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128 lines
6.3 KiB
128 lines
6.3 KiB
# DNS-over-TLS query forwarder design
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## Overview
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The DNS-over-TLS query forwarder consists of five classes:
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* `DnsTlsDispatcher`
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* `DnsTlsTransport`
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* `DnsTlsQueryMap`
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* `DnsTlsSessionCache`
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* `DnsTlsSocket`
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`DnsTlsDispatcher` is a singleton class whose `query` method is the DnsTls's
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only public interface. `DnsTlsDispatcher` is just a table holding the
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`DnsTlsTransport` for each server (represented by a `DnsTlsServer` struct) and
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network. `DnsTlsDispatcher` also blocks each query thread, waiting on a
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`std::future` returned by `DnsTlsTransport` that represents the response.
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`DnsTlsTransport` sends each query over a `DnsTlsSocket`, opening a
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new one if necessary. It also has to listen for responses from the
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`DnsTlsSocket`, which happen on a different thread.
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`IDnsTlsSocketObserver` is an interface defining how `DnsTlsSocket` returns
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responses to `DnsTlsTransport`.
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`DnsTlsQueryMap` and `DnsTlsSessionCache` are helper classes owned by `DnsTlsTransport`.
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`DnsTlsQueryMap` handles ID renumbering and query-response pairing.
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`DnsTlsSessionCache` allows TLS session resumption.
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`DnsTlsSocket` interleaves all queries onto a single socket, and reports all
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responses to `DnsTlsTransport` (through the `IDnsTlsObserver` interface). It doesn't
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know anything about which queries correspond to which responses, and does not retain
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state to indicate whether there is an outstanding query.
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## Threading
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### Overall patterns
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For clarity, each of the five classes in this design is thread-safe and holds one lock.
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Classes that spawn a helper thread call `thread::join()` in their destructor to ensure
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that it is cleaned up appropriately.
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All the classes here make full use of Clang thread annotations (and also null-pointer
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annotations) to minimize the likelihood of a latent threading bug. The unit tests are
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also heavily threaded to exercise this functionality.
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This code creates O(1) threads per socket, and does not create a new thread for each
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query or response. However, DnsProxyListener does create a thread for each query.
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### Threading in `DnsTlsSocket`
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`DnsTlsSocket` can receive queries on any thread, and send them over a
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"reliable datagram pipe" (`socketpair()` in `SOCK_SEQPACKET` mode).
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The query method writes a struct (containing a pointer to the query) to the pipe
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from its thread, and the loop thread (which owns the SSL socket)
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reads off the other end of the pipe. The pipe doesn't actually have a queue "inside";
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instead, any queueing happens by blocking the query thread until the
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socket thread can read the datagram off the other end.
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We need to pass messages between threads using a pipe, and not a condition variable
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or a thread-safe queue, because the socket thread has to be blocked
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in `poll()` waiting for data from the server, but also has to be woken
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up on inputs from the query threads. Therefore, inputs from the query
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threads have to arrive on a socket, so that `poll()` can listen for them.
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(There can only be a single thread because [you can't use different threads
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to read and write in OpenSSL](https://www.openssl.org/blog/blog/2017/02/21/threads/)).
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## ID renumbering
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`DnsTlsDispatcher` accepts queries that have colliding ID numbers and still sends them on
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a single socket. To avoid confusion at the server, `DnsTlsQueryMap` assigns each
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query a new ID for transmission, records the mapping from input IDs to sent IDs, and
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applies the inverse mapping to responses before returning them to the caller.
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`DnsTlsQueryMap` assigns each new query the ID number one greater than the largest
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ID number of an outstanding query. This means that ID numbers are initially sequential
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and usually small. If the largest possible ID number is already in use,
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`DnsTlsQueryMap` will scan the ID space to find an available ID, or fail the query
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if there are no available IDs. Queries will not block waiting for an ID number to
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become available.
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## Time constants
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`DnsTlsSocket` imposes a 20-second inactivity timeout. A socket that has been idle for
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20 seconds will be closed. This sets the limit of tolerance for slow replies,
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which could happen as a result of malfunctioning authoritative DNS servers.
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If there are any pending queries, `DnsTlsTransport` will retry them.
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`DnsTlsQueryMap` imposes a retry limit of 3. `DnsTlsTransport` will retry the query up
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to 3 times before reporting failure to `DnsTlsDispatcher`.
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This limit helps to ensure proper functioning in the case of a recursive resolver that
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is malfunctioning or is flooded with requests that are stalled due to malfunctioning
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authoritative servers.
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`DnsTlsDispatcher` maintains a 5-minute timeout. Any `DnsTlsTransport` that has had no
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outstanding queries for 5 minutes will be destroyed at the next query on a different
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transport.
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This sets the limit on how long session tickets will be preserved during idle periods,
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because each `DnsTlsTransport` owns a `DnsTlsSessionCache`. Imposing this timeout
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increases latency on the first query after an idle period, but also helps to avoid
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unbounded memory usage.
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`DnsTlsSessionCache` sets a limit of 5 sessions in each cache, expiring the oldest one
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when the limit is reached. However, because the client code does not currently
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reuse sessions more than once, it should not be possible to hit this limit.
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## Testing
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Unit tests for DoT are in `resolv_tls_unit_test.cpp`. They cover all the classes except
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`DnsTlsSocket` (which requires `CAP_NET_ADMIN` because it uses `setsockopt(SO_MARK)`) and
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`DnsTlsSessionCache` (which requires integration with libssl). These classes are
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exercised by the integration tests in `resolv_integration_test.cpp`.
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### Dependency Injection
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For unit testing, we would like to be able to mock out `DnsTlsSocket`. This is
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particularly required for unit testing of `DnsTlsDispatcher` and `DnsTlsTransport`.
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To make these unit tests possible, this code uses a dependency injection pattern:
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`DnsTlsSocket` is produced by a `DnsTlsSocketFactory`, and both of these have a
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defined interface.
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`DnsTlsDispatcher`'s constructor takes an `IDnsTlsSocketFactory`,
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which in production is a `DnsTlsSocketFactory`. However, in unit tests, we can
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substitute a test factory that returns a fake socket, so that the unit tests can
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run without actually connecting over TLS to a test server. (The integration tests
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do actual TLS.)
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## Reference
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* [BoringSSL API docs](https://commondatastorage.googleapis.com/chromium-boringssl-docs/headers.html)
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