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Comment by rootlocus | Hacker News Reader

How do the two processes listen to the same port?

Once the USR2 signal is received the master process forks, the child process inherits the parents file descriptors including listen(). One process stops accepting connections creating a queue in the kernel. The new process takes over and starts accepting connections.

You can follow the trail by searching for ngx_exec_new_binary in the nginx repo.

Just to add - Nginx normally spawns several worker processes that all process connections to the same port.

Correct but to clarify, only the master process binds to the ports. The master process creates socketpairs to the workers for interprocess communication. The workers accept connections over the shared socket.

https://www.nginx.com/blog/socket-sharding-nginx-release-1-9...

Page also has an example of how SO_REUSEPORT effects flow.

Oh, thanks! I didn't know that. I supposed it worked by inheriting the listening socket but I didn't check.

There’s an ioctl for this on FreeBSD and Linux — SO_REUSEPORT. You could also just leave the listening socket open when exec’ing the new httpd, or send it with a unix domain socket.

This article on how haproxy uses SO_REUSEPORT goes into some more detail: https://www.haproxy.com/blog/truly-seamless-reloads-with-hap...

Using socket option SO_REUSEPORT allows multiple processes to bind to same port.

is this what it's actually doing though? It doesn't say the reuseport option to the listen directive is required for this.

Is there any restrictions on this option? Eg only children of the same parent process are allowed to bind to same port. Otherwise how does the packet distribution work? And how does the response from that port work?

>So long as the first server sets this option before binding its socket, then any number of other servers can also bind to the same port if they also set the option beforehand. [...] To prevent unwanted processes from hijacking a port that has already been bound by a server using SO_REUSEPORT, all of the servers that later bind to that port must have an effective user ID that matches the effective user ID used to perform the first bind on the socket.

https://lwn.net/Articles/542629/

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