In order to run Hermes, you will need to have a configuration file.

The format supported for the configuration file is TOML.

By default, Hermes expects the configuration file to be located at $HOME/.hermes/config.toml.

This can be overridden by supplying the -c flag when invoking hermes, before the name of the command to run, eg. hermes -c my_config.toml query connection channels ibc-1 connection-1.

The current version of Hermes does not support managing the configuration file programmatically. You will need to use a text editor to create the file and add content to it.


Table of contents


The configuration file must have one global section, and one chains section for each chain.

Note: As of 0.6.0, the Hermes configuration file is self-documented. Please read the configuration file config.toml itself for the most up-to-date documentation of parameters.

By default, Hermes will relay on all channels available between all the configured chains. In this way, every configured chain will act as a source (in the sense that Hermes listens for events) and as a destination (to relay packets that others chains have sent).

For example, if there are only two chains configured, then Hermes will only relay packets between those two, i.e. the two chains will serve as a source for each other, and likewise as a destination for each other's relevant events. Hermes will ignore all events that pertain to chains which are unknown (ie. not present in config.toml).

To restrict relaying on specific channels, or uni-directionally, you can use packet filtering policies.

Adding private keys

For each chain configured you need to add a private key for that chain in order to submit transactions, please refer to the Keys sections in order to learn how to add the private keys that are used by the relayer.

Connecting via TLS

Hermes supports connection via TLS for use-cases such as connecting from behind a proxy or a load balancer. In order to enable this, you'll want to set the rpc_addr, grpc_addr, or websocket_addr parameters to specify a TLS connection via HTTPS using the following scheme (note that the port number 443 is just used for example):

rpc_addr = ''
grpc_addr = ''
websocket_addr = 'wss://'

Support for Interchain Accounts

As of version 0.13.0, Hermes supports relaying on Interchain Accounts channels.

If the packet_filter option in the chain configuration is disabled, then Hermes will relay on all existing and future channels, including ICA channels.

There are two kinds of ICA channels:

  1. The host channels, whose port is icahost
  2. The controller channels, whose port starts with icacontroller- followed by the owner account address. See the spec for more details.

If you wish to only relay on a few specific standard channels (here channel-0 and channel-1), but also relay on all ICA channels, you can specify the following packet filter:

Note the use of wildcards in the port and channel identifiers (['ica*', '*']) to match over all the possible ICA ports.

policy = 'allow'
list = [
  ['ica*', '*'], # allow relaying on all channels whose port starts with `ica`
  ['transfer', 'channel-0'],
  ['transfer', 'channel-1'],
  # Add any other port/channel pairs you wish to relay on

If you wish to relay on all channels but not on ICA channels, you can use the following packet filter configuration:

policy = 'deny'
list = [
  ['ica*', '*'], # deny relaying on all channels whose port starts with `ica`

Update the configuration without restarting Hermes

⚠️ This feature has been removed in Hermes v0.12.0.

Before Hermes 0.6.1, the only way to get Hermes to pick up a change in the configuration was to stop and restart Hermes.

As of version 0.6.1, Hermes will react to receiving a SIGHUP signal by reloading the [chains] section of the configuration, and stopping, starting or restarting the affected workers.

Warning: the configuration reload feature only supports adding, removing, or updating configuration of chains. It does not support dynamically changing global features, such as the filtering mechanism or logging level.

For example, say you start with the configuration given in the previous section in ~/.hermes/config.toml, ie. with two chains ibc-0 and ibc-1.

  1. Start three chains ibc-0, ibc-1 and ibc-2:

    ./scripts/dev-env ibc-0 ibc-1 ibc-2
  2. Start Hermes

    hermes start
  3. Add the configuration for the chain ibc-2 to the configuration file:

    id = 'ibc-2'
    rpc_addr = ''
    grpc_addr = ''
    websocket_addr = 'ws://'
    rpc_timeout = '10s'
    account_prefix = 'cosmos'
    key_name = 'testkey'
    store_prefix = 'ibc'
    max_gas = 20000000
    gas_price = { price = 0.001, denom = 'stake' }
    clock_drift = '5s'
    trusting_period = '14days'
  4. Change the configuration of the chain ibc-0, eg. the max_gas property.

  5. Send a SIGHUP signal to the hermes process:

    ⚠️ Warning: the command below will send a SIGHUP signal to the first process in the list emitted by ps aux which contains the string hermes. Alternatively, you can look up the process ID (PID) of the hermes process you want to target and use kill -SIGHUP PID.

    ps aux | rg hermes | awk '{ print $2 }' | head -n1 | xargs -I{} kill -SIGHUP {}
  6. Watch the output of Hermes, it will show that Hermes has picked up the changes in the config. Hermes is now relaying between the three chains and using the new maximum amount of gas specified for ibc-0.

    INFO reloading configuration (triggered by SIGHUP)
    INFO configuration successfully reloaded
    INFO updating existing chain
    INFO adding new chain

To make sure Hermes ends up in the expected state, check out the documentation on inspecting the relayer state.

Next steps

Now that you learned how to build the relayer and how to create a configuration file, you can go to the Two Chains tutorial to learn how to perform some local testing connecting the relayer to two local chains.