Set truncation when any required glue cannot fit

When generating a referral, a name server MUST include all available glue for in-domain name servers in the additional section, or MUST set TC=1 if message-size constraints prevent doing so (§3.1). “Some addresses were included” is not an exception.

• When generating a referral, a name server MUST include all available glue for in-domain name servers in the additional section, or MUST set TC=1 if message-size constraints prevent doing so (§3.1). “Some addresses were included” is not an exception.
• TC means Truncated. It is a flag in the DNS header, not a response-code value or an instruction to invent records. RFC 1035 §4.1.1 defines its position; the selected QR-only response flags change from hex 8000 to 8200 when TC is set. Setting TC is not an instruction to change unrelated header flags.
• Our chosen remaining glue budget is measured after space for the question, authority records and other required content. The illustration supplies already-sized whole blocks: A costs 28 bytes and AAAA costs 40. These are model inputs, not universal encoded record sizes. At 68 both fit; at 50 the A block fits but the AAAA block does not.
• A client receiving truncation SHOULD use another transport to retrieve the full response (§1). UDP-to-TCP retry is the example described in §3.1, not a guarantee that every retry succeeds or that TCP has unlimited space. The section reports a 65,535-byte TCP DNS-message limit and historical common UDP sizes; neither is our remaining-budget example.
• Other extensions can also be required, including OPT, TSIG and SIG(0) when used (§1). This lesson does not free space by silently discarding required protocol material, split a supplied record block, or claim to implement message compression or general truncation policy.