The transport module has a dedicated tracer logger which, when activated, logs incoming and out going requests. The log can be dynamically activated by settings the level of the transport.tracer logger to TRACE:

curl -XPUT localhost:9200/_cluster/settings -d '{
    "transient" : {
        "logger.transport.tracer" : "TRACE"
    }
}'

You can also control which actions will be traced, using a set of include and exclude wildcard patterns. By default every request will be traced except for fault detection pings:

curl -XPUT localhost:9200/_cluster/settings -d '{
    "transient" : {
        "transport.tracer.include" : "*"
        "transport.tracer.exclude" : "internal:discovery/zen/fd*"
    }
}'