DHCP server serves client with expected IP address and network, but unexpected gateway ("option routers")

Issue

• When requesting an IP address through DHCP, the DHCP client gets an IP address in expected subnet but the gateway from another subnet definition

  • Excerpt of the DHCP server configuration (/etc/dhcp/dhcpd.conf)

    subnet 10.0.1.0 netmask 255.255.255.0 {
        option subnet-mask 255.255.255.0;
        option routers 10.0.1.1;
        range 10.0.1.100 10.0.1.200;
        class "pxeclients" {
            match if substring(option vendor-class-identifier, 0,9) = "PXEClient";
            next-server 10.0.0.254;
            filename "pxelinux.0";
        }
    }
    
    subnet 10.0.99.0 netmask 255.255.255.0 {
        option subnet-mask 255.255.255.0;
        option routers 10.0.99.1;
        range 10.0.99.100 10.0.99.200;
        class "pxeclients" {
            match if substring(option vendor-class-identifier, 0,9) = "PXEClient";
            next-server 10.0.0.254;
            filename "pxelinux.0";
        }
    }
    

  With this configuration, DHCP clients in network 10.0.99.0/24 are supposed to get 10.0.99.1 as the default gateway.

  • DHCP client in network 10.0.99.0/24 booting with iPXE or PXE shows the following configuration (example, could be another output)

    Configuring (net1 52:54:00:34:c6:5a) ............. ok
    net1: 10.0.99.100/255.255.255.0 gw 10.0.1.1
    [...]
    

  Here we can see that the DHCP client in network 10.0.99.0/24 got an expected IP address, but wrong gateway (it got the gateway from another subnet).

  • Network capture on the DHCP server using tshark shows the following DHCP reply

    Frame 5: 342 bytes on wire (2736 bits), 342 bytes captured (2736 bits) on interface 0
    [...]
    Bootstrap Protocol
        Message type: Boot Reply (2)
        [...]
        Your (client) IP address: 10.0.99.100 (10.0.99.100)
        Next server IP address: 10.0.0.254 (10.0.0.254)
        Relay agent IP address: 0.0.0.0 (0.0.0.0)
        [...]
        Option: (3) Router
            Length: 4
            Router: 10.0.1.1 (10.0.1.1)
        [...]
    

• DHCP clients requesting an IP address through using the dhclient utility get the expected gateway