Cleaning Chrome DNS Cache

While playing with Mikrotik’s DNS I accidentally made a regular expression statement overly broad. Error was clear: I had “ana” in the DNS Rexexp field instead of “^ana$”. What this simple error did was to resolve everything with ana in the name to the machine on local network. I noticed that when I tried to access http://anandtech.com and got a timeout.

I fixed the erroneous entry and all was good when I checked it manually with nslookup. However, I still couldn’t access the web site. Interestingly, if I tried using Internet Explorer instead of my favorite Chrome, access worked. Yep, Chrome has its own internal DNS cache.

Cleaning Chrome’s cache is reasonably easy. Only thing needed is a visit to chrome://net-internals/#dns and hitting the Clear host cache button. However, my local erroneous address was back as soon as I tried accessing the site again.

Interestingly Windows themselves have also cached the incorrect IP address. Chrome using Windows API to resolve DNS name was catching the wrong one. Internet Explorer was unaffected as it made DNS query directly. Curious choices. :)

Cleaning Windows cache involved getting to elevated prompt. My favorite way is hitting Win+X and selecting Command Prompt (Admin) from the menu. Once in the prompt, we just execute:

ipconfig /flushdns
 Windows IP Configuration
 Successfully flushed the DNS Resolver Cache.

With this (and clearing Chrome’s cache again) I could browse anandtech.com again.

PS: For curious, Mikrotik supports extended POSIX regular expressions.

Mint Cacti for Mikrotik Queue

Mikrotik does routing beautifully but the same cannot be always said about its traffic monitoring facilities. While graphing does exist, its is as flexible as Trump supporter on immigration issues.

For me, one of the best ways to monitor router on the cheap is Cacti. Completely free and has built in SNMP support. Guess what else has built in SNMP support? Yep - Mikrotik.

To get Mikrotik’s SNMP working, just enable it from terminal window, adjusting firewall if necessary:

/snmp set enabled=yes

/ip firewall filter
add chain=input protocol=udp dst-port=161 in-interface=!ether1 action=accept place-before=0 comment="Allow local SNMP"

While we are playing with Mikrotik, we can also print OIDs for queues:

/queue simple
 print oid without-paging
 0    name=.1.3.6.1.4.1.14988.1.1.2.1.1.2.19
      bytes-in=.1.3.6.1.4.1.14988.1.1.2.1.1.8.19
      bytes-out=.1.3.6.1.4.1.14988.1.1.2.1.1.9.19
      packets-in=.1.3.6.1.4.1.14988.1.1.2.1.1.10.19
      packets-out=.1.3.6.1.4.1.14988.1.1.2.1.1.11.19
      queues-in=.1.3.6.1.4.1.14988.1.1.2.1.1.12.19
      queues-out=.1.3.6.1.4.1.14988.1.1.2.1.1.13.19
 ...

Just store this data somewhere are we are going to need bytes-in and bytes-out entries later.

To get Cacti running, I went with the latest Linux Mint distribution. Procedure is quite generic so you can select essentially any Linux. Just add a few packages:

sudo apt-get -y install lamp-server^
sudo apt-get -y install snmpd
sudo apt-get -y install cacti cacti-spine

During installation, some packages might have additional questions - especially password related - you might want to set. For the purpose of this exercise I just went with all defaults.

After all packages are installed, it is a good time to test if we get anything from Mikrotik:

snmpwalk -v 2c -c public ^^192.168.88.1^^

And yes, this command is going to show a lot. :)

Now that we know SNMP is working we can go further with Cacti setup. For that we go to http://127.0.0.1/cacti and answer a few questions - essentially just setting the admin password and confirming tool locations.

The next thing on Cacti’s Console page is selecting Devices and adding a new one. You need to enter Mikrotik’s IP address here and change SNMP version to 2. Once you create entry, you should see system name and uptime.

Now we can finally go to New Graph and create one based on SNMP - Generic OID Template. For the purpose of byte counting Maximum Value should be set to U and OID should be one belonging to Mikrotik’s queue byte count. In my case value .1.3.6.1.4.1.14988.1.1.2.1.1.8.19 is the one used for input bytes of my Internet queue. A few minutes afterward you can check your Graphs/Preview tab and you should see your data nicely displayed.

Of course, with Cacti’s seemingly infinite configurability, this is just a start. Feel free to snoop around and discover. :)

PS: To monitor router’s health, check out resource OIDs:

/system resource print oid
             uptime: .1.3.6.1.2.1.1.3.0
    total-hdd-space: .1.3.6.1.2.1.25.2.3.1.5.1
     used-hdd-space: .1.3.6.1.2.1.25.2.3.1.6.1
       total-memory: .1.3.6.1.2.1.25.2.3.1.5.2
        used-memory: .1.3.6.1.2.1.25.2.3.1.6.2
           cpu-load: .1.3.6.1.2.1.25.3.3.1.2.1

Escaping Backtick in (Perl)grep

It all started with a simple list of name='value' and name=`value` entries, all within the same line. My wish was to color a few matching entries for the devious purpose of making them more visible. First stab at solution was extremely easy:

something | grep --color=always -P "somename='.*?'"

Two things to note here: I had to use PERL-style grep as I needed a non-greedy matching and secondly this didn’t fulfill its task. Yep, it didn’t match backtick (`) character.

Simple regex adjustment one might think:

something | grep --color=always -P "somename=['`].*?['`]"

But no - backtick is a tricky one as it serves a special purpose in bash.

I tried a bunch of escaping methods before I remembered that hexadecimal characters are still a thing. Wouldn’t you know it - that worked. To match a backtick, instead of using character itself, one can always use its hexadecimal escape code:

something | grep --color=always -P "somename=['\x60].*?['\x60]"

OpenVPN or SSTP on Mikrotik?

As I have covered creating both OpenVPN and SSTP server on Mikrotik, one might rightfully wonder - which one is better?

Security-wise, on Mikrotik, they are pretty much even. Both use certificates, both can use AES, and both allow for the perfect forward secrecy. If you decide to stick with Windows 10 or you are willing to tweak Windows 7 a bit, SSTP can even be forced to use only TLS 1.2.

When it comes to connectivity, by default SSTP has a slight advantage as it defaults to port 443 which traverses pretty much any firewall. But it is not a big advantage as OpenVPN can offer exactly the same success rate if configured accordingly. Unfortunately both also support only TCP as the base protocol, by design in the case of SSTP and by Mikrotik’s choice in the case of OpenVPN. If you are on a lossy or even just slow link, TCP-over-TCP tunneling is going to make bad situation worse.

OpenVPN does have a bit of advantage when it comes to support across various platforms as you cannot find an OS without it. If you are dealing with Linux platforms (including Android), OpenVPN is probably the best route. While there are open source versions of SSTP for various platforms, it roots are on Windows and there it works flawlessly and out of the box. It is the VPN of choice if you need to get Windows machine on VPN without installing any additional software.

Guess what, performance of both protocols, if configured similarly, is also close. OpenVPN might seem a bit slower at the time but usually this is when different ciphers are selected. If you keep both at AES-128 (SSTP’s default in force-aes mode), you will see both as being equal. Mind you, neither is “cheap” as far as CPU usage goes. It is just that neither has advantage over the other.

Frankly, based on all things I cared about, either protocol will do a good job but neither is perfect nor supported on all devices. I personally keep both turned on with a common security profile so I can use the same user name and password for both. If I am connecting from Windows computer I go SSTP route just because it is so frictionless. For all non-Windows devices, including mobile phones, I go OpenVPN.

Unable to Execute File From NAS4Free Share

I use NAS4Free based server for all my file sharing needs. Give it a bit of ZFS turbo and you have a reliable setup.

However, I noticed one quirk if you will. From my computer I could execute installations located on the network share while my kids were greeted with “Windows cannot access…” error. Cause was clear. My kids had read-only access while I had full rights. And it was definitely that, as I could use chmod +x to make file executable and go on my marry way.

Doing it recursively over all directories would have solved the problem but not necessarily permanently as any newly created file would need the same adjustment. And yes, it would be possible to force inheritance of attributes from parent but that seemed overly restrictive - not even taking into consideration that my NAS4Free command line would suddenly auto complete every executable file in directory.

Fortunately, Samba 4.1 has a simple solution for it - acl allow execute always. This option than “fakes” executable attribute without setting it on the underlying system. It is as close as it gets to “have your cake and eat it too”.

NAS4Free unfortunately doesn’t have this option exposed to user. However, it does have free form field for configuration parameters. Solution is just finding Auxiliary parameters under Services, CIFS/SMB and adding acl allow execute always = true into a text box. Short restart later and any restricted Windows user will be able to execute files from the network share.

[2018-07-22: NAS4Free has been renamed to XigmaNAS as of July 2018]