#101961 - 2003-06-06 09:45 PM
Re: How to re-discover DHCP server
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Shawn
Administrator
Registered: 1999-08-13
Posts: 8611
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Not really changed. When i said "set to 255.255.255.255" i didn't mean broadcast that mask, I meant like in ipconfig /all example:
code:
Ethernet adapter El90x1:
Description . . . . . . . . : 3Com 3C90x Ethernet Adapter
DHCP Enabled. . . . . . . . : Yes
IP Address. . . . . . . . . : 0.0.0.0
Subnet Mask . . . . . . . . : 0.0.0.0
Default Gateway . . . . . . :
DHCP Server . . . . . . . . : 255.255.255.255
Q. How does one dynamically get the DHCP server back in business.
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#101964 - 2003-06-06 10:29 PM
Re: How to re-discover DHCP server
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Howard Bullock
KiX Supporter
Registered: 2000-09-15
Posts: 5809
Loc: Harrisburg, PA USA
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If you have a client on a standard LAN and need to perform those tasks you would use ipconfig.
code:
c:\data\scripts>ipconfig /?
Windows 2000 IP Configuration
USAGE:
ipconfig [/? | /all | /release [adapter] | /renew [adapter]
| /flushdns | /registerdns
| /showclassid adapter
| /setclassid adapter [classidtoset] ]
adapter Full name or pattern with '*' and '?' to 'match',
* matches any character, ? matches one character.
Options
/? Display this help message.
/all Display full configuration information.
/release Release the IP address for the specified adapter.
/renew Renew the IP address for the specified adapter.
/flushdns Purges the DNS Resolver cache.
/registerdns Refreshes all DHCP leases and re-registers DNS names
/displaydns Display the contents of the DNS Resolver Cache.
/showclassid Displays all the dhcp class IDs allowed for adapter.
/setclassid Modifies the dhcp class id.
The default is to display only the IP address, subnet mask and
default gateway for each adapter bound to TCP/IP.
For Release and Renew, if no adapter name is specified, then the IP address
leases for all adapters bound to TCP/IP will be released or renewed.
For SetClassID, if no class id is specified, then the classid is removed.
Examples:
> ipconfig ... Show information.
> ipconfig /all ... Show detailed information
> ipconfig /renew ... renew all adapaters
> ipconfig /renew EL* ... renew adapters named EL....
> ipconfig /release *ELINK?21* ... release all matching adapters,
eg. ELINK-21, myELELINKi21adapter.
VPN complicates things a somewhat.
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#101966 - 2003-06-07 07:51 PM
Re: How to re-discover DHCP server
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Shawn
Administrator
Registered: 1999-08-13
Posts: 8611
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Erik,
We went through a prototype phase for these "home worker" machines. With two variants:
1) One wkstn used a router to connect to DSL modem
2) One wkstn used a direct connect to DSL modem and PPP software driver.
They "the customer" along with some of "our" people (not me) made the descision to not use the router ... why ? I have not idea why. It also meant that we had to install a software firewall to protect the machine.
The root of the problem is that until the machine is connected to VPN, and because its still set to DHCP (which is how these machine are set in the office) ... the wkstn acts REALLY, REALLY sluggish during bootup, and during the initial login. In fact, because the bootup is so slow, things actually break (ie, startup tasks and services not starting properly) not all the time, but alot of times.
Once the machine is logged into VPN (after the second user logon for proper domain login &scripts) - the wkstn seems to pick-up steam.
The long part of this story is that we have tons of custom software on the wkstn (written by us) that is VERY network sensitive, therefore the hesitation to change anything. But, because we dont have a router serving-up IPs to the wkstn, I think I HAVE to convince the "customer" and "our guys" that going static IP is the only way to fix this.
In fact, I going to setup one of these wkstns on Monday and I am going to "slip-in" the static ip assignment and not tell anyone - if it breaks something, can always change back. If it works, we're laughing !
-Shawn
[ 07. June 2003, 19:52: Message edited by: Shawn ]
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#101967 - 2003-06-07 08:48 PM
Re: How to re-discover DHCP server
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Raceeend
Starting to like KiXtart
Registered: 2002-05-09
Posts: 129
Loc: The Netherlands
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When i work at home i also use VPN to connect it's over a cable connections but that can't that different. In my case the Cisco vpn concentrator acts as DHCP server but it is also possible to let the normal NT dhcp server give out ip addresses, then on the vpn concentrator DHCP forwarding must be activated. This option i do use for dail-up connections which works fine.
VPN uses UDP port 500 so your users be able to go through that port on there home adsl modem and laptop.
_________________________
regards,
Martijn
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#101968 - 2003-06-07 10:38 PM
Re: How to re-discover DHCP server
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Howard Bullock
KiX Supporter
Registered: 2000-09-15
Posts: 5809
Loc: Harrisburg, PA USA
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At home I have a D-Link Router/Gateway with DHCP server. I boot up and get an IP from the D-Link. I then VPN to a Cisco VPN concentrator which gives me another DHCP address. The local client is 192.168.0.102 then that gets nat'ed and the Cisco gives my VPN tunnel another IP that is internal to our corproate network.
[ 08. June 2003, 01:46: Message edited by: Howard Bullock ]
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#101970 - 2003-06-09 09:52 AM
Re: How to re-discover DHCP server
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Richard H.
Administrator
Registered: 2000-01-24
Posts: 4946
Loc: Leatherhead, Surrey, UK
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Your PC will broadcast for an IP address. That is the way DHCP works. If it doesn't get a response, it doesn't get an address.
This means that you must have something on it's LAN that will give a DHCP address.
All is not lost though ![[Smile]](images/icons/smile.gif)
What you need to do is configure the router to proxy the request for you. On IOS (Cisco) this parameter you are looking for is "ip helper-address" and is defined on the LAN interface.
Set this to the IP address of your DHCP server, and away you go - don't forget to define a DHCP range that matches the LAN interface of the router.
Of course, the router needs to be able to talk to your DHCP server, so if the VPN tunnel is created on demand it needs to be done quickly enough so that the client doesn't time-out.
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#101974 - 2003-06-09 04:57 PM
Re: How to re-discover DHCP server
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Richard H.
Administrator
Registered: 2000-01-24
Posts: 4946
Loc: Leatherhead, Surrey, UK
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If you have a DHCP address your machine will periodically attempt to renew the address from the server it was leased from.
The renewal happens when the DHCP client starts, or at half the lease period. IIRC it will contact the server directly, so a client with a reasonable lease time (say 30 days) should never actually lose it's lease as each time it is renewed it will get another 30 days.
You will only get problems if your lease is revoked or expired. This can happen when:
- It is manually revoked on the server
- It is manually released by the client (ipconfig /release)
- The DHCP server is not contactable when the client tries top renew it at the lease half-life, or the client is switched off.
Once the lease is gone (or if this is a client which has no lease) you will get problems.
This is because the DHCP request is a local broadcast - until it gets the DHCP information the client will not have any routing/gateway information, and will not in any case know what the IP addres of the DHCP server is.
As it is a local broadcast it will not be propogated across the WAN - unless you are operating in a bridged mode.
Your router/concentrator will see the broadcast, and relay or proxy the request directly to the DHCP server. It will add information about the LAN address that the request was relayed from, which will allow the DHCP server to identify which address pool to allocate the IP address from. The response packet is send back to the router, which directs it to the client.
Imagine trying to get some advice about a leak from a plumber at the other end of the 'phone. You cannot use the phone because you are too far away from it and are anyway holding on to the leaky pipe. Your long suffering partner (who told you to call a professional) relays the information across the 'phone.
No matter how loud or how frequently you shout you will never hear an answer - you need the relay to do it for you.
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