DNS issues - is it my isp or my network

Hi

I am not convinced Vodafone isn't having problems with their infrastructure.

This is because every time the internet stops working, it's only the DNS portion that doesn't work. I can still ping 8.8.8.8 which is googles dns server.

I'm using my trusty billion 8800nl instead of their [censored] router and it says the second dns is 90.255.255.255.

Isn't that address reserved for broadcasting?

thats like me putting my router's ip as 192.168.1.255

Edited by deleted (Mon 21-Oct-19 14:36:24)

That IP does seem correct for Voda DNS. If you have changed your primary to google then what reason did you not change the secondary to google secondary?

In reply to a post by picky_user:

Isn't that address reserved for broadcasting?

If part of a local LAN, generally yes...

but this is allocated to a service so you can technically use .0 or .255 if you want. Similar to that some broadband connections can be assigned .0 and .255 address. All perfectly valid.

Matt

Depending on the size of the netblock the network has been allocated from an IP address ending in .255 is potentially valid. Basically anything bigger than a /24 will have one valid .255 and .0 in it.

So for example 15.0.255.255 is a valid IP address somewhere inside the large block that HP has. Though depending on how they have carved their IP address range up internally it might not be valid.

That said 90.255.255.255 is not valid.

In reply to a post by jabuzzard:

That said 90.255.255.255 is not valid.

I don't believe you are correct. Broadcast addresses are only used within LANs. As this is a public Internet address it would not be used for broadcasting over the WAN and therefore is a perfectly valid address to use. I can't find anything solid to back this up but given this address works and can be pinged over the Internet I believe my understanding is correct.

Depends on the subnet size which depend on the subnet mask, if its set to a 255.255.255.0 mask it will only contains one 0 and one 255 in the range and they are both reserved but when the mask is set to 255.255.254.0 you get 2 x 255 (510) IP addresses so there are two 0 and two 255 in the range but only the first and last are reserved. you can increase the subnet size up and up by changing the subnet mask but there are issues with larger subnets. You can also reduced the size of the subnet down below 255 by changing the subnet mask e.g. 255.255.255.128 this will affect the reserved addresses.

In reply to a post by dect:

Depends on the subnet size which depend on the subnet mask, if its set to a 255.255.255.0 mask it will only contains one 0 and one 255 in the range and they are both reserved but when the mask is set to 255.255.254.0 you get 2 x 255 (510) IP addresses so there are two 0 and two 255 in the range but only the first and last are reserved. you can increase the subnet size up and up by changing the subnet mask but there are issues with larger subnets. You can also reduced the size of the subnet down below 255 by changing the subnet mask e.g. 255.255.255.128 this will affect the reserved addresses.

Ah ok. hahah A little rusty on calculating subnet masks and all that.
I never need to do it.

Am I correct in assuming it's vodafone being rubbish. Yes I have a rather complicated network with 3 vlans because I can and I am a nerd.


I am convinced it's them because when this happens I reset the modem then it works again but it's only been like this for a few days. I switched to Vodafone back in JULY!! Not had any problems apart from the usual initial teething issues.

When the DNS stops working again, Is there a way I can see where the dns request ends up? A bit like a traceroute but for dns?


If it does indeed end up as far as the modem then it's Vodafone's responsibility is it not?

I would try just using the VF router out of the box for a few days, in default config mode. There is not widespread vodafone users reporting no internet due to DNS issues, no.

Why do you need multiple VLANs? It sounds as though you are over-complicating your network despite not fully understanding how broadcast addresses work in relation to subnet masks, nor how DNS works.

Personally I wouldn't trust Vodafone DNS servers. Out of all the available public DNS servers you are probably best with 1.1.1.1 as they are less likely to be snooping on you compared to Google or Quad9.

It would be very easy to prove if it is the DNS side which is going wrong by opening nslookup in command prompt

then type
server <primary DNS server IP>
www.google.com
see if it resolves

then type <secondary DNS server IP>
www.google.com
see if it resolves

then type server 1.1.1.1
www.google.com
see if it resolves

For example:

Text

1 23 45 67 89 1011 1213 1415 1617 1819 2021 2223 2425 2627 2829 3031 3233

C:\Users\Admin>nslookup Default Server:  dns1.aa.net.ukAddress:  2001:8b0::2020  > server 2001:8b0::2020 Default Server:  dns1.aa.net.ukAddress:  2001:8b0::2020  > www.google.com Server:  dns1.aa.net.ukAddress:  2001:8b0::2020  Non-authoritative answer: Name:    www.google.comAddresses:  2a00:1450:4009:814::2004           216.58.210.228  > server 1.1.1.1Default Server:  one.one.one.one Address:  1.1.1.1  > www.google.comServer:  one.one.one.one Address:  1.1.1.1  Non-authoritative answer:Name:    www.google.com Addresses:  2a00:1450:4009:81c::2004          216.58.210.36  > exit  C:\Users\Admin>

Depending on what results you get from those 3 tests should point you in the direction of whether your ISP DNS servers are problematic or you have a wider connection issues.

Bear in mind 98% of Vodafone customers are probably using the provided [censored] routers or whatever so if their DNS service kept going down they would have their entire customer base complaining. So it is unlikely the issue is with their DNS servers if it is occurring frequently.

Try this, should list all the DNS: https://ipleak.net/

You can ping the broadcast address, try it

I am not aware of any routing protocol that allows one to route actual useful data packets to a broadcast address. Put another way how does the router route a packet to the network broadcast address? The next address range up (91.0.0.0/12) is owned by Deutsche Telekom AG so Vodafone are not combining net assignments.

You can test this quite simply I can ping the broadcast address, but the moment I try and connect for a DNS query I get a no route to host error.

[root@buzzard ~]# ping -c 3 90.255.255.255
PING 90.255.255.255 (90.255.255.255) 56(84) bytes of data.
64 bytes from 90.255.255.255: icmp_seq=1 ttl=54 time=6.65 ms
64 bytes from 90.255.255.255: icmp_seq=2 ttl=54 time=6.56 ms
64 bytes from 90.255.255.255: icmp_seq=3 ttl=54 time=6.62 ms

--- 90.255.255.255 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 6.562/6.613/6.652/0.037 ms
[root@buzzard ~]# telnet 90.255.255.255 53
Trying 90.255.255.255...
telnet: connect to address 90.255.255.255: No route to host

It looks like a mistake from Vodafone to me. I guess they could have done something special with their routers to make it work but it would not be standards compliant I think. Technically I think a UDP request on port 53 to 90.255.255.255 might be forwarded to all hosts the network with one or more responding. However it's highly none standard and would like break a lot of software not least because you can perfectly legitimately do DNS over TCP.

Personally all I take from my ISP is my IP address (which is actually fixed), network and gateway. The rest they can take a running jump.

Easiest just change it to something else anyway IMHO.

I've just used an internet tracert tool and can trace the route to 90.255.255.255 just fine. Are you sure it isn't the difference between LAN and WAN - you can do broadcast packets over a LAN but I am not sure you can over a WAN - the fact these are public WAN IPs means that the routers likely don't treat it as a broadcast address as it wouldn't make any sense to do so - therefore the address is available for allocation.

In reply to a post by jabuzzard:

You can ping the broadcast address, try it

That works better if you have a unix or linux based OS (e.g. macOS) than on Windows

90.255.255.255 is not a broadcast address (whois, using a tool which knows how to lookup IP assignments not just domains).

Vodafone have assigned all of 90.255.255.0/24 to anycast services, so a connection to an IP in this range is routed to the "best" of a group of hosts based on topology / geography or any other reasonable criteria (the exact method does not matter).

I won't go into a full explanation of anycast addressing here (one can readily find technical descriptions) suffice to say, for a client it won't really see any difference from connecting to a unicast address.

BTW just because an IP is pingable doesn't dictate which other protocols or services a given host will be offering or be reachable though the provider's firewall.
And telnet, while possible to hand-craft connecting to services which reply using human-readable text such as SMTP, will really only tell you about connecting to TCP ports which is not what vanilla DNS* uses . There are more generic tools for TCP/UDP and other protocols (e.g. nc, nmap and "port scanners").

Although I am arbitrarily replying to a post, this is more a general reply to the thread as a whole.

(*) not withstanding extensions such as EDNS and DNSSEC which do!

Edited by prlzx (Tue 22-Oct-19 22:00:28)

I'm currently with Vodafone FTTC. Vodafone's DNS servers go down regularly.
So I manually set my router to use google DNS 8.8.8.8 and 8.8.4.4
Never had a DNS problem since then.