I'm running a Debian Jessie mailserver using Postfix 2.11.3-1 with OpenSSL 1.0.2k-1. Recent changes to Debian policy have disabled several older insecure ciphers. Unfortunately some of the mailservers from which we receive mail are still running on an old version of Exchange (I think Exchange 2007 on Windows Server 2003), and they're now unable to connect due to TLS handshake failures. Here are the (very limited) ciphers supported by the sending server:
tls1: RC4-SHA tls1: RC4-MD5 tls1: DES-CBC3-SHA
I have no control over its configuration. According to http://www.postfix.org/TLS_README.html#server_cipher, "... Windows 2003 Microsoft Exchange servers have flawed implementations of DES-CBC3-SHA, which OpenSSL considers stronger than RC4-SHA. Enabling server cipher-suite selection may create interoperability issues with Windows 2003 Microsoft Exchange clients."
Here are the relevant lines of our Postfix config as produced by
postconf | grep smtpd_tls:
smtpd_tls_CAfile = smtpd_tls_CApath = smtpd_tls_always_issue_session_ids = yes smtpd_tls_ask_ccert = no smtpd_tls_auth_only = yes smtpd_tls_ccert_verifydepth = 9 smtpd_tls_cert_file = /path/to/cert smtpd_tls_ciphers = export smtpd_tls_dcert_file = smtpd_tls_dh1024_param_file = smtpd_tls_dh512_param_file = smtpd_tls_dkey_file = $smtpd_tls_dcert_file smtpd_tls_eccert_file = smtpd_tls_eckey_file = $smtpd_tls_eccert_file smtpd_tls_eecdh_grade = strong smtpd_tls_exclude_ciphers = smtpd_tls_fingerprint_digest = md5 smtpd_tls_key_file = /path/to/key smtpd_tls_loglevel = 0 smtpd_tls_mandatory_ciphers = medium smtpd_tls_mandatory_exclude_ciphers = smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3 smtpd_tls_protocols = !SSLv2, !SSLv3 smtpd_tls_received_header = yes smtpd_tls_req_ccert = no smtpd_tls_security_level = may smtpd_tls_session_cache_database = sdbm:/var/lib/postfix/smtpd_scache smtpd_tls_session_cache_timeout = 3600s smtpd_tls_wrappermode = no
At the moment it's looking like our only option might be to completely disable TLS until the sending servers get upgraded. Obviously I'm not keen to do that, but dropping the email is not an option. Is it possible to reconfigure Postfix to temporarily enable RC4-SHA*? If we can set it to the lowest priority, so much the better. It's a production mail server so the scope for testing is limited, I need a solution that I can be confident will work straight off so we don't lose mail.
Thanks in advance.
*On the grounds that it's better than RC4-MD5, and DES-CBC3-SHA won't work.