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dns_recv_response() checked for room using sizeof(struct dns_answer_s), but that structure is the 10-byte header plus a union holding the largest address it can carry. With IPv6 built the union is 16 bytes, so the check demanded 26 bytes where 10 were needed, and any answer sitting at the end of a response was rejected as truncated. An A record answer supplies 14 bytes, so whether a lookup worked depended on how much padding the server happened to send after it: $ dig +noedns @10.1.1.2 github.com A # ANSWER 1, AUTHORITY 0, ADDITIONAL 0 -> answer is last in the packet, 14 bytes remain, rejected $ dig +noedns @10.11.5.254 github.com A # ANSWER 1, AUTHORITY 13, ADDITIONAL 7 -> 26+ bytes remain, accepted On the board, before and after, against the first of those servers: nsh> nslookup apache.org [CPU1] dns_recv_response: DNS answer header truncated Host: apache.org Addr: 2a04:4e42::644 <- A record lost nsh> nslookup apache.org Host: apache.org Addr: 2a04:4e42::644 Host: apache.org Addr: 151.101.2.132 <- both returned The address that follows the header is already bounds checked separately, where its real length is known, so only the header check was wrong. The size is now a named constant next to the structure, since the rest of this function already used the literal 10 for the same quantity. Only IPv4-only builds escaped it, where sizeof happens to equal 14 and an A record fits exactly. Assisted-by: Claude:claude-opus-5 Signed-off-by: Justin Hammond <justin@dynam.ac> |
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