PXE启动报错PXE-99: Unexpected network error的排查求助

大家好，我配置了一台Kea DHCP服务器，用两个不同的网卡分别服务两个网段，具体配置如下：

{
  "Dhcp4": {
    "valid-lifetime": 4000,
    "renew-timer": 1000,
    "rebind-timer": 2000,
    "interfaces-config": {
      "interfaces": [ "eth0", "eth1" ]
    },
    "lease-database": {
      "type": "memfile",
      "persist": true,
      "name": "/var/lib/kea/kea-leases4.csv"
    },
    "subnet4": [
      {
        "subnet": "192.168.8.0/24",
        "pools": [
          {
            "pool": "192.168.8.2 - 192.168.8.200"
          }
        ]
      },
      {
        "subnet": "192.168.9.0/24",
        "pools": [
          {
            "pool": "192.168.9.2 - 192.168.9.200"
          }
        ]
      }
    ],
    "option-data": [
      {
        "name": "domain-name-servers",
        "data": "1.1.1.1,8.8.8.8"
      },
      {
        "name": "routers",
        "data": "192.168.8.1,192.168.9.1"
      }
    ],
    "client-classes": [
      {
        "name": "UEFI-32",
        "test": "substring(option[60].hex,0,20) == 'PXEClient:Arch:00006'",
        "boot-file-name": "ipxe/i386.efi"
      },
      {
        "name": "UEFI-64",
        "test": "substring(option[60].hex,0,20) == 'PXEClient:Arch:00007'",
        "boot-file-name": "ipxe/x86_64.efi"
      }
    ]
  }
}

我执行tcpdump -i eth0 -nn -vvvv抓包，能正常看到DHCP的请求和响应报文：

0.0.0.0.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request from 74:56:3c:83:a0:85, length 359, xid 0xb9760545, Flags [Broadcast] (0x8000)
Client-Ethernet-Address 74:56:3c:83:a0:85
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message (53), length 1: Request
Server-ID (54), length 4: 192.168.8.1
Requested-IP (50), length 4: 192.168.8.2
MSZ (57), length 2: 65280
Parameter-Request (55), length 35:
Subnet-Mask (1), Time-Zone (2), Default-Gateway (3), Time-Server (4)
IEN-Name-Server (5), Domain-Name-Server (6), Hostname (12), BS (13)
Domain-Name (15), RP (17), EP (18), RSZ (22)
TTL (23), BR (28), YD (40), YS (41)
NTP (42), Vendor-Option (43), Requested-IP (50), Lease-Time (51)
Server-ID (54), RN (58), RB (59), Vendor-Class (60)
TFTP (66), BF (67), GUID (97), Unknown (128)
Unknown (129), Unknown (130), Unknown (131), Unknown (132)
Unknown (133), Unknown (134), Unknown (135)
GUID (97), length 17: 0.0.192.165.218.38.183.238.17.128.0.116.86.60.131.160.133
NDI (94), length 3: 1.3.16
ARCH (93), length 2: 7
Vendor-Class (60), length 32: "PXEClient:Arch:00007:UNDI:003016"
END (255), length 0

22:54:47.774286 IP (tos 0x10, ttl 128, id 0, offset 0, flags [DF], proto UDP (17), length 318)
192.168.8.1.67 > 255.255.255.255.68: [udp sum ok] BOOTP/DHCP, Reply, length 290, xid 0xb9760545, Flags [Broadcast] (0x8000)
Your-IP 192.168.8.2
Client-Ethernet-Address 74:56:3c:83:a0:85
file "ipxe/x86_64.efi"
Vendor-rfc1048 Extensions
Magic Cookie 0x63825363
DHCP-Message (53), length 1: ACK
Subnet-Mask (1), length 4: 255.255.255.0
Default-Gateway (3), length 4: 192.168.9.1
Domain-Name-Server (6), length 8: 1.1.1.1,8.8.8.8
Lease-Time (51), length 4: 4000
Server-ID (54), length 4: 192.168.8.1
RN (58), length 4: 1000
RB (59), length 4: 2000
END (255), length 0

但奇怪的是，我完全看不到任何TFTP请求报文。客户端屏幕上会一闪而过显示下面的信息：

Station IP address is 192.168.8.2
Server IP address is 192.168.8.1
NBP filename is ipxe/x86_64.efi
NBP filesize 0 Bytes
PXE-E99: Unexpected network error

请问我到底哪里配置错了？

备注：内容来源于stack exchange，提问作者Keno Fischer