[bdNOG] Yahoo Mail can't communicate with my domain servers

Brian Candler brian at nsrc.org
Thu Dec 24 02:57:53 BDT 2015

On 23/12/2015 20:27, Anurag Bhatia wrote:
> Your reply is bit confusing from part below since I own and manage the 
> test server (server7) not Jasim's server. :)
> Keep in mind I am not running btraccl.net <http://btraccl.net> DNS 
> servers. It's Jasim who is having trouble. But yes I get hints you are 
> sharing for troubleshooting. :)
The confusion is entirely mine; I apologise for spreading it :-)

So, what you can do on your side is:

# in one window
sudo tcpdump -i eth0 -nn -s0 -v host or icmp
# in another window
dig @ <> btraccl.net 
<http://btraccl.net> mx

Probably you will just see your outbound packets, but you *might* see 
ICMP "Admin Prohibited" coming back. If so, that's hard evidence of 
filtering, and will show exactly where the filtering is taking place, 
which is extremely useful.

If you don't mind sharing your IP address (privately with Jasim if you 
prefer), then:

1. Jasim can try a traceroute to that address and see if it works. 
(Presumably it does, because of the end-to-end ping and traceroute you see)

2. Jasim can run tcpdump on the DNS server and look for traffic from 
your IP while you send a query, and see if the query arrives or not. It 
might be that the inbound packets are arriving, but the responses are 
being filtered.

I think you're right it's most likely a broken firewall somewhere, but I 
*have* seen ISPs intentionally block UDP 53/123 in a misguided attempt 
to "protect" their customers, and/or to force them to use the ISP's own 
caching resolver (for example because they are doing anti-porn or 
anti-warez filtering in DNS)

Another outside possibility is that there is a layer2 bonded pair of 
links in the path, and one of the links is faulty. These typically use a 
hash of source/destination IP and port to decide which link to send the 
packet over. This could mean that ICMP is taking one path but UDP port 
53 taking a different one.

Out of interest: if you do "telnet 25" from server7, does 
it connect successfully? This would imply that if Jasim gets a proper 
secondary DNS, the mail would flow happily (and also more strongly 
points the finger at UDP port 53 filtering)

I see:

$ telnet 25
Connected to
Escape character is '^]'.
<< long delay here; implies reverse DNS lookup problems at the server 
side >>
220-cp1.btraccl.net ESMTP Exim 4.86 #2 Thu, 24 Dec 2015 02:55:58 +0600
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.



P.S. For comparison, here's traceroute from a VM in the UK. The last few 
hops are the same as you see, but in this case it's not going via 
airtel.in. That *might* be where the problem is occuring.

$ traceroute -w1
traceroute to (, 30 hops max, 60 byte packets
  1  213-138-103-3.no-reverse-dns-set.uk0.bigv.io ( 0.578 
ms  0.575 ms  0.612 ms
  2  te3-4.cr2.man.bytemark.co.uk (  0.811 ms  0.952 ms 
1.073 ms
  3  te0-0-1-0.cr4.man.bytemark.co.uk (  6.914 ms 7.160 
ms  7.248 ms
  4  te0-0-1-2.cr1.lon.bytemark.co.uk (  7.090 ms 
te0-0-2-2.cr1.lon.bytemark.co.uk (  7.129 ms 
te0-0-1-2.cr1.lon.bytemark.co.uk (  7.215 ms
  5 (  7.186 ms  7.194 ms  7.198 ms
  6  ae-7.r23.londen03.uk.bb.gin.ntt.net (  28.439 ms 
12.812 ms  41.834 ms
  7  ae-0.r22.londen03.uk.bb.gin.ntt.net (  8.765 ms 8.743 
ms  8.735 ms
  8  ae-3.r00.sngpsi05.sg.bb.gin.ntt.net (  194.383 ms 
190.415 ms  190.434 ms
  9  ae-3.r00.sngpsi05.sg.bb.gin.ntt.net (  191.934 ms 
194.400 ms  192.356 ms
10 (  243.758 ms 
103-16-152-25-noc.bsccl.com (  241.136 ms  239.398 ms
11  103-16-152-25-noc.bsccl.com (  241.584 ms 
103-16-152-33-noc.bsccl.com (  241.600 ms 
103-16-152-25-noc.bsccl.com (  239.196 ms
12  103-16-155-26-noc.bsccl.com (  255.071 ms 
103-16-152-33-noc.bsccl.com (  243.217 ms  241.485 ms
13  103-16-155-26-noc.bsccl.com (  256.580 ms  256.585 ms 
po1-ar1-bn1-dh.equitel.com.bd (  259.161 ms
14  po1-ar1-bn1-dh.equitel.com.bd (  257.245 ms (  246.590 ms  248.499 ms
15 (  248.612 ms  249.879 ms 
(  246.982 ms

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