diff options
Diffstat (limited to 'dns.c')
| -rw-r--r-- | dns.c | 244 |
1 files changed, 244 insertions, 0 deletions
@@ -0,0 +1,244 @@ +#define _GNU_SOURCE +#include <stdio.h> +#include <stdlib.h> +#include <string.h> +#include <errno.h> +#include <sys/types.h> +#include <sys/socket.h> +#include <arpa/inet.h> +#include <netinet/in.h> +#include <signal.h> +#include <unistd.h> +#include "domain2name.c" // this is from http://git.sijanec.eu/sijanec/dnsfind +enum type { + A = 1, + Ns, + Md, + Mf, + Cname, + Soa, + Mb, + M, + Mr, + Null, + Wks, + Ptr, + Hinfo, + Minfo, + Mx, + Txt +}; +enum class { + In = 1, + Cs, + Ch, + He, + Any = 255 +}; +#define RESPONSE (1 << 15) +#define QUESTION (0 << 15) +#define QUERY (0 << 11) +#define IQUERY (1 << 11) +#define STATUS (1 << 12) +#define AA (1 << 10) +#define TC (1 << 9) +#define RD (1 << 8) +#define RA (1 << 7) +#define SUCCESS (0 << 0) +#define FORMAT_ERROR (1 << 0) +#define SERVFAIL (1 << 1) +#define NXDOMAIN (FORMAT_ERROR | SERVFAIL) +#define NI (1 << 2) +#define FORBIDDEN (NXDOMAIN | FORMAT_ERROR) +struct header { + uint16_t xid __attribute__((packed)); + uint16_t flags __attribute__((packed)); + uint16_t qdcount __attribute__((packed)); + uint16_t ancount __attribute__((packed)); + uint16_t nscount __attribute__((packed)); + uint16_t arcount __attribute__((packed)); + char data[]; /* ignored for char[] */ +} __attribute__((packed)); +struct rr { /* name is omitted, first byte of struct is first byte of type */ + uint16_t type __attribute__((packed)); + uint16_t class __attribute__((packed)); + uint32_t ttl __attribute__((packed)); + uint16_t len __attribute__((packed)); + char data[]; /* ignored for char[] */ +} __attribute__((packed)); +struct question { + uint16_t type __attribute__((packed)); + uint16_t class __attribute__((packed)); +} __attribute__((packed)); +enum dns_loglevel { + DNS_DEBUG, + DNS_INFO, + DNS_WARN, + DNS_ERROR +}; +typedef void (* dns_log_handler) (void * const, const enum dns_loglevel, const char * const, const char * const); +struct dns { + int fd; + char * domain; // this is as it appears in the packet + dns_log_handler log_handler; + void * log_userdata; + struct sockaddr_in sockaddr; +}; +typedef struct dns dns; +static void dns_default_log_handler (void * const u __attribute__((unused)), + const enum dns_loglevel l, const char * a, const char * m) { + char * n = "unspec"; + switch (l) { + case DNS_DEBUG: + n = "DEBUG"; + break; + case DNS_INFO: + n = "INFO"; + break; + case DNS_WARN: + n = "WARN"; + break; + case DNS_ERROR: + n = "ERROR"; + break; + } + fprintf(stderr, "%s %s %s\n", n, a, m); +} +struct dns * dns_init (void) { + struct dns * dns = calloc(1, sizeof(struct dns)); + dns->fd = -1; + dns->domain = strdup(" call dns_set_domain to set the domain"); + dns->domain[0] = strlen(dns->domain)-1; + dns->log_handler = dns_default_log_handler; + dns->sockaddr.sin_family = AF_INET; + dns->sockaddr.sin_port = htons(53); + dns->sockaddr.sin_addr.s_addr = INADDR_ANY; + return dns; +} +static void dns_set_domain (struct dns * dns, const char * domain) { // static functions are + int required = domain2name_len(domain, strlen(domain)); // visible inside the same + if (required <= 0) // translation unit - they + return; // are visible across + free(dns->domain); // included files + domain2name((dns->domain = malloc(required)), domain, strlen(domain)); +} +static void dns_set_log_handler (struct dns * dns, dns_log_handler log_handler) { + if (!log_handler) + log_handler = dns_default_log_handler; + dns->log_handler = log_handler; +} +static void dns_set_log_userdata (struct dns * dns, void * log_userdata) { + dns->log_userdata = log_userdata; +} +static void dns_set_port (struct dns * dns, int port) { + dns->sockaddr.sin_port = htons(port); +} +static void dns_set_ip (struct dns * dns, const char * ip) { + if (!ip) { + dns->sockaddr.sin_addr.s_addr = INADDR_ANY; + return; + } + inet_aton(ip, &dns->sockaddr.sin_addr); +} +static void dns_run_once (struct dns * dns) { +#define BUFLEN 4096 + char buf[BUFLEN]; + if (dns->fd == -1) { + if ((dns->fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) { + sprintf(buf, "socket failed with %s", strerror(errno)); + dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf); + return; + } + int ž = 1; + if (setsockopt(dns->fd, SOL_SOCKET, SO_BROADCAST, &ž, sizeof(ž)) == -1) { + sprintf(buf, "setsockopt failed with %s", strerror(errno)); + dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf); + close(dns->fd); + dns->fd = -1; + return; + } + if (bind(dns->fd, (struct sockaddr *) &dns->sockaddr, sizeof(struct sockaddr))) { + sprintf(buf, "bind failed with %s", strerror(errno)); + dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf); + close(dns->fd); + dns->fd = -1; + return; + } + } + struct sockaddr_in sender; + socklen_t sendl = sizeof sender; + int size = recvfrom(dns->fd, buf, 65535, MSG_DONTWAIT, (struct sockaddr *) &sender, &sendl); + if (size == -1) { + if (errno != EWOULDBLOCK) { + sprintf(buf, "recvfrom failed with %s", strerror(errno)); + dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf); + close(dns->fd); + dns->fd = -1; + return; + } + return; + } + struct header * header = (struct header *) buf; // time for some overwriting + header->flags = htons(RESPONSE | QUERY | AA | SUCCESS); + header->ancount = htons(1); // we keep question number intact, as we send all qs back + header->nscount = 0; + header->arcount = 0; + buf[BUFLEN-1] = '\0'; // strstr and strlen on untrusted data! + char * ouranswerisat = memmem(buf, size, "in-addr", strlen("in-addr")); + if (!ouranswerisat) { + dns->log_handler(dns->log_userdata, DNS_INFO, "dns", + "received request without 'in-addr' string ... weird."); + return; + } + ouranswerisat += 17; + struct rr * rr = (struct rr *) (ouranswerisat + strlen(header->data) + 1); + if (ouranswerisat + strlen(header->data) + 1 + sizeof(struct rr) + + strlen(dns->domain) + 1 >= buf + BUFLEN) { + dns->log_handler(dns->log_userdata, DNS_WARN, "dns", "sent packet would be to big"); + return; + } + strcpy(ouranswerisat, header->data); + rr->type = htons(Ptr); + rr->class = htons(In); + rr->ttl = 0; + rr->len = htons(strlen(dns->domain)+1); + strcpy(rr->data, dns->domain); + int len = rr->data - buf + strlen(dns->domain) + 1; + if (sendto(dns->fd, buf, len, 0, (struct sockaddr *) &sender, sizeof(sender)) == -1) { + sprintf(buf, "sendto failed with %s", strerror(errno)); + dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf); + close(dns->fd); + dns->fd = -1; + return; + } + char dst[INET_ADDRSTRLEN]; + const char * resp = inet_ntop(AF_INET, &sender.sin_addr, dst, INET_ADDRSTRLEN); + sprintf(buf, "successfully sent DNS reply to %s", resp ? resp : "[inet_ntop failed]"); + dns->log_handler(dns->log_userdata, DNS_ERROR, "dns", buf); +} +static void dns_free (struct dns * dns) { + if (dns->fd != -1) + close (dns->fd); + free(dns->domain); + free(dns); +} +#if __INCLUDE_LEVEL__ == 0 +int shouldexit = 0; +void handler (int signal __attribute__((unused))) { + shouldexit++; +} +int main (int argc, char ** argv) { + if (argc != 1+1) { + fprintf(stderr, "usage: %s respond.to.ptr.with.this.domain.\n", argv[0]); + return 1; + } + signal(SIGINT, handler); + signal(SIGTERM, handler); + dns * dns = dns_init(); + dns_set_domain(dns, argv[1]); + while (!shouldexit) { + dns_run_once(dns); + } + dns_free(dns); +} +#endif |