Artificially create a connection timeout error

Question

I've had a bug in our software that occurs when I receive a connection timeout. These errors are very rare (usually when my connection gets dropped by our internal network). How can I generate this kind of effect artificially so I can test our software?

If it matters the app is written in C++/MFC using CAsyncSocket classes.

Edit:

I've tried using a non-existent host, and I get the socket error:

WSAEINVAL (10022) Invalid argument

My next attempt was to use Alexander's suggestion of connecting to a different port, e.g. 81 (on my own server though). That worked great. Exactly the same as a dropped connection (60 second wait, then error). Thank you!

Answer 1

Connect to an existing host but to a port that is blocked by the firewall that simply drops TCP SYN packets. For example, www.google.com:81.

Answer 2

Connect to a non-routable IP address, such as 10.255.255.1.

Answer 3

If you are on a unix machine, you can start a port listening using netcat:

nc -l 8099

Then, modify you service to call whatever it usually does to that port e.g. http://localhost:8099/some/sort/of/endpoint

Then, your service will open the connection and write data, but will never get a response, and so will give you a Read Time Out (rather than Connection Refused)

Answer 4

Plenty of good answers but the cleanest solution seems to be this service

http://httpstat.us/504?sleep=60000

You can configure the timeout duration (up to 230 seconds) and eventual return code.

Answer 5

The following URL always gives a timeout, and combines the best of @Alexander and @Emu's answers above:

http://example.com:81

Using example.com:81 is an improvement on Alexander's answer because example.com is reserved by the DNS standard, so it will always be unreachable, unlike google.com:81, which may change if Google feels like it. Also, because example.com is defined to be unreachable, you won't be flooding Google's servers.

I'd say it's an improvement over @emu's answer because it's a lot easier to remember.

Answer 6

• 10.0.0.0
• 10.255.255.255
• 172.16.0.0
• 172.31.255.255
• 192.168.0.0
• 192.168.255.255

All these are non-routable.

Answer 7

You can use the Python REPL to simulate a timeout while receiving data (i.e. after a connection has been established successfully). Nothing but a standard Python installation is needed.

Python 2.7.4 (default, Apr  6 2013, 19:54:46) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)        
>>> s.bind(('localhost', 9000))
>>> s.listen(0)
>>> (clientsocket, address) = s.accept()

Now it waits for an incoming connection. Connect whatever you want to test to localhost:9000. When you do, Python will accept the connection and accept() will return it. Unless you send any data through the clientsocket, the caller's socket should time out during the next recv().

Answer 8

I would like to point everybody's attention to mitmproxy.

With a config (taken from their examples) of 200:b@100:dr you'll get a connection that randomly drops.

Answer 9

How about a software solution:

Install SSH server on the application server. Then, use socket tunnel to create a link between your local port and the remote port on the application server. You can use ssh client tools to do so. Have your client application connect to your mapped local port instead. Then, you can break the socket tunnel at will to simulate the connection timeout.

Answer 10

If you want to use an active connection you can also use http://httpbin.org/delay/#, where # is the time you want their server to wait before sending a response. As long as your timeout is shorter than the delay ... should simulate the effect. I've successfully used it with the python requests package.

You may want to modify your request if you're sending anything sensitive - no idea what happens to the data sent to them.

Answer 11

There are services available which allow you to artificially create origin timeouts by calling an API where you specify how long the server will take to respond. Server Timeout on macgyver is an example of such a service.

For example if you wanted to test a request that takes 15 seconds to respond you would simply make a post request to the macgyver API.

JSON Payload:

{
    "timeout_length": 15000
}

API Response (After 15 seconds):

{
    "response": "ok"
}

Server Timeout program on macgyver
https://askmacgyver.com/explore/program/server-timeout/3U4s6g6u

Answer 12

You might install Microsoft Loopback driver that will create a separate interface for you. Then you can connect on it to some service of yours (your own host). Then in Network Connections you can disable/enable such interface...

Answer 13

Despite it isn't completely clear which one the OP wants to test: there's a difference between attempting a connection to a non-existent host/port and a timeout of an already established connection. I would go with Rob and wait until the connection is working and then pull the cable. Or - for convenience - have a virtual machine working as the test server (with bridged networking) and just deactivating the virtual network interface once the connection is established.

Answer 14

The technique I use frequently to simulate a random connection timeout is to use ssh local port forwarding.

ssh -L 12345:realserver.com:80 localhost

This will forward traffic on localhost:12345 to realserver.com:80 You can loop this around in your own local machine as well, if you want:

ssh -L 12345:localhost:8080 localhost

So you can point your application at your localhost and custom port, and the traffic will get routed to the target host:port. Then you can exit out of this shell (you may also need to ctrl+c the shell after you exit) and it will kill the forwarding which causes your app to see a connection loss.

Answer 15

There are a couple of tactics I've used in the past to simulate networking issues;

1. Pull out the network cable
2. Switch off the switch (ideally with the switch that the computer is plugged into still being powered so the machine maintains it's "network connection") between your machine and the "target" machine
3. Run firewall software on the target machine that silently drops received data

One of these ideas might give you some means of artifically generating the scenario you need

Answer 16

Depending on what firewall software you have installed/available, you should be able to block the outgoing port and depending on how your firewall is setup it should just drop the connection request packet. No connection request, no connection, timeout ensues. This would probably work better if it was implemented at a router level (they tend to drop packets instead of sending resets, or whatever the equivalent is for the situation) but there's bound to be a software package that'd do the trick too.

Answer 17

The easiest thing would be to drop your connection using CurrPorts.

However, in order to unit test your exception handling code, perhaps you should consider abstracting your network connection code, and write a stub, mock or decorator which throws exceptions on demand. You will then be able to test the application error-handling logic without having to actually use the network.

Answer 18

I had issues along the same lines you do. In order to test the software behavior, I just unplugged the network cable at the appropriate time. I had to set a break-point right before I wanted to unplug the cable.

If I were doing it again, I'd put a switch (a normally closed momentary push button one) in a network cable.

If the physical disconnect causes a different behavior, you could connect your computer to a cheap hub and put the switch I mentioned above between your hub and the main network.

-- EDIT -- In many cases you'll need the network connection working until you get to a certain point in your program, THEN you'll want to disconnect using one of the many suggestions offered.

Answer 19

For me easiest way was adding static route on office router based on destination network. Just route traffic to some unresponsive host (e.g. your computer) and you will get request timeout.

Best thing for me was that static route can be managed over web interface and enabled/disabled easily.

Answer 20

Plug in your network cable into a switch which has no other connection/cables. That should work imho.

Answer 21

You can try to connect to one of well-known Web sites on a port that may not be available from outside - 200 for example. Most of firewalls work in DROP mode and it will simulate a timeout for you.