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Lotek 25th June 2009 06:02

Postfix TLS and Security
So I'm using gmail as my email relay with postfix and unfortunately it seems to have opened a large security hole for my server to be used for spam. To alleviate this I decided to use stmp_tls_security_level at the fingerprint level. I added in gmails sha1 key and I have no error in the logs, but I can't send mail. I seem to be able to receive it, but not send. Here's the output of my (sorry for the length of it)


# See /usr/share/postfix/ for a commented, more complete version

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtpd_tls_cert_file = /etc/postfix/ssl/smtpd.crt
smtpd_tls_key_file = /etc/postfix/ssl/smtpd.key
smtpd_use_tls = yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

myhostname =
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
relayhost = []:587
mynetworks =,
mailbox_command = procmail -a "$EXTENSION"
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
smtpd_sasl_local_domain =
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_sasl_authenticated_header = yes
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination
smtpd_tls_auth_only = no
smtp_use_tls = yes
smtp_tls_note_starttls_offer = yes
smtpd_tls_CAfile = /etc/postfix/ssl/cacert.pem
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_mechanism_filter = digest-md5

smtp_sasl_security_options =

# More security fixes: Disable if they interfere
smtp_tls_security_level = fingerprint
smtp_tls_fingerprint_digest = sha1
smtp_tls_fingerprint_cert_match = AB:BE:5E:B4:93:88:4E:E4:60:C6:EF:F8:EA:D4:B1:55:4B:C9:59:3C

virtual_maps = hash:/etc/postfix/virtusertable

mydestination = /etc/postfix/local-host-names

# Disable DNS Lookups
disable_dns_lookups = yes

Is it wrong somewhere? Am I doing something that I shouldn't be? I am no postfix guru by any stretch of the imagination, so any harsh, otherwise, criticisms are welcome. Thanks everyone!

till 25th June 2009 11:09


So I'm using gmail as my email relay with postfix and unfortunately it seems to have opened a large security hole for my server to be used for spam.
Why do you think that? Have you tested your server with an external mail relay check tool? Postfix setups are secure by default after you installed ISPConfig, so adding smtp_fingerprinting is not needed.

Lotek 25th June 2009 19:30

Thanks for the response Till.

To answer your question, yes actually I did check it with some standard security software I use to test common vulnerabilities. When I run my scan it turns up that my smtp is configured as a relay, which I assumed was because I'm using gmail to relay my mail. Once I enabled the fingerprint setting, however, the issue disappeared. I'm gathering that it's because it was not able to escalate privileges because there was that security enabled.

So do you have any suggestions?

till 25th June 2009 19:36

How did you test? I guess you tested from the local server which explains the wrong result. A mailserver always allows relaying from localhost, otherwise local shell and web scripts would not work. This does not mean that any third party can relay emails trough your server. Better use this for testing:

Lotek 25th June 2009 20:00

I actually tested from an outside machine to the outside ip address and received the results. When I tested from inside my network it did not return that it was running as a relay.

However, after running your test it appears that it is not acting as a relay. I'll have to find another way to test it to totally confirm that either test is correct now that I have two different results, but I'm glad you helped to shed a bit of light onto the subject.

Thanks again Till!

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