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    Windows 2019 DHCP stops serving IPs to LAN clients when OpenVPN site-to-site is on. [solved]

    DHCP and DNS
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    • S
      SuperVertrix
      last edited by johnpoz

      Hello Community,

      I do have a very weird problem that I can't seem to figure out.

      I have two sites connected via OpenVPN shared-key.

      The configuration:
      pfSense ver. on both ends: 2.4.5-RELEASE-p1

      Server/Site: A
      WAN: internet_ip_#
      LAN: 10.10.51.0/24
      OpenVPN Tunnel Network: 10.8.0.0/24
      Windows 2019 DHCP server, subnet range 10.10.51.30 to 10.10.51.240

      Client/Site: B
      WAN: internet_ip_#
      LAN: 10.10.40.0/24
      OpenVPN Tunnel Network: 10.8.0.0/24
      Windows 2019 DHCP server, subnet range 10.10.40.30 to 10.10.40.240

      The problem:
      Whenever I establish the OpenVPN tunnel, all the DHCP computer clients on the LAN side of Site A stops getting IPs from the Windows DHCP server on the same subnet, they try and fail.

      Site B computer clients on the LAN aside are able to obtain their IP's via their Windows DHCP server without a problem.

      I did a packet capture from Site A, LAN side, with tunnel turn on and off:

      Tunnel on: DHCP request is broadcasted, but Windows server just ignores it:

      22:39:35.027192 IP (tos 0x0, ttl 128, id 60430, offset 0, flags [none], proto UDP (17), length 328)
      0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:0c:29:67:bb:61, length 300, xid 0x2d5d54a4, Flags [none]
      Client-Ethernet-Address 00:0c:29:67:bb:61
      Vendor-rfc1048 Extensions
      Magic Cookie 0x63825363
      DHCP-Message Option 53, length 1: Discover
      Client-ID Option 61, length 7: ether 00:0c:29:67:bb:61
      Hostname Option 12, length 12: "WINDOWS10_VM"
      Vendor-Class Option 60, length 8: "MSFT 5.0"
      Parameter-Request Option 55, length 14:
      Subnet-Mask, Default-Gateway, Domain-Name-Server, Domain-Name
      Router-Discovery, Static-Route, Vendor-Option, Netbios-Name-Server
      Netbios-Node, Netbios-Scope, Option 119, Classless-Static-Route
      Classless-Static-Route-Microsoft, Option 252
      22:39:35.115947 IP (tos 0x0, ttl 128, id 60431, offset 0, flags [none], proto UDP (17), length 328)
      0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:0c:29:67:bb:61, length 300, xid 0xe17275e0, Flags [none]
      Client-Ethernet-Address 00:0c:29:67:bb:61
      Vendor-rfc1048 Extensions
      Magic Cookie 0x63825363
      DHCP-Message Option 53, length 1: Discover
      Client-ID Option 61, length 7: ether 00:0c:29:67:bb:61
      Hostname Option 12, length 12: "WINDOWS10_VM"
      Vendor-Class Option 60, length 8: "MSFT 5.0"
      Parameter-Request Option 55, length 14:
      Subnet-Mask, Default-Gateway, Domain-Name-Server, Domain-Name
      Router-Discovery, Static-Route, Vendor-Option, Netbios-Name-Server
      Netbios-Node, Netbios-Scope, Option 119, Classless-Static-Route
      Classless-Static-Route-Microsoft, Option 252
      22:39:39.978436 IP (tos 0x0, ttl 128, id 60432, offset 0, flags [none], proto UDP (17), length 328)
      0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:0c:29:67:bb:61, length 300, xid 0xe17275e0, Flags [none]
      Client-Ethernet-Address 00:0c:29:67:bb:61
      Vendor-rfc1048 Extensions
      Magic Cookie 0x63825363
      DHCP-Message Option 53, length 1: Discover
      Client-ID Option 61, length 7: ether 00:0c:29:67:bb:61
      Hostname Option 12, length 12: "WINDOWS10_VM"
      Vendor-Class Option 60, length 8: "MSFT 5.0"
      Parameter-Request Option 55, length 14:
      Subnet-Mask, Default-Gateway, Domain-Name-Server, Domain-Name
      Router-Discovery, Static-Route, Vendor-Option, Netbios-Name-Server
      Netbios-Node, Netbios-Scope, Option 119, Classless-Static-Route
      Classless-Static-Route-Microsoft, Option 252
      22:39:41.899921 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 169.254.85.78 tell 0.0.0.0, length 46
      22:39:42.899934 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 169.254.85.78 tell 0.0.0.0, length 46
      22:39:43.899924 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has 169.254.85.78 tell 0.0.0.0, length 46
      22:39:44.947106 IP (tos 0x0, ttl 128, id 60433, offset 0, flags [none], proto UDP (17), length 328)
      0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:0c:29:67:bb:61, length 300, xid 0xe17275e0, secs 1024, Flags [none]
      Client-Ethernet-Address 00:0c:29:67:bb:61
      Vendor-rfc1048 Extensions
      Magic Cookie 0x63825363
      DHCP-Message Option 53, length 1: Discover
      Client-ID Option 61, length 7: ether 00:0c:29:67:bb:61
      Hostname Option 12, length 12: "WINDOWS10_VM"
      Vendor-Class Option 60, length 8: "MSFT 5.0"
      Parameter-Request Option 55, length 14:
      Subnet-Mask, Default-Gateway, Domain-Name-Server, Domain-Name
      Router-Discovery, Static-Route, Vendor-Option, Netbios-Name-Server
      Netbios-Node, Netbios-Scope, Option 119, Classless-Static-Route
      Classless-Static-Route-Microsoft, Option 252

      Tunnel off: DHCP request is broadcasted, but Windows server replies to request with IP #:

      23:01:13.697351 IP (tos 0x0, ttl 128, id 3812, offset 0, flags [none], proto UDP (17), length 344)
      0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 00:0c:29:67:bb:61, length 316, xid 0xa0f0de30, Flags [none]
      Client-Ethernet-Address 00:0c:29:67:bb:61
      Vendor-rfc1048 Extensions
      Magic Cookie 0x63825363
      DHCP-Message Option 53, length 1: Request
      Client-ID Option 61, length 7: ether 00:0c:29:67:bb:61
      Requested-IP Option 50, length 4: 10.10.51.128
      Hostname Option 12, length 12: "WINDOWS10_VM"
      FQDN Option 81, length 15: "WINDOWS10_VM"
      Vendor-Class Option 60, length 8: "MSFT 5.0"
      Parameter-Request Option 55, length 14:
      Subnet-Mask, Default-Gateway, Domain-Name-Server, Domain-Name
      Router-Discovery, Static-Route, Vendor-Option, Netbios-Name-Server
      Netbios-Node, Netbios-Scope, Option 119, Classless-Static-Route
      Classless-Static-Route-Microsoft, Option 252
      23:01:13.698136 IP (tos 0x0, ttl 128, id 8959, offset 0, flags [none], proto UDP (17), length 328)
      10.10.51.5.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 300, xid 0xa0f0de30, Flags [none]
      Your-IP 10.10.51.128
      Client-Ethernet-Address 00:0c:29:67:bb:61
      Vendor-rfc1048 Extensions
      Magic Cookie 0x63825363
      DHCP-Message Option 53, length 1: ACK
      RN Option 58, length 4: 345600
      RB Option 59, length 4: 604800
      Lease-Time Option 51, length 4: 691200
      Server-ID Option 54, length 4: 10.10.51.5
      Subnet-Mask Option 1, length 4: 255.255.255.0
      Default-Gateway Option 3, length 4: 10.10.51.1
      Domain-Name-Server Option 6, length 4: 10.10.51.5
      Domain-Name Option 15, length 7: "my_domain_name.ad^@"

      Any idea why psfense OpenVPN running on a site-to-site configuration would be causing this?

      I'be been searching throught and I can't seem to find anybody else having this problem.

      Please help!

      Sincerely,
      SuperVertrix.

      S 1 Reply Last reply Reply Quote 0
      • S
        SteveITS Galactic Empire @SuperVertrix
        last edited by

        Just a guess, is the Windows server logging that it sees another DHCP server on the network and will stop processing DHCP requests? (it's an event, don't recall the event ID or exact wording)

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        S 1 Reply Last reply Reply Quote 1
        • S
          SuperVertrix @SteveITS
          last edited by

          @teamits

          Thank you, Steve, you somewhat pointed me in the right direction.
          The location of the events are located here:
          Applications and Services Logs > Microsoft > Windows > DHCP-Server

          I checked and I couldn't find the culprit, but there's a DHCP server log file here:
          %windir%\System32\Dhcp

          The log file revealed the following error while a client was asking for an IP and the OpenVPN tunnel was on:
          "Packet dropped because of Client ID hash has mismatch or standby server"

          Upon further search, it turns out that the Active Directory DHCP server on Site A was trying to contact an AD DHCP server on Site B that once was part of the AD Domain of Site A, but since was removed from, but some more cleanup was necessary on AD Site A to make it stop trying to contact that orphaned AD Domain Controller. The issue has been resolved, all clients are able to obtain IP #'s from Site A.

          Thanks so much for the hint.

          Stay safe my friends.

          SuperVertrix.

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