---
description: |
  Learn how to debug issues with Packer builds and plugins using `packer build`, logs, and other troubleshooting tools. 
page_title: Debugging Packer
---

# Debugging Packer Builds

Using `packer build -on-error=ask` allows you to inspect failures and try out
solutions before restarting the build.

For remote builds with cloud providers like Amazon Web Services AMIs, debugging
a Packer build can be eased greatly with `packer build -debug`. This disables
parallelization and enables debug mode.

Debug mode informs the builders that they should output debugging information.
The exact behavior of debug mode is left to the builder. In general, builders
usually will stop between each step, waiting for keyboard input before
continuing. This will allow you to inspect state and so on.

In debug mode once the remote instance is instantiated, Packer will emit to the
current directory an ephemeral private SSH key as a .pem file. Using that you
can `ssh -i <key.pem>` into the remote build instance and see what is going on
for debugging. The key will only be emitted for cloud-based builders. The
ephemeral key will be deleted at the end of the Packer run during cleanup.

For a local builder, the SSH session initiated will be visible in the detail
provided when `PACKER_LOG=1` environment variable is set prior to a build, and
you can connect to the local machine using the userid and password defined in
the kickstart or preseed associated with initializing the local VM.

It should be noted that one of the options `-on-error` is to `retry`, the retry
of the step in question has limitations:

- the template Packer is building is **not** reloaded from file.
- the resources specified from builders **are** reloaded from file.

Check the specifics on your builder to confirm their behavior.

### Windows

As of Packer 0.8.1 the default WinRM communicator will emit the password for a
Remote Desktop Connection into your instance. This happens following the
several minute pause as the instance is booted. Note a .pem key is still
created for securely transmitting the password. Packer automatically decrypts
the password for you in debug mode.

## Debugging Packer

Issues occasionally arise where certain things may not work entirely correctly,
or may not appear to work correctly. In these cases, it is sometimes helpful to
see more details about what Packer is actually doing.

Packer has detailed logs which can be enabled by setting the `PACKER_LOG`
environmental variable to any value but `""` (empty string) and `"0"` like this
`PACKER_LOG=1 packer build <config.json>`. This will cause detailed logs to
appear on stderr. The logs contain log messages from Packer as well as any
plugins that are being used. Log messages from plugins are prefixed by their
application name.

Note that because Packer is highly parallelized, log messages sometimes appear
out of order, especially with respect to plugins. In this case, it is important
to pay attention to the timestamp of the log messages to determine order.

In addition to simply enabling the log, you can set `PACKER_LOG_PATH` in order
to force the log to always go to a specific file when logging is enabled. Note
that even when `PACKER_LOG_PATH` is set, `PACKER_LOG` must be set in order for
any logging to be enabled.

### Debugging Plugins

Each packer plugin runs in a separate process and communicates with RPC over a
socket therefore using a debugger will not work (be complicated at least).

But most of the Packer code is really simple and easy to follow with PACKER_LOG
turned on. If that doesn't work adding some extra debug print outs when you have
homed in on the problem is usually enough.

### Debugging Packer in Powershell/Windows

In Windows you can set the detailed logs environmental variable `PACKER_LOG` or
the log variable `PACKER_LOG_PATH` using PowerShell environment variables. For
example:

```powershell
$env:PACKER_LOG=1
$env:PACKER_LOG_PATH="packerlog.txt"
```

If you find a bug with Packer, please include the detailed log by using a
service such as [gist](https://gist.github.com).

## Issues Installing Ubuntu Packages

Issues may arise using and building Ubuntu AMIs where common packages that
_should_ be installed from Ubuntu's Main repository are not found during a
provisioner step:

```text
amazon-ebs: No candidate version found for build-essential
amazon-ebs: No candidate version found for build-essential
```

This, obviously can cause problems where a build is unable to finish
successfully as the proper packages cannot be provisioned correctly. The
problem arises when cloud-init has not finished fully running on the source AMI
by the time that packer starts any provisioning steps.

Adding the following provisioner to the Packer template, allows for the
cloud-init process to fully finish before packer starts provisioning the source
AMI.

```json
{
  "type": "shell",
  "inline": [
    "cloud-init status --wait"
  ]
}
```

## Issues when using numerous Builders/Provisioners/Post-Processors

Packer uses a separate process for each builder, provisioner, post-processor,
and plugin. In certain cases, if you have too many of these, you can run out of
[file descriptors](https://en.wikipedia.org/wiki/File_descriptor). This results
in an error that might look like

```text
error initializing provisioner 'powershell': fork/exec /files/go/bin/packer:
too many open files
```

On Unix systems, you can check what your file descriptor limit is with
`ulimit -Sn`. You should check with your OS vendor on how to raise this limit.

## Issues when using long temp directory

Packer uses Unix sockets internally, which are created inside the default
directory for temporary files. Some operating systems place a limit on the
length of the socket name, usually between 80 and 110 characters. If you get an
error like this (for any builder, not just Docker):

```text
Failed to initialize build 'docker': error initializing builder 'docker': plugin exited before we could connect
```

you should try setting your temp directory to something shorter. This can be
done through the `TMPDIR` environment variable.
