Please Help: Dovecot ssl_ca selection based on remote IP address filtering not working.

[Sean Gallagher]

To follow up on my previous email,

It seems the lmtp deamon does NOT support ssl_ca selection based on 
remote IP. It also does NOT seem to support authorization.
The solution I arrived at was to create a separate CA, used solely for 
the purpose of authorizing LDAP clients.

openssl genpkey-algorithm  rsa-pkeyopt  rsa_keygen_bits:4096  -out  /etc/ssl/private/ca_dovecot_lmtps.key
openssl req-x509  -new  -subj  '/CN=Dovecot LMTP Authorization CA'  -key  /etc/ssl/private/ca_dovecot_lmtps.key-nodes  -sha256  -days  3650  -addext  'keyUsage=keyCertSign,cRLSign'  -out  /etc/ssl/private/ca_dovecot_lmtps.pem
  

This CA can then be used to sign certificates for LMTP clients to use 
when delivering mail.

export lmtpsrc=msa
openssl genpkey-algorithm  rsa-pkeyopt  rsa_keygen_bits:2048  -out  /etc/ssl/private/_lmtp.${lmtpsrc}.example.com.key
openssl req-new  -nodes  -key  /etc/ssl/private/_lmtp.${lmtpsrc}.example.com.key-sha256  -out  /etc/ssl/private/_lmtp.${lmtpsrc}.example.com.csr-subj  "/CN=${lmtpsrc}.example.com"
cat  <<EOF >/etc/ssl/private/_lmtp.${lmtpsrc}.example.com.ext
basicConstraints = critical, CA:FALSE
keyUsage = digitalSignature, keyEncipherment
extendedKeyUsage = serverAuth, clientAuth
subjectAltName = DNS:${lmtpsrc}.example.com
subjectKeyIdentifier =hash
authorityKeyIdentifier = keyid:always, issuer:always
EOF
openssl x509-req  -CA  /etc/ssl/private/ca_dovecot_lmtp.pem-CAkey  /etc/ssl/private/ca_dovecot_lmtp.key-sha256  -days  3650  -in  /etc/ssl/private/_lmtp.${lmtpsrc}.example.com.csr-extfile  /etc/ssl/private/_lmtp.${lmtpsrc}.example.com.ext-out  /etc/ssl/private/_lmtp.${lmtpsrc}.example.com.pem

The Dovecot configuration looks like this

protocol lmtp {
   mail_uid = nobody
   auth_ssl_require_client_cert = yes
   ssl_verify_client_cert = yes
   ssl_ca = </etc/ssl/private/ca_dovecot_lmtps.pem

   ssl = required
   ssl_cert = </etc/ssl/mda.example.com-chain.pem
   ssl_key = </etc/ssl/private/mda.example.com.key
   ssl_require_crl = no
}

This solution seems secure and manageable so long as the number of LMTP 
sources stays small, but overall, feels very unsatisfactory.

I would regard Dovecot's inability to inspect the LMTP certificate's 
subject name as a BUG that should be prioritized.

As it stands, I would not be surprised to find real-world deployments of 
Dovecot that are insecure, as _ANY_ host with a valid certificate could 
originate mail.
As a quick illustration, I created another certificate with an invalid 
name from my "/CN=Dovecot LMTP Authorization CA" authority. The mail was 
accepted without complaint. In "normal" setups, a certificate from any 
public CA would be accepted.

Feb 11 14:29:52 imaps mail.info dovecot: lmtp(16386): Received valid SSL certificate: /CN=Dovecot LMTP Authorization CA
Feb 11 14:29:52 imaps mail.info dovecot: lmtp(16386): Received valid SSL certificate: /CN=evil.example.com
Feb 11 14:29:52 imaps mail.debug dovecot: lmtp(16386): Debug: SSL: where=0x2001, ret=1: SSLv3/TLS read client certificate
Feb 11 14:29:52 imaps mail.debug dovecot: lmtp(16386): Debug: SSL: where=0x2001, ret=1: SSLv3/TLS read certificate verify
Feb 11 14:29:52 imaps mail.debug dovecot: lmtp(16386): Debug: SSL: where=0x2001, ret=1: SSLv3/TLS read finished
Feb 11 14:29:52 imaps mail.debug dovecot: lmtp(16386): Debug: SSL: where=0x20, ret=1: SSLv3/TLS write session ticket
Feb 11 14:29:52 imaps mail.debug dovecot: lmtp(16386): Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write session ticket
Feb 11 14:29:52 imaps mail.debug dovecot: lmtp(16386): Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write session ticket
Feb 11 14:29:52 imaps mail.debug dovecot: lmtp(16386): Debug: SSL: where=0x2002, ret=1: SSL negotiation finished successfully



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