diff options
Diffstat (limited to 'dns.c')
| -rw-r--r-- | dns.c | 89 |
1 files changed, 64 insertions, 25 deletions
@@ -10,6 +10,10 @@ #include <fcntl.h> +/* Entry point is dns_get. */ + + +/* Read about DNS in RFC 1034 & 1035. */ struct header { unsigned short id; @@ -48,18 +52,23 @@ struct resource_header { }; +/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * + * get-related + */ static void fill_header(char **writer) { struct header *head = (struct header *) *writer; memset(head, 0, sizeof(*head)); + /* PID is unique enough to use it as ID: */ head->id = (unsigned short) htons(getpid()); - head->a = 0b00000001; - head->questions = htons(1); - *writer += sizeof(*head); + head->a = 0b00000001; /* recursion desired */ + head->questions = htons(1); /* we have one question */ + *writer += sizeof(*head); } static void fill_name(char **writer, const char *host) -{ +{ /* In DNS names are always starting with dot, + * so 'example.com' need to become '.example.com'. */ char *last = *writer; do { if (*host == '.' || !*host) { @@ -82,7 +91,7 @@ static void fill_question(char **writer, int query_type) } static int askudp(const char *s, const char *h, int qt, struct sockaddr_in *a) -{ +{ /* Create socket, fill in the question and send it. */ int sent; int plen = sizeof(struct header) + strlen(h) + 2 + sizeof(struct question_header); @@ -96,10 +105,12 @@ static int askudp(const char *s, const char *h, int qt, struct sockaddr_in *a) a->sin_port = htons(53); a->sin_addr.s_addr = inet_addr(s); + /* Socket created, now fill in the question. */ fill_header(&writer); fill_name(&writer, h); fill_question(&writer, qt); + /* Question is filled in, so send it. */ sent = sendto(sock, packet, plen, 0, (struct sockaddr *) a, sizeof(*a)); free(packet); if (sent != plen) { @@ -109,14 +120,14 @@ static int askudp(const char *s, const char *h, int qt, struct sockaddr_in *a) } static int asktcp(const char *s, const char *h, int qt) -{ +{ /* Create socket, fill in the question and send it. */ struct sockaddr_in a; int sent; int plen = sizeof(struct header) + strlen(h) + 4 + sizeof(struct question_header); - char *packet = malloc(plen); - char *writer = packet; - int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); + char *packet = malloc(plen); + char *writer = packet; + int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); if (sock < 0) { return 0; } @@ -127,12 +138,15 @@ static int asktcp(const char *s, const char *h, int qt) return 0; } + /* Socket created, now fill in the question. + Note the difference from UDP: TCP has size in the start. */ writer += 2; fill_header(&writer); fill_name(&writer, h); fill_question(&writer, qt); *((unsigned short *) packet) = htons(writer - packet - 2); + /* Question is filled in, so send it. */ sent = write(sock, packet, plen); free(packet); if (sent != plen) { @@ -164,7 +178,7 @@ static char *gettcp(int sock) } static char *get(const char *s, const char *h, int qt) -{ +{ /* Get response in UDP, if it is truncated do it in TCP. */ struct sockaddr_in a; char *packet; socklen_t slen = sizeof(a); @@ -172,12 +186,12 @@ static char *get(const char *s, const char *h, int qt) if (!sock) { return 0; } - packet = malloc(512); + packet = malloc(512); /* Max size of UDP is 512 bytes. */ if (recvfrom(sock, packet, 512, 0, (struct sockaddr *) &a, &slen) < 0) { free(packet); close(sock); return 0; - } else if (packet[2] & 0b10) { + } else if (packet[2] & 0b10) { /* is truncated */ free(packet); close(sock); return gettcp(asktcp(s, h, qt)); @@ -187,6 +201,9 @@ static char *get(const char *s, const char *h, int qt) } +/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * + * parse-related + */ static void skip_questions(char **reader, int num) { for (; num; --num) { @@ -195,10 +212,14 @@ static void skip_questions(char **reader, int num) } } -static char *read_name(char **extrareader, char *packet) -{ +static char *read_name(char **extreader, char *packet) +{ /* Names are stored in binary format: + * length followed by name part (string between dots), + * length of zero means end of domain name, + * and length >= 192 means it will be followed by absolute link, + * name readed from link should be inserted instead of this part. */ char *name = malloc(256); /* cannot be more =) */ - char *reader = *extrareader; + char *reader = *extreader; char *writer = name; int jumped = 0; if (!*reader) { @@ -210,7 +231,7 @@ static char *read_name(char **extrareader, char *packet) if (len >= 192) { int jump = ((unsigned char) *reader) + (len - 192) * 256; if (!jumped) { - *extrareader += 2; + *extreader += 2; jumped = 1; } reader = packet + jump; @@ -220,43 +241,49 @@ static char *read_name(char **extrareader, char *packet) writer += len; reader += len; if (!jumped) { - *extrareader += len + 1; + *extreader += len + 1; } *writer++ = '.'; } if (!jumped) { - ++*extrareader; + ++*extreader; } *--writer = 0; return name; } static struct dns_answers *get_answers(char **reader, char *packet, int num) -{ +{ /* Read all (num) answers into res and return it. */ struct dns_answers *res; struct resource_header *head; - if (!num) { + if (!num) { /* no answers left to read */ return 0; } + /* Read one answer, starting with header: */ res = malloc(sizeof(*res)); res->host = read_name(reader, packet); head = (struct resource_header *) *reader; res->type = ntohs(head->type); res->size = ntohs(head->datalen); *reader += 10; + /* Analyse answer data based on answer type: */ if (res->type == dns_type_cname) { + /* For CNAME data is just domain name: */ res->size--; res->data = read_name(reader, packet); } else if (res->type == dns_type_mx) { + /* For MX data is record preference, followed by its name: */ char *name; unsigned short pref = ntohs(*((unsigned short *) *reader)); *reader += 2; name = read_name(reader, packet); - res->data = malloc(3 + strlen(name)); + res->data = malloc(2 + strlen(name) + 1); *((unsigned short *) res->data) = pref; memcpy(res->data + 2, name, strlen(name) + 1); free(name); } else if (res->type == dns_type_srv) { + /* For SRV data is record priority, weight and port + * followed by its name. */ char *name; unsigned short priority; unsigned short weight; @@ -275,7 +302,9 @@ static struct dns_answers *get_answers(char **reader, char *packet, int num) memcpy(res->data + 6, name, strlen(name) + 1); free(name); } else { + /* For other types just copy the data: */ if (res->type == dns_type_txt) { + /* Skipping leading length byte for TXT: */ res->size--; ++*reader; } @@ -283,6 +312,7 @@ static struct dns_answers *get_answers(char **reader, char *packet, int num) memcpy(res->data, *reader, res->size); *reader += res->size; } + /* Start recursion for the rest of answers and add them as next: */ res->next = get_answers(reader, packet, num - 1); return res; } @@ -295,9 +325,9 @@ static struct dns_answers *parse(char *packet) if (!packet) { return 0; } - head->questions = ntohs(head->questions); - head->answers = ntohs(head->answers); - reader += sizeof(*head); + head->questions = ntohs(head->questions); + head->answers = ntohs(head->answers); + reader += sizeof(*head); skip_questions(&reader, head->questions); res = get_answers(&reader, packet, head->answers); free(packet); @@ -305,6 +335,9 @@ static struct dns_answers *parse(char *packet) } +/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * + * MX utility + */ char *dns_mx_server(void *data) { return ((char *) data) + 2; @@ -316,6 +349,9 @@ int dns_mx_preference(void *data) } +/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * + * SRV utility + */ char *dns_srv_server(void *data) { return ((char *) data) + 6; @@ -337,8 +373,11 @@ int dns_srv_weight(void *data) } +/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * + * Constructor and destructor + */ struct dns_answers *dns_get(const char *server, const char *host, int query) -{ +{ /* Acquire response in get and parse it. */ return parse(get(server, host, query)); } |