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AWS SMTP Relay

SMTP server to relay emails via Amazon SES or Amazon Pinpoint using IAM roles.

Contents

Background

Amazon SES and Amazon Pinpoint both provide an API and an SMTP interface to send emails:

The SMTP interface is useful for applications that must use SMTP to send emails, but it requires providing a set of SMTP credentials:

For security reasons, using IAM roles is preferable, but only possible with the Email API and not the SMTP interface.

This is where this project comes into play, as it provides an SMTP interface that relays emails via SES or Pinpoint API using IAM roles.

Docker

This repository provides a sample Dockerfile to build and run the project in a container environment.

A prebuilt Docker image is also available on Docker Hub:

docker run blueimp/aws-smtp-relay --help

Installation

The aws-smtp-relay binary can be installed from source via go get:

go get github.com/blueimp/aws-smtp-relay

Usage

By default, aws-smtp-relay listens on port 1025 on all interfaces as open relay (without authentication) when started without arguments:

aws-smtp-relay

Options

Available options can be listed the following way:

aws-smtp-relay --help
Usage of aws-smtp-relay:
  -a string
        TCP listen address (default ":1025")
  -c string
        TLS cert file
  -d string
        Denied recipient emails regular expression
  -e string
        Amazon SES Configuration Set Name
  -h string
        Server hostname
  -i string
        Allowed client IPs (comma-separated)
  -k string
        TLS key file
  -l string
        Allowed sender emails regular expression
  -n string
        SMTP service name (default "AWS SMTP Relay")
  -r string
        Relay API to use (ses|pinpoint) (default "ses")
  -s    Require TLS via STARTTLS extension
  -t    Listen for incoming TLS connections only
  -u string
        Authentication username

Authentication

User

The supported user-based SMTP authentication mechanisms and their required configuration settings (see also RFC 4954):

Mechanism TLS User Hash Pass
LOGIN Yes Yes Yes No
PLAIN Yes Yes Yes No
CRAM-MD5 No Yes No Yes

Authentication can be enabled for LOGIN and PLAIN mechanisms by configuring TLS and a username and providing the bcrypt encrypted password as BCRYPT_HASH environment variable:

export BCRYPT_HASH=$(htpasswd -bnBC 10 '' password | tr -d ':\n')
export TLS_KEY_PASS="$PASSPHRASE"

aws-smtp-relay -c tls/default.crt -k tls/default.key -u username

If the password is provided as plain text PASSWORD environment variable, it will also enable the CRAM-MD5 authentication mechanism:

export PASSWORD=password
export TLS_KEY_PASS="$PASSPHRASE"

aws-smtp-relay -c tls/default.crt -k tls/default.key -u username

Without TLS configuration, only CRAM-MD5 will be enabled:

export PASSWORD=password

aws-smtp-relay -u username

Please note:

It is not recommended to provide the password as plain text environment variable, nor to configure the SMTP server without TLS support.

IP

To limit the allowed IP addresses, supply a comma-separated list via -i ips option:

aws-smtp-relay -i 127.0.0.1,::1

Please note:

To authorize their IP, clients must use a supported SMTP authentication mechanism, e.g. LOGIN or PLAIN via TLS or CRAM-MD5 on unencrypted connections.
This is required even if no user authentication is configured on the server, although in this case the credentials can be chosen freely by the client.

TLS

Configure TLS with the following steps:

Edit the openssl config file and change localhost to your server hostname.

Generate a self-signed certificate with a passphrase encrypted key:

openssl req -new -x509 -config tls/openssl.conf -days 24855 \
  -out tls/default.crt \
  -keyout /dev/stdout |
  openssl rsa -aes256 -out tls/default.key

Please note:

Encrypted key files are only supported if they contain a DEK-Info header, stating the encryption method used.
The openssl req command does not create this header if encryption is enabled, which is why we pipe the unencrypted key output to the openssl rsa command, which outputs an encrypted key file with the required DEK-Info header.

Provide the key file passphrase as TLS_KEY_PASS environment variable and the cert and key file as command-line arguments:

TLS_KEY_PASS="$PASSPHRASE" aws-smtp-relay -c tls/default.crt -k tls/default.key

Please note:

It is recommended to require TLS via STARTTLS extension (-s option flag) or to configure the server to listen for incoming TLS connections only (-t option flag).

Filtering

Senders

To limit the allowed sender email addresses, provide an allow list as regular expression via -l regexp option:

aws-smtp-relay -l '@example\.org$'

By default, all sender email addresses are allowed.

Recipients

To deny certain recipient email addresses, provide a deny list as regular expression via -d regexp option:

aws-smtp-relay -d 'admin@example\.org$'

By default, all recipient email addresses are allowed.

Region

The AWS_REGION must be set to configure the AWS SDK, e.g. by executing the following command before starting aws-smtp-relay:

export AWS_REGION=eu-west-1

Credentials

On EC2 or ECS, security credentials for the IAM role are automatically retrieved:

Logging

Requests are logged in JSON format to stdout with the Error property set to null:

{
  "Time": "2018-04-18T15:08:42.4388893Z",
  "IP": "172.17.0.1",
  "From": "[email protected]",
  "To": ["[email protected]"],
  "Error": null
}

Errors are logged in the same format to stderr, with the Error property set to a string value:

{
  "Time": "2018-04-18T15:08:42.4388893Z",
  "IP": "172.17.0.1",
  "From": "[email protected]",
  "To": ["[email protected]"],
  "Error": "MissingRegion: could not find region configuration"
}

Development

Build

First, clone the project and then switch into its source directory:

git clone https://github.com/blueimp/aws-smtp-relay.git
cd aws-smtp-relay

Please note:

This project relies on Go modules for automatic dependency resolution.

To build the project, run Make in the repository directory, which creates the aws-smtp-relay binary:

make

Lint

To lint the source code, first install staticcheck:

go install honnef.co/go/tools/cmd/staticcheck@latest

Then run the following command:

make lint

Test

All components come with unit tests, which can be executed the following way:

make test

Sending mails can also be tested with the provided mail shell script:

echo TEXT | ./mail.sh -p 1025 -f [email protected] -t [email protected]

Please note:

The provided shell script only supports the LOGIN authentication mechanism.

See also Testing Amazon SES Email Sending.

Install

The binary can also be built and installed in $GOPATH/bin/ with the following command:

make install

Uninstall

The uninstall command removes the binary from $GOPATH/bin/:

make uninstall

Clean

To remove any build artifacts, run the following:

make clean

Dependencies

License

Released under the MIT license.