aosp12/external/mdnsresponder/PrivateDNS.txt

Private DNS

Summary

Private DNS is an extension to standard Wide Area Bonjour that allows
for secure, encrypted, and authorized communications. Private data sent
from a client to a DNS server is encrypted using Transport Layer
Security (TLS), ensuring that the data is hidden from prying eyes, and
contains Transaction Signatures (TSIG), so the server can authorize the
request. TSIGs are typically associated with Dynamic Updates; we are
using them for standard and long-lived queries as well. Private DNS also
protects Dynamic Updates from eavesdropping, by wrapping the update in a
TLS communication channel if the server has been configured appropriately.

Architectural Overview

mDNSResponder has been modified to automatically issue a private query
when necessary. After receiving an NXDOMAIN error, mDNSResponder checks
in the system keychain to see if the user has a DNS query key (TSIG key)
for the name in question, or for a parent of that name. If a suitable
key is found, mDNSResponder looks up the zone data associated with the
name of the question. After determining the correct name server,
mDNSResponder looks up an additional SRV record "_dns-private._tcp". If
it finds this record, mDNSResponder will re-issue the query privately.
If either there is no _dns-private._tcp record, or there is no secret
key, the call fails as it initially did, with an NXDOMAIN error.

Once the secret key is found and the SRV record is looked up, mDNSResponder
opens a TLS connection to the server on the port specified in the SRV
record just looked up. After the connection succeeds, mDNSResponder
can proceed to use that communication channel to make requests of
the server. Every private packet must also have a TSIG record;
the DNS server uses this TSIG record to allow access to its data.

When setting up a long-lived query over TCP (with or without TLS)
TCP's standard three-way handshake makes the full four-packet LLQ setup
exchange described in <http://files.dns-sd.org/draft-sekar-dns-llq.txt>
unnecessary. Instead, when connecting over TCP, the client simply sends
a setup message and expects to receive ACK + Answers. The setup message
sent is formatted as described in the LLQ document, however there is
an additional TSIG' resource record added to the end of it. The TSIG
resource records looks and acts exactly as it does in a secure update.
So when the server receives an LLQ (or a standard query), it looks to
see if the zone that is being referenced is public or private. If it's
private, then it makes sure that the client is authorized to query that
zone (by using the TSIG signature) and returns the appropriate data.
When a zone is configured as private, the server will do this type of
authorization checking for every query except those queries that are
looking for SOA and NS records.

Implementation Issues

dnsextd

dnsextd has been modified to behave much like a DNS firewall. The "real"
DNS server is configured to listen on non-standard ports on the loopback
interface. dnsextd then listens on the standard DNS ports (TCP/UDP port
53) and intercepts all DNS traffic. It is responsible for determining
what zone a DNS request is associated with, determining whether the
client is allowed access to that zone, and returning the appropriate
information back to the caller. If the packet is allowed access, dnsextd
forwards the request to the "real" nameserver, and returns the result to
the caller.

It was tempting to use BIND9's facility for configuring TSIG enabled
queries while doing this work. However after proceeding down that path,
enough subtle interaction problems were found that it was not practical
to pursue this direction, so instead dnsextd does all TSIG processing
for queries itself. It does continue to use BIND9 for processing TSIG
enabled dynamic updates, though one minor downside with this is that
there are two configuration files (named.conf or dnsextd.conf) that have
the same secret key information. That seems redundant and error-prone,
and moving all TSIG processing for both queries and updates into dnsextd
would fix this.

All private LLQ operations are TSIG-enabled and sent over a secure
encrypted TLS channel. To accommodate service providers who don't want
to have to keep open a large number of TLS connections to a large number
of client machines, the server has the option of dropping the TLS
connection after initial LLQ setup and sending subsequent events and
refreshes using unencrypted UDP packets. This results in less load on
the server, at the cost of slightly lower security (LLQs can only be set
up by an authorized client, but once set up, subsequent change event
packets sent over unencrypted UDP could be observed by an eavesdropper).
A potential solution to this deficiency might be in using DTLS, which is
a protocol based on TLS that is capable of securing datagram traffic.
More investigation needs to be done to see if DTLS is suitable for
private DNS.

It was necessary to relax one of the checks that dnsextd performs during
processing of an LLQ refresh. Prior to these changes, dnsextd would
verify that the refresh request came from the same entity that setup the
LLQ by comparing both the IP Address and port number of the request
packet with the IP Address and port number of the setup packet. Because
of the preceding issue, a refresh request might be sent over two
different sockets. While their IP addresses would be the same, their
port numbers could potentially differ. This check has been modified to
only check that the IP addresses match.

When setting up a semi-private LLQ (where the request and initial answer
set is sent over TLS/TCP, but subsequent change events are sent over
unencrypted UDP), dnsextd uses the port number of the client's TCP
socket to determine the UDP event port number. While this eliminates the
need to pass the UDP event port number in the LLQ setup request
(obviating a potential data mismatch error), I think it does more harm
than good, for three reasons:

1) We are relying that all the routers out there implement the Port
   Mapping Protocol spec correctly.

2) Upon setup every LLQ must NAT map two ports. Upon tear down every LLQ
   must tear down two NAT mappings.

3) Every LLQ opens up two sockets (TCP and UDP), rather than just the
   one TCP socket.

All of this just to avoid sending two bytes in the LLQ setup packet
doesn't seem logical. The approach also necessitates creating an
additional UDP socket for every private LLQ, port mapping both the TCP
socket as well as the UDP socket, and moderately increasing the
complexity and efficiency of the code. Because of this we plan to allow
the LLQ setup packet to specify a different UDP port for change event
packets. This will allow mDNSResponder to receive all UDP change event
packets on a single UDP port, instead of a different one for each LLQ.

Currently, dnsextd is buggy on multi-homed hosts. If it receives a
packet on interface 2, it will reply on interface 1 causing an error in
the client program.

dnsextd doesn't fully process all of its option parameters.
Specifically, it doesn't process the keywords: "listen-on",
"nameserver", "private", and "llq". It defaults to expecting the "real"
nameserver to be listening on 127.0.0.1:5030.


mDNSResponder

Currently, mDNSResponder attempts to issue private queries for all
queries that initially result in an NXDOMAIN error. This behavior might
be modified in future versions, however it seems patently incorrect to
do this for reverse name lookups. The code that attempts to get the zone
data associated with the name will never find the zone for a reverse
name lookup, and so will issue a number of wasteful DNS queries.

mDNSResponder doesn't handle SERV_FULL or STATIC return codes after
setting up an LLQ over TCP. This isn't a terrible problem right now,
because dnsextd doesn't ever return them, but this should be fixed so
that mDNSResponder will work when talking to other servers that do
return these error codes.


Configuration:

Sample named.conf:

//
// Include keys file
//
include "/etc/rndc.key";
// Declares control channels to be used by the rndc utility.
//
// It is recommended that 127.0.0.1 be the only address used.
// This also allows non-privileged users on the local host to manage
// your name server.

//
// Default controls
//
controls
	{
	inet 127.0.0.1 port 54 allow { any; } keys { "rndc-key"; };
	};

options
	{
	directory "/var/named";
	/*
	 * If there is a firewall between you and nameservers you want
	 * to talk to, you might need to uncomment the query-source
	 * directive below. Previous versions of BIND always asked
	 * questions using port 53, but BIND 8.1 uses an unprivileged
	 * port by default.
	 */
	
	forwarders
			{
			65.23.128.2;
			65.23.128.3;
			};
	
	listen-on port 5030 { 127.0.0.1; };
	recursion true;
	};

// 
// a caching only nameserver config
// 
zone "." IN
	{
	type hint;
	file "named.ca";
	};

zone "localhost" IN
	{
	type master;
	file "localhost.zone";
	allow-update { none; };
	};

zone "0.0.127.in-addr.arpa" IN
	{
	type master;
	file "named.local";
	allow-update { none; };
	};

zone "hungrywolf.org." in
	{
	type master;
	file "db.hungrywolf.org";
	allow-update { key hungrywolf.org.; };
	};

zone "157.23.65.in-addr.arpa" IN
	{
	file "db.65.23.157";
	type master;
	};

zone "100.255.17.in-addr.arpa" IN
	{
	file "db.17.255.100";
	type master;
	};

zone "66.6.24.in-addr.arpa" IN
	{
	file "db.24.6.66";
	type master;
	};

key hungrywolf.org.
	{
	algorithm hmac-md5;
	secret "c8LWr16K6ju6KMO5zT6Tyg==";
	};

logging
	{
	category default { _default_log; };

	channel _default_log
		{
		file "/Library/Logs/named.log";
		severity info;
		print-time yes;
		};
	};


Sample dnsextd.conf:

options { };

key "hungrywolf.org."
	{
	secret "c8LWr16K6ju6KMO5zT6Tyg==";
	};

zone "hungrywolf.org."
	{
	type private;
	allow-query { key hungrywolf.org.; };
	};