As part of the
have addressed issue 20,
which is RPC authentication. Until this was implemented, all RPC calls to
were unauthenticated. To fix this, we've utilized
macaroons, which are similar
to cookies but more capable. This brief overview explains, at a basic level,
how they work, how we use them for
lnd authentication, and our future plans.
What are macaroons?
You can think of a macaroon as a cookie, in a way. Cookies are small bits of data that your browser stores and sends to a particular website when it makes a request to that website. If you're logged into a website, that cookie can store a session ID, which the site can look up in its own database to check who you are and give you the appropriate content.
A macaroon is similar: it's a small bit of data that a client (like
can send to a service (like
lnd) to assert that it's allowed to perform an
action. The service looks up the macaroon ID and verifies that the macaroon was
initially signed with the service's root key. However, unlike a cookie, you can
delegate a macaroon, or create a version of it that has more limited
capabilities, and then send it to someone else to use.
Just like a cookie, a macaroon should be sent over a secure channel (such as a TLS-encrypted connection), which is why we've also begun enforcing TLS for RPC requests in this release. Before SSL was enforced on websites such as Facebook and Google, listening to HTTP sessions on wireless networks was one way to hijack the session and log in as that user, gaining access to the user's account. Macaroons are similar in that intercepting a macaroon in transit allows the interceptor to use the macaroon to gain all the privileges of the legitimate user.
A macaroon is delegated by adding restrictions (called caveats) and an authentication code similar to a signature (technically an HMAC) to it. The technical method of doing this is outside the scope of this overview documentation, but the README in the macaroons package or the macaroon paper linked above describe it in more detail. The user must remember several things:
Sharing a macaroon allows anyone in possession of that macaroon to use it to access the service (in our case,
lnd) to do anything permitted by the macaroon. There is a specific type of restriction, called a "third party caveat," that requires an external service to verify the request; however,
lnddoesn't currently implement those.
If you add a caveat to a macaroon and share the resulting macaroon, the person receiving it cannot remove the caveat.
This is used in
lnd in an interesting way. By default, when
lnd starts, it
creates three files which contain macaroons: a file called
which contains a macaroon with no caveats, a file called
which is the same macaroon but with an additional caveat, that permits only
methods that don't change the state of
only has access to invoice related methods.
How macaroons are used by
lnd checks to see if the
invoice.macaroon files exist. If they don't exist,
lnd updates its
database with a new macaroon ID, generates the three files
invoice.macaroon, all with the same ID. The
readonly.macaroon file has an additional caveat which restricts the caller
to using only read-only methods and the
invoice.macaroon also has an
additional caveat which restricts the caller to using only invoice related
methods. This means a few important things:
You can delete the
admin.macaroonand be left with only the
readonly.macaroon, which can sometimes be useful (for example, if you want your
lndinstance to run in autopilot mode and don't want to accidentally change its state).
If you delete the data directory which contains the
macaroons.dbfile, this invalidates the
invoice.macaroonfiles. Invalid macaroon files give you errors like
cannot get macaroon: root key with id 0 doesn't existor
verification failed: signature mismatch after caveat verification.
You can also run
lnd with the
--no-macaroons option, which skips the
creation of the macaroon files and all macaroon checks within the RPC server.
This means you can still pass a macaroon to the RPC server with a client, but
it won't be checked for validity.
lnd requires macaroons by default in order to call RPC methods,
now reads a macaroon and provides it in the RPC call. Unless the path is
changed by the
lncli tries to read the macaroon from
the network directory of
lnd's currently active network (e.g. for simnet
lnddir/data/chain/bitcoin/simnet/admin.macaroon) by default and will error if
that file doesn't exist unless provided the
--no-macaroons option. Keep this
in mind when running
lncli will error out
unless called the same way or
lnd has generated a macaroon on a previous
run without this option.
lncli also adds a caveat which makes it valid for only 60 seconds by default
to help prevent replay in case the macaroon is somehow intercepted in
transmission. This is unlikely with TLS, but can happen e.g. when using a PKI
and network setup which allows inspection of encrypted traffic, and an attacker
gets access to the traffic logs after interception. The default 60 second
timeout can be changed with the
--macaroontimeout option; this can be
increased for making RPC calls between systems whose clocks are more than 60s
Using Macaroons with GRPC clients
When interacting with
lnd using the GRPC interface, the macaroons are encoded
as a hex string over the wire and can be passed to
lnd by specifying the
hex-encoded macaroon as GRPC metadata:
GET https://localhost:8080/v1/getinfo Grpc-Metadata-macaroon: <macaroon>
<macaroon> is the hex encoded binary data from the macaroon file itself.
A very simple example using
curl may look something like this:
curl --insecure --header "Grpc-Metadata-macaroon: $(xxd -ps -u -c 1000 $HOME/.lnd/data/chain/bitcoin/simnet/admin.macaroon)" https://localhost:8080/v1/getinfo
Have a look at the Java GRPC example for programmatic usage details.
Future improvements to the
lnd macaroon implementation
The existing macaroon implementation in
lncli lays the groundwork
for future improvements in functionality and security. We will add features
Improved replay protection for securing RPC calls
Macaroon database encryption
Root key rotation and possibly macaroon invalidation/rotation
Tools to allow you to easily delegate macaroons in more flexible ways
Additional restrictions, such as limiting payments to use (or not use) specific routes, channels, nodes, etc.
Accounting-based macaroons, which can make an instance of
lndact almost like a bank for apps: for example, an app that pays to consume APIs whose budget is limited to the money it receives by providing an API/service
Support for third-party caveats, which allows external plugins for authorization and authentication
With this new feature, we've started laying the groundwork for flexible
authentication and authorization for RPC calls to
lnd. We look forward to
expanding its functionality to make it easy to develop secure apps.