commit f2fec37de737faf303263bd93df206fd44ec42de Author: Nick Mathewson <nickm@torproject.org> Date: Fri Aug 2 12:50:43 2013 -0400 219-expanded-dns: Clarify the exact packet format. Also, make it a bit more extensible, in case we do get multi-packet responses working in the future. --- proposals/219-expanded-dns.txt | 60 +++++++++++++++++++++++++++++++++------- 1 file changed, 50 insertions(+), 10 deletions(-) diff --git a/proposals/219-expanded-dns.txt b/proposals/219-expanded-dns.txt index dd015a6..83bc42a 100644 --- a/proposals/219-expanded-dns.txt +++ b/proposals/219-expanded-dns.txt @@ -42,28 +42,68 @@ Status: Draft There will be two new cells, RELAY_DNS_BEGIN and RELAY_DNS_RESPONSE (we'll use DNS_BEGIN and DNS_RESPONSE for short below). +1.1.1. DNS_BEGIN + DNS_BEGIN payload: - DNS packet data (variable length) + FLAGS [2 octets] + DNS packet data (variable length, up to length of relay cell.) - The DNS packet must be generated internally by libunbound to avoid + The DNS packet must be generated internally by Tor to avoid fingerprinting users by differences in client resolvers' behavior. + [XXXX We need to specify the exact behavior here: saying "Just do what + Libunbound does!" would make it impossible to implement a + Tor-compatible client without reverse-engineering libunbound.] + + The FLAGS field is reserved, and should be set to 0 by all clients. + + Because of the maximum length of the RELAY cell, the DNS packet may + not be longer than 496 bytes. [XXXX Is this enough?] + + Some fields in the query must be omitted or set to zero: see section 3 + below. + +1.1.2. DNS_RESPONSE + DNS_RESPONSE payload: + STATUS [1 octet] + CONTENT [variable, up to length of relay cell] + + If the low bit of STATUS is set, this is the last DNS_RESPONSE that + the server will send in response to the given DNS_BEGIN. Otherwise, + there will be more DNS_RESPONSE packets. The other bits are reserved, + and should be set to zero for now. + + The CONTENT fields of the DNS_RESPONSE cells contain a DNS record, + split across multiple cells as needed, encoded as: + + total length (2 octets) data (variable) - Data contains the reply DNS packet or its part if packet would not fit into - the cell. Total length describes length of complete response packet, thus - one DNS_BEGIN may be answered by multiple DNS_RESPONSE cells. + So for example, if the DNS record R1 is only 300 bytes long, then it + is sent in a single DNS_RESPONSE cell with payload [01 01 2C] R1. But + if the DNS record R2 is 1024 bytes long, it's sent in 3 DNS_RESPONSE + cells, with contents: [00 04 00] R2[0:495], [00] R2[495:992], and + [01] R2[992:1024] respectively. + + [NOTE: I'm using the length field and the is-this-the-last-cell + field to allow multi-packet responses in the future.] + + AXFR and IXRF are not supported in this cell by design (see + specialized tool below in section 5). + +1.1.3. Matching queries to responses. - DNS_BEGIN must use a non-zero, distinct StreamID, corresponding DNS_RESPONSE - will use the same StreamID. Similarly to RELAY_RESOLVE(D), no actual stream - is created. + DNS_BEGIN must use a non-zero, distinct StreamID. The client MUST NOT + re-use the same stream ID until it has received a complete response + from the server or a RELAY_END cell. - AXFR and IXRF are not supported in this cell by design (see specialized tool - below). + The client may cancel a DNS_BEGIN request by sending a RELAY_END cell. + The server may refused to answer, or abort answering, a DNS_BEGIN cell + by sending a RELAY_END cell. 2. Interfaces to applications