From 5019022e13ac1f807d818ab4c8d6aafc1bd7d66d Mon Sep 17 00:00:00 2001
From: Ivan Nardi <12729895+IvanNardi@users.noreply.github.com>
Date: Mon, 31 Jul 2023 07:44:43 +0200
Subject: DNS: extract geolocation information, if available (#2065)

The option NSID (RFC5001) is used by Google DNS to report the
airport code of the metro where the DNS query is handled.

This option is quite rare, but the added overhead in DNS code is pretty
much zero for "normal" DNS traffic
---
 src/lib/protocols/dns.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

(limited to 'src/lib/protocols')

diff --git a/src/lib/protocols/dns.c b/src/lib/protocols/dns.c
index 6e4864405..07358468d 100644
--- a/src/lib/protocols/dns.c
+++ b/src/lib/protocols/dns.c
@@ -547,7 +547,8 @@ static int search_valid_dns(struct ndpi_detection_module_struct *ndpi_struct,
 	  u_int16_t num;
 
 	  for(num = 0; num < dns_header->additional_rrs; num++) {
-	    u_int16_t data_len;
+	    u_int16_t data_len, rdata_len, opt_code, opt_len;
+	    const unsigned char *opt;
 
 	    if((x+6) > packet->payload_packet_len) {
 	      break;
@@ -576,6 +577,34 @@ static int search_valid_dns(struct ndpi_detection_module_struct *ndpi_struct,
 	      printf("[DNS] [response] edns0_udp_payload_size: %u\n", flow->protos.dns.edns0_udp_payload_size);
 #endif
 	      x += 6;
+
+	      rdata_len = ntohs(*((u_int16_t *)&packet->payload[x]));
+#ifdef DNS_DEBUG
+	      printf("[DNS] [response] rdata len: %u\n", rdata_len);
+#endif
+	      if(rdata_len > 0 &&
+		 x + 6 <= packet->payload_packet_len) {
+		opt_code = ntohs(*((u_int16_t *)&packet->payload[x + 2]));
+		opt_len = ntohs(*((u_int16_t *)&packet->payload[x + 4]));
+		opt = &packet->payload[x + 6];
+		/* TODO: parse the TLV list */
+		if(opt_code == 0x03 &&
+		   opt_len <= rdata_len + 4 &&
+		   opt_len > 6 &&
+		   x + 6 + opt_len <= packet->payload_packet_len) {
+#ifdef DNS_DEBUG
+		  printf("[DNS] NSID: [%.*s]\n", opt_len, opt);
+#endif
+		  if(memcmp(opt, "gpdns-", 6) == 0) {
+#ifdef DNS_DEBUG
+		    printf("[DNS] NSID Airport code [%.*s]\n", opt_len - 6, opt + 6);
+#endif
+		    memcpy(flow->protos.dns.geolocation_iata_code, opt + 6,
+			   ndpi_min(opt_len - 6, (int)sizeof(flow->protos.dns.geolocation_iata_code) - 1));
+		  }
+		}
+
+	      }
 	    } else {
 	      x += 6;
 	    }
-- 
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