RFC1101, network info in DNS. Usefull ?

Marc_Lampo at hotmail.com Marc_Lampo at hotmail.com
Mon Jan 24 15:46:56 UTC 2000


Thanks, an interesting example and easy to see the benifit in the output
of "netstat -r".
However, you seem to be defining A-records at the subnet-level itself.
Whereas rfc1101 attaches the A-record to the surrounding subnet.

According to the complicated example from rfc1101, I think it should
read :
;	RFC1101 name and netmask entry for local net0
0.242.6.62.in-addr.arpa.	IN	PTR	net0.rfc1035.com.
0.242.6.62.in-addr.arpa.	IN	A	255.255.255.248
;	RFC1101 name and netmask entry for local net1
8.242.6.62.in-addr.arpa.	IN	PTR	net1.rfc1035.com.
;8.242.6.62.in-addr.arpa.	IN	A	255.255.255.248
;	RFC1101 name and netmask entry for local net2
16.242.6.62.in-addr.arpa.	IN	PTR	net2.rfc1035.com.
;16.242.6.62.in-addr.arpa.	IN	A	255.255.255.248
;	RFC1101 name and netmask entry for local net3
24.242.6.62.in-addr.arpa.	IN	PTR	net3.rfc1035.com.
;24.242.6.62.in-addr.arpa.	IN	A	255.255.255.248

Since the .8, .16 and .24 are not further subnetted, they do not require
additional A-records (but kind of "inherit" the A-record from the
surrounding "class C" (0.242.6.62.in-addr.arpa)
Further according to that RFC1101, an A-record would be required at
surrounding net :
0.0.0.62.in-addr.arpa.  A  255.255.255.0

The potential problem I see and to which I was referring at in my
original posting, rises when that 62.0.0.0/8 network is variably length
subnetted.  eg
62.8.0.0/16 to organization1
62.6.242.0/27 to organization2
This would require 2 A-records at "top level"
0.0.0.62.in-addr.arpa.  A  255.255.0.0
                        A  255.255.255.224

In the example from rfc1101, the "subnet 0" is not used.  So the A-RR at
that level helps to locate the other subnets.  But what would happen if
that subnet 0 were itself subnetted is left "undefined".

Marc

>
> Well the resolver code in BIND can use it. If there's an A record for
> the net's subnet mask in the reverse zone file, you get a /N appended
> to the netname in the output from netstat -r. This /N is the number of
> bits in the netmask.
>
> Here's an actual example from my own DNS data. I've only been assigned
> 32 addresses (62.6.242.0/27), but tell lies to my local name servers
> by pretending to be master for the whole 242.6.62.in-addr.arpa
> zone. The real reverse zone belongs to my ISP and only has CNAMEs, so
> I couldn't put PTR and A records there for my internal nets.
>
> Here are the relevant bits of the (internal) zone file:
>
> 	;	RFC1101 name and netmask entry for local net0
> 	0.242.6.62.in-addr.arpa.	IN	PTR
net0.rfc1035.com.
> 	0.242.6.62.in-addr.arpa.	IN	A	255.255.255.248
> 	;	RFC1101 name and netmask entry for local net1
> 	8.242.6.62.in-addr.arpa.	IN	PTR
net1.rfc1035.com.
> 	8.242.6.62.in-addr.arpa.	IN	A	255.255.255.248
> 	;	RFC1101 name and netmask entry for local net2
> 	16.242.6.62.in-addr.arpa.	IN	PTR
net2.rfc1035.com.
> 	16.242.6.62.in-addr.arpa.	IN	A	255.255.255.248
> 	;	RFC1101 name and netmask entry for local net3
> 	24.242.6.62.in-addr.arpa.	IN	PTR
net3.rfc1035.com.
> 	24.242.6.62.in-addr.arpa.	IN	A	255.255.255.248
>
> And here's what netstat has to say for itself on a host that's
> connected to two subnets:
>
> 	% netstat -r
> 	Routing tables
>
> 	Internet:
> 	Destination               Gateway                  Flags    MTU
If
> 	default                   router                   UGS     1500
ne0
> 	net0.rfc1035.com/29       link#2                   UC      1500
ne0
> 	router                    0:d0:58:e0:3c:bd         UHLc    1500
ne0
> 	gromit                    0:20:18:3a:fb:13         UHLc    1500
lo0
> 	net1.rfc1035.com/29       link#1                   UC      1500
exp0
> 	gromit                    0:a0:c9:e5:ea:8b         UHLc    1500
lo0
> 	wallace                   0:10:5a:ac:73:b6         UHLc    1500
exp0
> 	loopback-net              localhost                UGRS    4352
lo0
> 	localhost                 localhost                UH      4352
lo0
> 	base-address.mcast.net/8  link#1                   UC      1500
exp0
>
>


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