TP-Link TL-MR6400 and DNS

I bought a new GSM router to have DNS where I cannot get Internet through otherwise. The TP-Link TL-MR6400 has much better connectivity and hence is faster than using my phone as hotspot. Only trouble, it has some DNS issues when trying to access the Internet from a raspberry running Linux. In the following, I will see how far I can get to identify the problem.

TP-Link Nameserver

Time to understand what is going. strace is a tool that shows all the system calls made by a program which allows us to get an idea what the program is doing. Let’s do a strace ping yahoo.com. The end of the output gives the following:

...
socket(AF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, 16) = 0
poll([{fd=5, events=POLLOUT}], 1, 0)    = 1 ([{fd=5, revents=POLLOUT}])
sendmmsg(5, [{msg_hdr={msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\242\225\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", iov_len=31}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, msg_len=31}, {msg_hdr={msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\206\350\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1", iov_len=31}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, msg_len=31}], 2, MSG_NOSIGNAL) = 2
poll([{fd=5, events=POLLIN}], 1, 5000)  = 1 ([{fd=5, revents=POLLIN}])
ioctl(5, FIONREAD, [97])                = 0
recvfrom(5, "\242\225\201\200\0\1\0\3\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1\300"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, [28->16]) = 97
poll([{fd=5, events=POLLIN}], 1, 4937

And after some time, we get the result of the last poll operation. Essentially, the operation times out and ping tries to resend the request.

)  = 0 (Timeout)
poll([{fd=5, events=POLLOUT}], 1, 0)    = 1 ([{fd=5, revents=POLLOUT}])
send(5, "\242\225\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", 31, MSG_NOSIGNAL) = 31
poll([{fd=5, events=POLLIN}], 1, 5000

After a couple of retries, ping tries another strategy which again fails.

close(5)                                = 0
socket(AF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, 16) = 0
poll([{fd=5, events=POLLOUT}], 1, 0)    = 1 ([{fd=5, revents=POLLOUT}])
send(5, "8\365\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", 31, MSG_NOSIGNAL) = 31
poll([{fd=5, events=POLLIN}], 1, 5000)  = 1 ([{fd=5, revents=POLLIN}])
ioctl(5, FIONREAD, [106])               = 0
recvfrom(5, "8\365\201\200\0\1\0\3\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1\300"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, [28->16]) = 106
poll([{fd=5, events=POLLOUT}], 1, 4998) = 1 ([{fd=5, revents=POLLOUT}])
send(5, "O\373\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1", 31, MSG_NOSIGNAL) = 31
poll([{fd=5, events=POLLIN}], 1, 4997)  = 0 (Timeout)

Eventually, ping tries a third approach which is successful. However, we do not always want to wait for all these timeouts to occur.

close(5)                                = 0
socket(AF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, 16) = 0
poll([{fd=5, events=POLLOUT}], 1, 0)    = 1 ([{fd=5, revents=POLLOUT}])
send(5, "8\365\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", 31, MSG_NOSIGNAL) = 31
poll([{fd=5, events=POLLIN}], 1, 5000)  = 1 ([{fd=5, revents=POLLIN}])
ioctl(5, FIONREAD, [106])               = 0
recvfrom(5, "8\365\201\200\0\1\0\3\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1\300"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, [28->16]) = 106
close(5)                                = 0
socket(AF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, 16) = 0
poll([{fd=5, events=POLLOUT}], 1, 4997) = 1 ([{fd=5, revents=POLLOUT}])
send(5, "O\373\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1", 31, MSG_NOSIGNAL) = 31
poll([{fd=5, events=POLLIN}], 1, 4997)  = 1 ([{fd=5, revents=POLLIN}])
ioctl(5, FIONREAD, [121])               = 0
recvfrom(5, "O\373\201\200\0\1\0\3\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1\300"..., 65536, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.1")}, [28->16]) = 121
close(5)                                = 0

What is the difference between the different approaches?

  • In the first approach, send multiple messages (sendmmsg) is used to send the following two messages: {iov_base="\242\225\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", iov_len=31} and {iov_base="\206\350\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1", iov_len=31}. For each timeout, the second message is resent: send(5, "\242\225\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", 31, MSG_NOSIGNAL) = 31,
  • The second approach right away instead of using send multiple messages, sends the two messages as two individual messges: send(5, "8\365\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", 31, MSG_NOSIGNAL) = 31 and send(5, "O\373\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1", 31, MSG_NOSIGNAL) = 31.
  • The third approach, like the second approach sends the two messages as separate messages but inbetween closes the connection to the DNS server and recreates the connection for the second message.
    Essentially, the problem seems to be that the TP-Link DNS server does not answer multiple messages on the same connection.

Another Nameserver

Now for a quick and dirty solution. The problem seems to be the DNS of the TP-Link acting in a way that the Linux resolver does not like it. Let’s replace the TP-Link name server listed in /etc/resolv.conf with a nameserver from Google (8.8.8.8) and redo the ping operation.

...
socket(AF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("8.8.8.8")}, 16) = 0
poll([{fd=5, events=POLLOUT}], 1, 0)    = 1 ([{fd=5, revents=POLLOUT}])
sendmmsg(5, [{msg_hdr={msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\222\344\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1", iov_len=31}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, msg_len=31}, {msg_hdr={msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\317\205\1\0\0\1\0\0\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1", iov_len=31}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, msg_len=31}], 2, MSG_NOSIGNAL) = 2
poll([{fd=5, events=POLLIN}], 1, 5000)  = 1 ([{fd=5, revents=POLLIN}])
ioctl(5, FIONREAD, [97])                = 0
recvfrom(5, "\222\344\201\200\0\1\0\3\0\0\0\0\3www\5yahoo\3com\0\0\1\0\1\300"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("8.8.8.8")}, [28->16]) = 97
poll([{fd=5, events=POLLIN}], 1, 4958)  = 1 ([{fd=5, revents=POLLIN}])
ioctl(5, FIONREAD, [121])               = 0
recvfrom(5, "\317\205\201\200\0\1\0\3\0\0\0\0\3www\5yahoo\3com\0\0\34\0\1\300"..., 65536, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("8.8.8.8")}, [28->16]) = 121
close(5)                                = 0

Essentially, the Google nameservers are happy to receive the two messages as part of a send multiple messages invocation.

My Solution

I would prefer to configure my local system to stick to the TP-Link DNS server and simply recreate the connection to the DNS server. I don’t believe that running to Google’s DNS server whenever there is a problem is the right solution.
Anyway, I have to admit after having spent enough time, this is the “solution”, I am sticking to for the time being. What is the proper way of adding the Google nameservers. After looking at /etc/resolv.conf, there is a nice message that on my system, this file is managed by resolvconf. After looking at man resolvconf and man resolvconf.conf, it turns out the proper way of specifying the nameservers is by adding the following line to /etc/resolvconf.conf:

name_servers="8.8.4.4 8.8.8.8"

Figuring out the proper syntax required looking into /sbin/resolvconf which fortunately is a shell script. To make the changes take affect, run the following command:

prompt# resolvconf -u
Too few arguments.
Too few arguments.

Yes there are two errors but /etc/resolv.cong has been updated and I am happy for now. If you are interested on the two nameservers, here is what Google says about them.
Hope this helps, if there are any comments or ideas of how to solve it better please let me know. And yes, bug report for resolvconf and request for documentation update has been sent to the maintainers of my distribution.

Posted in Uncategorized | Tagged , , , | Leave a comment

Uwatec SmartTRAK and Aladin TEC2G

All one needs to understand the Uwatec TEC2G is a bit of information, MDB Tools, and libdivecomputer. In this article, I will show how to understand and get data out of SmartTRAK logs as well as how to get dive data from the Aladin TEC2G, among others the watertype of a given dive data that otherwise is missing in libdivecomputer.
Continue reading

Posted in Diving, Technology | Leave a comment

Windows Registry and Policy Keys

I have a Samba network providing an NT domain to my network. My machines are Windows 7, however. Here I share a few links to make this configuration nicer to work with. Yes, probably, it would be best to migrate my Samba setup to a proper Active Directory domain. But that I leave for some other time. As always, it is your responsibility to verify the information given below. I am not responsible for any damages independently of whether any information contained herein looks reasonable or not. Please, be aware that any mistakes in the commands below may render your system unusable.

Continue reading

Posted in Technology, Uncategorized, Windows | Tagged , , | Leave a comment

Cheap Underfloor Heating Thermostat

byc Thermostat

byc Thermostat

Last month, I bough a few cheap underfloor heating thermostats with touch screen. These thermostats are available from a number of sources under different names such as BTSKY, Jago, Prodeals, ProTouch, SM-PC, etc. The thermostats work with electrical and hydronic or water-based underfloor heating systems (the latter is what I am using it for). Because I am very happy with those thermostats and because they are fairly common and available through various channels, I decided to write about these thermostats. In total, I ordered 4 devices of which I have 2 sets of 2 identical devices (byc07 and byc71). Judging from pictures on the web, there may be other variants of the devices available as well.

Continue reading

Posted in Miscellaneous, Technology, Uncategorized | Tagged | 30 Comments

Zotac ZBOX CA320 Driver Installation

Zotac ZBOX

Zotac ZBOX CA320

Since I needed a small and cheap computer to run Windows 7, I got the Zotac ZBOX CA320. It is equipped with an AMD A6-1450 quad-core processor which is clocked at 1GHz. Certainly, this is not a powerful device but it is good enough for what I use it for.

After installation of Windows 7, I installed the display drivers from the Zotac web site by running setup from the amd_vga directory which installed the Microsoft .NET framework in four different flavours (current version 32bit, current version 64bit, 2012 version 32bit, 2012 version 64bit), makes about 1.2GB. Plus it installs a useless Catalyst Center which takes ages to start and all it can tweak is to turn the power of the antenna on and off as well as minimum and maximum speed of the processor. I am underwhelmed (and this is an overstatement).  In the following, I will show how to install the drivers without that bloatware.

Continue reading

Posted in Technology, Windows | Tagged , | Leave a comment

UEFI Secure Boot: Hands-on Experience

If you want to create your own secure boot loader, you should have a look at the shim UEFI boot loader. It implements all the requirements set force by Microsoft. Even Microsoft says that this is a good starting point. While the instructions presented here are specific to Ubuntu 14.04, they should be easy to adapt to other distributions.

Continue reading

Posted in Technology | Tagged , , , | Leave a comment

UEFI Secure Boot Resources

In this blog entry, I will summarize interesting articles I found on the net that explain all the different aspects of Secure Boot. If you are interested in UEFI Secure Boot and are wondering what it is, how it can help you, or simply would like to experiment with it, you are interesting in the following articles, blog entries etc. If you find other useful articles that should be mentioned please mention them in the comments below.

Continue reading

Posted in Technology | Tagged , , | 1 Comment

OSX: Installing and Running Emacs

If you prefer the standard Emacs without any extra thrills, the easiest way is to download the sources from its official web site (GNU Emacs) and to compile it yourself. In the following, we will have a look on how to accomplish this.

Continue reading

Posted in OSX, Technology | Tagged , | Leave a comment

Generating Custom Reports with pywws

Sample Rain Chart

Generated Rain Chart (mm over time)

In the previous blog entry, I have set up my weather station with pywws. In this part, I will show how to create custom reports.

Continue reading

Posted in Raspberry, Technology | Tagged , , | Leave a comment

Installation of pywws on a Raspberry Pi

Weather StationToday, I will show how to use the Conrad weather station with a Raspberry Pi running Raspbian. We will discuss how to obtain weather data and create some charts. These are pre-requisites if you later want to upload the data to Weather Underground, or to other weather services.

Continue reading

Posted in Raspberry, Technology | Tagged , , | 2 Comments