Google App Engine custom IP port in Go

I want to create a web app on GAE in Go and I need it to listen to a custom port. How do I set it up? I tried to use http.ListenAndServe(":12345", nil) in the sandbox, but got this in my console:

WARNING  2011-10-07 20:01:01,252 urlfetch_stub.py:108] No ssl package found. urlfetch 

will not be able to validate SSL certificates.
INFO     2011-10-07 20:01:01,847 appengine_rpc.py:159] Server: appengine.google.com
INFO     2011-10-07 20:01:01,855 appcfg.py:463] Checking for updates to the SDK.
INFO     2011-10-07 20:01:04,625 appcfg.py:480] The SDK is up to date.
WARNING  2011-10-07 20:01:04,625 datastore_file_stub.py:512] Could not read datastore data from /var/folders/e0/e0U5NtMXFHOB6jVwkVhsnE+++TM/-Tmp-/dev_appserver.datastore
INFO     2011-10-07 20:01:04,627 rdbms_sqlite.py:58] Connecting to SQLite database '' with file '/var/folders/e0/e0U5NtMXFHOB6jVwkVhsnE+++TM/-Tmp-/dev_appserver.rdbms'
WARNING  2011-10-07 20:01:04,633 dev_appserver.py:4748] Could not initialize images API; you are likely missing the Python "PIL" module. ImportError: No module named _imaging
INFO     2011-10-07 20:01:04,641 dev_appserver_multiprocess.py:637] Running application tpbitblock on port 8080: http://localhost:8080
INFO     2011-10-07 20:01:08,448 __init__.py:365] building _go_app
INFO     2011-10-07 20:01:11,105 __init__.py:351] running _go_app
throw: init rescheduling

runtime.throw+0x40 /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/runtime.c:102
    runtime.throw(0x278c7f, 0xdc73)
schedule+0x3e /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:535
    schedule(0x10803000, 0x10803000)
runtime.mcall+0x3a /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/386/asm.s:174
    runtime.mcall(0x10803000, 0x0)

goroutine 2 [1]:
net.*pollServer·Run /private/tmp/appengine/google_appengine/goroot/src/pkg/net/fd.go:213
    net.*pollServer·Run(0x10801c90, 0x0)
runtime.goexit /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:178
    runtime.goexit()
----- goroutine created by -----
net.newPollServer+0x2dd /private/tmp/appengine/google_appengine/goroot/src/pkg/net/newpollserver.go:39

goroutine 1 [4]:
runtime.gosched+0x4d /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:603
    runtime.gosched()
runtime.chanrecv+0x14d /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/chan.c:361
    runtime.chanrecv(0x1081d900, 0x30800e6c, 0x0, 0x0, 0x0, ...)
runtime.chanrecv1+0x3f /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/chan.c:424
    runtime.chanrecv1(0x1081d900, 0x1081e580)
net.*pollServer·WaitRead+0x47 /private/tmp/appengine/google_appengine/goroot/src/pkg/net/fd.go:255
    net.*pollServer·WaitRead(0x10801c90, 0x1081e580, 0x0)
net.*netFD·accept+0x2ae /private/tmp/appengine/google_appengine/goroot/src/pkg/net/fd.go:605
    net.*netFD·accept(0x1081e580, 0x45716, 0x0, 0x0, 0x0, ...)
net.*TCPListener·AcceptTCP+0x53 /private/tmp/appengine/google_appengine/goroot/src/pkg/net/tcpsock.go:272
    net.*TCPListener·AcceptTCP(0x108008b8, 0x30800f00, 0x0, 0x0)
net.*TCPListener·Accept+0x36 /private/tmp/appengine/google_appengine/goroot/src/pkg/net/tcpsock.go:282
    net.*TCPListener·Accept(0x108008b8, 0x0, 0x0, 0x0, 0x0, ...)
http.*Server·Serve+0x96 /private/tmp/appengine/google_appengine/goroot/src/pkg/http/server.go:868
    http.*Server·Serve(0x10839d20, 0x10839e20, 0x108008b8, 0x0, 0x0, ...)
http.*Server·ListenAndServe+0x9c /private/tmp/appengine/google_appengine/goroot/src/pkg/http/server.go:855
    http.*Server·ListenAndServe(0x10839d20, 0x10839d20, 0x406)
http.ListenAndServe+0x52 /private/tmp/appengine/google_appengine/goroot/src/pkg/http/server.go:920
    http.ListenAndServe(0x155640, 0x6, 0x0, 0x0, 0x0, ...)
BitBlock.init·1+0x174 BitBlock/BitBlock.go:43
    BitBlock.init·1()
BitBlock.init+0x70 BitBlock/BitBlock.go:0
    BitBlock.init()
main.init+0x39 _go_main.go:0
    main.init()
runtime.mainstart+0x5 /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/386/asm.s:91
    runtime.mainstart()
runtime.goexit /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:178
    runtime.goexit()
----- goroutine created by -----
_rt0_386+0xbf /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/386/asm.s:80
ERROR    2011-10-07 20:01:12,196 dev_appserver.py:4200] Exception encountered handling request
Traceback (most recent call last):
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 4143, in _HandleRequest
    self._Dispa开发者_如何学运维tch(dispatcher, self.rfile, outfile, env_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 4049, in _Dispatch
    base_env_dict=env_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 616, in Dispatch
    base_env_dict=base_env_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 3120, in Dispatch
    self._module_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 2972, in ExecuteCGI
    env, infile, outfile)
  File "/Google App Engine/google/appengine/ext/go/__init__.py", line 399, in execute_go_cgi
    GO_APP.make_and_run()
  File "/Google App Engine/google/appengine/ext/go/__init__.py", line 362, in make_and_run
    wait_until_go_app_ready(self.proc.pid)
  File "/Google App Engine/google/appengine/ext/go/__init__.py", line 285, in wait_until_go_app_ready
    raise Exception('unable to start ' + GO_APP_NAME)
Exception: unable to start _go_app
INFO     2011-10-07 20:01:12,242 dev_appserver.py:4247] "GET / HTTP/1.1" 500 -
INFO     2011-10-07 20:01:12,505 __init__.py:351] running _go_app
throw: init rescheduling

runtime.throw+0x40 /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/runtime.c:102
    runtime.throw(0x278c7f, 0xdc73)
schedule+0x3e /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:535
    schedule(0x10803000, 0x10803000)
runtime.mcall+0x3a /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/386/asm.s:174
    runtime.mcall(0x10803000, 0x0)

goroutine 2 [1]:
net.*pollServer·Run /private/tmp/appengine/google_appengine/goroot/src/pkg/net/fd.go:213
    net.*pollServer·Run(0x10801c90, 0x0)
runtime.goexit /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:178
    runtime.goexit()
----- goroutine created by -----
net.newPollServer+0x2dd /private/tmp/appengine/google_appengine/goroot/src/pkg/net/newpollserver.go:39

goroutine 1 [4]:
runtime.gosched+0x4d /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:603
    runtime.gosched()
runtime.chanrecv+0x14d /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/chan.c:361
    runtime.chanrecv(0x1081d900, 0x30800e6c, 0x0, 0x0, 0x0, ...)
runtime.chanrecv1+0x3f /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/chan.c:424
    runtime.chanrecv1(0x1081d900, 0x1081e580)
net.*pollServer·WaitRead+0x47 /private/tmp/appengine/google_appengine/goroot/src/pkg/net/fd.go:255
    net.*pollServer·WaitRead(0x10801c90, 0x1081e580, 0x0)
net.*netFD·accept+0x2ae /private/tmp/appengine/google_appengine/goroot/src/pkg/net/fd.go:605
    net.*netFD·accept(0x1081e580, 0x45716, 0x0, 0x0, 0x0, ...)
net.*TCPListener·AcceptTCP+0x53 /private/tmp/appengine/google_appengine/goroot/src/pkg/net/tcpsock.go:272
    net.*TCPListener·AcceptTCP(0x108008b8, 0x30800f00, 0x0, 0x0)
net.*TCPListener·Accept+0x36 /private/tmp/appengine/google_appengine/goroot/src/pkg/net/tcpsock.go:282
    net.*TCPListener·Accept(0x108008b8, 0x0, 0x0, 0x0, 0x0, ...)
http.*Server·Serve+0x96 /private/tmp/appengine/google_appengine/goroot/src/pkg/http/server.go:868
    http.*Server·Serve(0x10839d20, 0x10839e20, 0x108008b8, 0x0, 0x0, ...)
http.*Server·ListenAndServe+0x9c /private/tmp/appengine/google_appengine/goroot/src/pkg/http/server.go:855
    http.*Server·ListenAndServe(0x10839d20, 0x10839d20, 0x406)
http.ListenAndServe+0x52 /private/tmp/appengine/google_appengine/goroot/src/pkg/http/server.go:920
    http.ListenAndServe(0x155640, 0x6, 0x0, 0x0, 0x0, ...)
BitBlock.init·1+0x174 BitBlock/BitBlock.go:43
    BitBlock.init·1()
BitBlock.init+0x70 BitBlock/BitBlock.go:0
    BitBlock.init()
main.init+0x39 _go_main.go:0
    main.init()
runtime.mainstart+0x5 /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/386/asm.s:91
    runtime.mainstart()
runtime.goexit /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/proc.c:178
    runtime.goexit()
----- goroutine created by -----
_rt0_386+0xbf /private/tmp/appengine/google_appengine/goroot/src/pkg/runtime/386/asm.s:80
ERROR    2011-10-07 20:01:13,530 dev_appserver.py:4200] Exception encountered handling request
Traceback (most recent call last):
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 4143, in _HandleRequest
    self._Dispatch(dispatcher, self.rfile, outfile, env_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 4049, in _Dispatch
    base_env_dict=env_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 616, in Dispatch
    base_env_dict=base_env_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 3120, in Dispatch
    self._module_dict)
  File "/Google App Engine/google/appengine/tools/dev_appserver.py", line 2972, in ExecuteCGI
    env, infile, outfile)
  File "/Google App Engine/google/appengine/ext/go/__init__.py", line 399, in execute_go_cgi
    GO_APP.make_and_run()
  File "/Google App Engine/google/appengine/ext/go/__init__.py", line 362, in make_and_run
    wait_until_go_app_ready(self.proc.pid)
  File "/Google App Engine/google/appengine/ext/go/__init__.py", line 285, in wait_until_go_app_ready
    raise Exception('unable to start ' + GO_APP_NAME)
Exception: unable to start _go_app
INFO     2011-10-07 20:01:13,531 dev_appserver.py:4247] "GET /favicon.ico HTTP/1.1" 500 -

You can't do that. App Engine doesn't allow custom http ports. App Engine environment is heavily sandboxed and not everything that you can do in pure Go (or Java or Python) is available in their respective App Engine environments.

From http://code.google.com/appengine/docs/go/overview.html:

As with the Java and Python environments, not all the standard library's functionality is available inside the sandbox. For example, attempts to open a socket or write to a file will return an os.EINVAL error.

Go apps run inside a secure "sandbox" environment with a reduced set of libraries. For instance, an app cannot write data to the local file system or make arbitrary network connections. Instead, apps use scalable services provided by App Engine to store data and communicate over the Internet.

It is indeed possible, all you need to do is goapp serve --port=3000 (or whatever port you'd like).

If you're trying to access your goapp on a VM (parallels), like I was, you can do goapp serve --host=0.0.0.0

Documentation: https://cloud.google.com/appengine/docs/go/tools/devserver#Go_Command-line_arguments