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94a83d126f
3proxy/src/dnspr.c
Vladimir Dubrovin 94a83d126f dnspr: only the answer to the query asked is relayed 
dnspr relayed to the client whatever came first to the socket it asked
the name server from, from any address, under any id. Now:

- the query goes to the name server under a random id of its own, the
  client's put back in what is relayed to it;
- an answer over UDP is taken only from the address and port of the
  name server asked, anything else read past until DNS_TO runs out;
- the answer must have that id, be a response, and carry the question
  asked byte for byte, over UDP and TCP alike (a TCP answer that does
  not is error 834).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-22 12:04:18 +03:00

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/*
3APA3A simplest proxy server
(c) 2002-2026 by Vladimir Dubrovin <vlad@3proxy.org>
please read License Agreement
*/
#include "proxy.h"
#ifndef UDP
#define UDP
#endif
#define RETURN(xxx) { param->res = xxx; goto CLEANRET; }
#define BUFSIZE 16384
/* An answer to the query asked: our id, a response, and the question
byte for byte. */
static int dnsanswer(const unsigned char *buf, int len, const unsigned char *q, int qend){
return len >= qend && buf[0] == q[0] && buf[1] == q[1] && (buf[2] & 0x80) &&
buf[4] == q[4] && buf[5] == q[5] && !memcmp(buf + 12, q + 12, qend - 12);
}
void * dnsprchild(struct clientparam* param) {
uint32_t ip = 0;
unsigned char *bbuf;
unsigned char *buf, *s1, *s2;
char * host = NULL;
unsigned char c;
SASIZETYPE size;
int res, i;
int len;
int semlocked = 1;
unsigned type=0;
uint32_t ttl;
unsigned char addr[16];
#ifdef _WIN32
unsigned long ul = 1;
#endif
if(!(bbuf = malloc(BUFSIZE+2))){
RETURN (21);
}
buf = bbuf+2;
size = sizeof(param->sinsl);
getsockname(param->srv->srvsock, (struct sockaddr *)&param->sincl, &size);
i = param->srv->udplen;
if(i > BUFSIZE) i = BUFSIZE;
memcpy(buf, param->srv->udpbuf, i);
_3proxy_sem_unlock(udpinit);
semlocked = 0;
#if defined(_WIN32) && !defined(SHARE_UDP_SOCKET)
if((param->clisock=param->srv->so._socket(param->sostate, AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == INVALID_SOCKET) {
RETURN(818);
}
ioctlsocket(param->clisock, FIONBIO, &ul);
if(param->srv->so._setsockopt(param->sostate, param->clisock, SOL_SOCKET, SO_REUSEADDR, (char *)&ul, sizeof(int))) {RETURN(820);};
if(param->srv->so._bind(param->sostate, param->clisock,(struct sockaddr *)&param->sincl,SASIZE(&param->sincl))) {
RETURN(822);
}
#else
/* The reply has to come from the address the query was sent to, which
is the listening socket. */
param->clisock = param->srv->srvsock;
#endif
if(i < 0) {
RETURN(813);
}
buf[BUFSIZE - 1] = 0;
if(i<=13 || i>1000){
RETURN (814);
}
param->operation = DNSRESOLVE;
if((res = (*param->srv->authfunc)(param))) {RETURN(res);}
if(buf[4]!=0 || buf[5]!=1) RETURN(816);
for(len = 12; len<i; len+=(c+1)){
c = buf[len];
if(!c)break;
buf[len] = '.';
}
if(len > (i-4)) {RETURN(817);}
host = strdup((char *)buf+13);
if(!host) {RETURN(21);}
for(s2 = buf + 12; (s1 = (unsigned char *)strchr((char *)s2 + 1, '.')); s2 = s1)*s2 = (unsigned char)((s1 - s2) - 1);
*s2 = (len - (int)(s2 - buf)) - 1;
type = ((unsigned)buf[len+1])*256 + (unsigned)buf[len+2];
if(type==0x01 && param->srv->fakeip){
ip = 1;
ttl = 3600;
*(uint32_t *)addr = param->srv->fakeip;
}
#ifndef NOIPV6
else if(type==0x1c && *(uint32_t *)param->srv->fakeip6){
ip = 1;
ttl = 3600;
memcpy(addr, param->srv->fakeip6, 16);
}
#endif
else if((type==0x01 || type==0x1c) && !param->srv->s_option){
ip = udpresolve((type==0x1c)?AF_INET6:AF_INET, (unsigned char *)host, addr, &ttl, param, 0);
}
len+=5;
if(ip){
buf[2] = 0x85;
buf[3] = 0x80;
buf[6] = 0;
buf[7] = 1;
buf[8] = buf[9] = buf[10] = buf[11] = 0;
memset(buf+len, 0, 16);
buf[len] = 0xc0;
buf[len+1] = 0x0c;
buf[len+3] = type;
buf[len+5] = 1;
ttl = htonl(ttl);
memcpy(buf + len + 6, &ttl, 4);
buf[len+11] = type==1? 4:16;
memcpy(buf+len+12,(void *)&addr,type==1? 4:16);
len+=(type==1?16:28);
}
else if(type == 0x0c) {
unsigned a = 0, b = 0, c = 0, d = 0;
if(sscanf(host, "%u.%u.%u.%u", &a, &b, &c, &d) != 4) a = b = c = d = 0;
ip = htonl((d<<24) ^ (c<<16) ^ (b<<8) ^ a);
if(*SAFAMILY(&param->sincl) == AF_INET && ip == *(uint32_t *)SAADDR(&param->sincl)){
buf[2] = 0x85;
buf[3] = 0x80;
buf[6] = 0;
buf[7] = 1;
buf[8] = buf[9] = buf[10] = buf[11] = 0;
memset(buf+len, 0, 20);
buf[len] = 0xc0;
buf[len+1] = 0x0c;
buf[len+3] = 0x0c;
buf[len+5] = 1;
ttl = htonl(3600);
memcpy(buf + len + 6, &ttl, 4);
buf[len+11] = 7;
buf[len+12] = 6;
memcpy(buf+len+13,(void *)"3proxy",6);
len+=20;
}
else ip = 0;
}
if(!ip && numservers){
/* the question, to check the answer against; the name server is
asked under an id of our own, the client's put back in its answer */
unsigned char question[1024], cid[2];
int qend = (len < i)? len : i;
unsigned short id = (unsigned short)myrand();
PROXYSOCKADDRTYPE from;
time_t deadline;
int to;
cid[0] = buf[0];
cid[1] = buf[1];
buf[0] = (unsigned char)(id >> 8);
buf[1] = (unsigned char)(id & 0xff);
memcpy(question, buf, qend);
if((param->remsock=param->srv->so._socket(param->sostate, SASOCK(&nservers[0].addr), nservers[0].usetcp? SOCK_STREAM:SOCK_DGRAM, nservers[0].usetcp?IPPROTO_TCP:IPPROTO_UDP)) == INVALID_SOCKET) {
RETURN(818);
}
memset(&param->sinsl, 0, sizeof(param->sinsl));
*SAFAMILY(&param->sinsl) = *SAFAMILY(&nservers[0].addr);
if(bindwithrange(param, param->remsock, &param->sinsl, param->extport)) {
RETURN(819);
}
param->sinsr = nservers[0].addr;
if(nservers[0].usetcp) {
if(connectwithpoll(param, param->remsock,(struct sockaddr *)&param->sinsr,SASIZE(&param->sinsr),conf.timeouts[CONNECT_TO])) RETURN(830);
buf-=2;
*(unsigned short*)buf = htons(i);
i+=2;
}
else {
#ifdef _WIN32
ioctlsocket(param->remsock, FIONBIO, &ul);
#else
fcntl(param->remsock, F_SETFL, O_NONBLOCK | fcntl(param->remsock, F_GETFL));
#endif
}
if(socksendto(param, param->remsock, (struct sockaddr *)&param->sinsr, buf, i, conf.timeouts[SINGLEBYTE_L]*1000) != i){
RETURN(820);
}
param->statscli64 += i;
param->nwrites++;
/* over UDP only an answer from the name server asked is taken, any
other read past while the time for it lasts */
deadline = time(NULL) + conf.timeouts[DNS_TO];
for(;;){
to = (int)(deadline - time(NULL));
if(to <= 0) RETURN(821);
len = sockrecvfrom(param, param->remsock, (struct sockaddr *)&from, buf, BUFSIZE, to*1000);
if(len <= 13) {
RETURN(821);
}
if(nservers[0].usetcp) break;
if(*SAFAMILY(&from) == *SAFAMILY(&param->sinsr) &&
!memcmp(SAADDR(&from), SAADDR(&param->sinsr), SAADDRLEN(&param->sinsr)) &&
*SAPORT(&from) == *SAPORT(&param->sinsr) &&
dnsanswer(buf, len, question, qend)) break;
}
param->statssrv64 += len;
param->nreads++;
if(nservers[0].usetcp) {
unsigned short us;
us = ntohs(*(unsigned short *)buf);
if(us > 4096) RETURN(833);
buf += 2;
len -= 2;
if(len < us) len += sockgetlinebuf(param, SERVER, buf+len, us - len, 0, conf.timeouts[SINGLEBYTE_L]);
if(len != us) RETURN(832);
if(!dnsanswer(buf, len, question, qend)) RETURN(834);
}
buf[0] = cid[0];
buf[1] = cid[1];
if(buf[6] || buf[7]){
if(socksendto(param, param->clisock, (struct sockaddr *)&param->sincr, buf, len, conf.timeouts[SINGLEBYTE_L]*1000) != len){
RETURN(822);
}
RETURN(0);
}
}
if(!ip) {
buf[2] = 0x85;
buf[3] = 0x83;
}
res = socksendto(param, param->clisock, (struct sockaddr *)&param->sincr, buf, len, conf.timeouts[SINGLEBYTE_L]*1000);
if(res != len){RETURN(819);}
if(!ip) {RETURN(888);}
CLEANRET:
if(semlocked) _3proxy_sem_unlock(udpinit);
if(param->res!=813){
sprintf((char *)buf, "%04x/%s/",
(unsigned)type,
host?host:"");
if((ip && type == 0x01) || type == 0x1c){
myinet_ntop(type == 0x01? AF_INET:AF_INET6, addr, (char *)buf+strlen((char *)buf), 64);
}
dolog(param, buf);
}
if(bbuf)free(bbuf);
if(host)free(host);
#if !defined(_WIN32) || defined(SHARE_UDP_SOCKET)
/* The socket belongs to the service, so the caller must not close it. */
param->clisock = INVALID_SOCKET;
#endif
return (NULL);
}
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