
  // encodes a MDNS packet from the given mdns_pkt struct into a buffer
  // returns the size of the entire MDNS packet
  size_t mdns_encode_pkt(struct mdns_pkt *answer, uint8_t *pkt_buf, size_t pkt_len) {
  	struct name_comp *comp;
  	uint8_t *p = pkt_buf;
  	//uint8_t *e = pkt_buf + pkt_len;
  	size_t off;
  	int i;
  	struct rr_list *rr_set[3];
  
  	assert(answer != NULL);
  	assert(pkt_len >= 12);
  
  	if (p == NULL)
  		return -1;
  
  	// this is an Answer - number of qns should be zero
  	assert(answer->num_qn == 0);
  
  	p = mdns_write_u16(p, answer->id);
  	p = mdns_write_u16(p, answer->flags);
  	p = mdns_write_u16(p, answer->num_qn);
  	p = mdns_write_u16(p, answer->num_ans_rr);
  	p = mdns_write_u16(p, answer->num_auth_rr);
  	p = mdns_write_u16(p, answer->num_add_rr);
  
  	off = p - pkt_buf;
  
  	// allocate list for name compression
  	comp = malloc(sizeof(struct name_comp));
  	if (comp == NULL)
  		return -1;
  	memset(comp, 0, sizeof(struct name_comp));
  
  	// dummy entry
  	comp->label = (uint8_t *) "";
  	comp->pos = 0;
  
  	// skip encoding of qn
  	rr_set[0] =	answer->rr_ans;
  	rr_set[1] = answer->rr_auth;
  	rr_set[2] =	answer->rr_add;
  
  	// encode answer, authority and additional RRs
  	for (i = 0; i < sizeof(rr_set) / sizeof(rr_set[0]); i++) {
  		struct rr_list *rr = rr_set[i];
  		for (; rr; rr = rr->next) {
  			size_t l = mdns_encode_rr(pkt_buf, pkt_len, off, rr->e, comp);
  			off += l;
  
  			if (off >= pkt_len) {
  				DEBUG_PRINTF("packet buffer too small\n");
  				return -1;
  			}
  		}
  
  	}
  
  	// free name compression list
  	while (comp) {
  		struct name_comp *c = comp->next;
  		free(comp);
  		comp = c;
  	}
  
  	return off;
  }
  
  