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TOPIC: ipconfig /release_all works on which layer ?

ipconfig /release_all works on which layer ? 12 years 9 months ago #2666

  • naddyboy
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hi people

I use a dialup connection to connect to the internet and i wanted to check if ipconfig /release_all /renew_all switches work with the ISP, Even tried ipconfig /renew 0 /release 0 but each time i try doing this i get the same response of "ipconfig" and the adapter does'nt let go the IP address assigned to it.

My questions are:

1. Is this some sort of rule on the firewall or is it an access list on some router thats preventing me from renewing the IP address ? or what is it ???
2. When I'm connecting to the ISP which device am I connecting to ?
3. At which layer does the ipconfig /release_all switch work on ?


thanx in advance :)

Cheers,
Syed
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Re: ipconfig /release_all works on which layer ? 12 years 9 months ago #2668

  • sahirh
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The adapter 'letting go' of an address is a DHCP release.. you can only get reassigned (renew) an IP address if there is a DHCP server at your ISP end that is willing to give you an IP.. if you have a static IP, none of this makes any difference.

There is no rule as such or access-list that prevents you from renewing the IP, however the DHCP server will have to be in the same subnet as you, or the router should be configured to be a DHCP relay-agent and pass on DHCP requests.

When you're connecting to your ISP, what you connect to depends on what sort of connection you have... here we move into telecom territory, something I am (unfortunately) not very well versed with.. the hardware on the other end can differ, but invariably it will be some sort of analog to digital converter..

As a simple example, if you use dial-up, you're connecting to a modem at the other end, once you connect to this modem, it establishes a data-link protocol (such as PPP), once this connection is established, and you have successfully logged in, a DHCP server will provide your machine with an IP address.

IPconfig deals with IP addresses... in other words, Internet Protocol, which resides at the network (3rd layer) of the OSI model.


Hope that answers everything.
Sahir Hidayatullah.
Firewall.cx Staff - Associate Editor & Security Advisor
tftfotw.blogspot.com
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Re: ipconfig /release_all works on which layer ? 12 years 9 months ago #2682

  • naddyboy
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Sahir,

ofcourse, logically you'd never be using the release/renew switches if u have a static ip ... my isp assigns the address from a dhcp scope ... i'm doing a little probing in these switches on how they work ... agreed that ipconfig is a layer 3 command since it deals with ip addresses ... but the /release/renew switch is a request made by this command to the dhcp server ... and since i wanted to know why i was'nt able to renew by adapter ... or do a dhcp release .... i need to find out how these ipconfig switches interact with the modem or whateva device on the other end ... does it use the udp protocol ? tcp ? or what ? ... and if its one of these protocols .... then is it an access list thats blocking the way .... this is only my perception so i may not be right ... i have'nt really gone to such a depth .... can u add some light to this ?

Syed
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Re: ipconfig /release_all works on which layer ? 12 years 9 months ago #2683

  • jackeen
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hi naddyboy,

i once was very interested in this as well,i also didnt go into too much depth either i just took it that it was something to do with access lists too,

pls post any findings you come across.
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Re: ipconfig /release_all works on which layer ? 12 years 9 months ago #2686

  • sahirh
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Fire up a sniffer and then use the switches to see what happens.
You'll find the protocol is UDP, the DHCP client communicates from UDP port 68, and the server listens on UDP port 67.

The protocol is technically BOOTP or the bootstrap protocol, of which DHCP is an extension. You can read the RFC's here :

www.ietf.org/rfc/rfc2132.txt?number=2131
www.ietf.org/rfc/rfc2132.txt?number=2132

A quick google for 'DHCP packet format' should give you the whole packet structure, however if you look at the decode in a decent sniffer (I recommend ethereal) then you'll see its all pretty self explanatory.

It doesn't interact with the hardware at all.. its fully layer 4 UDP running over layer3 IP. Most of the client requests are in the form of broadcasts.

Also remember, the routers must be configured as dhcp-relay-agents.. because by default they drop inter subnet DHCP queries.

Hope that helps. Perhaps we'll get round to covering the protocol.. however its one of those nice ones that 'just works' if yknow what I mean ;)
Sahir Hidayatullah.
Firewall.cx Staff - Associate Editor & Security Advisor
tftfotw.blogspot.com
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