jruby 1.6.8 does not have this problem:

$ jruby-1.6.8 -rsocket -e " p RUBY_DESCRIPTION, Socket.new(::Socket::AF_INET, ::Socket::SOCK_STREAM, ::Socket::IPPROTO_TCP)" 
"jruby 1.6.8 (ruby-1.8.7-p357) (2013-06-03 fffffff) (OpenJDK 64-Bit Server VM 1.7.0_21) [linux-amd64-java]"
#<Socket:0x6fdb48f5>

Error is platform-dependent. On Mac OS X, the unknown protocol is PF_NS. The error arises because we are using the IP protocol value (6 in this case) to look up from the protocol family table. https://github.com/jruby/jruby/blob/1.7.4/src/org/jruby/ext/socket/RubySocket.java#L393

The third parameter should be looked up in ProtocolFamily (https://github.com/jnr/jnr-constants/blob/master/src/main/java/jnr/constants/platform/ProtocolFamily.java) or IPProto (https://github.com/jnr/jnr-constants/blob/master/src/main/java/jnr/constants/platform/IPProto.java) depending on the first parameter.

@BanzaiMan, do you know which AddressFamily's should trigger looking up a ProtocolFamily versus a IPProto? My guess is it switches based on this if statement or this switch statement:

https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/ext/socket/RubySocket.java#L359-L365
https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/ext/socket/RubySocket.java#L571-L579

So when AddressFamily is INET, INET6, or UNSPEC then use IPProto and when AddressFamily is UNIX or LOCAL use ProtocolFamily? What about when AddressFamily is none of the above?

@atambo It is not clear to me, to be honest. I'd consult @wmeissner first.

A side question: why use Socket instead of TCPSocket? We are somewhat limited in what features of Socket we can provide since JDK does not provide a raw non-TCP socket for us to customize.

@headius TCPSocket automatically connects on instantiation. If you don't want that, that's one reason to use Socket instead. I've actually been running into this issue as well.

@jlee-r7 as far as I can tell this issue is no longer reproducible. @BanzaiMan does allude to it being platform dependent but jnr-constants and JRuby have changed quite a bit in the past few years :)

I'm not able to repro this (on Linux though), so I'm going to close this. If you can still reproduce it with 9k please re-open!