From 1dc1890ca0316a2271c7b4d36a86680e0591d83e Mon Sep 17 00:00:00 2001 From: Matthias Gerstner Date: Thu, 27 Oct 2022 12:32:49 +0200 Subject: dnsproxy: refactor parse_response() - add a descriptive comment to make clear what the function does - use const pointers and size_t where possible - move stack variables into more localized scopes - use named constants over literal numbers where applicable --- src/dnsproxy.c | 110 ++++++++++++++++++++++++++++++--------------------------- 1 file changed, 57 insertions(+), 53 deletions(-) diff --git a/src/dnsproxy.c b/src/dnsproxy.c index 37bc472a..b2ec7d42 100644 --- a/src/dnsproxy.c +++ b/src/dnsproxy.c @@ -921,7 +921,7 @@ static int get_name(int counter, static int parse_rr(const unsigned char *buf, const unsigned char *start, const unsigned char *max, - unsigned char *response, unsigned int *response_size, + unsigned char *response, size_t *response_size, uint16_t *type, uint16_t *class, int *ttl, uint16_t *rdlen, const unsigned char **end, char *name, size_t max_name) @@ -982,55 +982,67 @@ static bool check_alias(GSList *aliases, const char *name) return false; } -static int parse_response(unsigned char *buf, int buflen, - char *question, int qlen, +/* + * Parses the DNS response packet found in 'buf' consisting of 'buflen' bytes. + * + * The parsed question label, response type and class, ttl and number of + * answer sections are output parameters. The response output buffer will + * receive all matching resource records to be cached. + * + * Return value is < 0 on error (negative errno) or zero on success. + */ +static int parse_response(const unsigned char *buf, size_t buflen, + char *question, size_t qlen, uint16_t *type, uint16_t *class, int *ttl, - unsigned char *response, unsigned int *response_len, + unsigned char *response, size_t *response_len, uint16_t *answers) { struct domain_hdr *hdr = (void *) buf; struct domain_question *q; - const unsigned char *ptr; - uint16_t qdcount = ntohs(hdr->qdcount); - uint16_t ancount = ntohs(hdr->ancount); - int err, i; - uint16_t qtype, qclass; - const unsigned char *next = NULL; - unsigned int maxlen = *response_len; - GSList *aliases = NULL, *list; - char name[NS_MAXDNAME + 1]; - - if (buflen < 12) + uint16_t qtype; + int err = -ENOMSG; + uint16_t ancount, qclass; + GSList *aliases = NULL; + const size_t maxlen = *response_len; + + *response_len = 0; + *answers = 0; + + if (buflen < DNS_HEADER_SIZE) return -EINVAL; + const uint16_t qdcount = ntohs(hdr->qdcount); + const unsigned char *ptr = buf + DNS_HEADER_SIZE; + const unsigned char *eptr = buf + buflen; + debug("qr %d qdcount %d", hdr->qr, qdcount); /* We currently only cache responses where question count is 1 */ if (hdr->qr != 1 || qdcount != 1) return -EINVAL; - ptr = buf + sizeof(struct domain_hdr); - - strncpy(question, (char *) ptr, qlen); + /* + * NOTE: currently the *caller* ensures that the `question' buffer is + * always zero terminated. + */ + strncpy(question, (const char *) ptr, MIN(qlen, buflen - DNS_HEADER_SIZE)); qlen = strlen(question); ptr += qlen + 1; /* skip \0 */ + if ((eptr - ptr) < sizeof(struct domain_question)) + return -EINVAL; + q = (void *) ptr; qtype = ntohs(q->type); /* We cache only A and AAAA records */ - if (qtype != 1 && qtype != 28) + if (qtype != DNS_TYPE_A && qtype != DNS_TYPE_AAAA) return -ENOMSG; - qclass = ntohs(q->class); - - ptr += 2 + 2; /* ptr points now to answers */ - - err = -ENOMSG; - *response_len = 0; - *answers = 0; + ptr += sizeof(struct domain_question); /* advance to answers section */ - memset(name, 0, sizeof(name)); + ancount = ntohs(hdr->ancount); + qclass = ntohs(q->class); /* * We have a bunch of answers (like A, AAAA, CNAME etc) to @@ -1038,7 +1050,8 @@ static int parse_response(unsigned char *buf, int buflen, * resource records. Only A and AAAA records are cached, all * the other records in answers are skipped. */ - for (i = 0; i < ancount; i++) { + for (uint16_t i = 0; i < ancount; i++) { + char name[NS_MAXDNAME + 1] = {0}; /* * Get one address at a time to this buffer. * The max size of the answer is @@ -1046,24 +1059,27 @@ static int parse_response(unsigned char *buf, int buflen, * 4 (ttl) + 2 (rdlen) + addr (16 or 4) = 28 * for A or AAAA record. * For CNAME the size can be bigger. + * TODO: why are we using the MAXCDNAME constant as buffer + * size then? */ - unsigned char rsp[NS_MAXCDNAME]; - unsigned int rsp_len = sizeof(rsp) - 1; - int ret; + unsigned char rsp[NS_MAXCDNAME] = {0}; + size_t rsp_len = sizeof(rsp) - 1; + const unsigned char *next = NULL; uint16_t rdlen; - memset(rsp, 0, sizeof(rsp)); - - ret = parse_rr(buf, ptr, buf + buflen, rsp, &rsp_len, + int ret = parse_rr(buf, ptr, buf + buflen, rsp, &rsp_len, type, class, ttl, &rdlen, &next, name, sizeof(name) - 1); if (ret != 0) { err = ret; - goto out; + break; } + /* set pointer to the next RR for the next iteration */ + ptr = next; + /* - * Now rsp contains compressed or uncompressed resource + * Now rsp contains a compressed or an uncompressed resource * record. Next we check if this record answers the question. * The name var contains the uncompressed label. * One tricky bit is the CNAME records as they alias @@ -1075,8 +1091,6 @@ static int parse_response(unsigned char *buf, int buflen, * looking for. */ if (*class != qclass) { - ptr = next; - next = NULL; continue; } @@ -1101,7 +1115,7 @@ static int parse_response(unsigned char *buf, int buflen, * address of ipv6.l.google.com. For caching purposes this * should not cause any issues. */ - if (*type == 5 && strncmp(question, name, qlen) == 0) { + if (*type == DNS_TYPE_CNAME && strncmp(question, name, qlen) == 0) { /* * So now the alias answered the question. This is * not very useful from caching point of view as @@ -1125,8 +1139,6 @@ static int parse_response(unsigned char *buf, int buflen, name, sizeof(name) - 1, &name_len); if (ret != 0) { /* just ignore the error at this point */ - ptr = next; - next = NULL; continue; } @@ -1138,12 +1150,8 @@ static int parse_response(unsigned char *buf, int buflen, */ aliases = g_slist_prepend(aliases, g_strdup(name)); - ptr = next; - next = NULL; continue; - } - - if (*type == qtype) { + } else if (*type == qtype) { /* * We found correct type (A or AAAA) */ @@ -1160,7 +1168,7 @@ static int parse_response(unsigned char *buf, int buflen, */ if (*response_len + rsp_len > maxlen) { err = -ENOBUFS; - goto out; + break; } memcpy(response + *response_len, rsp, rsp_len); *response_len += rsp_len; @@ -1168,13 +1176,9 @@ static int parse_response(unsigned char *buf, int buflen, err = 0; } } - - ptr = next; - next = NULL; } -out: - for (list = aliases; list; list = list->next) + for (GSList *list = aliases; list; list = list->next) g_free(list->data); g_slist_free(aliases); @@ -1380,7 +1384,7 @@ static int cache_update(struct server_data *srv, unsigned char *msg, char question[NS_MAXDNAME + 1]; unsigned char response[NS_MAXDNAME + 1]; unsigned char *ptr; - unsigned int rsplen; + size_t rsplen; bool new_entry = true; time_t current_time; -- cgit v1.2.1