Issue #10203 has been updated by Lucas Nussbaum.


Indeed, bindv6only=1 seems to change this. However, bindv6only=1 is kind-of the default on all systems (except some Linux distros).

My goal was to implement the following:
try to bind port 10001 ; if already taken, bind port 10002
That's not possible with the current behaviour.

I don't think it's an OS behaviour. I straced the ruby process, and saw:

first TCPServer call:
    [pid 16280] bind(7, {sa_family=AF_INET, sin_port=htons(10001), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
second TCPServer call:
    [pid 16280] bind(8, {sa_family=AF_INET, sin_port=htons(10001), sin_addr=inet_addr("0.0.0.0")}, 16) = -1 EADDRINUSE (Address already in use)
    [pid 16280] bind(8, {sa_family=AF_INET6, sin6_port=htons(10001), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 0
third TCPServer call:
    [pid 16280] bind(9, {sa_family=AF_INET, sin_port=htons(10001), sin_addr=inet_addr("0.0.0.0")}, 16) = -1 EADDRINUSE (Address already in use)
    [pid 16280] bind(9, {sa_family=AF_INET6, sin6_port=htons(10001), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = -1 EADDRINUSE (Address already in use)

So the second bind() on AF_INET port 10001 fails correctly. It seems that Ruby implements a fallback mechanism that makes it bind the corresponding AF_INET6 port. I think that's wrong. But maybe it's just a documentation issue.

----------------------------------------
Bug #10203: TCPServer.new has strange behaviour when EADDRINUSE without specifying hostname
https://bugs.ruby-lang.org/issues/10203#change-49408

* Author: Lucas Nussbaum
* Status: Feedback
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
* ruby -v: ruby 2.1.2p95
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
after:
irb(main):003:0> TCPServer::new(10001)
=> #<TCPServer:fd 9>
I get irb listening to port 10001 using IPv4, but not IPv6:
tcp        0      0 0.0.0.0:10001           0.0.0.0:* LISTEN      1000       376068      24437/irb

a second creation also works, but only binds the IPv6 address:
irb(main):004:0> TCPServer::new(10001)
=> #<TCPServer:fd 10>
tcp6       0      0 :::10001                :::* LISTEN      1000       376098      24437/irb

=> I would have expected the first creation to bind to both IPv4 and IPv6, not just IPv4, and the second attempt to fail.

Trying once again, the creation fails with a strange exception:
irb(main):007:0> TCPServer::new(10001)
TypeError: no implicit conversion of nil into String
	from (irb):7:in `initialize'
	from (irb):7:in `new'
	from (irb):7
	from /usr/bin/irb:11:in `<main>'


Binding explicitely to 0.0.0.0 avoids this:
irb(main):005:0> TCPServer::new('0.0.0.0', 10002)
=> #<TCPServer:fd 11>
irb(main):006:0> TCPServer::new('0.0.0.0', 10002)
Errno::EADDRINUSE: Address already in use - bind(2) for "0.0.0.0" port 10002
	from (irb):6:in `initialize'
	from (irb):6:in `new'
	from (irb):6
	from /usr/bin/irb:11:in `<main>'



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