Postfix "Trusted TLS connection established" but "Server certificate not verified"
I´m using a Postfix TLS Policy to enforce TLS for outgoing email. Unfortunately in some cases the certificate verification fails and I don´t know why.
For instance, this is an excerpt of my TLS Policy
#/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA
facebook.com secure ciphers=high
hearst.com secure match=gslb.pphosted.com ciphers=high
fastmail.fm secure ciphers=high
All this 3 providers use the same Root CA. I can send emails to facebook.com without any problems. In the case of hearst.com I have to specific a CN match because the certificate doesn't have the proper SAN field. What I don´t understand is why I have to also add a match CN for fastmail.fm. Otherwise the certifcate validation fails. The certificate is trusted, the target server name is smtp.messagingengine.com and the certificate has a SAN field that matches it (*.messagingengine.com)
Feb 25 21:57:22 mail postfix/smtp[25291]: Trusted TLS connection established to in1-smtp.messagingengine.com[66.111.4.74]:25: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
Feb 25 21:57:22 mail postfix/smtp[25291]: D33A02504112: to=<[email protected]>, relay=in1-smtp.messagingengine.com[66.111.4.74]:25, delay=8.4, delays=0.02/0/8.4/0, dsn=4.7.5, status=deferred (Server certificate not verified)
Does anyone know why the certificate is not accepted? Any sugestion to enforce a "secure" TLS Policy without having to specifiy the match rule?
Version details
root@mail:/etc/postfix# uname -a
Linux mail.EXAMPLE.com 3.13.0-65-generic #106-Ubuntu SMP Fri Oct 2 22:08:27 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
root@mail:/etc/postfix# postconf -d | grep mail_version
mail_version = 2.11.0
milter_macro_v = $mail_name $mail_version
Extended log
Feb 25 21:57:22 mail postfix/smtp[25291]: setting up TLS connection to in1-smtp.messagingengine.com[66.111.4.74]:25
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: TLS cipher list "aNULL:-aNULL:ALL:!EXPORT:!LOW:!MEDIUM:+RC4:@STRENGTH:!MD5:!DES:!ADH:!RC4:!PSD:!SRP:!3DES:!eNULL:!aNULL"
Feb 25 21:57:22 mail postfix/smtp[25291]: looking for session smtp&fastmail.fm&in1-smtp.messagingengine.com&66.111.4.74&&FC83E1ADCEDFC581CE5F87CAF6E49FEFFF83CD0F9EBC0B57C4A19ED3DC3416EC in smtp cache
Feb 25 21:57:22 mail postfix/tlsmgr[25292]: lookup smtp session id=smtp&fastmail.fm&in1-smtp.messagingengine.com&66.111.4.74&&FC83E1ADCEDFC581CE5F87CAF6E49FEFFF83CD0F9EBC0B57C4A19ED3DC3416EC
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:before/connect initialization
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:unknown state
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 read server hello A
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: depth=2 verify=1 subject=/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: depth=1 verify=1 subject=/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: depth=0 verify=1 subject=/C=AU/ST=Victoria/L=Melbourne/O=FastMail Pty Ltd/CN=*.messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 read server certificate A
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 read server key exchange A
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 read server done A
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 write client key exchange A
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 write change cipher spec A
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 write finished A
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 flush data
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 read server session ticket A
Feb 25 21:57:22 mail postfix/smtp[25291]: SSL_connect:SSLv3 read finished A
Feb 25 21:57:22 mail postfix/smtp[25291]: save session smtp&fastmail.fm&in1-smtp.messagingengine.com&66.111.4.74&&FC83E1ADCEDFC581CE5F87CAF6E49FEFFF83CD0F9EBC0B57C4A19ED3DC3416EC to smtp cache
Feb 25 21:57:22 mail postfix/tlsmgr[25292]: put smtp session id=smtp&fastmail.fm&in1-smtp.messagingengine.com&66.111.4.74&&FC83E1ADCEDFC581CE5F87CAF6E49FEFFF83CD0F9EBC0B57C4A19ED3DC3416EC [data 1788 bytes]
Feb 25 21:57:22 mail postfix/tlsmgr[25292]: write smtp TLS cache entry smtp&fastmail.fm&in1-smtp.messagingengine.com&66.111.4.74&&FC83E1ADCEDFC581CE5F87CAF6E49FEFFF83CD0F9EBC0B57C4A19ED3DC3416EC: time=1456433842 [data 1788 bytes]
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: subjectAltName: *.messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: subjectAltName: messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: subjectAltName: mail.messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: subjectAltName: dav.messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: subjectAltName: caldav.messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: subjectAltName: carddav.messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25 CommonName *.messagingengine.com
Feb 25 21:57:22 mail postfix/smtp[25291]: in1-smtp.messagingengine.com[66.111.4.74]:25: subject_CN=*.messagingengine.com, issuer_CN=DigiCert SHA2 High Assurance Server CA, fingerprint=D8:F5:7E:43:A8:DA:29:22:6B:7E:90:A6:31:86:C8:CD, pkey_fingerprint=49:07:46:E5:F1:35:C2:96:75:09:67:BE:D9:FE:DB:46
Feb 25 21:57:22 mail postfix/smtp[25291]: Trusted TLS connection established to in1-smtp.messagingengine.com[66.111.4.74]:25: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
Feb 25 21:57:22 mail postfix/smtp[25291]: D33A02504112: to=<[email protected]>, relay=in1-smtp.messagingengine.com[66.111.4.74]:25, delay=8.4, delays=0.02/0/8.4/0, dsn=4.7.5, status=deferred (Server certificate not verified)
main.cf
# See /usr/share/postfix/main.cf.dist for a commented, more complete version
smtpd_banner = mail.EXAMPLE.com ESMTP $mail_name (nou)
biff = no
# appending .domain is the MUA's job.
append_dot_mydomain = no
# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h
readme_directory = no
# TLS parameters
smtpd_tls_cert_file = /etc/ssl/certs/EXAMPLE.com.crt
smtpd_tls_key_file = /etc/ssl/private/EXAMPLE.com.key
smtpd_use_tls = yes
smtpd_tls_auth_only = yes
smtpd_tls_security_level = may
smtpd_tls_mandatory_ciphers = high
smtp_tls_mandatory_exclude_ciphers = MD5, DES, ADH, RC4, PSD, SRP, 3DES, eNULL
smtpd_tls_mandatory_exclude_ciphers = MD5, DES, ADH, RC4, PSD, SRP, 3DES, eNULL
smtpd_tls_exclude_ciphers = MD5, DES, ADH, RC4, PSD, SRP, 3DES, eNULL
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtp_tls_protocols = !SSLv2, !SSLv3
smtpd_tls_protocols = !SSLv2, !SSLv3
smtpd_starttls_timeout = 300s
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_tls_dh1024_param_file = /etc/postfix/dh_1024.pem
smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem
smtpd_tls_eecdh_grade = strong
tls_preempt_cipherlist = yes
#smtp_tls_note_starttls_offer = yes
#smtp_tls_per_site = may
# Logging
smtp_tls_loglevel = 2
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = mail.EXAMPLE.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = mail.EXAMPLE.com, localhost.contabo.host, localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
# Handing off local delivery to Dovecot's LMTP
virtual_transport = lmtp:unix:private/dovecot-lmtp
#Enabling SMTP for authenticated users, and handing off authentication to Dovecot
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
smtp_tls_security_level = may
# Force TLS for outgoing server connection
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
smtp_tls_CApath = /etc/postfix/rootcas/
#Virtual domains, users, and aliases
virtual_mailbox_domains = /etc/postfix/virtual_mailbox_domains
virtual_mailbox_base = /var/mail/vhosts
virtual_mailbox_maps = hash:/etc/postfix/vmaps
virtual_uid_maps = static:1001
virtual_gid_maps = static:1001
virtual_alias_maps = hash:/etc/postfix/valias
# DKIM
milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891
content_filter = smtp-amavis:[127.0.0.1]:10024
message_size_limit = 0
master.cf
smtp inet n - - - - smtpd
submission inet n - - - - smtpd
-o smtpd_tls_security_level=may
pickup unix n - - 60 1 pickup
-o content_filter=
-o receive_override_options=no_header_body_checks
cleanup unix n - - - 0 cleanup
qmgr unix n - n 300 1 qmgr
#qmgr unix n - n 300 1 oqmgr
tlsmgr unix - - - 1000? 1 tlsmgr
rewrite unix - - - - - trivial-rewrite
bounce unix - - - - 0 bounce
defer unix - - - - 0 bounce
trace unix - - - - 0 bounce
verify unix - - - - 1 verify
flush unix n - - 1000? 0 flush
proxymap unix - - n - - proxymap
proxywrite unix - - n - 1 proxymap
smtp unix - - - - - smtp
relay unix - - - - - smtp
# -o smtp_helo_timeout=5 -o smtp_connect_timeout=5
showq unix n - - - - showq
error unix - - - - - error
retry unix - - - - - error
discard unix - - - - - discard
local unix - n n - - local
virtual unix - n n - - virtual
lmtp unix - - - - - lmtp
anvil unix - - - - 1 anvil
scache unix - - - - 1 scache
maildrop unix - n n - - pipe
uucp unix - n n - - pipe
flags=Fqhu user=uucp argv=uux -r -n -z -a$sender - $nexthop!rmail ($recipient)
ifmail unix - n n - - pipe
flags=F user=ftn argv=/usr/lib/ifmail/ifmail -r $nexthop ($recipient)
bsmtp unix - n n - - pipe
flags=Fq. user=bsmtp argv=/usr/lib/bsmtp/bsmtp -t$nexthop -f$sender $recipient
scalemail-backend unix - n n - 2 pipe
flags=R user=scalemail argv=/usr/lib/scalemail/bin/scalemail-store ${nexthop} ${user} ${extension}
mailman unix - n n - - pipe
flags=FR user=list argv=/usr/lib/mailman/bin/postfix-to-mailman.py
${nexthop} ${user}
smtp-amavis unix - - - - 2 smtp
-o smtp_data_done_timeout=1200
-o smtp_send_xforward_command=yes
-o disable_dns_lookups=yes
-o max_use=20
127.0.0.1:10025 inet n - - - - smtpd
-o content_filter=
-o local_recipient_maps=
-o relay_recipient_maps=
-o smtpd_restriction_classes=
-o smtpd_delay_reject=no
-o smtpd_client_restrictions=permit_mynetworks,reject
-o smtpd_helo_restrictions=
-o smtpd_sender_restrictions=
-o smtpd_recipient_restrictions=permit_mynetworks,reject
-o smtpd_data_restrictions=reject_unauth_pipelining
-o smtpd_end_of_data_restrictions=
-o mynetworks=127.0.0.0/8
-o smtpd_error_sleep_time=0
-o smtpd_soft_error_limit=1001
-o smtpd_hard_error_limit=1000
-o smtpd_client_connection_count_limit=0
-o smtpd_client_connection_rate_limit=0
-o receive_override_options=no_header_body_checks,no_unknown_recipient_checks
with secure level you are asking postfix to verify the relation of recipient and server, but in a secure way (without relying on DNS data)
It correctly initiates trusted TLS connection (the certificate is signed by a CA you know/trust)
Then it tries to securely verify the server/recipient, if any CN/SAN match fastmail.fm - and they don't. So the message is deferred in the local queue.
certificates of messagingengine.com/gslb.pphosted.com do not vouch for other domains they accept. facebook.com validates itself.
you modified secure with an added match of the MX - that's what verify actually does. so you can drop down to verify, or just keep adding the matches.
smtp_tls_security_level
-
mayTLS? good. no TLS? good. -
encryptaccept any invalid server certificate, demands encryption. -
verifyaccept trusted server certificate (do I trust the CA? does the CN match the MX?), demands encryption. -
secureonly accept trusted certificate if CN/SAN matches the recipient domain - and ignore insecure (MX) information for validation.
the postfix docs are a little unclear in explaining the differences.
in the /etc/postfix/main.cf file you could place this directive:
smtp_tls_secure_cert_match = hostname, nexthop, dot-nexthop
putting this in the main.cf will allow for a hostname match from the DNS MX records to be allowed for the secure policy level. All this in essence does is convert the secure policy level to the verify policy level, so you might as well just change your tls policy level to verify or lower. To answer your question on what is causing it to fail your original recipient is
when you set your tls policy to secure the controlling variable is
smtp_tls_secure_cert_match (default: nexthop, dot-nexthop)
and nexthop unless it has an override in a transport table is determined as so discussed in the man 5 postconf page posted below.
the next-hop domain, which is either the recipient domain, or the transport next-hop configured for the domain
your next-hop in secure policy with that email address is fastmail.fm while the only domain that is in the MX record is that of messagingengine.com, it will never match under the secure policy. However looking at the MX record of fastmail.fm.
[[email protected] openssl]# host -t MX fastmail.fm
fastmail.fm mail is handled by 10 in1-smtp.messagingengine.com.
fastmail.fm mail is handled by 20 in2-smtp.messagingengine.com.
if you were to relax the secure cert match directive to include hostname (again the exact same thing as smtp_tls_security_level = verify) the certificate would verify based on the MX record which points to in1-smtp.messagingengine.com which matches against the CommonName *.messagingengine.com. This also happens with [email protected], and im sure many others, so just like they say in the man 5 page for postconf
smtp_tls_security_level (default: empty)
verify Mandatory TLS verification. At this security level, DNS MX lookups are trusted to be secure enough, and the name verified in the server certificate is usually
obtained indirectly via unauthenticated DNS MX lookups. The smtp_tls_verify_cert_match parameter controls how the server name is verified. In practice
explicit control over matching is more common at the "secure" level, described below. This security level is not an appropriate default for systems delivering
mail to the Internet.
secure Secure-channel TLS. At this security level, DNS MX lookups, though potentially used to determine the candidate next-hop gateway IP addresses, are not trusted
to be secure enough for TLS peername verification. Instead, the default name verified in the server certificate is obtained from the next-hop domain as speci‐
fied in the smtp_tls_secure_cert_match configuration parameter. The default matching rule is that a server certificate matches when its name is equal to or is
a sub-domain of the nexthop domain. This security level is not an appropriate default for systems delivering mail to the Internet.
This security level is not an appropriate default for systems delivering mail to the Internet.
then again it says that as well for tls policy verify as well, but this is why it will always fail.
for reference the postconf man5 page for the two directives referenced above:
smtp_tls_secure_cert_match (default: nexthop, dot-nexthop)
How the Postfix SMTP client verifies the server certificate peername for the "secure" TLS security level. In a "secure" TLS policy table ($smtp_tls_policy_maps)
entry the optional "match" attribute overrides this main.cf setting.
This parameter specifies one or more patterns or strategies separated by commas, whitespace or colons. In the policy table the only valid separator is the colon
character.
For a description of the pattern and strategy syntax see the smtp_tls_verify_cert_match parameter. The "hostname" strategy should be avoided in this context, as in
the absence of a secure global DNS, using the results of MX lookups in certificate verification is not immune to active (man-in-the-middle) attacks on DNS.
Sample main.cf setting:
smtp_tls_secure_cert_match = nexthop
Sample policy table override:
example.net secure match=example.com:.example.com
.example.net secure match=example.com:.example.com
This feature is available in Postfix 2.3 and later.
it also references smtp_tls_verify_cert_match for better understanding
smtp_tls_verify_cert_match (default: hostname)
How the Postfix SMTP client verifies the server certificate peername for the "verify" TLS security level. In a "verify" TLS policy table ($smtp_tls_policy_maps)
entry the optional "match" attribute overrides this main.cf setting.
This parameter specifies one or more patterns or strategies separated by commas, whitespace or colons. In the policy table the only valid separator is the colon
character.
Patterns specify domain names, or domain name suffixes:
example.com
Match the example.com domain, i.e. one of the names the server certificate must be example.com, upper and lower case distinctions are ignored.
.example.com
Match subdomains of the example.com domain, i.e. match a name in the server certificate that consists of a non-zero number of labels followed by a .exam‐
ple.com suffix. Case distinctions are ignored.
Strategies specify a transformation from the next-hop domain to the expected name in the server certificate:
nexthop
Match against the next-hop domain, which is either the recipient domain, or the transport next-hop configured for the domain stripped of any optional socket
type prefix, enclosing square brackets and trailing port. When MX lookups are not suppressed, this is the original nexthop domain prior to the MX lookup, not
the result of the MX lookup. For LMTP delivery via UNIX-domain sockets, the verified next-hop name is $myhostname. This strategy is suitable for use with the
"secure" policy. Case is ignored.
dot-nexthop
As above, but match server certificate names that are subdomains of the next-hop domain. Case is ignored.
hostname
Match against the hostname of the server, often obtained via an unauthenticated DNS MX lookup. For LMTP delivery via UNIX-domain sockets, the verified name is
$myhostname. This matches the verification strategy of the "MUST" keyword in the obsolete smtp_tls_per_site table, and is suitable for use with the "verify"
security level. When the next-hop name is enclosed in square brackets to suppress MX lookups, the "hostname" strategy is the same as the "nexthop" strategy.
Case is ignored.
Sample main.cf setting:
smtp_tls_verify_cert_match = hostname, nexthop, dot-nexthop
Sample policy table override:
example.com verify match=hostname:nexthop
.example.com verify match=example.com:.example.com:hostname