Source of IP packet from lan via rtr

[maryodriscoll]

Hi

Can someone confirm that the source ip of an ip packet when communicating with a central dhcp server will be the source ip of workstation?  IP helper-addressing configured on lRouter Lan interface so incoming dhcp request will be forwarded on as a result of helper-address to remote Central DHCP Server but I just want to ensure that the router doesn't remark the IP SRC IP to its own WAN int? Or is the incoming ip packet source ip address amended to router ethernet ip where the helper-address is configured? What happens if there is secondary addressing configured on the same ethernet?

Rgards

Mary

[Giuseppe Larosa · Accepted solution]

Hello Mary,

there is a specific internal field called gatway in DHCP message that allows to identify the correct vlan on which original DHCP request was recevied so the real ip source of packet can be different without causing issues.

the field is filled with router lan interface ip address during ip packet manipulation triggered by ip helper address

http://www.tcpipguide.com/free/t_DHCPMessageFormat.htm

>> What happens if secondary address is configured?

to be investigated..Hope to help

Giuseppe

[maryodriscoll]

Thanks Giuseppe, the link helped

[Jon Marshall · Accepted solution]

maryodriscoll wrote:
Hi
Can someone confirm that the source ip of an ip packet when communicating with a central dhcp server will be the source ip of workstation?  IP helper-addressing configured on lRouter Lan interface so incoming dhcp request will be forwarded on as a result of helper-address to remote Central DHCP Server but I just want to ensure that the router doesn't remark the IP SRC IP to its own WAN int? Or is the incoming ip packet source ip address amended to router ethernet ip where the helper-address is configured? What happens if there is secondary addressing configured on the same ethernet?
Rgards
Mary

Mary

Until the client has been assigned an IP it can't be the client IP because it doesn't have one. The IP address will be the LAN interface IP address of the router ie. the interface where the ip helper-address is configured.

The router will use the primary address because it has no way of knowing that a client without an IP wants one of the secondary addresses.

Jon

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