DNS Telegram

The concierge's question travels as a telegram lesson 1's story, one level closer — what's actually written on the note that gets mailed. still no scary bytes. what your laptop wrote: TO: the concierge EXAMPLE (7 letters) · COM (3 letters) · END no dots! the wire spells names like a telegram: say how many letters are coming, then the chunk, then a full stop. (the dot is for humans — the machines were never told about it) what came back: WHO: 〃 (ditto — same as above. why repeat it?) HOUSE No: 104.20.23.154 SPARE DOOR: 172.66.147.243 VALID FOR: 32 seconds the ditto mark is a real trick in the message — repeating names wastes space, and space is precious (see right →) why so frugal? the whole exchange must fit ONE postcard each way (lesson 3!). small message = one trip = fast internet. and if an answer ever outgrows the postcard? it comes back stamped like this → and your laptop simply redials the same question as a phone call instead. TOO LONG — CALL ME two doors, one shop door 1 104.20.23.154 door 2 172.66.147.243 handing out two addresses splits the crowd naturally — half the visitors walk to each door. crowd control with zero machinery. big sites do exactly this, at scale. where you feel this: a site that stays up while millions visit at once — and gamers switching DNS to 1.1.1.1 hoping for "faster internet" (now you know what they mean) the jargon, now that you already own the story: letter-counting = "length-prefixed labels" 〃 ditto mark = "compression pointer" the expiry = "TTL" "too long — call me" stamp = the "TC bit" postcard vs phone call = UDP vs TCP want the actual bytes with these exact values? the x-ray view is one click away: DNS Wire Format THE HEADER & FLAG BITS -- the structure behind the telegram a DNS message = 5 sections: HEADER (12 bytes) QUESTION (what you asked) ANSWER (the records) AUTHORITY (nameservers) ADDITIONAL (extras) your telegram story lives in QUESTION + ANSWER. the 12-byte HEADER, field by field: ID 2 bytes FLAGS 2 bytes QDCOUNT 2 bytes ANCOUNT 2 bytes NSCOUNT 2 bytes ARCOUNT 2 bytes the 2 FLAGS bytes, bit by bit: QR OPCODE AA TC RD RA Z RCODE TC = your 'TOO LONG, CALL ME' stamp. RD = 'please recurse'. QR = query(0)/response(1). RCODE = error (0=OK, 3=NXDOMAIN). the 'ditto' on the wire = 0xC0 0x0C: the bits '11' mean 'pointer', the rest = offset 12 -> reuse the name already written in QUESTION.