更新DNS响应包RDATA自定义IP后无法生效的技术问询

Let's break down why your custom DNS response isn't being accepted—there are a few common gotchas when manually tweaking DNS packets that you might have overlooked:

1. You Didn't Recalculate the Packet Checksums #

DNS typically runs over UDP, and network stacks will silently drop packets with invalid checksums. When you modified the RDATA field, you changed the packet's content, which invalidates both the UDP checksum and (if using IPv4) the IP header checksum. You must recalculate these after making your edits:

• IPv4 Header Checksum: Set the checksum field to 0 first, then compute the 16-bit one's complement of the one's complement sum of all 16-bit words in the header.
• UDP Checksum: This includes the UDP header, payload, and a pseudo-header (source IP, destination IP, protocol, UDP length). Skipping this step is almost certainly why your response is being ignored.

2. Key DNS Header Fields Don't Match the Query #

Clients only accept responses that directly correspond to their original query. Double-check these critical fields:

• ID Field: The ID in your modified response must exactly match the ID from the DNS query. If they don't line up, the client will discard the response entirely.
• Flags: Ensure the QR bit is set to 1 (marks this as a response), RCODE is 0 (no error), and other flags like AA (Authoritative Answer) are set appropriately for your use case.
• Question Section: The question in the response (domain name, record type, class) must be identical to the one in the query. Any mismatches here will make the client reject the response.

3. You Might Be Editing the Wrong Bytes #

You mentioned targeting the four bytes after data_len, but DNS packets have a variable structure that can trip you up:

• After the header comes the question section, followed by answer records. Each answer record may use a pointer to the question name (to save space) before the type, class, TTL, data length, and RDATA fields.
• Use Wireshark to capture the original and modified packets side-by-side—this will let you confirm you're actually editing the RDATA section of the A record, not some other part of the packet.

4. Minor Metadata or Caching Issues #

While less likely, these could also cause repeated queries:

• A very short TTL might make the client re-query immediately. Try setting a reasonable TTL (like 300 seconds) in the answer record.
• Some clients cache old DNS responses. Clear the client's DNS cache to ensure it's not ignoring your new response in favor of an old one.

Quick Action Steps to Test #

1. Capture and Compare Packets: Use Wireshark to check for differences in checksum, ID, flags, and RDATA position between the original and modified response.
2. Add Checksum Recalculation: Here's a simplified pseudocode example for recalculating the UDP checksum (you'll need to adapt this to your codebase):

   void update_udp_checksum(struct iphdr *ip_header, struct udphdr *udp_header) {
       udp_header->check = 0;
       // Build UDP pseudo-header
       struct pseudo_header {
           uint32_t src_ip;
           uint32_t dst_ip;
           uint8_t zero;
           uint8_t protocol;
           uint16_t udp_length;
       } pseudo_hdr;
       
       pseudo_hdr.src_ip = ip_header->saddr;
       pseudo_hdr.dst_ip = ip_header->daddr;
       pseudo_hdr.zero = 0;
       pseudo_hdr.protocol = IPPROTO_UDP;
       pseudo_hdr.udp_length = udp_header->len;
       
       // Calculate sum of pseudo-header, UDP header, and payload
       uint32_t sum = checksum((uint16_t *)&pseudo_hdr, sizeof(pseudo_hdr));
       sum += checksum((uint16_t *)udp_header, ntohs(udp_header->len));
       
       // Fold sum to 16-bit checksum
       while (sum >> 16) sum = (sum & 0xFFFF) + (sum >> 16);
       udp_header->check = htons(~sum);
   }
   

3. Validate Header Mismatches: Confirm the response ID matches the query ID, and all required flags are set correctly.

Fixing these issues should make your modified DNS response get accepted by the client.

内容的提问来源于stack exchange，提问作者user707901