[tor-dev] DNS/DNSSEC resolving in Tor (PoC implementation)

Ondrej Mikle ondrej.mikle at gmail.com
Thu Jan 26 21:42:53 UTC 2012


I decided to give it a shot in implementing full DNS/DNSSEC resolution support
for Tor, here's the branch:


ATM the biggest limitation is that reply DNS packet must fit in a single cell
(i.e. max size is RELAY_PAYLOAD_SIZE).

How it's implemented:

There's new command SOCKS_COMMAND_RESOLVE_FULL for SOCKS interface and new cells
RELAY_COMMAND_RESOLVE(D)_FULL. The RESOLVE_FULL cell contains query string and
RR-type, RESOLVED_FULL just the DNS packet in wire format.

Resolving is implemented via libunbound on the relay's side, ldns parses packet
on client's side.

The tor-resolve now uses the new SOCKS command and accepts -t parameter with
RR-type (numeric, default 1 - RR-type 'A'), e.g.:

./src/tools/tor-resolve -t 28 lupa.cz localhost:10050

Packet size: 319
Flags: qr: 1, aa: 0, tc: 0, rd: 1, cd: 0, ra: 1, ad: 1
-- Rcode: NOERROR
-- Opcode: QUERY
-- Question section
lupa.cz.	IN	AAAA
-- Answer section
lupa.cz.	600	IN	AAAA	2001:67c:68::7b
lupa.cz.	600	IN	RRSIG	AAAA 5 2 600 20121022235305 20111023235305 8130 lupa.cz.
-- Authority section
lupa.cz.	600	IN	NS	ns.iinfo.cz.
lupa.cz.	600	IN	NS	ns6.adminit.cz.
lupa.cz.	600	IN	RRSIG	NS 5 2 600 20121022235305 20111023235305 8130 lupa.cz.
-- Additional section

Other implementation notes:
- some checks like whether private address is resolved are missing (also a
whitelist of allowed RR-types might be implemented)
- in the SOCKS5 request, RR-type is hacked onto port number
- in SOCKS5 reply, high byte of length is hacked onto SOCKS5 reserved byte
- libunbound supports async resolving, for now synchronous is used
- there are more details, grep FIXDNS in code
- Makefile.am's have -lldns and -lunbound hardwired
- new code may not be pretty at some places (getting to know Tor code)

Also, this seems to be a bug in
relay.c:connection_edge_process_relay_cell_not_open(), the

    answer_len = cell->payload[RELAY_HEADER_SIZE+1];
    if (rh->length < 2 || answer_len+2>rh->length) {...}

Payload is accessed before checking bounds.


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