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emailsslpostfix-mtaclient-certificates
Ivan Voras
Ivan Voras

Reputation: 2014

Postfix SMTP relay: client does not offer TLS client certificate to the server?

I have two machines, one running Ubuntu and one runing Debian, both running Postfix. The intent is that machine#2 becomes a SMTP relay/smarthost for machine#1. I have created a CA and issued certificates for both of the machines: a server certificate for #2 and a client certificate for #1.

When sending e-mail from #1 (by having the MUA talk to Postfix on localhost:25 with the intent that it relays e-mail to #2), the basic things work fine: the machines can talk to each other and an attempt to relay is actually made. The idea is to allow relaying on #2 if a valid client-side SSL/TLS certificate is presented from #1.

The relevant configuration for #2 is:

smtpd_tls_received_header = yes
smtpd_tls_loglevel = 2
smtpd_use_tls = yes
smtpd_tls_cert_file = /etc/ssl/private/cert2.pem
smtpd_tls_key_file = /etc/ssl/private/key2-d.pem
smtpd_tls_CAfile = /etc/ssl/certs/cacert.pem
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_tls_mandatory_protocols = SSLv3, TLSv1
smtpd_tls_mandatory_ciphers = medium
smtpd_tls_auth_only = yes
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination permit_tls_all_clientcerts

The configuration on #1 is:

smtp_tls_CAfile = /etc/ssl/certs/cacert.pem
smtp_tls_cert_file = /etc/ssl/private/cert1.pem
smtp_tls_key_file = /etc/ssl/private/key1-d.pem
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_tls_security_level = verify
smtp_tls_loglevel = 2

Machine#1 connects to #2, enables STARTTLS, the log files show that it successfuly verifies the certificate from #2, and attempts to relay the message. However, it appears not to send the client certificate to #2, and #2 refuses to relay the message.

Log entries from #1:

Apr 17 01:18:14 mail1 postfix/smtp[30250]: Verified TLS connection established to mail2[x.x.x.x]:25: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
Apr 17 01:18:14 mail1 postfix/smtp[30244]: 8A2328BDB4: to=<[email protected]>, relay=mail2[x.x.x.x]:25, delay=3488, delays=3486/0.41/0.85/0.19, dsn=4.7.1, status=deferred (host mail2[x.x.x.x] said: 454 4.7.1 <[email protected]>: Relay access denied (in reply to RCPT TO command))

Log entries from #2:

Apr 17 01:18:13 mail2 postfix/smtpd[28798]: Anonymous TLS connection established from unknown[y.y.y.y]: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
Apr 17 01:18:13 mail2 postfix/smtpd[28798]: NOQUEUE: reject: RCPT from unknown[y.y.y.y]: 454 4.7.1 <[email protected]>: Relay access denied; from=<addr@mail1> to=<[email protected]> proto=ESMTP helo=<mail1>

Any ideas? I'm basing my assumption that #1 didn't send its client cert on the "Anonymous TLS connection established" part in the logs from mail2.

Upvotes: 2

Views: 11610

Answers (2)

Moho
Moho

Reputation: 1

add your server adress ( server1.domaine.com ) in the postfix conf file main.cf mynetworks = 127.0.0.1/8

Upvotes: 0

Steffen Ullrich
Steffen Ullrich

Reputation: 123441

A TLS server must request a certificate from the client, the client will not send it by its own. Try to add

smtpd_tls_ask_ccert=yes

on the server side

Upvotes: 3

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