dhcp breaks when renew ACKs don't contain router and netmask

[Dirbaio]

Had some fun with a customer's Mikrotik HAP AC2 router today...

Its DHCP server includes netmask, router and DNS only in the ACK to the initial request, and not in the ACK to renew requests: pcap, text

This causes DHCP to deconfigure the interface on first renew. Afterwards, it tries to renew with client_ip: 0.0.0.0, which doesn't even seem to hit the wire.

2796 TRACE DHCP send discover to 255.255.255.255:67: Repr { message_type: Discover,
    transaction_id: 2,
    client_hardware_address: Address([2, 173, 57, 2, 48, 41]),
    client_ip: 0.0.0.0,
    your_ip: 0.0.0.0,
    server_ip: 0.0.0.0,
    router: None,
    subnet_mask: None,
    relay_agent_ip: 0.0.0.0,
    broadcast: true,
    requested_ip: None,
    client_identifier: Some(Address([2, 173, 57, 2, 48, 41])),
    server_identifier: None,
    parameter_request_list: None,
    dns_servers: None,
    max_size: Some(900) }  smoltcp::dhcp::clientv4::{{impl}}::egress @ /home/dirbaio/akiles/smoltcp/src/macros.rs:10
3037 DEBUG DHCP recv Offer from 172.29.47.1 (172.29.47.1): Repr { message_type: Offer,
    transaction_id: 2,
    client_hardware_address: Address([2, 173, 57, 2, 48, 41]),
    client_ip: 0.0.0.0,
    your_ip: 192.168.1.56,
    server_ip: 172.29.47.1,
    router: None,
    subnet_mask: None,
    relay_agent_ip: 0.0.0.0,
    broadcast: true,
    requested_ip: None,
    client_identifier: None,
    server_identifier: Some(172.29.47.1),
    parameter_request_list: None,
    dns_servers: None,
    max_size: None }  smoltcp::dhcp::clientv4::{{impl}}::ingress @ /home/dirbaio/akiles/smoltcp/src/macros.rs:11
3047 TRACE DHCP send request to 255.255.255.255:67 = Repr { message_type: Request,
    transaction_id: 3,
    client_hardware_address: Address([2, 173, 57, 2, 48, 41]),
    client_ip: 0.0.0.0,
    your_ip: 0.0.0.0,
    server_ip: 0.0.0.0,
    router: None,
    subnet_mask: None,
    relay_agent_ip: 0.0.0.0,
    broadcast: false,
    requested_ip: Some(192.168.1.56),
    client_identifier: Some(Address([2, 173, 57, 2, 48, 41])),
    server_identifier: Some(172.29.47.1),
    parameter_request_list: Some([1, 3, 6]),
    dns_servers: None,
    max_size: Some(900) }  smoltcp::dhcp::clientv4::{{impl}}::egress @ /home/dirbaio/akiles/smoltcp/src/macros.rs:10
3292 DEBUG DHCP recv Ack from 172.29.47.1 (172.29.47.1): Repr { message_type: Ack,
    transaction_id: 3,
    client_hardware_address: Address([2, 173, 57, 2, 48, 41]),
    client_ip: 0.0.0.0,
    your_ip: 192.168.1.56,
    server_ip: 172.29.47.1,
    router: Some(192.168.1.1),
    subnet_mask: Some(255.255.255.0),
    relay_agent_ip: 0.0.0.0,
    broadcast: false,
    requested_ip: None,
    client_identifier: None,
    server_identifier: Some(172.29.47.1),
    parameter_request_list: None,
    dns_servers: Some([Some(193.22.119.22), Some(217.18.237.74), Some(8.8.8.8)]),
    max_size: None }  smoltcp::dhcp::clientv4::{{impl}}::ingress @ /home/dirbaio/akiles/smoltcp/src/macros.rs:11
3306 INFO  IPv4 address: 0.0.0.0/32 -> 192.168.1.56/24  embassy_net::stack::{{impl}}::poll_configurator::{{closure}} @ /home/dirbaio/.cargo/git/checkouts/embassy-net-5fb2bb5f0f16ef02/9bee576/embassy-net/src/stack.rs:85
3308 INFO  IPv4 gateway: None -> Some(192.168.1.1)  embassy_net::stack::{{impl}}::poll_configurator::{{closure}} @ /home/dirbaio/.cargo/git/checkouts/embassy-net-5fb2bb5f0f16ef02/9bee576/embassy-net/src/stack.rs:96

....

1969306 TRACE DHCP send renew to 172.29.47.1:67: Repr { message_type: Request,
    transaction_id: 4,
    client_hardware_address: Address([2, 173, 57, 2, 48, 41]),
    client_ip: 192.168.1.56,
    your_ip: 0.0.0.0,
    server_ip: 0.0.0.0,
    router: None,
    subnet_mask: None,
    relay_agent_ip: 0.0.0.0,
    broadcast: false,
    requested_ip: None,
    client_identifier: Some(Address([2, 173, 57, 2, 48, 41])),
    server_identifier: None,
    parameter_request_list: None,
    dns_servers: None,
    max_size: Some(900) }  smoltcp::dhcp::clientv4::{{impl}}::egress @ /home/dirbaio/akiles/smoltcp/src/macros.rs:10
1969546 DEBUG DHCP recv Ack from 172.29.47.1 (172.29.47.1): Repr { message_type: Ack,
    transaction_id: 4,
    client_hardware_address: Address([2, 173, 57, 2, 48, 41]),
    client_ip: 192.168.1.56,
    your_ip: 192.168.1.56,
    server_ip: 172.29.47.1,
    router: None,
    subnet_mask: None,
    relay_agent_ip: 0.0.0.0,
    broadcast: false,
    requested_ip: None,
    client_identifier: None,
    server_identifier: Some(172.29.47.1),
    parameter_request_list: None,
    dns_servers: None,
    max_size: None }  smoltcp::dhcp::clientv4::{{impl}}::ingress @ /home/dirbaio/akiles/smoltcp/src/macros.rs:11
1969556 INFO  IPv4 address: 192.168.1.56/24 -> 0.0.0.0/32  embassy_net::stack::{{impl}}::poll_configurator::{{closure}} @ /home/dirbaio/.cargo/git/checkouts/embassy-net-5fb2bb5f0f16ef02/9bee576/embassy-net/src/stack.rs:85
1969559 INFO  IPv4 gateway: Some(192.168.1.1) -> None  embassy_net::stack::{{impl}}::poll_configurator::{{closure}} @ /home/dirbaio/.cargo/git/checkouts/embassy-net-5fb2bb5f0f16ef02/9bee576/embassy-net/src/stack.rs:96

...

3935614 TRACE DHCP send renew to 172.29.47.1:67: Repr { message_type: Request,
    transaction_id: 5,
    client_hardware_address: Address([2, 173, 57, 2, 48, 41]),
    client_ip: 0.0.0.0,
    your_ip: 0.0.0.0,
    server_ip: 0.0.0.0,
    router: None,
    subnet_mask: None,
    relay_agent_ip: 0.0.0.0,
    broadcast: false,
    requested_ip: None,
    client_identifier: Some(Address([2, 173, 57, 2, 48, 41])),
    server_identifier: None,
    parameter_request_list: None,
    dns_servers: None,
    max_size: Some(900) }  smoltcp::dhcp::clientv4::{{impl}}::egress @ /home/dirbaio/akiles/smoltcp/src/macros.rs:10