Why can't I bind to 127.0.0.1 on Mac OS X?

Posted by Noah Lavine on Server Fault See other posts from Server Fault or by Noah Lavine
Published on 2011-02-06T20:53:17Z Indexed on 2011/02/06 23:28 UTC
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Hello,

I'm trying to set up a simple web server on Mac OS X, and I keep getting an error when I run bind. Here's what I'm running (this transcript uses GNU Guile, but just as a convenient interface to posix).

(define addr (inet-aton "127.0.0.1"))                     ; get internal representation of 127.0.0.1
(define sockaddr (make-socket-address AF_INET addr 8080)) ; make a struct sockaddr
(define sock (socket PF_INET SOCK_STREAM 0))              ; make a socket
(bind sock sockaddr)                                      ; bind the socket to the address

That gives me the error In procedure bind: can't assign requested address.

So I tried it again allowing any address.

(define anyaddr (make-socket-address AF_INET INADDR_ANY 8080)) ; allow any address
(bind sock anyaddr)

And that works fine. But it's weird, because ifconfig lo0 says

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
  inet6 ::1 prefixlen 128 
  inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1 
  inet 127.0.0.1 netmask 0xff000000 

So the loopback device is assigned to 127.0.0.1. So my question is, why can't I bind to that address?

Thanks.

Update: the output of route get 127.0.0.1 is

   route to: localhost
destination: localhost
  interface: lo0
      flags: <UP,HOST,DONE,LOCAL>
 recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu     expire
   49152     49152         0         0         0         0     16384         0 

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