Linux – How to find DNS servers for each PPP session in linux

linuxpoint-to-point-protocol

How to get the DNS server address which are specific to each ppp session. For example, I have two ppp sessions each one connected different service provider, how do I find out the DNS server IP address which are assigned by each service provider? In general, the /etc/ppp/resolv.conf file will be overwritten with the new DNS IP address whenever a new ppp session is established. Is there any way how we can maintain separate files for DNS servers for each service provider?

Best Answer

man pppd:

   usepeerdns
         Ask the peer for up to 2 DNS server addresses.  The addresses supplied 
         by the peer (if any) are passed to the /etc/ppp/ip-up script in the  
         environment  variables  DNS1  and DNS2, and the environment variable 
         USEPEERDNS will be set to 1.  In addition, pppd will create an /etc/ppp/
         resolv.conf file containing one or two nameserver lines with the
         address(es) supplied by the peer.

So just evaluate DNS1 and DNS2 in your ip-up script (which is run once after the PPP connection is established) and if you need this information later on, store it somewhere.

But as mailq has written, there is really only little use for this kind of feature.

Related Solutions

Xl2tpd: Provide L2TP/IPsec VPN connectivity to an entire subnet without using a default route

As I suspected the problem is that I'm a bit of a numpty. In case anyone finds this question and is wondering the same thing, these are the key things you should know:

nodefaultroute doesn't go in your VPN server config. It is something for a PPP client. PPP clients are responsible for deciding whether or not to create a default route. This is the "Send all traffic" checkbox on iOS.

Your PPP client will always create a route to the remote gateway, plus a route for the remote subnet. Provided that you have IP forwarding enabled on your VPN server (/proc/sys/net/ipv4/ip_forward = 1 plus appropriate iptables rules on linux) your client will be able to send traffic to hosts on the remote subnet.

You must specify proxyarp in the ppp options.xl2tpd file for hosts on the VPN server's LAN to be able to send traffic back to clients. This will create an entry in the arp table and enable the proxy_arp option in the kernel. This allows the VPN server to accept packets on behalf of VPN clients using its own ethernet interface. If you don't want to do this or need to cross routers it appears that putting the VPN clients behind NAT is a popular solution, though I didn't test this myself.

Use the ms-dns ppp option to provide a nameserver for clients if they are going to use the VPN as a default route. Otherwise an iPad will complain that it's not connected to the internet.

Linux server not receiving default IPv6 route over PPP link

I've got a debian Linux box doing almost the same thing (I don't use a dhcpv6 server and I'm using dibbler as the ipv6 dhcp client for the ppp interface), in my PPP config I've got:

defaultroute
replacedefaultroute
+ipv6
ipv6cp-accept-local

if you add:

debug

to the ppp config you'll get the details of the ipv6 negotiation. I suspect you're missing the defaultroute option

Best Answer

Related Solutions

Xl2tpd: Provide L2TP/IPsec VPN connectivity to an entire subnet without using a default route

Linux server not receiving default IPv6 route over PPP link

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