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Parsing of mdns packets #4

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80f7a7c
mdns new function to initialize nay kind of packet
mchalain Aug 3, 2020
d6aa42a
parse_rr and parse_qn request a list of entries
mchalain Aug 3, 2020
46bb2c7
parse_rr supports the SRV code.
mchalain Aug 3, 2020
e05e71c
full parsing of the packet with all entries.
mchalain Aug 3, 2020
c7861eb
the rr encoding must check pointer before to use it
mchalain Aug 3, 2020
a66bda3
allows mdns_encode_pkt to encode questions too.
mchalain Aug 3, 2020
0312c35
display all data of received messages and announcement
mchalain Aug 3, 2020
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100 changes: 73 additions & 27 deletions mdns.c
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Original file line number Diff line number Diff line change
Expand Up @@ -601,14 +601,13 @@ uint32_t mdns_read_u32(const uint8_t *ptr) {
((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) {
void mdns_init_pkt(struct mdns_pkt *pkt, uint16_t id) {
// copy transaction ID
pkt->id = id;

// response flags
pkt->flags = MDNS_FLAG_RESP | MDNS_FLAG_AA;
// question flags
pkt->flags = 0;

rr_list_destroy(pkt->rr_qn, 0);
rr_list_destroy(pkt->rr_ans, 0);
Expand All @@ -626,6 +625,14 @@ void mdns_init_reply(struct mdns_pkt *pkt, uint16_t id) {
pkt->num_add_rr = 0;
}

// initialize the packet for reply
void mdns_init_reply(struct mdns_pkt *pkt, uint16_t id) {
mdns_init_pkt(pkt, id);

// response flags
pkt->flags = MDNS_FLAG_RESP | MDNS_FLAG_AA;
}

// destroys an mdns_pkt struct, including its contents
void mdns_pkt_destroy(struct mdns_pkt *p) {
rr_list_destroy(p->rr_qn, 1);
Expand All @@ -640,13 +647,11 @@ void mdns_pkt_destroy(struct mdns_pkt *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) {
struct rr_list **list) {
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));

Expand All @@ -661,15 +666,15 @@ static size_t mdns_parse_qn(uint8_t *pkt_buf, size_t pkt_len, size_t off,
rr->rr_class = mdns_read_u16(p) & ~0x80;
p += sizeof(uint16_t);

rr_list_append(&pkt->rr_qn, rr);
rr_list_append(list, 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) {
struct rr_list **list) {
const uint8_t *p = pkt_buf + off;
const uint8_t *e = pkt_buf + pkt_len;
struct rr_entry *rr;
Expand All @@ -678,10 +683,10 @@ static size_t mdns_parse_rr(uint8_t *pkt_buf, size_t pkt_len, size_t off,
struct rr_data_txt *txt_rec;
int parse_error = 0;

assert(pkt != NULL);

if (off > pkt_len)
if (off > pkt_len) {
DEBUG_PRINTF("error length %ld %ld\n", off, pkt_len);
return 0;
}

rr = malloc(sizeof(struct rr_entry));
memset(rr, 0, sizeof(struct rr_entry));
Expand All @@ -704,6 +709,13 @@ static size_t mdns_parse_rr(uint8_t *pkt_buf, size_t pkt_len, size_t off,
rr_data_len = mdns_read_u16(p);
p += sizeof(uint16_t);

if (rr_data_len == 0)
{
DEBUG_PRINTF("no data %ld\n", p - (pkt_buf + off));
rr_list_append(list, rr);
return p - (pkt_buf + off);
}

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);
Expand Down Expand Up @@ -778,6 +790,18 @@ static size_t mdns_parse_rr(uint8_t *pkt_buf, size_t pkt_len, size_t off,
}
break;

case RR_SRV:
srv_rec = &rr->data.SRV;
srv_rec->priority = mdns_read_u16(p);
p += sizeof(uint16_t);
srv_rec->weight = mdns_read_u16(p);
p += sizeof(uint16_t);
srv_rec->port = mdns_read_u16(p);
p += sizeof(uint16_t);
srv_rec->target = uncompress_nlabel(pkt_buf, pkt_len, p - pkt_buf);
p += label_len(pkt_buf, pkt_len, p - pkt_buf);
break;

default:
// skip to end of RR data
p = e;
Expand All @@ -789,7 +813,7 @@ static size_t mdns_parse_rr(uint8_t *pkt_buf, size_t pkt_len, size_t off,
return 0;
}

rr_list_append(&pkt->rr_ans, rr);
rr_list_append(list, rr);

return p - (pkt_buf + off);
}
Expand Down Expand Up @@ -818,7 +842,7 @@ struct mdns_pkt *mdns_parse_pkt(uint8_t *pkt_buf, size_t pkt_len) {

// parse questions
for (i = 0; i < pkt->num_qn; i++) {
size_t l = mdns_parse_qn(pkt_buf, pkt_len, off, pkt);
size_t l = mdns_parse_qn(pkt_buf, pkt_len, off, &pkt->rr_qn);
if (! l) {
DEBUG_PRINTF("error parsing question #%d\n", i);
mdns_pkt_destroy(pkt);
Expand All @@ -830,7 +854,7 @@ struct mdns_pkt *mdns_parse_pkt(uint8_t *pkt_buf, size_t pkt_len) {

// parse answer RRs
for (i = 0; i < pkt->num_ans_rr; i++) {
size_t l = mdns_parse_rr(pkt_buf, pkt_len, off, pkt);
size_t l = mdns_parse_rr(pkt_buf, pkt_len, off, &pkt->rr_ans);
if (! l) {
DEBUG_PRINTF("error parsing answer #%d\n", i);
mdns_pkt_destroy(pkt);
Expand All @@ -840,7 +864,29 @@ struct mdns_pkt *mdns_parse_pkt(uint8_t *pkt_buf, size_t pkt_len) {
off += l;
}

// TODO: parse the authority and additional RR sections
// parse authority RRs
for (i = 0; i < pkt->num_auth_rr; i++) {
size_t l = mdns_parse_rr(pkt_buf, pkt_len, off, &pkt->rr_auth);
if (! l) {
DEBUG_PRINTF("error parsing authority #%d\n", i);
mdns_pkt_destroy(pkt);
return NULL;
}

off += l;
}

// parse additional RRs
for (i = 0; i < pkt->num_add_rr; i++) {
size_t l = mdns_parse_rr(pkt_buf, pkt_len, off, &pkt->rr_add);
if (! l) {
DEBUG_PRINTF("error parsing additional #%d\n", i);
mdns_pkt_destroy(pkt);
return NULL;
}

off += l;
}

return pkt;
}
Expand Down Expand Up @@ -936,10 +982,12 @@ static size_t mdns_encode_rr(uint8_t *pkt_buf, size_t pkt_len, size_t off,
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);
if (rr->data.PTR.name != NULL || rr->data.PTR.entry != NULL) {
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:
Expand Down Expand Up @@ -996,17 +1044,14 @@ size_t mdns_encode_pkt(struct mdns_pkt *answer, uint8_t *pkt_buf, size_t pkt_len
//uint8_t *e = pkt_buf + pkt_len;
size_t off;
int i;
struct rr_list *rr_set[3];
struct rr_list *rr_set[4];

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);
Expand All @@ -1027,9 +1072,10 @@ size_t mdns_encode_pkt(struct mdns_pkt *answer, uint8_t *pkt_buf, size_t pkt_len
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;
rr_set[0] = answer->rr_qn;
rr_set[1] = answer->rr_ans;
rr_set[2] = answer->rr_auth;
rr_set[3] = answer->rr_add;

// encode answer, authority and additional RRs
for (i = 0; i < sizeof(rr_set) / sizeof(rr_set[0]); i++) {
Expand Down
1 change: 1 addition & 0 deletions mdns.h
View file
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Original file line number Diff line number Diff line change
Expand Up @@ -169,6 +169,7 @@ struct mdns_pkt {
struct mdns_pkt *mdns_parse_pkt(uint8_t *pkt_buf, size_t pkt_len);

void mdns_init_reply(struct mdns_pkt *pkt, uint16_t id);
void mdns_init_pkt(struct mdns_pkt *pkt, uint16_t id);
size_t mdns_encode_pkt(struct mdns_pkt *answer, uint8_t *pkt_buf, size_t pkt_len);

void mdns_pkt_destroy(struct mdns_pkt *p);
Expand Down
114 changes: 104 additions & 10 deletions mdnsd.c
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Original file line number Diff line number Diff line change
Expand Up @@ -90,6 +90,8 @@ struct mdns_service {
struct rr_list *entries;
};

static int display_mdns_segment(struct mdnsd *svr, struct rr_list *segm, const char *name);

/////////////////////////////////


Expand Down Expand Up @@ -277,6 +279,93 @@ static void announce_srv(struct mdnsd *svr, struct mdns_pkt *reply, uint8_t *nam

// additional records for additional records
add_related_rr(svr, reply->rr_add, reply);

display_mdns_segment(svr, reply->rr_ans, "announce ans");
display_mdns_segment(svr, reply->rr_add, "announce add");
}

static void query_svr(struct mdnsd *svr, struct mdns_pkt *query) {
mdns_init_pkt(query, 0);

struct rr_list *n;
n = svr->queries;
for (; n; n = n->next) {
rr_list_append(&query->rr_qn, n->e);
query->num_qn++;
}
}

static int display_rr_ptr(struct mdnsd *svr, struct rr_entry *ptr) {
int ret = -1;
char *namestr = nlabel_to_str(ptr->name);
DEBUG_PRINTF("type %s (%02x) %s - ", rr_get_type_name(ptr->type), ptr->type, namestr);
switch (ptr->type) {
case RR_PTR:
if (ptr->data.PTR.name != NULL) {
char *namestr = nlabel_to_str(ptr->data.PTR.name);
DEBUG_PRINTF("name: %s ", namestr);
free(namestr);
} else if (ptr->name != NULL) {
char *namestr = nlabel_to_str(ptr->name);
DEBUG_PRINTF("name: %s ", namestr);
free(namestr);
}
if (ptr->data.PTR.entry != NULL) {
DEBUG_PRINTF("ptr: ");
display_rr_ptr(svr, ptr->data.PTR.entry);
}
break;
case RR_SRV:
DEBUG_PRINTF("port: %d - %d - %d", ptr->data.SRV.port, ptr->data.SRV.weight, ptr->data.SRV.priority);
if (ptr->data.SRV.target != NULL) {
char *namestr = nlabel_to_str(ptr->data.SRV.target);
DEBUG_PRINTF(" target: %s", namestr);
free(namestr);
}
break;
case RR_TXT: {
struct rr_data_txt *txte;
DEBUG_PRINTF("txt");
for (txte = &ptr->data.TXT; txte; txte = txte->next) {
char *namestr = nlabel_to_str(txte->txt);
DEBUG_PRINTF(" %s -", namestr);
free(namestr);
}
}
break;
case RR_A: {
struct in_addr in;
in.s_addr = ptr->data.A.addr;
DEBUG_PRINTF("address: %s", inet_ntoa(in));
}
default:
break;
}
struct rr_entry *e = rr_entry_match(svr->queries, ptr);
if (e != NULL) {
DEBUG_PRINTF("receive answer for query %s\n", namestr);
if (ret == -1)
ret = 0;
}
free(namestr);
return ret;
}

static int display_mdns_segment(struct mdnsd *svr, struct rr_list *segm, const char *name) {
int i;
struct rr_list *ans;
int ret = -1;

ans = NULL; i = 0;
for (ans = segm; ans; ) {
struct rr_list *next = ans->next;
DEBUG_PRINTF("%s #%d: ", name, i);
ret = display_rr_ptr(svr, ans->e);
DEBUG_PRINTF("\n");
ans = next;
i++;
}
return ret;
}

// processes the incoming MDNS packet
Expand All @@ -285,20 +374,22 @@ static int process_mdns_pkt(struct mdnsd *svr, struct mdns_pkt *pkt, struct mdns
int i;
struct rr_list *qnl;
struct rr_list *ans, *prev_ans;
int ret = 0;

assert(pkt != NULL);

DEBUG_PRINTF("id = %04x, flags = %04x, qn = %d, ans = %d, add = %d %p\n",
pkt->id,
pkt->flags,
pkt->num_qn,
pkt->num_ans_rr,
pkt->num_add_rr, pkt->rr_add);

// is it standard query?
if ((pkt->flags & MDNS_FLAG_RESP) == 0 &&
MDNS_FLAG_GET_OPCODE(pkt->flags) == 0) {
mdns_init_reply(reply, pkt->id);

DEBUG_PRINTF("flags = %04x, qn = %d, ans = %d, add = %d\n",
pkt->flags,
pkt->num_qn,
pkt->num_ans_rr,
pkt->num_add_rr);

// loop through questions
qnl = pkt->rr_qn;
for (i = 0; i < pkt->num_qn; i++, qnl = qnl->next) {
Expand Down Expand Up @@ -357,12 +448,15 @@ static int process_mdns_pkt(struct mdnsd *svr, struct mdns_pkt *pkt, struct mdns
// additional records for additional records
add_related_rr(svr, reply->rr_add, reply);

DEBUG_PRINTF("\n");

return reply->num_ans_rr;
ret = reply->num_ans_rr;
}

return 0;
display_mdns_segment(svr, pkt->rr_ans, "ans");
display_mdns_segment(svr, pkt->rr_auth, "auth");
display_mdns_segment(svr, pkt->rr_add, "add");
DEBUG_PRINTF("\n");

return ret;
}

int create_pipe(int handles[2]) {
Expand Down