where does IPS assign IP address of DNS for dial-up users ?

[MahVincent]

Hi everyone,

I have found out that when I connect directly to my IPS to use Internet, I always assign TCP/IP parameters such as gateway's IP address, DNS's IP address ... but when I setup dial-up networking for using Internet, I don't setup these parameters. So I wonder where my IPS assign them for dial-up users, in the dial-up access server or radius/tacacs ... or something like that?

Please tell me about this setting and give me an extract of assignment codes for DNS/gateway (TCP/IP) in above mentioned devices.

Thanks in advance for your answer.

Mah Vincent

 

[ifincham]

Hi,

It's all done with DHCP (Dynamic Host Configuration Protocol). If an interface is configured to use dhcp it broadcasts a DHCPDISCOVER message (or DHCPREQUEST to renew) and should eventually receive the offer of a lease from the dhcp server's pool. The interface is then configured accordingly by the dhcp client s/w. Various options can be specified as well as the IP address, including the dns addresses. The following example dhcpd.conf file will give you a rough idea:

# Sample /etc/dhcpd.conf
# (add your comments here)
default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.1.255;
option routers 192.168.1.254;
option domain-name-servers 192.168.1.1, 192.168.1.2;
option domain-name "mydomain.org"; subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.10 192.168.1.100;
range 192.168.1.150 192.168.1.200;
}

See -->

http://www.isc.org/products/DHCP/

.

Regards

 

[MahVincent]

Hi Ifincham,

First of all, thanks so much for your prompt answer. But, I'm afraid to say that there's still something vague. As far I'm concerned, I know my IPS has Cisco AS 5800 access server, and this access server authenticates based on RADIUS server (running under Solaris). And when I setup Dial-up networking icon (under Win98) to connect to my ISP, I'm sure I not use DHCP. (As you see, when you setup dial-up networking icon, you just put telephone number, account's password, choose COM port, speed... and leave everything related to TCP/IP parametersas default, of course it's not based on DHCP all ). Therefore, I have a question where they assign TCP/IP parameters. According to your answer, they use DHCP setup, so could you tell me where they setup this protocol, in RADIUS server? or Access Server ? or somewhere else ? Give me more detailed explanation. ( Forgive me if I say something wrong )

Thanks sincerely.

Mah Vincent

 

[ifincham]

Hi,

Well if the ISP was Prodigy or AOL then the protocol on the dial-up would not be conventional TCP/IP so that would be different. I'm not familiar enough with the products concerned but the way I understand it is that the IOS would effectively 'call' the radius server for authenication and possibly accounting purposes only. See -->

http://www.cisco.com/warp/public/707/32.html

and

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fsecur_c/fsecsp/scfrad.htm

.

Anyway, if you are using a 'normal' browser (vs AOL or suchlike) then the browser s/w is going to seek name resolution before it even attempts to get the packets routed. This would require that the client machine has a tcp stack configured (with dns ip addresses) to enable it to contact dns servers for such resolution. Unless a proprietary protocol was in use then this would be via tcp/udp port 53 and the interface would need to be configured accordingly - either statically or dynamically via dhcp. If there were no dns servers configured what address would be put in the dns query packet ?

Regards