mailtos3 MDA

Config
Usage
Installation with Postfix

A light message delivery agent that can use AWS s3 buckets as mailbox storage instead of local storage or physical drive. This can be used with postfix or other MTA (message transfer agent) instead of standard MDA that rely on local disks.

Why use s3 as mail storage?

• unlimited mailbox size
• unlimited number of mailboxes
• journaling
• feed for forensics / surveillance systems

This is only MDA with no MRA capabilities, in order to retrieve messages you have to use other means of retrieving bucket objects

The only dependencies are the standard library and aws-sdk-go

To build locally (not system wide) clone this repo cd into dir where main.go is, get all dependencies and build:

go get ./...
go build -o mailtos3

Config

mailtos3 uses local config for mapping of user email addresses to s3 buckets, you can also specify CATCH ALL route for all calls without valid email address ( in order to use catch_all do not pass -a / -address flag).

requestConfig consists of:

• region string bucket region
• timeout (optional) int time out setting for all put requests ( default is 0 )
• endpoint (optional) bool if true all put request will use s3.<region>.amazonaws.com ( you cannot use this setting when testing locally )

Each mailbox mapping consists of:

• address string email address that the message is addressed to
• bucket string bucket name to store the massage in
• cmkKeyArn (optional) string if specified mailtos3 will use server side encryption before storing the message in the bucket.
  Without the key, message body will be saved in plain text.
• prefix (optional) string prefix to be added to objects of particular mailbox. Any string can be added and also GO time.Now() formatted just like regular date time layout
  example: "prefix": "dateTimeFormat(20060102)" object will be saved as 20200811/objectKey

{
    "requestConfig": {
        "region": "eu-west-1",
        "timeout": 10,
        "endpoint": false
    },
    "mailboxes": [
        {
            "address": "user@host.co.uk",
            "bucket": "my_bucket",
            "cmkKeyArn": "arn:aws:kms:eu-west-1:123456:key/123456-123456-123456" // cmkKey is optional
            "prefix": "user"
        },
        {
            "address": "user2@host.co.uk",
            "bucket": "my_bucket2",
            "cmkKeyArn": "arn:aws:kms:eu-west-1:123456:key/123456-123456-123456" // cmkKey is optional
            "prefix": "dateTimeFormat(20060102)"
        }
    ]
}

Usage

To pass message body from MTA to s3: ( this is the same for local testing )

mailtos3 -a=user@host.co.uk -f=from@host.co.uk "message body"
// or from pipe
echo "message body" | mailtos3 -a=user@host.co.uk -f=from@host.co.uk

Postfix master.cf

mailtos3  unix  -       n       n       -       -       pipe
      flags=ODRhu user=mailtos3 argv=/usr/local/bin/mailtos3/mailtos3 -a=${recipient} -f=${sender}

this will store message for user@host.co.uk in my_bucket

Installation with Postfix

append to the bottom of /etc/postfix/main.cf

default_privs=mailtos3
mailtos3_destination_recipient_limit = 1
virtual_mailbox_domains = <hostname>.co.uk
virtual_transport = mailtos3
virtual_mailbox_maps = hash:/etc/postfix/virtual_mailbox

append to the bottom of /etc/postfix/master.cf

mailtos3  unix  -       n       n       -       -       pipe
      flags=ODRhu user=mailtos3 argv=/usr/local/bin/mailtos3/mailtos3 -a ${recipient}

create file /etc/postfix/virtual_mailbox and add the recipient addresses for which mailtos3 will be triggered
in the form of:
user@domain some_text or @domain catch_all

user1@<hostname>.co.uk ( text here does not matter but must be present )
user2@<hostname>.co.uk ( text here does not matter but must be present )

Create service user mailtos3 this is the user postfix will assume to use mailtos3, we cannot use postfix user or root.
Create log file and change permissions to mailtos3 user

sudo touch /var/log/mailtos3.log
sudo chown mailtos3:mailtos3 /var/log/mailtos3.log

Create folder for the binaries and config, and place your files in there:

sudo mkdir /usr/local/bin/mailtos3
sudo chown mailtos3:mailtos3 /usr/local/bin/mailtos3

Copy build artifacts to /usr/local/bin/mailtos3, and create your config.json file in the same dir. Reload virtual maps and postfix

sudo postmap /etc/postfix/virtual_mailbox
sudo service postfix restart