Tuesday, September 17, 2013

DHCPNAK messages in log file

When I was checking log files I spotted the following log entries that were strange:
Sep  7 11:32:20 srv dhcpd: DHCPREQUEST for from 00:40:5a:18:83:56 via eth0
Sep  7 11:32:20 srv dhcpd: DHCPACK on to 0:4:5:1:8:5 via eth0
Sep  7 11:32:20 srv dhcpd: DHCPREQUEST for from 0:4:5:1:8:5 via
Sep  7 11:32:20 srv dhcpd: DHCPACK on to 0:4:5:1:8:5 via
Sep  7 11:32:20 srv dhcpd: DHCPREQUEST for from 0:4:5:1:8:5 via wrong network.
Sep  7 11:32:20 srv dhcpd: DHCPNAK on to 0:4:5:1:8:5 via
The problem is that DHCP request is received three times, on two of which the answer is positive (DHCPACK) while one received negative response (DHCPNAK) and dhcpd logged the error message 'wrong network'.

The important thing is the network configuration in this specific scenario, which looks something like follows:
  +----+            +-----+              +----+
  |    |------------|     |--------------|    |
  +----+            +-----+              +----+
  Client      Firewall/DHCP relay      DHCP server
Looking into log entries, not much can be inferred. The only thing that can be seen is that third DHCPREQUEST came from which isn't on the same network with a client requesting IP address. Sniffing the network gave a bit more information on what's happening. Analyzing the network trace the following were conclusions:

  1. There are three DHCPREQUEST messages with the same transaction ID, the same destination (, i.e. DHCP server) and also client IP address field within DHCP request is set to
  2. The first DHCPREQUEST comes directly from the client. It has source IP, and there is no relay field (i.e. the value is Also, client MAC address field within DHCP request has MAC address of a given client. 
  3. The second DHCP request comes from DHCP relay on the firewall. It has source set to, and relay field is properly set to, i.e. the IP address from the client's network,.
  4. The third DHCP request also comes from DHCP relay on the firewall, but this time relay field is set to This contradicts client's IP address and DHCP server rejects this request.
So, the conclusion is that client sends request to This request is forwarded by the firewall to the server, but also intercepted by DHCP relay on the firewall that creates two proxy requests and sends them to DHCP server too, one of which is rejected.

The interesting thing, not visible in logs, is that DHCP relay upon receiving NAK from the DCHP server, generates new NAK that is broadcasted on the network where DHCP server lives. 

So, the conclusion is that firewall is wrongly configured. It should not forward DHCP requests if there is a relay agent running. Furthermore, those NAKs aren't seen by the client, only by DHCP relay that reflects them back to DHCP servers.

Thursday, September 12, 2013

Adding Zimbra disclaimer using shell scripts...

While Zimbra 8 (and 7, too) have domain wide disclaimer support built in, there are two shortcomings that forced me to fall back to the old way of doing it:
  1. There is no support for not adding disclaimer if it already exists, and
  2. No support to exclude some addresses from adding disclaimer.
The second problem I managed to solve by patching Amavis script. That approach adds extra effort for maintainability (primarily during the upgrades), but it works. To solve the first problem the same way was too much work that I wasn't prepared to invest so I had to abandon domain wide disclaimer provided by Zimbra. There was also a third problem. Namely, for all mail messages sent from Outlook, Zimbra added two extra characters at the end of a HTML disclaimer, namely characters "= A". Why is this, I don't have slightest clue. I suspect it has something to do with encoding and decoding messages while going through the mail system, but exact reasons are unknown to me.

So, I went to solve all those problems and first I tried the old way, namely modifying postfix subsystem. It turned out that it didn't work. Just for a reference, at the end of this post, I described what I did. Next, option was modifying amavis. But that turned out to be too complicated and error prone - as I said in the introduction paragraph. Finally, I decided to put a proxy script in front of altermime that will be called by amavis and that will check if there is already disclaimer. If it isn't, then it calls altermime. Note that in this way there was no need to change amavis, and that means a lot from the maintenance perspective. So, here is what I did.

First, I created the following simple script in /opt/zimbra/altermime directory:
echo "`date +%Y%m%d%H%M%S` $@" >> /tmp/altermime-args
exec /opt/zimbra/altermime-0.3.10/bin/altermime-bin "$@"
What it does is it just logs how it was called and then it calls altermime. Note one more important thing here. In order to be able to put this script before altermime, I had to call it altermime, and altermime binary I renamed to altermime-bin. If you are doing this on a live system be very careful how you do this switch. I suggest that you first create script called altermime.sh, check that it works, and then use the following command to make a switch:
mv altermime altermime-bin && mv altermime.sh altermime
Ok, in this way I was able to find out how altermime is actually called. This is what I saw in /tmp/altermime-args file:
20130912100915 --input=/opt/zimbra/data/amavisd/tmp/amavis-20130912T100229-30384-pc8afS_K/email-repl.txt --verbose --disclaimer=/opt/zimbra/data/altermime/global-default.txt --disclaimer-html=/opt/zimbra/data/altermime/global-default.html
That's just one line of the output. As it can be seen, the first argument specifies file with mail message, and the rest specify disclaimer to be added. So, in order not to add disclaimer, if there is already one, I modified the altermime.sh script to have the following content:
grep "DISCLAIMER:" ${1#--input=} > /dev/null 2>&1
if [ ! "$?" = 0 ]; then
    exec /opt/zimbra/altermime-0.3.10/bin/altermime-bin "$@"
Again, be careful if you are modifying this script on a live system.

Now, in order to control where disclaimer is added, you can modify this simple shell script. One more thing you should be aware of, this approach impacts performance as, instead of running one process, it now runs at least 3 per mail message, and there are few extra file accesses. 

Finally, as a side note, I managed to get rid of those strange characters added to Outlook's email messages. I just edited a little bit html file that contains disclaimer, and those characters were gone. That's definitely a bug somewhere, but who knows where...

The old way that didn't work

As I said, the first approach I tried is to use the procedure from Wiki. But it didn't work. Anyway, for a reference, here is what I tried to do. Note that, as Zimbra already ships with altermime, there is no need to install it. The altermime is in /opt/zimbra/altermime/bin directory and you can safely use it. Ok, now to changes:

First, change a line in master.cf.in that reads
smtp    inet  n       -       n       -       -       smtpd
smtp    inet  n       -       n       -       -       smtpd        -o content_filter=dfilt:
and also add the following two lines:
dfilt   unix  -       n       n       -       -       pipe
        flags=Rq user=filter argv=/opt/zimbra/postfix/conf/disclaimer.sh -f ${sender} -- ${recipient}
Note that by this last line you specified that your script is called disclaimer.sh and that it is placed in /opt/zimbra/postfix/conf directory. This script, when run, should be run with a user privileges filter. Also, be careful where you put those lines. Namely, put them after the following three lines:
%%uncomment SERVICE:opendkim%%  -o content_filter=scan:[%%zimbraLocalBindAddress%%]:10030
%%uncomment LOCAL:postjournal_enabled%% -o smtpd_proxy_filter=[%%zimbraLocalBindAddress%%]:10027
%%uncomment LOCAL:postjournal_enabled%% -o smtpd_proxy_options=speed_adjust
The reason is that those line logically belong to the first smtp line, and if you add dfilt in front of it, you'll mess things, probably very badly, depending on your luck!

If you had Zimbra's domain wide disclaimer enabled, then disable it using:
zmprov mcf zimbraDomainMandatoryMailSignatureEnabled FALSE
as a zimbra user, and then restart amavis:
zmamavisdctl restart
still as a zimbra user.

Finally, to active custom script to add disclaimer run the following command as zimbra user:
zmmtactl restart
After I did all that, it didn't work. :D But, then I realized that there are two content_filter options to smtp which might not work, and so I resorted to proxying altermime.

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scientist, consultant, security specialist, networking guy, system administrator, philosopher ;)

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