HLD: DHCPv4 - Specify dhcp relay's Gateway explicitly with Primary address. #1470
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pushpraj
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venkatmahalingam
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prsunny
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Can you please explain more about this problem? DHCP relay is enabled on a given VLAN/Interface and the 'giaddr' is populated using the IP address assigned to that VLAN/interface. So where are the "multiple interfaces" coming from? Are you referring to a scenario whether a given VLAN/interface can have multiple IP addresses (one primary and several secondary)? |
Yes. this is the scenario where one VLAN can have multiple IP addresses. We have updated the design to mark such additional addresses as 'secondary' now.
Yes. |
doc/DHCPv4_Gateway/DHCPv4_gateway.md
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| 1. Support a new member 'secondary' of VLAN_INTERFACE in config_db. | ||
| 2. Support parsing and assignment of subnets from minigraph/json to config_db. | ||
| 3. Support specifying non-secondary interfaces' gateway address to command line arguments to /usr/sbin/dhcrelay as -g. | ||
| 4. isc-dhcp/dhcrelay to support '-g gateway' argument, by porting an existing patch. |
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Hope we handle any on-the-fly primary/secondary flag changes in CONFIG_DB for VLAN interface? i.e restarting the DHCP relay to pickup right primary IP address.
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Add relay address by cli would cause dhcp_relay container to restart https://github.com/sonic-net/sonic-buildimage/blob/c71fb3a30f3d384438f2e4861c80c28aea398fef/dockers/docker-dhcp-relay/cli/config/plugins/dhcp_relay.py#L77
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Community review recording https://zoom.us/rec/share/LiTYydJg8GJCntz_HL1DZk-sFQxMdcAA89Mj35l3zBAUYtLgxNR4XZyq4p9e_xqC.eAJyKMK62ORcaaFZ |
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@venkatmahalingam can you please review this hld, and respective PRs too? thank you. |
This was referenced Nov 22, 2023
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This is change taken as part of the HLD: sonic-net/SONiC#1470 and this is a follow up on the PR #16827 where in the docker-dhcp we pick the value of primary gateway of the interface from the VLAN_Interface table which has "secondary" flag set in the config_db Microsoft ADO (number only): 16784946 How did I do it - Changes in the j2 file to add a new "-pg" parameter in the dhcpv4-relay.agents.j2, the ip would be retrieved from the config db's vlan_interface table such that the interface which are picked will have secondary field set. - Changes in isc-dhcp to re-order the addresses of the discovered interface and which has the ip which has the passed parameter.
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This is change taken as part of the HLD: sonic-net/SONiC#1470. In this PR we add the logic to parse the SecondarySubnets field in the minigraph and add a flag in "secondary" in the vlan_interface table of the config db. Microsoft ADO (number only): 16784946 How I did it Made changes in the minigraph.py to parse the xml entry and add the parsed value to the config db How to verify it Added python tests in the sonic-config-engine folder to test the config db entries.
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This is change taken as part of the HLD: sonic-net/SONiC#1470 and this is a follow up on the PR #16827 where in the docker-dhcp we pick the value of primary gateway of the interface from the VLAN_Interface table which has "secondary" flag set in the config_db Microsoft ADO (number only): 16784946 How did I do it - Changes in the j2 file to add a new "-pg" parameter in the dhcpv4-relay.agents.j2, the ip would be retrieved from the config db's vlan_interface table such that the interface which are picked will have secondary field set. - Changes in isc-dhcp to re-order the addresses of the discovered interface and which has the ip which has the passed parameter.
yxieca
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This is change taken as part of the HLD: sonic-net/SONiC#1470. In this PR we add the logic to parse the SecondarySubnets field in the minigraph and add a flag in "secondary" in the vlan_interface table of the config db. Microsoft ADO (number only): 16784946 How I did it Made changes in the minigraph.py to parse the xml entry and add the parsed value to the config db How to verify it Added python tests in the sonic-config-engine folder to test the config db entries.
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@pushpraj I tried back-porting this feature on 202111. I see that it is working. But I observed one weird behaviour. Issue: During the normal scenario it works fine as expected. But after reboot of the device, dhcp-relay honours the primary address and forwards the dhcp request via the correct subnet / gateway (172.19.5.x). But when it receives the dhcp reply from the server, dhcp-relay forwards the IP back to the client via 1.1.1.x gateway (secondary) in place of 172.19.5.x. LOGS: Are you also seeing similar behaviour? |
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@prsunny @venkatmahalingam can you please help to approve this PR if you are ok? Thanks. |
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@pushpraj I am wondering if it's possible to specify an IP address as the primary address for the DHCP relay service. In the current design, the 'secondary' parameter's meaning is applied under the interface, encompassing the entire interface view. However, this feature is intended only for the DHCP relay, not for the entire interface |
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@prsunny @venkatmahalingam are you ok to merge this PR? Thanks. |
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yes @zhangyanzhao , we can merge this PR |
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This document describes the High Level Design of 'Secondary' interfaces of vlan. These secondary interfaces are also excluded in use in dhcpv4 relay.
A vlan can have more than one subnet assigned to it. Such additional interfaces should be tagged as 'secondary'. Sonic OS should support dhcpv4 over non-secondary interfaces.
Use case: We want to increase the IP addresses range supported by a live switch. These additional IPs are to be used for new scenarios on existing physical server by additional IP addresses.
Challenge: DHCPv4 is affected when sonic switch is configured with multiple interfaces in a vlan. When a dhcpv4 packet is forwarded via dhcprelayagent, it embeds its own gateway address as 'giaddr' for return communication.
In case of multiple interfaces, dhcprelayagent randomizes giaddr among all interfaces.
This behavior is fixed by marking additional subnets as 'secondary' and explicitly specifying the primary interface as gateway for dhcpv4.