Posts Tagged ‘sendmail’

Implemented stronger Diffie-Hellman encryption thanks to “Logjam”

Well, I haven’t written anything in a while so I figured I’d put this into WP. I recently upgraded a couple of my iOS devices to 8.4 and sure enough, I couldn’t send emails. Come to find out, my services at home were using weaker DH encryption and I needed to fix them if I wanted to send email ever again from my iPhone.

First I worked on sendmail. I needed to first create a DH 2048 bit file using openssl:

openssl dhparam -out dh_2048.pem -2 2048

This produced a file in my /etc/pki/tls/certs folder which I can now configure to use via adding this line:

define(`confDH_PARAMETERS', `/etc/pki/tls/certs/dh_2048.pem')

Next you can do a ‘make -C /etc/mail’ or simply restart sendmail as it will detect the changes and do it for you (did for me at least.) Email was now working as expected and I’m no longer seeing this in my /var/log/maillog folder:

Aug 1 11:01:10 Sauron sendmail[11796]: t71F0eh9011796: [] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA
Aug 1 11:01:11 Sauron sendmail[11803]: STARTTLS=server, error: accept failed=0, SSL_error=5, errno=0, retry=-1
Aug 1 11:01:11 Sauron sendmail[11803]: t71F1Ae8011803: [] did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA
Aug 1 11:01:11 Sauron sendmail[11804]: STARTTLS=server, error: accept failed=0, SSL_error=5, errno=0, retry=-1
Aug 1 11:01:11 Sauron sendmail[11804]: t71F1BBU011804: [] did not issue MAIL/EXPN/VRFY/ETRN during connection to TLSMTA

Now to take a look at Apache. I’m using an older version, I think 2.2.3-91 w/ CentOS so there’s only so much I can do regarding MITM attacks apparently. But I can explicitly tell Apache to NOT use weaker encryption protocols even though I can’t use the SSLOpenSSLConfCmd DHParameters “{path to dhparams.pem}” option and specify my DH key.

Here’s what I did put in my ssl.conf file:

SSLProtocol all -SSLv2 -SSLv3
SSLHonorCipherOrder on


Which will at least help for now.


Here’s a couple of useful links:



Sendmail’s tweakable settings in the file

Recently had a question at work about the maximum number of recipients Sendmail would allow thru our in-house server. I wasn’t sure but knew I hadn’t personally configured a hard limit. Did some quick searching and stumbled upon confMAX_RCPT_PER_MESSAGE as the answer. I haven’t set it to anything in our config but didn’t know what the default limit was in Sendmail so I did some digging. Stumbled upon the following links of value (to me at least!)

Answer: infinite number. Sendmail doesn’t have a limit (now it does!)

Also stumbled upon’s version 5 Deployment Guide on running the m4 macro command to build a new file.


DNSBL, Sendmail and my Aunt…

So this past weekend I went to visit my Aunt & Uncle in Harrisonburg, VA. While there, my Aunt tells me she can’t send email thru my server which I was fairly certain was once again, “user error.” Come to find out. It was a misconfiguration on my part! Ah ha!

Long story short, I had reconfigured my email server about two months ago and added a DNSBL ( to my sendmail features and it was blocking her from relaying email thru my sendmail server. I simply had to add…


… to my file and restart the service. (NOTE: On CentOS 5.0 if you restart the sendmail service it will check your & files and do a ‘make -C /etc/mail’ for you if the is newer.)

Now, Sendmail will wait on the DNSBL and give the user a chance to authenticate first which will allow them to then bypass the DNSBL. Seems her residential comcast IP Address was added to which is commonly done to blocks of residential user IP address spaces to prevent spam.

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