Which DHCPOFFER the client accept when there are two dhcp servers on the same physical network?

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP











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I am not completely oblivious when it comes to networks but here is something I can't wrap my head around.



Imagine we have a network like in the picture. 6 hosts on one layer 2 network, no VLANs. The network is supposed to be segmented into 2 subnets, with one DHCP server each. The DHCP servers have fixed IP addresses, so they know in which subnet they belong, obviously.



Then new clients get plugged in. They know nothing about which subnet they're supposed to be in and send their DHCPDISCOVER to the ethernet broadcast 255.255.255.255, so it goes to both DHCP servers. Both servers reply with an offer. Now here's my question: How does the client know, which DHCPOFFER it is supposed to accept?



DHCP situation










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  • Compare this question and answers there.
    – Kamil Maciorowski
    11 hours ago










  • Here is another related question.
    – kasperd
    8 hours ago










  • "the ethernet broadcast 255.255.255.255" -- That's the IP broadcast address for the local network, not an Ethernet address. The initial DHCP DISCOVER messages also are very likely to use the Ethernet broadcast address ff:ff:ff:ff:ff:ff, but those really aren't the same thing.
    – ilkkachu
    46 mins ago














up vote
10
down vote

favorite
1












I am not completely oblivious when it comes to networks but here is something I can't wrap my head around.



Imagine we have a network like in the picture. 6 hosts on one layer 2 network, no VLANs. The network is supposed to be segmented into 2 subnets, with one DHCP server each. The DHCP servers have fixed IP addresses, so they know in which subnet they belong, obviously.



Then new clients get plugged in. They know nothing about which subnet they're supposed to be in and send their DHCPDISCOVER to the ethernet broadcast 255.255.255.255, so it goes to both DHCP servers. Both servers reply with an offer. Now here's my question: How does the client know, which DHCPOFFER it is supposed to accept?



DHCP situation










share|improve this question























  • Compare this question and answers there.
    – Kamil Maciorowski
    11 hours ago










  • Here is another related question.
    – kasperd
    8 hours ago










  • "the ethernet broadcast 255.255.255.255" -- That's the IP broadcast address for the local network, not an Ethernet address. The initial DHCP DISCOVER messages also are very likely to use the Ethernet broadcast address ff:ff:ff:ff:ff:ff, but those really aren't the same thing.
    – ilkkachu
    46 mins ago












up vote
10
down vote

favorite
1









up vote
10
down vote

favorite
1






1





I am not completely oblivious when it comes to networks but here is something I can't wrap my head around.



Imagine we have a network like in the picture. 6 hosts on one layer 2 network, no VLANs. The network is supposed to be segmented into 2 subnets, with one DHCP server each. The DHCP servers have fixed IP addresses, so they know in which subnet they belong, obviously.



Then new clients get plugged in. They know nothing about which subnet they're supposed to be in and send their DHCPDISCOVER to the ethernet broadcast 255.255.255.255, so it goes to both DHCP servers. Both servers reply with an offer. Now here's my question: How does the client know, which DHCPOFFER it is supposed to accept?



DHCP situation










share|improve this question















I am not completely oblivious when it comes to networks but here is something I can't wrap my head around.



Imagine we have a network like in the picture. 6 hosts on one layer 2 network, no VLANs. The network is supposed to be segmented into 2 subnets, with one DHCP server each. The DHCP servers have fixed IP addresses, so they know in which subnet they belong, obviously.



Then new clients get plugged in. They know nothing about which subnet they're supposed to be in and send their DHCPDISCOVER to the ethernet broadcast 255.255.255.255, so it goes to both DHCP servers. Both servers reply with an offer. Now here's my question: How does the client know, which DHCPOFFER it is supposed to accept?



DHCP situation







networking dhcp






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share|improve this question













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share|improve this question








edited 33 mins ago









Braiam

3,99631850




3,99631850










asked 14 hours ago









Michael Niemand

1607




1607











  • Compare this question and answers there.
    – Kamil Maciorowski
    11 hours ago










  • Here is another related question.
    – kasperd
    8 hours ago










  • "the ethernet broadcast 255.255.255.255" -- That's the IP broadcast address for the local network, not an Ethernet address. The initial DHCP DISCOVER messages also are very likely to use the Ethernet broadcast address ff:ff:ff:ff:ff:ff, but those really aren't the same thing.
    – ilkkachu
    46 mins ago
















  • Compare this question and answers there.
    – Kamil Maciorowski
    11 hours ago










  • Here is another related question.
    – kasperd
    8 hours ago










  • "the ethernet broadcast 255.255.255.255" -- That's the IP broadcast address for the local network, not an Ethernet address. The initial DHCP DISCOVER messages also are very likely to use the Ethernet broadcast address ff:ff:ff:ff:ff:ff, but those really aren't the same thing.
    – ilkkachu
    46 mins ago















Compare this question and answers there.
– Kamil Maciorowski
11 hours ago




Compare this question and answers there.
– Kamil Maciorowski
11 hours ago












Here is another related question.
– kasperd
8 hours ago




Here is another related question.
– kasperd
8 hours ago












"the ethernet broadcast 255.255.255.255" -- That's the IP broadcast address for the local network, not an Ethernet address. The initial DHCP DISCOVER messages also are very likely to use the Ethernet broadcast address ff:ff:ff:ff:ff:ff, but those really aren't the same thing.
– ilkkachu
46 mins ago




"the ethernet broadcast 255.255.255.255" -- That's the IP broadcast address for the local network, not an Ethernet address. The initial DHCP DISCOVER messages also are very likely to use the Ethernet broadcast address ff:ff:ff:ff:ff:ff, but those really aren't the same thing.
– ilkkachu
46 mins ago










2 Answers
2






active

oldest

votes

















up vote
17
down vote



accepted










Simplest answer - first come first served.



If you had multiple VLANs and 10.10.10.0/24 was on a different VLAN to 10.10.20.0/24 - the broadcast wouldn't cross VLANs.



If the DHCP Server was on a separate VLAN to the clients, an iphelper on the routing interface between vlans would direct the broadcast onto the correct location.



In your scenario where you have 2 separate networks within the same VLAN (or lack thereof) serving up different subnets - its a race.



DHCP Serves up using the following transactions:



  1. DHCP Discovery (DHCPDISCOVER) - Client Broadcast - "Is there a DHCP
    Server out there?"

  2. DHCP Offer (DHCPOFFER) - Server to Client - "Yeah, I'm here and available!"

  3. DHCP Request (DHCPREQUEST) - Client to Server "Awesome, Can I have an address please?"

  4. DHCP Acknowledgement (DHCPACK) - Server to client "Sure, here's an IP, a mask, a gateway, some DNS/WINS Servers, a Time Server, and all the other stuff configured for your scope"

All of this happens on UDP Ports 67 for the server and 68 for the client.



As soon as Step 2 is reached - the client will stop "listening" to other DHCP Servers responses - its happy dealing with the first Server to give it some attention.



As a side note - there is actually a well known series of DoS (Denial of Service) attacks which abuse this right. An attacker plugs in a device which responds and sends out DHCPOFFER packets and then doesn't send DHCPACK out when asked... over and over and over again. There is also another DoS attack where "fake" DHCP Servers offer out addresses that can't be routed or that conflict with other IPs it's sniffed to mess with networks.






share|improve this answer
















  • 14




    And therefore the short answer to "But then how do I run multiple subnets on a single Layer-2 segment?" is "You don't." (Yes, there are ways, but it's not something you should generally do. One layer-2 domain = one subnet.)
    – grawity
    13 hours ago










  • Thank you guys, that really clicked with me. I always wondered how this would be possible, but it simply isn't. So the take away is: Have a router / layer 3 switch between subnets or segment with VLANs, am I right?
    – Michael Niemand
    13 hours ago






  • 3




    In general, yes, you need either VLANs or physical segmentation. Sharing a L2 domain would be doable only if both of your DHCP servers were restricted to handling "known" clients (e.g. by list of 'static leases' with allowed MAC addresses).
    – grawity
    12 hours ago






  • 2




    I think you could give each DHCP server a whitelist of MAC addresses and control which client gets an address from which server that way.
    – Darren
    10 hours ago










  • @grawity, you can easily run multiple IP subnets on the same layer-2 segment, if the subnets are equal, and you don't care which client gets an address from which subnet. You just have one DHCP server that gives addresses from both blocks, and one router that acts as a gateway for both blocks (with an address in each). Done. Saying just "you don't" is plain wrong.
    – ilkkachu
    44 mins ago

















up vote
4
down vote













The existing answer from @Fazer87 is broadly correct in practice and I recommend upvoting and accepting it. This answer explores a minor detail a little more precisely.




Both DHCP servers may respond with a DHCPOffer message.



A DHCP client may accept them on a "first come, first serve" basis. However, it is not required to take that approach.



RFC2131 specifies:




The client receives one or more DHCPOFFER messages from one or more
servers. The client may choose to wait for multiple responses.
The client chooses one server from which to request configuration
parameters, based on the configuration parameters offered in the
DHCPOFFER messages.




So, if second DHCP server offered a longer IP address reservation, or offered a time-server where the other didn't, or maybe had a custom field that the client had been programmed to prefer, it may accept the second offer.



Typically, a "first come, first served" approach is going to get you the offer that hasn't been through several hops across devices (BOOTP rebroadcasts) so it is a good protocol to follow if you have no reason to care.



I was on one project where a custom device would prefer a DHCPOffer that included a TFTP server where updated firmware could be found.






share|improve this answer




















  • ...or if one server offered an address that the client had already used before and wanted to keep
    – ilkkachu
    42 mins ago










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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
17
down vote



accepted










Simplest answer - first come first served.



If you had multiple VLANs and 10.10.10.0/24 was on a different VLAN to 10.10.20.0/24 - the broadcast wouldn't cross VLANs.



If the DHCP Server was on a separate VLAN to the clients, an iphelper on the routing interface between vlans would direct the broadcast onto the correct location.



In your scenario where you have 2 separate networks within the same VLAN (or lack thereof) serving up different subnets - its a race.



DHCP Serves up using the following transactions:



  1. DHCP Discovery (DHCPDISCOVER) - Client Broadcast - "Is there a DHCP
    Server out there?"

  2. DHCP Offer (DHCPOFFER) - Server to Client - "Yeah, I'm here and available!"

  3. DHCP Request (DHCPREQUEST) - Client to Server "Awesome, Can I have an address please?"

  4. DHCP Acknowledgement (DHCPACK) - Server to client "Sure, here's an IP, a mask, a gateway, some DNS/WINS Servers, a Time Server, and all the other stuff configured for your scope"

All of this happens on UDP Ports 67 for the server and 68 for the client.



As soon as Step 2 is reached - the client will stop "listening" to other DHCP Servers responses - its happy dealing with the first Server to give it some attention.



As a side note - there is actually a well known series of DoS (Denial of Service) attacks which abuse this right. An attacker plugs in a device which responds and sends out DHCPOFFER packets and then doesn't send DHCPACK out when asked... over and over and over again. There is also another DoS attack where "fake" DHCP Servers offer out addresses that can't be routed or that conflict with other IPs it's sniffed to mess with networks.






share|improve this answer
















  • 14




    And therefore the short answer to "But then how do I run multiple subnets on a single Layer-2 segment?" is "You don't." (Yes, there are ways, but it's not something you should generally do. One layer-2 domain = one subnet.)
    – grawity
    13 hours ago










  • Thank you guys, that really clicked with me. I always wondered how this would be possible, but it simply isn't. So the take away is: Have a router / layer 3 switch between subnets or segment with VLANs, am I right?
    – Michael Niemand
    13 hours ago






  • 3




    In general, yes, you need either VLANs or physical segmentation. Sharing a L2 domain would be doable only if both of your DHCP servers were restricted to handling "known" clients (e.g. by list of 'static leases' with allowed MAC addresses).
    – grawity
    12 hours ago






  • 2




    I think you could give each DHCP server a whitelist of MAC addresses and control which client gets an address from which server that way.
    – Darren
    10 hours ago










  • @grawity, you can easily run multiple IP subnets on the same layer-2 segment, if the subnets are equal, and you don't care which client gets an address from which subnet. You just have one DHCP server that gives addresses from both blocks, and one router that acts as a gateway for both blocks (with an address in each). Done. Saying just "you don't" is plain wrong.
    – ilkkachu
    44 mins ago














up vote
17
down vote



accepted










Simplest answer - first come first served.



If you had multiple VLANs and 10.10.10.0/24 was on a different VLAN to 10.10.20.0/24 - the broadcast wouldn't cross VLANs.



If the DHCP Server was on a separate VLAN to the clients, an iphelper on the routing interface between vlans would direct the broadcast onto the correct location.



In your scenario where you have 2 separate networks within the same VLAN (or lack thereof) serving up different subnets - its a race.



DHCP Serves up using the following transactions:



  1. DHCP Discovery (DHCPDISCOVER) - Client Broadcast - "Is there a DHCP
    Server out there?"

  2. DHCP Offer (DHCPOFFER) - Server to Client - "Yeah, I'm here and available!"

  3. DHCP Request (DHCPREQUEST) - Client to Server "Awesome, Can I have an address please?"

  4. DHCP Acknowledgement (DHCPACK) - Server to client "Sure, here's an IP, a mask, a gateway, some DNS/WINS Servers, a Time Server, and all the other stuff configured for your scope"

All of this happens on UDP Ports 67 for the server and 68 for the client.



As soon as Step 2 is reached - the client will stop "listening" to other DHCP Servers responses - its happy dealing with the first Server to give it some attention.



As a side note - there is actually a well known series of DoS (Denial of Service) attacks which abuse this right. An attacker plugs in a device which responds and sends out DHCPOFFER packets and then doesn't send DHCPACK out when asked... over and over and over again. There is also another DoS attack where "fake" DHCP Servers offer out addresses that can't be routed or that conflict with other IPs it's sniffed to mess with networks.






share|improve this answer
















  • 14




    And therefore the short answer to "But then how do I run multiple subnets on a single Layer-2 segment?" is "You don't." (Yes, there are ways, but it's not something you should generally do. One layer-2 domain = one subnet.)
    – grawity
    13 hours ago










  • Thank you guys, that really clicked with me. I always wondered how this would be possible, but it simply isn't. So the take away is: Have a router / layer 3 switch between subnets or segment with VLANs, am I right?
    – Michael Niemand
    13 hours ago






  • 3




    In general, yes, you need either VLANs or physical segmentation. Sharing a L2 domain would be doable only if both of your DHCP servers were restricted to handling "known" clients (e.g. by list of 'static leases' with allowed MAC addresses).
    – grawity
    12 hours ago






  • 2




    I think you could give each DHCP server a whitelist of MAC addresses and control which client gets an address from which server that way.
    – Darren
    10 hours ago










  • @grawity, you can easily run multiple IP subnets on the same layer-2 segment, if the subnets are equal, and you don't care which client gets an address from which subnet. You just have one DHCP server that gives addresses from both blocks, and one router that acts as a gateway for both blocks (with an address in each). Done. Saying just "you don't" is plain wrong.
    – ilkkachu
    44 mins ago












up vote
17
down vote



accepted







up vote
17
down vote



accepted






Simplest answer - first come first served.



If you had multiple VLANs and 10.10.10.0/24 was on a different VLAN to 10.10.20.0/24 - the broadcast wouldn't cross VLANs.



If the DHCP Server was on a separate VLAN to the clients, an iphelper on the routing interface between vlans would direct the broadcast onto the correct location.



In your scenario where you have 2 separate networks within the same VLAN (or lack thereof) serving up different subnets - its a race.



DHCP Serves up using the following transactions:



  1. DHCP Discovery (DHCPDISCOVER) - Client Broadcast - "Is there a DHCP
    Server out there?"

  2. DHCP Offer (DHCPOFFER) - Server to Client - "Yeah, I'm here and available!"

  3. DHCP Request (DHCPREQUEST) - Client to Server "Awesome, Can I have an address please?"

  4. DHCP Acknowledgement (DHCPACK) - Server to client "Sure, here's an IP, a mask, a gateway, some DNS/WINS Servers, a Time Server, and all the other stuff configured for your scope"

All of this happens on UDP Ports 67 for the server and 68 for the client.



As soon as Step 2 is reached - the client will stop "listening" to other DHCP Servers responses - its happy dealing with the first Server to give it some attention.



As a side note - there is actually a well known series of DoS (Denial of Service) attacks which abuse this right. An attacker plugs in a device which responds and sends out DHCPOFFER packets and then doesn't send DHCPACK out when asked... over and over and over again. There is also another DoS attack where "fake" DHCP Servers offer out addresses that can't be routed or that conflict with other IPs it's sniffed to mess with networks.






share|improve this answer












Simplest answer - first come first served.



If you had multiple VLANs and 10.10.10.0/24 was on a different VLAN to 10.10.20.0/24 - the broadcast wouldn't cross VLANs.



If the DHCP Server was on a separate VLAN to the clients, an iphelper on the routing interface between vlans would direct the broadcast onto the correct location.



In your scenario where you have 2 separate networks within the same VLAN (or lack thereof) serving up different subnets - its a race.



DHCP Serves up using the following transactions:



  1. DHCP Discovery (DHCPDISCOVER) - Client Broadcast - "Is there a DHCP
    Server out there?"

  2. DHCP Offer (DHCPOFFER) - Server to Client - "Yeah, I'm here and available!"

  3. DHCP Request (DHCPREQUEST) - Client to Server "Awesome, Can I have an address please?"

  4. DHCP Acknowledgement (DHCPACK) - Server to client "Sure, here's an IP, a mask, a gateway, some DNS/WINS Servers, a Time Server, and all the other stuff configured for your scope"

All of this happens on UDP Ports 67 for the server and 68 for the client.



As soon as Step 2 is reached - the client will stop "listening" to other DHCP Servers responses - its happy dealing with the first Server to give it some attention.



As a side note - there is actually a well known series of DoS (Denial of Service) attacks which abuse this right. An attacker plugs in a device which responds and sends out DHCPOFFER packets and then doesn't send DHCPACK out when asked... over and over and over again. There is also another DoS attack where "fake" DHCP Servers offer out addresses that can't be routed or that conflict with other IPs it's sniffed to mess with networks.







share|improve this answer












share|improve this answer



share|improve this answer










answered 13 hours ago









Fazer87

10k12439




10k12439







  • 14




    And therefore the short answer to "But then how do I run multiple subnets on a single Layer-2 segment?" is "You don't." (Yes, there are ways, but it's not something you should generally do. One layer-2 domain = one subnet.)
    – grawity
    13 hours ago










  • Thank you guys, that really clicked with me. I always wondered how this would be possible, but it simply isn't. So the take away is: Have a router / layer 3 switch between subnets or segment with VLANs, am I right?
    – Michael Niemand
    13 hours ago






  • 3




    In general, yes, you need either VLANs or physical segmentation. Sharing a L2 domain would be doable only if both of your DHCP servers were restricted to handling "known" clients (e.g. by list of 'static leases' with allowed MAC addresses).
    – grawity
    12 hours ago






  • 2




    I think you could give each DHCP server a whitelist of MAC addresses and control which client gets an address from which server that way.
    – Darren
    10 hours ago










  • @grawity, you can easily run multiple IP subnets on the same layer-2 segment, if the subnets are equal, and you don't care which client gets an address from which subnet. You just have one DHCP server that gives addresses from both blocks, and one router that acts as a gateway for both blocks (with an address in each). Done. Saying just "you don't" is plain wrong.
    – ilkkachu
    44 mins ago












  • 14




    And therefore the short answer to "But then how do I run multiple subnets on a single Layer-2 segment?" is "You don't." (Yes, there are ways, but it's not something you should generally do. One layer-2 domain = one subnet.)
    – grawity
    13 hours ago










  • Thank you guys, that really clicked with me. I always wondered how this would be possible, but it simply isn't. So the take away is: Have a router / layer 3 switch between subnets or segment with VLANs, am I right?
    – Michael Niemand
    13 hours ago






  • 3




    In general, yes, you need either VLANs or physical segmentation. Sharing a L2 domain would be doable only if both of your DHCP servers were restricted to handling "known" clients (e.g. by list of 'static leases' with allowed MAC addresses).
    – grawity
    12 hours ago






  • 2




    I think you could give each DHCP server a whitelist of MAC addresses and control which client gets an address from which server that way.
    – Darren
    10 hours ago










  • @grawity, you can easily run multiple IP subnets on the same layer-2 segment, if the subnets are equal, and you don't care which client gets an address from which subnet. You just have one DHCP server that gives addresses from both blocks, and one router that acts as a gateway for both blocks (with an address in each). Done. Saying just "you don't" is plain wrong.
    – ilkkachu
    44 mins ago







14




14




And therefore the short answer to "But then how do I run multiple subnets on a single Layer-2 segment?" is "You don't." (Yes, there are ways, but it's not something you should generally do. One layer-2 domain = one subnet.)
– grawity
13 hours ago




And therefore the short answer to "But then how do I run multiple subnets on a single Layer-2 segment?" is "You don't." (Yes, there are ways, but it's not something you should generally do. One layer-2 domain = one subnet.)
– grawity
13 hours ago












Thank you guys, that really clicked with me. I always wondered how this would be possible, but it simply isn't. So the take away is: Have a router / layer 3 switch between subnets or segment with VLANs, am I right?
– Michael Niemand
13 hours ago




Thank you guys, that really clicked with me. I always wondered how this would be possible, but it simply isn't. So the take away is: Have a router / layer 3 switch between subnets or segment with VLANs, am I right?
– Michael Niemand
13 hours ago




3




3




In general, yes, you need either VLANs or physical segmentation. Sharing a L2 domain would be doable only if both of your DHCP servers were restricted to handling "known" clients (e.g. by list of 'static leases' with allowed MAC addresses).
– grawity
12 hours ago




In general, yes, you need either VLANs or physical segmentation. Sharing a L2 domain would be doable only if both of your DHCP servers were restricted to handling "known" clients (e.g. by list of 'static leases' with allowed MAC addresses).
– grawity
12 hours ago




2




2




I think you could give each DHCP server a whitelist of MAC addresses and control which client gets an address from which server that way.
– Darren
10 hours ago




I think you could give each DHCP server a whitelist of MAC addresses and control which client gets an address from which server that way.
– Darren
10 hours ago












@grawity, you can easily run multiple IP subnets on the same layer-2 segment, if the subnets are equal, and you don't care which client gets an address from which subnet. You just have one DHCP server that gives addresses from both blocks, and one router that acts as a gateway for both blocks (with an address in each). Done. Saying just "you don't" is plain wrong.
– ilkkachu
44 mins ago




@grawity, you can easily run multiple IP subnets on the same layer-2 segment, if the subnets are equal, and you don't care which client gets an address from which subnet. You just have one DHCP server that gives addresses from both blocks, and one router that acts as a gateway for both blocks (with an address in each). Done. Saying just "you don't" is plain wrong.
– ilkkachu
44 mins ago












up vote
4
down vote













The existing answer from @Fazer87 is broadly correct in practice and I recommend upvoting and accepting it. This answer explores a minor detail a little more precisely.




Both DHCP servers may respond with a DHCPOffer message.



A DHCP client may accept them on a "first come, first serve" basis. However, it is not required to take that approach.



RFC2131 specifies:




The client receives one or more DHCPOFFER messages from one or more
servers. The client may choose to wait for multiple responses.
The client chooses one server from which to request configuration
parameters, based on the configuration parameters offered in the
DHCPOFFER messages.




So, if second DHCP server offered a longer IP address reservation, or offered a time-server where the other didn't, or maybe had a custom field that the client had been programmed to prefer, it may accept the second offer.



Typically, a "first come, first served" approach is going to get you the offer that hasn't been through several hops across devices (BOOTP rebroadcasts) so it is a good protocol to follow if you have no reason to care.



I was on one project where a custom device would prefer a DHCPOffer that included a TFTP server where updated firmware could be found.






share|improve this answer




















  • ...or if one server offered an address that the client had already used before and wanted to keep
    – ilkkachu
    42 mins ago














up vote
4
down vote













The existing answer from @Fazer87 is broadly correct in practice and I recommend upvoting and accepting it. This answer explores a minor detail a little more precisely.




Both DHCP servers may respond with a DHCPOffer message.



A DHCP client may accept them on a "first come, first serve" basis. However, it is not required to take that approach.



RFC2131 specifies:




The client receives one or more DHCPOFFER messages from one or more
servers. The client may choose to wait for multiple responses.
The client chooses one server from which to request configuration
parameters, based on the configuration parameters offered in the
DHCPOFFER messages.




So, if second DHCP server offered a longer IP address reservation, or offered a time-server where the other didn't, or maybe had a custom field that the client had been programmed to prefer, it may accept the second offer.



Typically, a "first come, first served" approach is going to get you the offer that hasn't been through several hops across devices (BOOTP rebroadcasts) so it is a good protocol to follow if you have no reason to care.



I was on one project where a custom device would prefer a DHCPOffer that included a TFTP server where updated firmware could be found.






share|improve this answer




















  • ...or if one server offered an address that the client had already used before and wanted to keep
    – ilkkachu
    42 mins ago












up vote
4
down vote










up vote
4
down vote









The existing answer from @Fazer87 is broadly correct in practice and I recommend upvoting and accepting it. This answer explores a minor detail a little more precisely.




Both DHCP servers may respond with a DHCPOffer message.



A DHCP client may accept them on a "first come, first serve" basis. However, it is not required to take that approach.



RFC2131 specifies:




The client receives one or more DHCPOFFER messages from one or more
servers. The client may choose to wait for multiple responses.
The client chooses one server from which to request configuration
parameters, based on the configuration parameters offered in the
DHCPOFFER messages.




So, if second DHCP server offered a longer IP address reservation, or offered a time-server where the other didn't, or maybe had a custom field that the client had been programmed to prefer, it may accept the second offer.



Typically, a "first come, first served" approach is going to get you the offer that hasn't been through several hops across devices (BOOTP rebroadcasts) so it is a good protocol to follow if you have no reason to care.



I was on one project where a custom device would prefer a DHCPOffer that included a TFTP server where updated firmware could be found.






share|improve this answer












The existing answer from @Fazer87 is broadly correct in practice and I recommend upvoting and accepting it. This answer explores a minor detail a little more precisely.




Both DHCP servers may respond with a DHCPOffer message.



A DHCP client may accept them on a "first come, first serve" basis. However, it is not required to take that approach.



RFC2131 specifies:




The client receives one or more DHCPOFFER messages from one or more
servers. The client may choose to wait for multiple responses.
The client chooses one server from which to request configuration
parameters, based on the configuration parameters offered in the
DHCPOFFER messages.




So, if second DHCP server offered a longer IP address reservation, or offered a time-server where the other didn't, or maybe had a custom field that the client had been programmed to prefer, it may accept the second offer.



Typically, a "first come, first served" approach is going to get you the offer that hasn't been through several hops across devices (BOOTP rebroadcasts) so it is a good protocol to follow if you have no reason to care.



I was on one project where a custom device would prefer a DHCPOffer that included a TFTP server where updated firmware could be found.







share|improve this answer












share|improve this answer



share|improve this answer










answered 4 hours ago









Oddthinking

544217




544217











  • ...or if one server offered an address that the client had already used before and wanted to keep
    – ilkkachu
    42 mins ago
















  • ...or if one server offered an address that the client had already used before and wanted to keep
    – ilkkachu
    42 mins ago















...or if one server offered an address that the client had already used before and wanted to keep
– ilkkachu
42 mins ago




...or if one server offered an address that the client had already used before and wanted to keep
– ilkkachu
42 mins ago

















 

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