various typos and improvements

This commit is contained in:
Christoph Haas 2025-12-17 01:43:18 +01:00
parent 912492287d
commit 4da8f077e6
5 changed files with 322 additions and 9 deletions

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@ -23,7 +23,8 @@ server.
limit is the size of your disk. limit is the size of your disk.
- Have a **webmail** interface so users can access their emails securely from any location using a web browser. - Have a **webmail** interface so users can access their emails securely from any location using a web browser.
- Let your users fetch email using **IMAP** and send email through your servers using **SMTP**. - Let your users fetch email using **IMAP** and send email through your servers using **SMTP**.
- Add automatic cryptographic signatures using DKIM to outgoing emails to prove that you are the owner of your domain. - Add automatic cryptographic signatures using **DKIM** to outgoing emails to prove that you are the owner of your
domain.
- Allow users to manage server-based **filter rules**. Distribute incoming emails to different folders. Forward copies. - Allow users to manage server-based **filter rules**. Distribute incoming emails to different folders. Forward copies.
Or send out-of-office notifications. Or send out-of-office notifications.
- Mitigate **brute force** attacks. - Mitigate **brute force** attacks.
@ -51,9 +52,10 @@ server.
- An **internet domain** (or several) to receive emails for. You need to be able to set A, MX and TXT records for that - An **internet domain** (or several) to receive emails for. You need to be able to set A, MX and TXT records for that
domain. You should also be able to set PTR records for your IP address because some mail servers on the internet domain. You should also be able to set PTR records for your IP address because some mail servers on the internet
require you to have matching forward and reverse DNS records. require you to have matching forward and reverse DNS records.
- **Patience**. We will proceed slowly and after every step ensure that are still on track. Dont hurry and skip parts - **Patience**. We will proceed slowly and after every step ensure that you are still on track. Dont hurry and skip
even if they appear confusing at first. If you get lost just submit your question at the bottom of any page throughout parts even if they appear confusing at first. If you get lost just submit your question at the bottom of any page
this guide and help is on the way. Or join the [chat channel](https://riot.im/app/#/room/#ispmail:matrix.org). throughout this guide and help is on the way. Or join the
[chat channel](https://riot.im/app/#/room/#ispmail:matrix.org).
## What this is not about ## What this is not about

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@ -46,7 +46,7 @@ CREATE DATABASE mailserver;
## Create the database users ## Create the database users
In this section you will create the basic database `mailserver` and two users: In this section you will create the basic database called `mailserver` and two users:
| User | Permissions | Purpose | | User | Permissions | Purpose |
| :--------- | :---------- | :--------------------------------------------------------- | | :--------- | :---------- | :--------------------------------------------------------- |

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@ -8,8 +8,313 @@ sidebar:
import { Aside } from "@astrojs/starlight/components"; import { Aside } from "@astrojs/starlight/components";
<Aside type="tip" title="Coming soon"> <Aside type="tip" title="Optional feature">
This page is currently rewritten. It will be online in a couple of days. This feature is completely optional. If you are eager to get finished then skip this page and maybe come back later.
</Aside> </Aside>
![Under construction](images/under-construction.jpg) Quotas are size limits for users. You can make sure that users do not waste arbitrary amounts of disk space but are
forced to clean up old emails every now and then.
The magic happens in two places:
1. Postfix needs to reject new emails if the users mailbox is over quota.
2. Dovecot needs to keep track of the quota and how much the user has already used up of it.
---
https://doc.dovecot.org/2.4.2/core/plugins/quota.html#quota-service
https://sys4.de/en/blog/postfix-dovecot-mailbox-quota/
---
https://www.postfix.org/SMTPD_POLICY_README.html
telnet 127.0.0.1 13373 Trying 127.0.0.1... Connected to 127.0.0.1. Escape character is '^]'.
recipient=chris@auenland.workaround.org
action=554 5.2.2 Quota exceeded (mailbox for user is full)
---
```swaks --from nonexistens@example.org --to chris@auenland.workaround.org
=== Trying auenland.workaround.org:25...
=== Connected to auenland.workaround.org.
<- 220 auenland ESMTP Postfix (Debian)
-> EHLO minty
<- 250-auenland
<- 250-PIPELINING
<- 250-SIZE 10240000
<- 250-VRFY
<- 250-ETRN
<- 250-STARTTLS
<- 250-ENHANCEDSTATUSCODES
<- 250-8BITMIME
<- 250-DSN
<- 250-SMTPUTF8
<- 250 CHUNKING
-> MAIL FROM:<nonexistens@example.org>
<- 250 2.1.0 Ok
-> RCPT TO:<chris@auenland.workaround.org>
<** 554 5.2.2 <chris@auenland.workaround.org>: Recipient address rejected: Quota exceeded (mailbox for user is full)
-> QUIT
<- 221 2.0.0 Bye
=== Connection closed with remote host.
```
---
```
cat 99-ispmail-quota.conf
mail_plugins {
quota = yes
}
quota "User quota" {
storage_size = 100K
storage_grace = 0
warning warn-95 {
quota_storage_percentage = 95
execute quota-warning {
args = 95 %{user}
}
}
warning warn-80 {
quota_storage_percentage = 80
execute quota-warning {
args = 80 %{user}
}
}
}
service quota-status {
executable = quota-status -p postfix
inet_listener quota-status {
port = 13373
}
client_limit = 1
}
# Example quota-warning service. The unix listener's permissions should be
# set in a way that mail processes can connect to it. Below example assumes
# that mail processes run as vmail user. If you use mode=0666, all system users
# can generate quota warnings to anyone.
service quota-warning {
executable = script /usr/local/bin/ispmail-quota-warning.sh
user = dovecot
unix_listener quota-warning {
user = vmail
}
}
##
## Quota backends
##
# Multiple backends are supported:
# count: Default and recommended, quota driver tracks the quota internally within Dovecot's index files.
# maildir: Maildir++ quota
# fs: Read-only support for filesystem quota
#quota "User quota" {
# driver = count
#}
```
---
```
cat /usr/local/bin/ispmail-quota-warning.sh
#!/bin/sh
PERCENT=$1
USER=$2
cat << EOF | /usr/lib/dovecot/dovecot-lda -d $USER -o quota_enforce=no
From: postmaster@domain.com
Subject: quota warning
Your mailbox is now $PERCENT% full.
EOF
```
---
### Dovecot quota policy service
Lets start with Dovecot. Find the file `/etc/dovecot/conf.d/90-quota.conf` and edit it. There are several `plugin {}`
sections. Take one and make it look like:
```
plugin {
quota = count:User quota
quota_vsizes = yes
quota_status_success = DUNNO
quota_status_nouser = DUNNO
quota_status_overquota = "452 4.2.2 Mailbox is full and cannot receive any more emails"
}
```
The first line defines that you want to calculate the used space in a users _maildir_. There are several
[backends](https://doc.dovecot.org/configuration_manual/quota_plugin/) like that but the
_[count](https://doc.dovecot.org/configuration_manual/quota/quota_count/#quota-backend-count)_ is the best choice in
this context. (Previous guides used _maildir_ here.) The string “User quota” is just an arbitrary string that may be
queried from a mail user agent.
The lines starting with “`quota_status_…`” set return values for the service that you will set up in a minute. It will
tell Postfix that it will not interfere (_DUNNO_ colloquial way to say “I dont know”). And it will return a string
with a return code 452 if the user is over quota. Codes starting with “4” mean temporary errors. It will tell the
sending party that it is worth retrying at a later time. However if the user does not resolve the issue it will lead to
a _bounce_ error email after three days.
In the same file (_90-quota.conf_) add another section:
```
service quota-status {
executable = /usr/lib/dovecot/quota-status -p postfix
unix_listener /var/spool/postfix/private/quota-status {
user = postfix
}
}
```
This creates a new [Dovecot service](https://doc.dovecot.org/configuration_manual/service_configuration/) responding to
requests from other processes. You surely recognize that we put it into the jail that Postfix runs in
(_/var/spool/postfix_), so that Postfix can access it.
Time to restart Dovecot:
```
systemctl restart dovecot
```
Take a look at the /var/spool/postfix/private directory. If all went as intended you will find a socket file called
`quota-status` there. Otherwise please check the `/var/log/mail.log` file for errors.
### Postfix recipient restrictions
If we stopped here, then Dovecot would reject emails for users who have no space left. However Postfix would still
happily receive new emails and attempt to forward them to Dovecot via LMTP. Dovecot however will deny that. It will then
keep the email in its queue and retry for a while. In the end it will send a _bounce_ back to the sender telling them
about the problem. So why is this bad?
1. The sender will assume that the email was delivered while it is stuck in the queue for up to three days.
2. Spam emails use forged senders. So at the time that Postfix generates the _bounce email_ it will likely send it to an
innocent person. This is called _backscatter_ and considered a mail server misconfiguration. Such a problem may get
your mail server blacklisted. You dont want that.
So the next logical step is to make Postfix check whether a mailbox is over quota whenever a new email arrives. Lets
hook up into the “RCPT TO” phase of the SMTP dialog when a new email comes in. Postfix checks its
_smtpd_recipient_restrictions_ configuration at this stage. Run this command in the shell:
```
postconf smtpd_recipient_restrictions=reject_unauth_destination, \
"check_policy_service unix:private/quota-status"
```
This adds two checks:
1. `reject_unauth_destination` checks whether the mail server is the final destination for the recipients email
address. This is pretty much the default behavior if you do not define any restrictions.
2. `check_policy_service` connects to the socket file at `/var/spool/postfix/private/quota-status` that was put there by
Dovecot. It will use it to ask Dovecot whether the user is over quota in which case the email would get rejected.
### Test it
If you are curious to see this working, then set Johns mailbox quota to 5 KB:
```sql
# mariadb mailserver
mysql> update virtual_users set quota=4000 where email='john@example.org';
```
Send him a few emails using the swaks tool:
```
swaks --server localhost --to john@example.org
```
After a few emails you will see the rejection message:
```
-> RCPT TO:john@example.org
<** 452 4.2.2 john@example.org: Recipient address rejected: Mailbox is full and cannot receive any more emails
```
### Troubleshooting
These are things you should consider if quotas do not seem to work properly:
- Check if you have enabled “quota” in the “mail_plugins” in the 10-mail.conf file.
- Your users may complain that they have deleted many emails but are still over quota. Let them check if they actually
emptied the _Trash_ folder. Of course emails in that folder also contribute to the disk space usage. Once the Trash
folder is expunged the problem should be gone. You may also allow your users more space in the Trash folder. Thats
explained in the [Dovecot documentation](https://doc.dovecot.org/configuration_manual/quota/#quota-rules).
- If you directly remove files from a users Maildir instead of properly accessing the mailbox using IMAP then you will
screw up the quota calculation. In that case let Dovecot recalculate the quota:
`doveadm quota recalc -u john@example.org`
### Automatic warning emails
The last step is to inform the poor users if they accidentally went over quota. After all they do not necessarily
recognize that on their own. Lets do that by sending them an email with a warning. Yes, we will make sure that the
email gets through even if the quota is reached.
Edit the `90-quota.conf` file again. Add this section to the file (derived from the
[Dovecot documentation](https://doc.dovecot.org/configuration_manual/quota/#quota-warning-scripts)):
```
plugin {
quota_warning = storage=95%% quota-warning 95 %u
quota_warning2 = storage=80%% quota-warning 80 %u
}
service quota-warning {
executable = script /usr/local/bin/quota-warning.sh
unix_listener quota-warning {
user = vmail
group = vmail
mode = 0660
}
}
```
This section defines two automatic quota warnings. The first (quota_warning) is triggered if the user reaches 95% of the
quota. The second (quota_warning2) at 80%. These lines follow this schema:
- **Trigger** (e.g. “storage=95%”). The “%” sign needs to be used twice if you want to emit a literal percent sign. So
this is not a typo.
- The **socket** you want to call in that case. Our socket is the “service quota-warning” that calls a shell script.
- Additional **parameters** that are passed to the shell script in our case. They tell the script the percentage that
has been reached (e.g. 95) and the address of the user who should get the warning.
Apparently we need the script to run. So please create a new file at `/usr/local/bin/quota-warning.sh` and put these
lines into it:
```
#!/bin/sh
PERCENT=$1
USER=$2
cat << EOF | /usr/lib/dovecot/dovecot-lda -d $USER -o "plugin/quota=maildir:User quota:noenforcing"
From: postmaster@webmail.example.org
Subject: Quota warning - $PERCENT% reached
Your mailbox can only store a limited amount of emails.
Currently it is $PERCENT% full. If you reach 100% then
new emails cannot be stored. Thanks for your understanding.
EOF
```
Make this file executable:
```
chmod +x /usr/local/bin/quota-warning.sh
```
Time to restart Dovecot again:
```
systemctl restart dovecot
```
Dovecots quota limits can be configured in many ways. If you have special needs then give
[their documentation](https://doc.dovecot.org/configuration_manual/quota/) a look.

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@ -36,6 +36,12 @@ ALTER USER 'mailserver'@'127.0.0.1' IDENTIFIED BY '${PW_MAILSERVER}';
FLUSH PRIVILEGES; FLUSH PRIVILEGES;
EOF EOF
# Delete example data
mariadb <<EOF
DELETE FROM virtual_domains where name='example.org';
EOF
# Print the passwords for us to write down # Print the passwords for us to write down
echo "mailadmin password: $PW_MAILADMIN" echo "mailadmin password: $PW_MAILADMIN"
echo "mailserver password: $PW_MAILSERVER" echo "mailserver password: $PW_MAILSERVER"