# Compose environment variables reference **Note:** Environment variables are no longer the recommended method for connecting to linked services. Instead, you should use the link name (by default, the name of the linked service) as the hostname to connect to. See the [docker-compose.yml documentation](compose-file.md#links) for details. Compose uses [Docker links] to expose services' containers to one another. Each linked container injects a set of environment variables, each of which begins with the uppercase name of the container. To see what environment variables are available to a service, run `docker-compose run SERVICE env`. name\_PORT
Full URL, e.g. `DB_PORT=tcp://172.17.0.5:5432` name\_PORT\_num\_protocol
Full URL, e.g. `DB_PORT_5432_TCP=tcp://172.17.0.5:5432` name\_PORT\_num\_protocol\_ADDR
Container's IP address, e.g. `DB_PORT_5432_TCP_ADDR=172.17.0.5` name\_PORT\_num\_protocol\_PORT
Exposed port number, e.g. `DB_PORT_5432_TCP_PORT=5432` name\_PORT\_num\_protocol\_PROTO
Protocol (tcp or udp), e.g. `DB_PORT_5432_TCP_PROTO=tcp` name\_NAME
Fully qualified container name, e.g. `DB_1_NAME=/myapp_web_1/myapp_db_1` [Docker links]: http://docs.docker.com/userguide/dockerlinks/ ## Related Information - [User guide](index.md) - [Installing Compose](install.md) - [Command line reference](./reference/index.md) - [Compose file reference](compose-file.md)