| io | Package io provides basic interfaces to I/O primitives. |
| net | Package net provides a portable interface for network I/O, including TCP/IP, UDP, domain name resolution, and Unix domain sockets. |
| net/http | Package http provides HTTP client and server implementations. |
| net/http/httputil | Package httputil provides HTTP utility functions, complementing the more common ones in the net/http package. |
| os | Package os provides a platform-independent interface to operating system functionality. |
| runtime | Package runtime contains operations that interact with Go's runtime system, such as functions to control goroutines. |
| sync | Package sync provides basic synchronization primitives such as mutual exclusion locks. |
| testing | Package testing provides support for automated testing of Go packages. |