# PBM: No name resolution for local machines with Sagem ADSL

## VinzC

Hi all.

I have a Sagem F@st ADSL modem (3464AB) delivered by my ISP, which fails name resolution for hosts on the local network, not for Internet hosts. I have /etc/resolv.conf indeed list "search mydomain.local" but I get a name resolution error whenever I want to ping my laptop by its name (either FQDN or not). I booted my laptop

Can anyone tell me how I can debug this? Or even point to my ISP there is a problem with their router? (In how the embedded DHCP server works, probably.)

Thanks in advance for any hint/suggestion.

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## Bones McCracker

It sounds like you have no local dns server (for your LAN).  Do you have one running?  Is the ADSL modem supposed to be running one?

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## krinn

I only saw DSL modem that were identifying dhcp client by their netbios name. 

You can do bonekracker said, or put the dns name in /etc/hosts and fix your ip.

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## VinzC

Well, the router forwards DNS requests, which I am sure of. However I've scanned the open ports and no port 53 was reported by nmap  :Shocked:  . Strangely enough, my vDSL2 router is listed as a name server in resolv.conf!

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## Bones McCracker

 *VinzC wrote:*   

> Well, the router forwards DNS requests, which I am sure of. However I've scanned the open ports and no port 53 was reported by nmap  . Strangely enough, my vDSL2 router is listed as a name server in resolv.conf!

 

I'm assuming you are using IPv4.  A router forwarding DNS requests will not result in you having local name resolution on your LAN (because of Network Address Translation, the DNS server out there on the Internet has no visibility of your local machines).  To get local name resolution (e.g. so you can type "ssh hobbit" and it connects to the machine on your LAN named "hobbit"), you would need to have a DNS server on your LAN (or, alternatively, you could implement some form of zero-configuration networking such as are built into Gnome, KDE, MacOS X, and Windows 7 these days).

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## VinzC

 *VinzC wrote:*   

> Well, the router forwards DNS requests, which I am sure of. However I've scanned the open ports and no port 53 was reported by nmap  . Strangely enough, my vDSL2 router is listed as a name server in resolv.conf!

 

 *BoneKracker wrote:*   

> I'm assuming you are using IPv4.  A router forwarding DNS requests will not result in you having local name resolution on your LAN (because of Network Address Translation, the DNS server out there on the Internet has no visibility of your local machines).  To get local name resolution (e.g. so you can type "ssh hobbit" and it connects to the machine on your LAN named "hobbit"), you would need to have a DNS server on your LAN (or, alternatively, you could implement some form of zero-configuration networking such as are built into Gnome, KDE, MacOS X, and Windows 7 these days).

 

Yes, I knew that DNS tech'. I was not asking *how* I can have local DNS resolution. Just that I expect a DNS forwarder to also listen to UDP port 53 at the very least. There doesn't seem to be such listening service on the router, according to my nmap results. Or is it just a set of firewalling rules that blindly forward DNS packets to upward servers? I can hardly believe that. My initial question was in fact to query if there's anyone who knows that kind of router and can optionally confirm that a DNS forwarder implies a service listening to port 53.

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## Bones McCracker

 *VinzC wrote:*   

>  *VinzC wrote:*   Well, the router forwards DNS requests, which I am sure of. However I've scanned the open ports and no port 53 was reported by nmap  . Strangely enough, my vDSL2 router is listed as a name server in resolv.conf! 
> 
>  *BoneKracker wrote:*   I'm assuming you are using IPv4.  A router forwarding DNS requests will not result in you having local name resolution on your LAN (because of Network Address Translation, the DNS server out there on the Internet has no visibility of your local machines).  To get local name resolution (e.g. so you can type "ssh hobbit" and it connects to the machine on your LAN named "hobbit"), you would need to have a DNS server on your LAN (or, alternatively, you could implement some form of zero-configuration networking such as are built into Gnome, KDE, MacOS X, and Windows 7 these days). 
> 
> Yes, I knew that DNS tech'. I was not asking *how* I can have local DNS resolution. Just that I expect a DNS forwarder to also listen to UDP port 53 at the very least. There doesn't seem to be such listening service on the router, according to my nmap results. Or is it just a set of firewalling rules that blindly forward DNS packets to upward servers? I can hardly believe that. My initial question was in fact to query if there's anyone who knows that kind of router and can optionally confirm that a DNS forwarder implies a service listening to port 53.

 

As far as I know a DNS forwarder (i.e., a caching forward-only nameserver) would indeed listen on port 53, on the internal interface (or interfaces) of the router.  This won't provide local name resolution though.

A quick glance at the single page of documentation available at the website shows that it claims to provide "DNS relay" (whatever that means).  I would assume that to mean that's providing a caching, forward-only nameserver (and I too would expect it to be listening on port 53).  When you checked with nmap, did you scan only for TCP?  DNS works on both TCP and UDP.  Maybe it's listening on UDP and not TCP?

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## VinzC

I didn't set UDP explicitly as I believed nmap was doing UDP and TCP scans by default. My bad. I'll try to figure out how to do a UDP probe on port 53.

BTW from what you read in the small doc page, if that's a relay, I expect it to serve no local DNS request, which kind of pisses me of as it's Linux inside and for the same price they could have at least embedded dnsmasq; I know it, it's perfect for the job! No, better charge the customer for a more expensive model...

<sighs>

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## Bones McCracker

 *VinzC wrote:*   

> I didn't set UDP explicitly as I believed nmap was doing UDP and TCP scans by default. My bad. I'll try to figure out how to do a UDP probe on port 53.
> 
> BTW from what you read in the small doc page, if that's a relay, I expect it to serve no local DNS request, which kind of pisses me of as it's Linux inside and for the same price they could have at least embedded dnsmasq; I know it, it's perfect for the job! No, better charge the customer for a more expensive model...
> 
> <sighs>

 

I haven't played with nmap in a while.  For all I know it scans both TCP and UDP by default.

That router does have some nice other features on it, though.  Have you checked to see whether you can put 3rd-party firmware on it (not likely, but worth a check)?

One thing you could do is simply turn off it's DHCP server and set up dnsmasq on one of your other machines, if you have one that's on all the time.  Or, you could use zero-config networking (something I am still avoiding like herpes).

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## VinzC

Thanks for your suggestion. I hadn't checked if 3rd-party firmwares fit. My favourite is DD-WRT but this router is not supported alas. I already have a linksys router flashed with DD-WRT so I won't need to leave one machine on all the time. I know that one works.

Do you know other 3rd party firmwares? I only know OpenWrt by name and have never tried it, for instance.

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