General tips for using Compute Engine

This page describes tips that you might find helpful if you run into problems using Compute Engine.

For help troubleshooting specific issues, see one of the following sections:

Viewing different response formats

The gcloud command-line tool performs most of its actions by making REST API calls. The pretty-printed results show only the most important information returned by any specific command. To see the different response formats, use the --format flag which displays the response in different output formats, including json, yaml, and text. For example, to see a list of instances in JSON, use --format json:

gcloud compute instances list --format json

Viewing gcloud compute logs

The gcloud tool creates and stores logs in a log file that you can query, located at $HOME/.config/gcloud/logs. To see the latest log file on a Linux-based operating system, run:

$ less $(find ~/.config/gcloud/logs | sort | tail -n 1)

The log file includes information about all requests and responses made using the gcloud compute tool.

Selecting resource names

When selecting names for your resources, keep in mind that these friendly-names may be visible on support and operational dashboards within Compute Engine. For this reason, it is recommended that resource names that do not expose any sensitive information.

Communicating to the internet

An instance has direct internet access only if both of the following conditions are true:

Instances can also access the internet indirectly, by connecting through Cloud NAT or an instance-based proxy. For additional considerations, including firewall rule configuration, see Internet access requirements.

Idle connections

Google Cloud VPC networks implement 10-minute connection tracking for IP protocols that have a concept of a connection (TCP for example). This means that inbound packets associated with an established connection are permitted as long as at least one packet is sent or received for the connection within the last 10 minutes. If no packets for the connection have been sent or received for 10 minutes or longer, the idle connection's tracking entries are removed. After the connection's tracking entries have been removed, Google Cloud does not permit additional inbound packets until at least one new outbound packet has been sent. This connection tracking applies to all sources and destinations – both internal and external IP addresses .

To prevent idle connections, do the following:

  • Set operating system TCP keep-alive parameters to a time frame of less than 10 minutes. This ensures that at least one packet is sent within the time frame.

  • Ensure applications that open TCP connections do so with the SO_KEEPALIVE option enabled.

The following examples demostrate how to set operating system TCP keep-alive parameters with an interval value of one minute. Consult your application or software library's documentation to determine how to configure it to use SO_KEEPALIVE.


Run the following command:

$ sudo /sbin/sysctl -w net.ipv4.tcp_keepalive_time=60 net.ipv4.tcp_keepalive_intvl=60 net.ipv4.tcp_keepalive_probes=5
To ensure that the settings survive a reboot, add the settings to your /etc/sysctl.conf file.

See Linux TCP Keepalive HOWTO for additonal information.


Run the following command:

$ sudo sysctl -w net.inet.tcp.always_keepalive=1 net.inet.tcp.keepidle=60000 net.inet.tcp.keepinit=60000 net.inet.tcp.keepintvl=60000


Under the registry path HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\, add the following settings, using the DWORD data type, or edit the values if the settings already exist:

KeepAliveInterval: 1000
KeepAliveTime: 60000
TcpMaxDataRetransmissions: 10

Accessing Compute Engine as a different SSH user

By default, the gcloud compute command-line tool uses the $USER variable to add users to the /etc/passwd file for connecting to virtual machine instances using SSH. You can specify a different user using the --ssh-key-file PRIVATE_KEY_FILE flag when running the gcloud compute ssh command. For example:

gcloud compute ssh example-instance --ssh-key-file my-private-key-file

See the gcloud reference documentation for more information.

Interacting with the serial console

You can enable interactive access to an instance's serial console so you can connect and troubleshoot instances through the serial console.

To learn more, read Interacting with the Serial Console.

Avoiding packet fragmentation to instances built from custom images

The VPC network has a default maximum transmission unit (MTU) of 1460 bytes for Linux images and Windows Server images. However, the network MTU can be changed. For details, see Maximum transmission unit (MTU) in the VPC documentation.

When creating client applications that communicate with Compute Engine instances over UDP sockets, you can avoid fragmentation if you set the maximum size of the UDP datagram's data to 28 bytes less than the network MTU. For example, if the network's MTU is 1460 bytes, you can send up to 1432 bytes of UDP data per packet without fragmentation. If the network's MTU is 1500 bytes, you can send up to 1472 bytes of UDP data without fragmentation. The 28 bytes are used for an IPv4 packet header (20 bytes) and a UDP datagram header (8 bytes).