HTTP Server on random available port in Go

Imagine you want to start an HTTP server without specifying the port. For example, a temporary server for testing. How do you do that?

The answer is straightforward. Specify the port number to 0 like:

l, err := net.Listen("tcp", ":0")

It’s an easy rule provided by the net package, but do you understand how Go binds a random port allocation. I’ll give you a clear understanding of what Go does inside the net package.

The presentation contains the following topics.

net, net/http packages
File descriptor
TCP, UDP
System calls
I think this presentation will give audiences not only a knowledge of Go, but also one of system calls.

Transcript

1. HTTP Server on random available port in Go Kazuki Higashiguchi March 31, 2022 @ Conf42: Golang 2022
2. About Me Kazuki Higashiguchi Backend Engineer at Autify. No-code AI-powered software testing automation platform Follow @hgsgtk on Twitter
3. Autify Solution No code unlocks automation at scale AI automatically maintenance test scripts
4. Autify for Web / for Mobile We are taking demo requests https://autify.com/ Autify for Web Autify for Mobile
5. Available ports HTTP server on a random available port 50123 60012 45342 ????? go run main.go No port specified HTTP server ????? Port: ????? Bind & Listen
6. Implementation using Go using port: 57645 using port: 57473 using port: 57464 …
7. net.Listen "If the port in the address parameter is empty or "0", as in "127.0.0.1:" or "[::1]:0", a port number is automatically chosen” https://pkg.go.dev/net#Listen
8. net/http/httptest invokes net.Listen with “0” net/http/httptest/server.go https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/net/http/httptest/server.go;drc=refs%2Ftags%2Fgo1.18;l=60
9. Dive into the standard libraries https://unsplash.com/photos/AN2SypyyOnA
10. Signature: “network”
12. 1: net.Listen -> net.ListenConfig.Listen https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/net/dial.go;l=710 net/ipsock.go
13. 2. net.ListenConfig.Listen -> net.DefaultResolver https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/net/dial.go;l=625 DefaultResolver resolves the network IP address net/dial.go
14. 3. net.DefaultResolver -> net.LookupPort LookupPort looks up the port for the given network e.g. LookupPort(“127.0.0.1”, “0”) -> returned port: 0 net/ipsock.go https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/net/ipsock.go;l=260;drc=refs%2Ftags%2Fgo1.18
15. 4: net.ListenConfig.Listen -> sysListener.listenTCP https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/net/dial.go;l=639;drc=refs%2Ftags%2Fgo1.18 net/dial.go
16. 5: sysListener.listenTCP -> internalsocket https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/net/tcpsock_posix.go;l=167;drc=refs%2Ftags%2Fgo1.18 net/tcpsocket_posix.go Return a network file descriptor
17. socket type: SOCK_STREAM Network type Socket type Description “tcp” (Transmission Control Protocol) SOCK_STREAM Stream-oriented Sequenced, reliable, two-way, connection-base byte streams. “unix” (Unix domain sockets) “udp” (User Datagram Protocol) SOCK_DGRAM Datagram-oriented. Connectionless, unreliable messages “unixgram” “unixpacket” SOCK_SEQPACKET Datagram-oriented. Sequenced, reliable, two-way, connection-base byte streams.
18. 6: internalsocket -> net.listenStream https://cs.opensource.google/go/go/+/refs/tags/go1.18:src/net/sock_posix.g o;l=56;drc=refs%2Ftags%2Fgo1.18 net/sock_posix.go
19. 7: execute three system calls https://cs.opensource.google/go/go/+/refs/tag s/go1.18:src/net/sock_posix.go;drc=refs%2Ftag s%2Fgo1.18;l=175 net/sock_posix.go
20. bind, listen, getsockname Bind Listen Getsockname Assign the socket address to the socket referred to by the file descriptor Mark the socket as a passive socket Return the current socket address
21. Bind to an ephemeral port Bind Ephemeral port (Dynamic port) HTTP server Port: 54563 Bind Choose one from ephemeral ports
22. Specification of bind OS When a port number is zero Port range Linux Attempt to bind to an ephemeral port (Basically) 49152 - 65535 Windows Assign a unique port from the dynamic client port range (On Windows Vista and later) 49152 - 65535 (Windows Server 2003 and earlier) 1025 - 5000 IANA defines the port range is 49152 - 65535
23. Key consideration 1. Confirm the “bind” specification 2. Check your infrastructure can use an ephemeral port 3. Check the range of ephemeral port
24. Thank you See more detail in the article on dev,to.