Unable to browse from external network in SplitDNS

Thu Mar 19 03:42:23 UTC 2020

There are three(3) cases as mentioned below.

Case I
Request from DMZ host(SNat to 172.28.0.2) to Internal of split DNS(172.28.0.11).
We are able to NSLOOKUP for "registry.npmjs.org".
We are able to wget/browse "https://registry.npmjs.org"

So, No issues in this.

Case II
Request from DMZ host(SNat to 196.1.113.242) to Public of split
DNS(196.1.113.248).
We are able to NSLOOKUP for "registry.npmjs.org".
We are NOT able to wget/browse "https://registry.npmjs.org"

So, this we want to fix.

Observation: In the TCP dump on the interface with IP address "196.1.113.248",
we see that the DMZ host is trying to re-transmitting SYN packets to DNS server
multiple times.
We cannot do telnet(TCP) from DMZ host to 196.1.113.248 and thats the expected
behaviour.

The question is why it is switching from UDP to TCP while we try to wget/browse
and not the same is happening in Case I.

Case III
Executed for Troubleshooting.

Request from DMZ host(SNat to 196.1.113.242) to Google DNS(8.8.8.8).
We are able to do NSLOOKUP for "registry.npmjs.org".
We are able to wget/browse "https://registry.npmjs.org"

So, No issues in this.

Hope the above gives more insight into the issue.

Regards,

Purva Rawan

On March 18, 2020 at 7:05 PM Warren Kumari <warren at kumari.net> wrote:

> 
> 
>  On Wed, Mar 18, 2020 at 9:03 AM Purva Rawan < purvar at cdac.in
> <mailto:purvar at cdac.in> > wrote:
>    > > 
> >    Hello ,
> > 
> >    We have configured splitDNS .Bind version is 9.9.2.We are able to lookup
> > and browse to particular URL( e.g.https://registry.npmjs.org
> > <https://registry.npmjs.org/> ) from internal network but the same URL when
> > we tried from external network ,it failed to browse ,but able to do
> > nslookup.We checked tcpdump logs and observed that DNS protocol switched
> > from udp to tcp.
> > 
> >    Tcpdump logs for reference
> > 
> >    17:39:28.380918 ARP, Request who-has 196.1.113.242 tell 196.1.113.248,
> > length 28
> > 
> >    17:39:28.381205 ARP, Reply 196.1.113.242 is-at 00:09:0f:09:00:1a, length
> > 46
> > 
> >    17:39:30.395995 IP 196.1.113.242.54930 > 196.1.113.248.domain: Flags [S],
> > seq 2177054283, win 14600, options [mss 1460,sackOK,TS val 2512104 ecr
> > 0,nop,wscale 7], length 0
> > 
> >    17:39:38.420575 IP 196.1.113.242.54930 > 196.1.113.248.domain: Flags [S],
> > seq 2177054283, win 14600, options [mss 1460,sackOK,TS val 2520128 ecr
> > 0,nop,wscale 7], length 0
> > 
> >    17:39:54.451991 IP 196.1.113.242.54930 > 196.1.113.248.domain: Flags [S],
> > seq 2177054283, win 14600, options [mss 1460,sackOK,TS val 2536160 ecr
> > 0,nop,wscale 7], length 0
> > 
> >    17:40:26.483591 IP 196.1.113.242.54930 > 196.1.113.248.domain: Flags [S],
> > seq 2177054283, win 14600, options [mss 1460,sackOK,TS val 2568192 ecr
> > 0,nop,wscale 7], length 0
> > 
> >    Kindly help to resolve the same.
> > 
> >  >  You appear to have network / firewall, not DNS issues -- 196.1.113.242
> >  > is sending SYN (open a connection) packets to  ns1.cdac.in
> >  > <http://ns1.cdac.in> , but is not getting any reply packets from it
> >  > (assuming you included all of the tcpdump output) - this either means
> >  > that  ns1.cdac.in <http://ns1.cdac.in> was down, or, more likely, that
> >  > 196.1.113.242 cannot send packets to it on port 53.
>  As a quick and dirty test, can you telnet from 196.1.113.242 to port 53 on
> 196.1.113.248?
> 
>  W
> 
> 
>    > >    Regards,
> > 
> >    Purva Rawan
> > 
> > 
> >    [150th Anniversary Mahatma Gandhi]
> > 
> > 
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>     ---maf
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