add 2048 bits of DH params for nginx, postfix, dovecot

nginx/postfix use a new pre-generated dh2048.pem file. dovecot generates the bits on its own.

ssllabs.com reports that TLS_DHE ciphers went from 1024 to 2048 bits as expected. The ECDHE ciphers remain at 256 bits --- no idea what that really means. (This tests nginx only. I haven't tested postfix/dovecot.)

see https://discourse.mailinabox.email/t/fips-ready-for-ssl-dhec-key-exchange/76/3

conf/nginx-ssl.conf

@@ -55,7 +55,7 @@ spdy_headers_comp 0;
 #   openssl dhparam -outform pem -out dhparam2048.pem 2048
 # 
 # Note: raising the bits to 2048 excludes Java 6 clients. Comment out if a problem.
-#ssl_dhparam /path/to/dhparam2048.pem;
+ssl_dhparam STORAGE_ROOT/ssl/dh2048.pem;
 
 
 # OCSP stapling - means nginx will poll the CA for signed OCSP responses, 

setup/mail-dovecot.sh

@@ -43,10 +43,14 @@ tools/editconf.py /etc/dovecot/conf.d/10-auth.conf \
 
 # Enable SSL, specify the location of the SSL certificate and private key files,
 # and allow only good ciphers per http://baldric.net/2013/12/07/tls-ciphers-in-postfix-and-dovecot/.
+# Set a higher default Diffie Hellman key size. The default is 1024. Higher than
+# 2048 takes considerably longer to regenerate, which dovecot does automatically
+# apparently weekly. See http://wiki2.dovecot.org/SSL/DovecotConfiguration for more.
 tools/editconf.py /etc/dovecot/conf.d/10-ssl.conf \
 	ssl=required \
 	"ssl_cert=<$STORAGE_ROOT/ssl/ssl_certificate.pem" \
 	"ssl_key=<$STORAGE_ROOT/ssl/ssl_private_key.pem" \
+	"ssl_dh_parameters_length=2048" \
 	"ssl_cipher_list=TLSv1+HIGH !SSLv2 !RC4 !aNULL !eNULL !3DES @STRENGTH"
 
 # Disable in-the-clear IMAP and POP because we're paranoid (we haven't even

setup/mail-postfix.sh

@@ -69,11 +69,13 @@ cp conf/postfix_outgoing_mail_header_filters /etc/postfix/outgoing_mail_header_f
 # Enable TLS on these and all other connections (i.e. ports 25 *and* 587) and
 # require TLS before a user is allowed to authenticate. This also makes
 # opportunistic TLS available on *incoming* mail.
+# Set stronger DH parameters, which via openssl tend to default to 1024 bits.
 tools/editconf.py /etc/postfix/main.cf \
 	smtpd_tls_security_level=may\
 	smtpd_tls_auth_only=yes \
 	smtpd_tls_cert_file=$STORAGE_ROOT/ssl/ssl_certificate.pem \
 	smtpd_tls_key_file=$STORAGE_ROOT/ssl/ssl_private_key.pem \
+	smtpd_tls_dh1024_param_file=$STORAGE_ROOT/ssl/dh2048.pem \
 	smtpd_tls_received_header=yes
 
 # Prevent non-authenticated users from sending mail that requires being

setup/ssl.sh

@@ -43,3 +43,8 @@ if [ ! -f $STORAGE_ROOT/ssl/ssl_certificate.pem ]; then
 	  -in $STORAGE_ROOT/ssl/ssl_cert_sign_req.csr -signkey $STORAGE_ROOT/ssl/ssl_private_key.pem -out $STORAGE_ROOT/ssl/ssl_certificate.pem
 fi
 
+# For nginx and postfix, pre-generate some better DH bits. They seem to
+# each rely on openssl's default of 1024 bits.
+if [ ! -f $STORAGE_ROOT/ssl/dh2048.pem ]; then
+	openssl dhparam -out $STORAGE_ROOT/ssl/dh2048.pem 2048
+fi

setup/web.sh

@@ -9,9 +9,11 @@ apt_install nginx php5-fpm
 
 rm -f /etc/nginx/sites-enabled/default
 
-# copy in a nginx configuration file for common and best-practices
-# SSL settings from @konklone
-cp conf/nginx-ssl.conf /etc/nginx/nginx-ssl.conf
+# Copy in a nginx configuration file for common and best-practices
+# SSL settings from @konklone. Replace STORAGE_ROOT so it can find
+# the DH params.
+sed "s#STORAGE_ROOT#$STORAGE_ROOT#" \
+	conf/nginx-ssl.conf > /etc/nginx/nginx-ssl.conf
 
 # Fix some nginx defaults.
 # The server_names_hash_bucket_size seems to prevent long domain names?