Apple - Cannot access localhost after upgrade to Mavericks but can access 127.0.0.1

Everything seems to be working now. I can access localhost again, and my app is running Sinatra on Thin as it was before. Thanks to bmike, I did a bit of searching on why my loopback was unreachable and came across this article. I moved my old hosts file (/etc/hosts) to hosts.old and made a new one in its place that simply contained:

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1           localhost
255.255.255.255     broadcasthost
::1                 localhost
fe80::1%lo0         localhost

Then, I ran dscacheutil -flushcache and rebooted my computer*. After that, things appear to be working normally again. I can only guess that something weird happened with my old hosts file. I also took JakeGould's advice and blew away ruby 2.0.0 (I still had ruby 1.8.7--you cannot uninstall ruby entirely on a mac, since it uses ruby for other things) and rvm and reinstalled those; but I'm not sure that had anything to do with my success, as I still had the same issues until I followed the steps outlined above.

Note: the above hosts file contains virtually the same thing that I say my hosts file contained before (see original question); however, there were other things in my original hosts file that I did not share in my question (I just shared what I thought was the relevant part), so perhaps something there got borked when I upgraded--I wish I knew for sure.

*Rebooting is probably not necessary. I actually had this problem after setting up a new user from a backup drive on my machine at work (which is running Mountain Lion) and (just today) followed the same steps above (minus blowing away ruby and rebooting the machine), and it worked. This leads me to believe that the heart of the issue was a borked /etc/hosts file.

I have no such problems connecting to a web server when it's actually listening on port 80.

If you have no web server running, here is what you should get from terminal to show that name resolution is working on all network interfaces before the telnet gives up connecting to port 80:

mac:~ me$ telnet localhost 80
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
telnet: connect to address 127.0.0.1: Connection refused
Trying fe80::1...
telnet: connect to address fe80::1: Connection refused
telnet: Unable to connect to remote host

Should you get different results, I would use scutil to determine reachability for your network destination:

mac:~ me$ scutil -r localhost
Reachable,Local Address
mac:~ me$ scutil -r loopback
Reachable
mac:~ me$ scutil -r 127.0.0.1
Reachable,Local Address

It might be worth checking if you have LittleSnitch or another firewall that has rules that are blocking network access or reboot into Safe mode to test if some kernel extensions or other conflict is preventing the default reachability and access to local ports. Also, you will most certainly want to check whatever web server you are running to ensure it's really listening to port 80:

mac:~ me$ sudo lsof -i :80 | egrep "PID|LISTEN"
Password:
COMMAND   PID USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
httpd    1645 root    8u  IPv6 0x513627eaa81c8205      0t0  TCP *:http (LISTEN)
httpd   15851 _www    8u  IPv6 0x513627eaa81c8205      0t0  TCP *:http (LISTEN)
httpd   72969 _www    8u  IPv6 0x513627eaa81c8205      0t0  TCP *:http (LISTEN)

Also, quit all web browsers or be prepared to filter those network connections from the above lsof command as it shows all connections over port 80.