Setting up a DHCP relay agent
Hello,
I'm trying to setup a relay agent for an XP client to obtain configuration through 2 routers on a VM LAB
I have 3 Segments/subnets 1,2 and 3
the topology is the following:
1- server 2008 R2 AD DS DC on subnet 1 (192.168.1.0) and a DHCP server with 2 scopes
Internal 192.168.1.0 (subnet1)
external 192.168.3.0 (subnet 3)
2- server 2008 R2 with RRAS installed
Interface 1 pointing to the internal subnet1 192.168.1.0
Interface 2 pointing to subnet2 192.168.2.0
3- server 2003 with RRAS installed
Interface 1 pointing to subnet2 192.168.2.0
Interface 2 pointing to subnet3 192.168.3.0
relay agent installed on Interface 2
(servers/Routers 2 and 3 running RIP v2)
4- XP client on subnet3 (192.168.3.0) and the client trying to obtain config.
The XP client is unable to contact the DHCP to obtain config.
server 2003 relay agent receiving requests with no replies.
How do i get this to work?
Hi,
First, let’s see how DHCP relay agent works:
1. The DHCP client broadcasts a DHCPDISCOVER packet.
2. The DHCP relay agent on the client’s subnet forwards the DHCPDISCOVER message to the DHCP server by using unicast.
3. The DHCP server uses unicast to send a DHCPOFFER message to the DHCP relay agent.
4. The DHCP relay agent broadcasts the DHCPOFFER packet to the DHCP client’s subnet.
5. The DHCP client broadcasts a DHCPREQUEST packet.
6. The DHCP relay agent on the client’s subnet forwards the DHCPREQUEST message to the DHCP server by using unicast.
7. The DHCP server uses unicast to send a DHCPACK message to the DHCP relay agent.
8. The DHCP relay agent broadcasts the DHCPACK to the DHCP client’s subnet.
We can see in the second step, DHCP relay agent send unicast to DHCP server after receiving DHCP request. So confirm unicast communication between DHCP server
and DHCP relay agent works fine. At least ping should be working. You can use the following commend to add the route entry.
Add a static IP route
http://technet.microsoft.com/en-us/library/cc757323(v=ws.10).aspx
The result should be based on your test. If it doesn’t work it just indicates that we cannot configure another DHCP relay agent behind a relay agent.
Hope this helps.
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A DHCP relay agent is any host that forwards DHCP packets between clients and servers. Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams are switched between networks somewhat transparently. By contrast, relay agents receive DHCP messages and then generate a new DHCP message to send out on another interface. The relay agent sets the gateway address (giaddr field of the DHCP packet) and, if configured, adds the relay agent information option in the packet and forwards it to the DHCP server. The reply from the server is forwarded back to the client after removing option .
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Windows DHCP Server and Linux DHCP Relay Agent
We are trying to organize a VLAN (say VLAN 1) for guests who must be assigned IP addresses from a DHCP server in a different VLAN (VLAN 2). This DHCP server is configured with two scopes - 172.16.0.0/24 (for VLAN 2) and 172.16.4.0/24 (for the Guests
VLAN 1). The DHCP server successfully distributes addresses to clients in its VLAN (it has the IP address 172.16.0.2). For the clients in the other VLAN a DHCP Relay Agent has been setup on the router. It is DHCRELAY running on Linux (CentOS) which has
been configured to accept the DHCPDISCOVER broadcasts coming on the VLAN1 interface of the router and forward these to the DHCP server. The IP address of the VLAN1 interface of the router is 172.16.4.254 and on the VLAN2 interface - 172.16.0.254
The problem is that the DHCP server won't respond with a DHCPOFFER message to the relay agent. I have traced the frames on the router and on the DHCP server. They arrive on the DHCP server with the correct GIADDR of the relay agent. According to all documentation,
if a scope has been configured on the DHCP server and it receives a unicast message with the GIADDR set by a relay agent that matches one of the configured scopes, the DHCP server must send a unicast DHCPOFFER to the relay agent. But it doesn't.
Here is what Wireshark reports (ignore the Destination port unreachable messages, the DHCP service was stopped at the time Wireshark was running)
When the service is running, there are just DHCPDISCOVERs - no OFFER. You can see that the server has the two scopes configured:
The relay agent seems to work normally - it forwards the DHCPDISCOVERs to the server continuously (tried many times with ipconfig /renew on the client).
I read many posts about this problem. Some users had other services running on the DHCP server that used the DHCP port, but I don't have such an issue (you see that when the service is stopped, an ICMP port unreachable is sent which is correct). Others however
did not find a solution. Am I missing something? Is there something specific when using the DHCRELAY agent from DHCPD? Can I turn on some verbose logging to track this down? Thanks in advance.WIth DHCP, there is really nothing to configure. If the Relay Agent/IP Helper is pointing to it, and the VLAN subnet exactly matches the scope subnet, then it should just work.
What I've seen in the VLAN config is either a static route back to the subnet the DHCP server itself is sitting on is not configured or incorrectly configured, or there are ports blocked (need UDP, too, since that's what DHCP uses to pass the OFFER), and
other necessary ports are opened, then it should just work.
Sometimes NIC teaming on the DHCP server will cause it. Not sure. Microsoft doesn't support teaming prior to Windwos 2012, but it doesn't mean that it doesn't work. Don't get me wrong, teaming works nicely, but they just don't support it because they never
certified the drivers, that's all.
The issues I've seen with DHCP relays and VLANs in the forums are usually based on misconfigs in the VLAN or ports blocked. Sometimes we'll refer to call Microsoft Support for specific, hands-on assistance. And searching the threads, from what
I've found that if they did call support, they've never posted back what the problem was based on or the resolution. I can post a couple of them for you to read through, but there were never any response with the actual resolution.
If you like, you also have the option to contact Microsoft Support. Here's a list of phone numbers if you choose this option:
http://support.microsoft.com/contactus/
Ace Fekay
MVP, MCT, MCITP/EA, MCTS Windows 2008/R2 & Exchange 2007, Exchange 2010 EA, MCSE & MCSA 2003/2000, MCSA Messaging 2003
Microsoft Certified Trainer
Microsoft MVP - Directory Services
Technical Blogs & Videos: http://www.delawarecountycomputerconsulting.com/
This post is provided AS-IS with no warranties or guarantees and confers no rights. -
I have a 1941W and an external DHCP server. Wireless clients should get their IPs the same way when they are wired. I have checked the documentation but no help. Someone share such a document or help on how it can be done.
still no luck
Thanks for the video..lets me check it out.
on the ap:
dot11 syslog
dot11 ssid TEST
vlan 600
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on the router
interface Embedded-Service-Engine0/0
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no ip redirects
no ip unreachables
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ip flow ingress
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Relay Agent Information(option82) :windows server 2012
Hi,
I am trying to integrate a wireless AP which have option82 inbuilt and act as DHCP relay agent ; but I don't know where to find the relay agent information(the hexa decimal value for the AP). If any one know where to find or how can I get this relay agent
information(hexdec value of the AP) will be very help full.Hi,
There might be some misunderstanding.
Option82 is not necessary for DHCP relay. It is used for DHCP policy in windows server 2012. Since this option is
inserted by a DHCP relay agent, the relay agent information should be set on the DHCP relay agent.
The relay agent information which you configure in the windows DHCP server is used to recognize the relay agent. Then DHCP server can
give customized IP address and configuration options using this option.
How to get the relay agent information
may vary from product to product. You should contact your AP vendor or manufacturer.
Besides, the DHCP Relay Agent is compliant with RFC 1542, "Clarifications and Extensions for the Bootstrap Protocol.". You may refer to the link below,
Clarifications and Extensions for the Bootstrap Protocol
http://tools.ietf.org/html/rfc1542#page-13
Notice: The relay agent information which is configured on relay agent may not be
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hexadecimal value.
Hope this helps.
Steven Lee
TechNet Community Support -
Dhcp relay and option 82 on solaris 10
Hi
I cannot seem to find any information on this topic, there are lots of references to it for linux but not solaris.
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Wireshark with captured dhcp discover broadcast packet. Dhcp part:
As You can see - there is no 82 option.
All interfaces are trusted (dhcp snooping).
Should I configure anything else to make it work ?
regards, -
SGE2010P - DHCP Relay - offer packet not reaching host
I have an SGE2010P in L3 mode. I set up 2 vlans, 1 & 5. Vlan 1 has an interface of 10.0.3.252; Vlan 5 is at 10.0.10.9. I have defined the DHCP at 10.0.3.4 (on vlan 1) and enabled DHCP Interface for vlan 5. Put switch port g28 in access mode on vlan 5 and connected client machine. Set up wireshark on both DHCP a server and client. DHCP server is receiving the DHCP discovery packet with the correct relay agent address (10.0.10.9), and it sends back an offer packet to that address, but it is never received by the host machine. What am I missing?
Hello,
DHCP server typically is on subnet of one of the switch IP interface. Thus there si no need to indicate where the server is however you have to indicate from which VLAN DHCP requests are relayed. For example:
VLAN 1 - switch IP 192.168.1.25/24
DHCP server is on VLAN 1 with IP 192.168.1.1/24
Other VLANs:
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Regards,
Aleksandra -
WRVS4400Nv2 DHCP Relay on 2nd VLAN
Hi,
Here's what I'm trying to figure out:
My network is set up such that I have a Wireless Network in VLAN 1, which is the primary network that we use. The subnet is 10.5.1.x.
My goal is to set up a completely isolated Guest Wireless Network, however it would work best. What I am trying to do now is I created a seperate VLAN (VLAN 2, IP range 10.5.2.x) and turned on DHCP on the WRVS4400N. However, in the Guest Network, it is always picking up a 10.5.1.x IP which is handed out by the DHCP server (10.5.1.5, Win 2003) and still routing all of the traffic to/from our private network.
Here's What I have set:
Wireless>Security Settings>Guest Network (SSID 2)
Wireless Isolation (between SSID w/o VLAN): Enabled
Wireless Isolation (within SSID): Enabled
Setup>LAN>VLAN 1
Router IP 10.5.1.1, WLAN IP 10.5.1.3
DHCP Relay for 10.5.1.5
Setup>LAN>VLAN 2
Router IP 10.5.2.1
DHCP Enabled for 10.5.2.x subnet
DHCP Relay option is grayed out (not sure why)
Setup>Advanced Routing
Inter-VLAN Routing: Disabled
Any way to solve this would be fine. I just do not want traffic routing through our internal network. Ideally, if I could get the Windows server to hand out 10.5.2.x addresses, that would be perfect, but I'm not sure how to configure it for such.
If anyone has any ideas, that'd be great- thanks!
MattYes...here's an answer I got from Cisco's Engineering support:
The issue you reported is a know issue.
Engineering and development are aware of this issue, and have provided the following information:
PROBLEM DESCRIPTION:
If the WRVS4400N is configured with multiple VLANs, and these VLANs are mapped to different SSID, the user cannot use an external DHCP server to provide IP scopes for these VLANs.
Hosts connected to both SSID will obtain IP address from native DHCP server only.
The workaround for this is to use the embedded DHCP server for all VLANs defined on the WRVS4400N.
Note: This is not considered a bug but rather a product limitation. The developer has confirmed the WRVS4400N is functioning as designed.
Regarding a fix:
Due to wireless and trunk switch port using different chip set, it is not possible to provide a fix for this issue.
In future product, Engineering & Dev teams will strive to use the same chip set (same vendor).
This functionality has been targeted for next new Product. No fix will be made on the current hardware.
Note: If this feature/function is mission critical to your deployment, and you would like to recover the cost of the WRVS4400N, please forward the serial number and a copy of the proof of purchase, and we will gladly provide a refund.
Best regards,
Alex Delano -
DHCP Relay through another firewall
Hello,
I have set up two ISA Servers in my lab (learning environment).
One faces the internet, the other one is in between the lan and the perimetral network.
Sort of:
LAN <--- ISA2---> DMZ <---ISA1---> INTERNET.
The thing is that the other office connects to ISA1 through a VPN-Site-to-Site, and it goes all fine except that ISA1 cannot take any ip from the dhcp server standing on the lan.
I have read this: http://technet.microsoft.com/en-us/library/cc302680.aspx
But in my lab, there are two isa servers , so I don't know how to send dhcp requests from isa1 to isa2 excepto for broadcast 255.255.255.255 , but the isa1 does not know how to forward that to the dhcp server in the lan.
I am mixed up because I am not an expert and am learning now about this dhcp relay thing now.
Thanks in advance!!
Luis Olías Técnico/Admon Sistemas . Sevilla (España - Spain)Thanks a lot Keith.
I was all wrong. The relation between perimeter and lan (in ISA2) is route. And as far as I have read, that is how it should be, when no external network is involved.
I have a question, I feel doubtful about: When ISA1 takes the DHCP request, does he make a broadcast petition to 255.255.255.255 or a unicast request directly to dhcp-server machine ? , because the ISA1 machine is a DHCP relay so, when I configured it I
was asked to write the ip of the dhcp server, then maybe it just knows where to send the dhcp requests from the client and does not perfom multicast 255.255.255.255 but unicast to the dhcp server ?
EDITION: In the statistics of dhcp-server, there is no dhcp requests, so the traffic is not reaching it.
I am still digging in. This is difficult for me.
Luis Olías Técnico/Admon Sistemas . Sevilla (España - Spain) -
Sonicwall DHCP relay not working
I recently set up a new vlan and am trying to get the dhcp server on the existing vlan to issue IPs. I have no trouble getting dhcp working with the sonicwall as the server, but I can't get it to pass it along to the actual server. I've set up the IP of the server as a trusted DHCP relay host but I don't see anything in the logs for the relay for the test host on the vlan. I tagged the port the server is connected to for the new vlan so I'm lost as to why it isn't working. DNS also isn't working using that server for the lookup. I am able to ping it by IP though. I'm sure it's something simple I'm missing but vlans are new to me so I'm still learning.
This topic first appeared in the Spiceworks CommunityI recently set up a new vlan and am trying to get the dhcp server on the existing vlan to issue IPs. I have no trouble getting dhcp working with the sonicwall as the server, but I can't get it to pass it along to the actual server. I've set up the IP of the server as a trusted DHCP relay host but I don't see anything in the logs for the relay for the test host on the vlan. I tagged the port the server is connected to for the new vlan so I'm lost as to why it isn't working. DNS also isn't working using that server for the lookup. I am able to ping it by IP though. I'm sure it's something simple I'm missing but vlans are new to me so I'm still learning.
This topic first appeared in the Spiceworks Community -
SG300 won't insert option82 during DHCP relay
Hey guys, anyone having trouble getting an SG300 series switch to insert option82 information? I have DHCP relay working successfully between two VLANs, however, I would like to identify where the client is connected in order to set their routing preferences optimally.
switch-20-0#show ip dhcp relay DHCP relay is EnabledOption 82 is EnabledMaximum number of supported VLANs without IP Address is 256Number of DHCP Relays enabled on VLANs without IP Address is 1DHCP relay is not configured on any port.DHCP relay is enabled on Vlans: 20Active: 20Inactive: Servers: 192.168.0.2
I have to missing something simple. Any help, or even "hey did you try" type answers very much appreciated!http://www.cloudshark.org/captures/f1dbc2e0e9a6
I had already done this at some point, but I tried again this morning after a cup of coffee. At some point this started working!
Now I just need Dnsmasq to recognize this info! -
We need to check our network performance for DHCP operation,to cover SLA through CPE(DHCP client)-send DHCPDISCOVER broadcast ,PE router relay agent-send DHCP operation packets to main and backup DHCP servers simultaneously(backup and main servers work standalone,they are communicate with each other and sends reply to DHCPDISCOVER message by DHCPOFFER),server vrrp,server (DHCP server).For this we need to create broadcast message from CPE end using cisco ipsla.Is there any possibility to check this.
Thaks & RegardsCan we configure 12 DHCP pools for LAN users and wireless users in the Cisco WS-3750X-24S-S switch?
I believe you can configure this much DHCP pools.
Can It Handle the DHCP Load of the LAN users and Wireless users together?
It can. DHCP is not CPU intensive. Plus you have lease time.
Besides, it's a small network. Should work. The main thing though is this: Why so many VLANs?
Downside is you don't have much features. You can't trace a MAC address from weeks ago. DHCP profiling is also not supported. -
DHCP Relay forwarded to Secondary when Scope is not available in Primaray
Two ip helper-addresses (let suppose DHCPServer1 and DHCPServer2) are defined on each of the branch router, and customer want to divide the load of DHCP request on two different DHCP Servers. The propose solution by customer is to disable some scope from one DHCP Server (DHCPServer1) and define the similar scope in second DHCP Server (DHCPServer2). Does the DHCP Relay request would be forwarded to secondary server (DHCPServer2), if the scope is disabled on first DHCP Server (DHCPServer1)?
The DHCP request is forwarded to all the addresses defined with the ip helper-address command.
So if you have 2 ip helper-addresses then the DHCP request is sent to both at the same time by the router. First one to respond is usually the one accepted by the client.
Jon -
ASA 5512-X - VPN & local clients DHCP relaying (DHCP Proxy vs. DHCP Relay conflict)
Hey all,
I have ASA-5512-X serving as general firewall/router. It also serves as AnyConnect SSL VPN gateway (webvpn).
It has ~10 VLANs connected over 1 trunk port. One of the VLANs has DHCP server that shall serve all the VLANs (192.168.16.2).
I'm trying to have the ASA relay DHCP requests from all VLANs to the DHCP server and to also serve VPN clients.
However, according to bug https://tools.cisco.com/bugsearch/bug/CSCsd22469 both DHCP Proxy (webvpn) and DHCP Relay (local interfaces) can't be enabled at the same time.
As VPN clients connect to the same VLANs as local users (eg. VLAN 2 - 192.168.2.0/24) I want to have the very same DHCP server serving both, otherwise it's gonna become a mess.
Note: if I configure DHCP Relay functionality and disable DHCP Proxy - local clients are served fine. If I configure DHCP Proxy (webvpn) and disable DHCP Relay VPN clients are served fine. I therefore consider setup to be correct, just the ASA limitation won't allow me to make it serve both.
Can DHCP Relay also serve VPN clients (no DHCP Proxy enabled)? did I miss something?
Thanks!Hi,
The only workaround for this issue is to configure the ASA itself to act as DHCP server for vpn clients. You also have the flexibility of using local pool and AAA server. Why exactly do you want to use the same DHCP server for both?
AM -
Hi,
I've create 2 vlan in a cisco SG500X-24 and a DHCP server on vlan 2. I just want to dhcp server assign ip to devices on vlan 3. I've configured the vlan and dhcp server relay commands.
ip dhcp relay address 192.168.1.11
ip dhcp relay enable
ip dhcp information option
interface vlan 2
ip address 192.168.1.250 255.255.255.0
interface vlan 3
ip address 192.168.51.254 255.255.255.0
ip dhcp relay enable
The dhcp server gets the request from pc, and sends a new address, but the offer packet not comes to device. With Wireshark a see like offer dhcp packet can't jump to vlan 3.
It's the first time a work with SMB series, and this never happens with catalyst. I'm turning crazy.
Anyone can help me? Thank you in advances.
Victor.Hi,
Yes, also I configured ip dhcp relay on intefaces.
Yesterday I found the trouble. I was using the tftpd32 dhcp server, and I tested with a Windows DHCP server and everything works like a champ. I didn't know what have do, and i turned crazy, so I began to change every element on solution, finally the DHCP server that it was the key.
Thank you so much for your answer.
KR!
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