I need to wait for a specific condition to be met a network interface is considered "up". Upstart is sending its net-device-up signal before this condition is met, which cases the scripts that depend on this condition to fail. How do I delay Upstart's net-device-up signal until a condition is met?


One solution is to put a script in /etc/network/if-up.d/ that executes before any other if-up.d script and waits for a specific condition to be met or a timeout occurs. For example, here's a script to wait for an non-link-local IPv6 address on an IPv6 interface configured with the "auto" method:



if [ "$ADDRFAM" = "inet6" -a "$METHOD" = "auto" ]; then
        echo "Auto method detected, waiting for IP address" >> $LOG_FILE

        #wait until we have an IP address, or a timeout occurs
        TIMEOUT=50 #5 second timeout--each iteration sleeps for 1/10th of a second
        #get the number of addresses, excluding link-local addresses
        ADDRESS_COUNT=`ip addr show dev eth0 | grep inet6 | grep -v fe80 | wc -l`

        until [ $TIMEOUT -eq 0 ]; do
                NEW_ADDRESS_COUNT=`ip addr show dev eth0 | grep inet6 | grep -v fe80 | wc -l`

                # break out of the loop if more IPv6 addresses have been assigned
                # this is implementation is naive: no guarantee is given that the new addresses
                # are the result of the autoconfiguration request
                if [ "$NEW_ADDRESS_COUNT" -gt "$ADDRESS_COUNT" ]; then
                        echo "Detected new IP address, exiting" >> $LOG_FILE
                        ifconfig >> $LOG_FILE
                        exit 0;

                sleep .1

        echo "Timeout waiting for IP address" >> $LOG_FILE
        exit 1;

It's important this "wait-for-condition" script run before the if-up.d Upstart script runs (the Upstart if-up.d script is where the net-device-up signals are generated). The if-up.d scripts are executed by run-parts in lexical sort order (the same order as the default output of the ls command), so a simple technique for guaranteeing order is to name the script "000wait-for-condition".

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