Postfix

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Postfix är en mail transfer agent(MTA) som kan skicka och ta emot email. Postfix används ofta tillsammans med Dovecot. Postfix kom redan 1986 men skrevs om 1997, ca 32% av dagens publika mailservrar kör Postfix.

Förberedelse

Öppna port i brandväggen.

  • 25 (SMTP)

Och skapa ett A och MX record i DNSen som spekar mot mailservern.

Installation 

apt-get install postfix postfix-pcre

Om man vill kunna skicka testmails(swaks) och enkelt styra mailboxen(mutt). 

apt-get install mutt swaks

Konfiguration

Konfigurations filerna finns under /etc/postfix

LDAP

Man kan enkelt koppla Postfix mot en LDAP server för autentisering, vilken mail och mailalias en användare ska ha och hur mycket lagring användare får.

Förberedelse

En uppsatt OpenLDAP server med ett postfix schema inlagt. Och en Dovecot server som också är LDAP ansluten.

Installation 

apt-get install postfix-ldap

main.cf Detta är main konfigurations filen. 

#!/bin/bash
###################################################################################################
### Base Settings ###
#####################

# Listen on all interfaces
inet_interfaces = all

# Use TCP IPv4
inet_protocols = ipv4

# Greet connecting clients with this banner
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)

# Fully-qualified hostname
myhostname = mail.example.com

# Do not append domain part to incomplete addresses (this is the MUA's job)
append_dot_mydomain = no

# Trusted networks/hosts (these are allowed to relay without authentication)
mynetworks =
    # Local
    127.0.0.0/8
    # External
    1.2.3.4/32


###################################################################################################
### Local Transport ###
#######################

# Disable local transport (so that system accounts can't receive mail)
local_transport = error:Local Transport Disabled

# Don't use local alias maps
alias_maps = 

# Local domain (could be omitted, since it is automatically derived from $myhostname)
mydomain = example.com

# Mails for these domains will be transported locally
mydestination =
    $myhostname
    localhost.$mydomain
    localhost


###################################################################################################
### Virtual Transport ###
#########################

# Deliver mail for virtual recipients to Dovecot
virtual_transport = dovecot

# Process one mail at one time
dovecot_destination_recipient_limit = 1

# Valid virtual domains
virtual_mailbox_domains = hash:/etc/postfix/virtual_domains

# Valid virtual recipients
virtual_mailbox_maps = proxy:ldap:/etc/postfix/ldap_virtual_recipients.cf

# Virtual aliases
virtual_alias_maps = proxy:ldap:/etc/postfix/ldap_virtual_aliases.cf


###################################################################################################
### ESMTP Settings ###
######################

### SASL ###

# Enable SASL (required for SMTP authentication)
smtpd_sasl_auth_enable = yes

# Enable SASL for Outlook-Clients as well
broken_sasl_auth_clients = yes

### TLS ###

# Enable TLS (required to encrypt the plaintext SASL authentication)
smtpd_tls_security_level = may

# Only offer SASL in a TLS session
smtpd_tls_auth_only = yes

# Certification Authority
smtpd_tls_CAfile = /etc/postfix/certs/example-cacert.pem

# Public Certificate
smtpd_tls_cert_file = /etc/postfix/certs/mail_public_cert.pem

# Private Key (without passphrase)
smtpd_tls_key_file = /etc/postfix/certs/mail_private_key.pem

# Randomizer for key creation
tls_random_source = dev:/dev/urandom

# TLS related logging (set to 2 for debugging)
smtpd_tls_loglevel = 0

# Avoid Denial-Of-Service-Attacks
smtpd_client_new_tls_session_rate_limit = 10

# Activate TLS Session Cache
smtpd_tls_session_cache_database = btree:/etc/postfix/smtpd_session_cache

# Deny some TLS-Ciphers
smtpd_tls_exclude_ciphers =
        EXP
        EDH-RSA-DES-CBC-SHA
        ADH-DES-CBC-SHA
        DES-CBC-SHA
        SEED-SHA

# Diffie-Hellman Parameters for Perfect Forward Secrecy
# Can be created with:
# openssl dhparam -2 -out dh_512.pem 512
# openssl dhparam -2 -out dh_1024.pem 1024
smtpd_tls_dh512_param_file = ${config_directory}/certs/dh_512.pem
smtpd_tls_dh1024_param_file = ${config_directory}/certs/dh_1024.pem


###################################################################################################
### Connection Policies ###
###########################

# Reject Early Talkers
postscreen_greet_action = enforce


###################################################################################################
### Session Policies ###
########################

# Recipient Restrictions (RCPT TO related)
smtpd_recipient_restrictions =
        # Allow relaying for SASL authenticated clients and trusted hosts/networks
        # This can be put to smtpd_relay_restrictions in Postfix 2.10 and later 
        permit_sasl_authenticated
        permit_mynetworks
        reject_non_fqdn_recipient
        reject_unknown_recipient_domain
        reject_unauth_destination
        # Reject the following hosts
        check_sender_ns_access cidr:/etc/postfix/drop.cidr
        check_sender_mx_access cidr:/etc/postfix/drop.cidr
        # Additional blacklist
        reject_rbl_client ix.dnsbl.manitu.net
        # Finally permit (relaying still requires SASL auth)
        permit

# Reject the request if the sender is the null address and there are multiple recipients
smtpd_data_restrictions = reject_multi_recipient_bounce

# Sender Restrictions
smtpd_sender_restrictions =
        reject_non_fqdn_sender
        reject_unknown_sender_domain

# HELO/EHLO Restrictions
smtpd_helo_restrictions =
		permit_mynetworks
        check_helo_access pcre:/etc/postfix/identitycheck.pcre
        #reject_non_fqdn_helo_hostname
        reject_invalid_hostname

# Deny VRFY recipient checks
disable_vrfy_command = yes

# Require HELO
smtpd_helo_required = yes

# Reject instantly if a restriction applies (do not wait until RCPT TO)
smtpd_delay_reject = no

# Client Restrictions (IP Blacklist)
smtpd_client_restrictions = check_client_access cidr:/etc/postfix/drop.cidr

virtual_domains Innehåller vilka domäner server tar emot mail för. 

#!/bin/bash
# Domain		Anything
example.com		OK

ldap_virtual_recipients.cf LDAP fråga för att validera mottagaren. 

#!/bin/bash
bind = yes
bind_dn = uid=postfix,ou=services,dc=example,dc=com
bind_pw = secret
server_host = ldap://127.0.0.1:389
search_base = ou=people,dc=example,dc=com
domain = example.com
query_filter = (&(mail=%s)(mailEnabled=TRUE))
result_attribute = mail

ldap_virtual_aliases.cf LDAP fråga för att få fram aliases och forwarding adress. 

#!/bin/bash
bind = yes
bind_dn = uid=postfix,ou=services,dc=example,dc=com
bind_pw = secret
server_host = ldap://127.0.0.1:389
search_base = ou=people,dc=example,dc=com
domain = example.com
query_filter = (&(mailAlias=%s)(mailEnabled=TRUE))
result_attribute = mail, email

identitycheck.pcre Regexp för att blocka klienter som använder ditt hostnamn. 

#!/bin/bash
# Identity (RegEx)		Action

/^(mail\.example\.com)$/	REJECT Hostname Abuse: $1
/^(1\.2\.3\.4)$/		REJECT Hostname Abuse: $1
/^(\[1\.2\.3\.4\])$/		REJECT Hostname Abuse: $1

drop.cidr Innehåller svartlistade IP-adresser. 

#!/bin/bash
# IP/CIDR			Action

1.2.3.0/24			REJECT Blacklisted

Temporärt kommentera ut följande rader eftersom att Dovecot och TLS inte är konfigurerat i main.cf:

  • dovecot_destination_recipient_limit = 1
  • smtpd_tls_security_level = may
  • smtpd_tls_auth_only = yes
  • smtpd_tls_CAfile = /etc/postfix/certs/example-cacert.pem
  • smtpd_tls_cert_file = /etc/postfix/certs/mail_public_cert.pem
  • smtpd_tls_key_file = /etc/postfix/certs/mail_private_key.pem

Skapa en postmap db fil för din domän. 

postmap hash:/etc/postfix/virtual_domains

Starta postfix och anslut mot servern med telnet mot port 25. Prova att skicka EHLO client, då ska du få följande svar: 

Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
220 mail.example.com ESMTP Postfix (Ubuntu)
EHLO client
250-mail.example.com
250-PIPELINING
250-SIZE 10240000
250-ETRN
250-AUTH DIGEST-MD5 NTLM CRAM-MD5 LOGIN PLAIN
250-AUTH=DIGEST-MD5 NTLM CRAM-MD5 LOGIN PLAIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
QUIT
221 2.0.0 Bye

Testa att ställa en LDAP fråga. 

postmap -q user@example.com ldap:/etc/postfix/ldap_virtual_recipients.cf
postmap -q postmaster@example.com ldap:/etc/postfix/ldap_virtual_aliases.cf

Båda frågorna bör ge user@example.com som svar.

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