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I am building a library to simulate some faults in a system. One of the faults, is to simulate a network failure, which would forbid any connection. Currently, I am using this Kotlin code to disable the container network interface:

Runtime.getRuntime().exec("ifconfig eth0 down")
// wait some time
Runtime.getRuntime().exec("ifconfig eth0 up")

When the interface is re-enabled, I am not able to restore connections with the container. I tried it on the command line, and the effect is the same:

docker run --privileged -it alpine:latest sh
/ # apk add curl
...
OK: 7 MiB in 18 packages
/ # curl google.com

<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>301 Moved</TITLE></HEAD><BODY>
<H1>301 Moved</H1>
The document has moved
<A HREF="http://www.google.com/">here</A>.
</BODY></HTML>

/ # ifconfig eth0 down
/ # ifconfig eth0 up
/ # curl google.com
curl: (6) Could not resolve host: google.com

Does anybody have any idea why it occurs inside a docker container?

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1 Answer 1

Reset to default
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The problem was that Docker was losing the default gateway address. I just added another command to reset the gateway address after restarting the interface and everything worked again:

route add default gw ${this.defaultGatewayAddress}

At the end, I got this working Kotlin code:

data class NetworkInterface(val name: String) {

    private var defaultGatewayAddress: String

    init {
        this.defaultGatewayAddress = getDefaultGatewayIpAddress().address
    }

    fun disable() {
        Environment.runCLICommand("ifconfig $name down")
    }

    fun enable() {
        Environment.runCLICommand("ifconfig $name down")
        Environment.runCLICommand("route add default gw ${this.defaultGatewayAddress}")
    }

    fun getDefaultGatewayIpAddress(): IpAddress {
        val command = "netstat -nr | awk '{print $2}' | head -n3 | tail -n1"
        return IpAddress(Environment.runCLICommand(command).trim())
    }
}
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