Strange UDP problem - BSD sockets

This is a problem with BSD sockets on Ubuntu. I hope this is not the wrong place to ask this.

I have a piece of code that should listen on DNS port 53 of the local machine, and handle DNS queries (responses and replies). If the code gets back a response with 0 answer sections, the code re-issues a query for a different DNS RR type. The code re-issues a query asking for a custom RR type. This packet is seen on the issuing machine's tcpdump, but not on the tcpdump of the machine running the resolver. All packets are sent using UDP.

Two observations:

1. If I let my piece of code sleep a second before sending the packet, all's successful! (I've commented out sleep in the code below).
2. If the application requesting a DNS query requests for the custom RR type (what should have been re-issued by my piece of code), the response is retrieved.

Please let me know if additional information would be useful

Thank you for your time,
Arun Madhavan

Code:

Socket creation: m_udp_fd = socket(PF_INET, SOCK_DGRAM, 0); if (m_udp_fd >= 0) { struct sockaddr_in sin; sin.sin_family = AF_INET; sin.sin_addr.s_addr = INADDR_ANY; sin.sin_port = htons(DNS_PORT); // allow the socket to re-bind to the same port do { int ignore = 0; if (setsockopt(m_udp_fd, SOL_SOCKET, SO_REUSEADDR, (char *)&ignore, sizeof(ignore)) < 0) { fprintf(stderr, "setsockopt with SO_REUSEADDR failed with error %d\n", GetLastError()); return false; } } while(0); // Windows non-blocking is done at create #ifndef _MSC_VER set_nonblocking(m_udp_fd); #endif /* _MSC_VER */ // bind to the port if (bind(m_udp_fd, (struct sockaddr *)&sin, sizeof(sin)) == 0) { eprintf("PANZER: port %d\n", sin.sin_port); select_loop(); // this can fall through } } void Resolver::set_nonblocking(int fd) { long flags; if ((flags = fcntl(fd, F_GETFL)) < 0) err(1, "fcntl(F_GETFL)"); if (fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) err(1, "fcntl(F_SETFL)"); } Entry point of code: if ((r = recvfrom(m_udp_fd, (char*)buf, sizeof(buf), 0, (struct sockaddr *)&sin, &sz)) > 0) { DnsPacket *p ; p = new DnsPacket(r); p->add_bytes(buf, r); if (p->parse()) { read_packet(sin, *p); } else { delete p; } } } void Resolver::read_packet(struct sockaddr_in &from, DnsPacket &p) { DnsHeader& h = p.header(); // response packet if (h.response()) { int id = h.id(); DnsQuery *q = m_queries.get(id); if (q == NULL) { delete &p; } else { q->add_response(p); // sleep(1); // forward packet write_packet(q->dest(), q->packet_to_forward()); if (q->done()) { int id = h.id(); m_queries.remove(id); } } } // packet is a question else { // silently drop packets that aren't from localhost if (from.sin_addr.s_addr == ntohl(INADDR_LOOPBACK)) { DnsQuery *q = new DnsQuery(from, p); // add the query to the LRU int id = p.header().id(); m_queries.add(id, *q); size_t len; u_char *b = p.get_bytes(len); write_packet(m_tRes, b, len); } else { delete &p; } } } void Resolver::write_packet(struct sockaddr_in &dst, DnsPacket &p) { size_t size; u_char *bytes = p.get_bytes(size); write_packet(dst, bytes, size); } void Resolver::write_packet(struct sockaddr_in &dst, u_char *b, size_t len) { bool sent = false; //printf("trying to send to %x:%d\n", dst.sin_addr.s_addr, ntohs(dst.sin_port)); // attempt to write the packet if the queue is empty if (m_send_q.empty()) { sent = try_send(dst, b, len); } if (!sent) { enqueue(dst, b, len); } } bool Resolver::try_send(struct sockaddr_in &dst, u_char *b, size_t len) { SSIZE_T w; do { w = sendto(m_udp_fd, (char*)b, len, 0, (struct sockaddr *)&dst, sizeof(dst)); if (w > 0) { eprintf("sent to %x:%d\n", dst.sin_addr.s_addr, ntohs(dst.sin_port)); delete [] b; } } while (w < 0 && errno == EINTR); if (w == 0) { eprintf("socket closed while writing\n"); exit(1); } if (w < 0 && errno != EAGAIN) { int errorNum = GetLastError(); eprintf("error on socket while writing: %s/%d\n", strerror(errorNum), errorNum); exit(1); } // return true only if we wrote the packet if (w > 0) { return true; } return false; }