123 lines
5.0 KiB
Markdown
123 lines
5.0 KiB
Markdown
# Block and Transaction Broadcasting With ZeroMQ
|
|
|
|
[ZeroMQ](http://zeromq.org/) is a lightweight wrapper around TCP
|
|
connections, inter-process communication, and shared-memory, providing
|
|
various message-oriented semantics such as publish/subscribe,
|
|
request/reply, and push/pull.
|
|
|
|
The Zcash daemon can be configured to act as a trusted "border
|
|
router", implementing the Zcash wire protocol and relay, making
|
|
consensus decisions, maintaining the local blockchain database,
|
|
broadcasting locally generated transactions into the network, and
|
|
providing a queryable RPC interface to interact on a polled basis for
|
|
requesting blockchain related data. However, there exists only a
|
|
limited service to notify external software of events like the arrival
|
|
of new blocks or transactions.
|
|
|
|
The ZeroMQ facility implements a notification interface through a set
|
|
of specific notifiers. Currently there are notifiers that publish
|
|
blocks and transactions. This read-only facility requires only the
|
|
connection of a corresponding ZeroMQ subscriber port in receiving
|
|
software; it is not authenticated nor is there any two-way protocol
|
|
involvement. Therefore, subscribers should validate the received data
|
|
since it may be out of date, incomplete or even invalid.
|
|
|
|
ZeroMQ sockets are self-connecting and self-healing; that is,
|
|
connections made between two endpoints will be automatically restored
|
|
after an outage, and either end may be freely started or stopped in
|
|
any order.
|
|
|
|
Because ZeroMQ is message oriented, subscribers receive transactions
|
|
and blocks all-at-once and do not need to implement any sort of
|
|
buffering or reassembly.
|
|
|
|
## Prerequisites
|
|
|
|
The ZeroMQ feature in Zcash requires the ZeroMQ API >= 4.0.0
|
|
[libzmq](https://github.com/zeromq/libzmq/releases), which you will
|
|
need to install if you are not using the depends system.
|
|
Typically, it is packaged by distributions as something like
|
|
*libzmq5-dev*. The C++ wrapper for ZeroMQ is *not* needed.
|
|
|
|
In order to run the example Python client scripts in contrib/ one must
|
|
also install *python3-zmq*, though this is not necessary for daemon
|
|
operation.
|
|
|
|
## Security WARNING
|
|
|
|
Enabling this feature even on the loopback interface only (e.g. binding
|
|
it to localhost or 127.0.0.1) will still expose it to the wilds of the
|
|
Internet, because of an attack vector called DNS rebinding. DNS
|
|
rebinding allows an attacker located remotely on the Internet to trick
|
|
applications that you're running on the same computer as Zcashd to
|
|
contact your supposedly localhost-only ZMQ port, then, depending on the
|
|
program they may be able to attempt to attack it.
|
|
|
|
Do not enable this feature unless you are sure that you know what you
|
|
are doing, and that you have a strong reason for thinking that you are
|
|
not vulnerable to this type of attack.
|
|
|
|
## Enabling
|
|
|
|
By default, the ZeroMQ feature is automatically compiled in if the
|
|
necessary prerequisites are found. To disable, use --disable-zmq
|
|
during the *configure* step of building zcashd:
|
|
|
|
$ ./configure --disable-zmq (other options)
|
|
|
|
To actually enable operation, one must set the appropriate options on
|
|
the commandline or in the configuration file.
|
|
|
|
## Usage
|
|
|
|
Currently, the following notifications are supported:
|
|
|
|
-zmqpubhashtx=address
|
|
-zmqpubhashblock=address
|
|
-zmqpubrawblock=address
|
|
-zmqpubrawtx=address
|
|
|
|
The socket type is PUB and the address must be a valid ZeroMQ socket
|
|
address. The same address can be used in more than one notification.
|
|
|
|
For instance:
|
|
|
|
$ zcashd -zmqpubhashtx=tcp://127.0.0.1:28332 \
|
|
-zmqpubrawtx=ipc:///tmp/zcashd.tx.raw
|
|
|
|
Each PUB notification has a topic and body, where the header
|
|
corresponds to the notification type. For instance, for the
|
|
notification `-zmqpubhashtx` the topic is `hashtx` (no null
|
|
terminator) and the body is the hexadecimal transaction hash (32
|
|
bytes).
|
|
|
|
These options can also be provided in zcash.conf.
|
|
|
|
ZeroMQ endpoint specifiers for TCP (and others) are documented in the
|
|
[ZeroMQ API](http://api.zeromq.org/4-0:_start).
|
|
|
|
Client side, then, the ZeroMQ subscriber socket must have the
|
|
ZMQ_SUBSCRIBE option set to one or either of these prefixes (for
|
|
instance, just `hash`); without doing so will result in no messages
|
|
arriving. Please see `contrib/zmq/zmq_sub.py` for a working example.
|
|
|
|
## Remarks
|
|
|
|
From the perspective of zcashd, the ZeroMQ socket is write-only; PUB
|
|
sockets don't even have a read function. Thus, there is no state
|
|
introduced into zcashd directly. Furthermore, no information is
|
|
broadcast that wasn't already received from the public P2P network.
|
|
|
|
No authentication or authorization is done on connecting clients; it
|
|
is assumed that the ZeroMQ port is exposed only to trusted entities,
|
|
using other means such as firewalling.
|
|
|
|
Note that when the block chain tip changes, a reorganisation may occur
|
|
and just the tip will be notified. It is up to the subscriber to
|
|
retrieve the chain from the last known block to the new tip.
|
|
|
|
There are several possibilities that ZMQ notification can get lost
|
|
during transmission depending on the communication type you are
|
|
using. Zcashd appends an up-counting sequence number to each
|
|
notification which allows listeners to detect lost notifications.
|