Days ago we got some issue with Amazon SES and decide to make our own SMTP relay service. I tried the combinations of sendmail with dim-milter but for some reason I could not make it work so I start another server from scratch which did work this time. This post mainly focus on configuring OpenDKIM as Postfix is fairly straightforward.
The first thing to do is disabling sendmail which is the default SMTP client on Amazon Linux:
service sendmail stop chkconfig sendmail off
And then installing Postfix and configuring it:
yum install postfix cp /etc/postfix/main.cf /etc/postfix/main.cf.original vi /etc/postfix/main.cf (appendix 01)
Configuration mainly involves adding DKIM settings (such as milter socket info) and modifying receptions restrictions. In the new setting we define sender_access file to contain the list of senders who can relay through our SMTP service.
cat "DOMAIN.COM OK" >> /etc/postfix/sender_access postmap /etc/postfix/sender_access
Now we are done with Postfix so it is better to check by sending a test mail.
service postfix start chkconfig postfix on
The main concern of this post, DKIM, is to cryptographically validate the sender is really from that domain (i.e. domain.com). The validation process starts with the SMTP server signing the email using its private key and then the destination mail server tries to match the private key with the public key obtained from senders’ claimed domain DNS. Once the private and public key matched then it means the SMTP server is from the domain it claims to be.
To install the DKIM we need some API from sendmail and openssl:
yum install sendmail-devel openssl-devel
openDKIM is not available in default repository so we will add it and then install it:
rpm -Uvh http://mirror.pnl.gov/epel/6/i386/epel-release-6-8.noarch.rpm yum --disablerepo=* --enablerepo=epel install opendkim
Once installation finished we proceed to configurations. There are 3 main files to configure: opendkim.conf contains the main configs such as address of signing table file, key table file and the socket address to listen. The key table file contains the list of keys. The signing table defines which domains should be signed by which key. You will also need to add trusted IP addresses of senders in /etc/opendkim/TrustedHosts to grant them access to SMTP.
cp /etc/opendkim.conf /etc/opendkim.conf.original vi /etc/opendkim.conf (appendix 02) vi /etc/opendkim/KeyTable (appendix 03) vi /etc/opendkim/SigningTable (appendix 04)
Now we need to generate a pair of keys (private and public) which the public key will be added into DNS records of send domain:
mkdir /etc/opendkim/keys/DOMAIN.COM opendkim-genkey -D /etc/opendkim/keys/DOMAIN.COM/ -d DOMAIN.COM -s default mv /etc/opendkim/keys/DOMAIN.COM/default.private /etc/opendkim/keys/DOMAIN.COM/default chown -R opendkim:opendkim /etc/opendkim/keys/DOMAIN.COM cat /etc/opendkim/keys/DOMAIN.COM/default.txt
Then start the service:
service opendkim start chkconfig opendkim on
Finally, you have to add a TXT record in your DNS dashboard. The record name should be default._domainkey.DOMAIN.COM and it should contains something like the following (based on the /etc/opendkim/keys/DOMAIN.COM/default.txt):
“v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDHY7Zl+n3SUldTYRUEU1BErHkKN0Ya52gazp1R7FA7vN5RddPxW/sO9JVRLiWg6iAE4hxBp42YKfxOwEnxPADbBuiELKZ2ddxo2aDFAb9U/lp47k45u5i2T1AlEBeurUbdKh7Nypq4lLMXC2FHhezK33BuYR+3L7jxVj7FATylhwIDAQAB”
/etc/postfix/main.cf (appendix 01)
smtpd_milters = inet:127.0.0.1:2525 non_smtpd_milters = $smtpd_milters milter_default_action = accept smtpd_error_sleep_time = 1s smtpd_soft_error_limit = 10 smtpd_hard_error_limit = 20 smtpd_recipient_restrictions = permit_mynetworks check_sender_access hash:/etc/postfix/sender_access reject_unauth_destination queue_directory = /var/spool/postfix command_directory = /usr/sbin daemon_directory = /usr/libexec/postfix data_directory = /var/lib/postfix mail_owner = postfix inet_interfaces = all inet_protocols = all mydestination = $myhostname, localhost.$mydomain, localhost unknown_local_recipient_reject_code = 550 alias_maps = hash:/etc/aliases alias_database = hash:/etc/aliases debug_peer_level = 2 debugger_command = PATH=/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin ddd $daemon_directory/$process_name $process_id & sleep 5 sendmail_path = /usr/sbin/sendmail.postfix newaliases_path = /usr/bin/newaliases.postfix mailq_path = /usr/bin/mailq.postfix setgid_group = postdrop html_directory = no manpage_directory = /usr/share/man sample_directory = /usr/share/doc/postfix-2.6.6/samples readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES
/etc/opendkim.conf (appendix 02)
Canonicalization relaxed/relaxed ExternalIgnoreList refile:/etc/opendkim/TrustedHosts InternalHosts refile:/etc/opendkim/TrustedHosts KeyTable refile:/etc/opendkim/KeyTable LogWhy Yes MinimumKeyBits 1024 Mode sv PidFile /var/run/opendkim/opendkim.pid SigningTable refile:/etc/opendkim/SigningTable Socket inet:email@example.com Syslog Yes SyslogSuccess Yes TemporaryDirectory /var/tmp UMask 022 UserID opendkim:opendkim
/etc/opendkim/KeyTable (appendix 03)
/etc/opendkim/SigningTable (appendix 04)