FortiGate DNS failover behavior when using "failover" as server-select-method

Description

How "retry" and "timeout" values impact the DNS failover when using "failover" as the server-select-method.

Scope

FortiGate.

Solution

With the default configuration, the failover happens to the secondary DNS server after the firewall retries 2 times after every 5 seconds of timeout.

krbfgt (dns) # show full-configuration 
config system dns
    set primary 10.1.1.1
    set secondary 10.1.1.2
    set server-select-method failover  
    set timeout 5   
    set retry 2
end

Debug commands:

 
Initial query to login.windows.net:

2025-06-02 13:49:20 [worker 0] dns_local_lookup()-2529: vfid=0, real_vfid=0, qname=login.windows.net, qtype=1, qclass=1, offset=35, map#=3 max_sz=131072
2025-06-02 13:49:20 [worker 0] dns_lookup_aa_zone()-608: vfid=0, fqdn=login.windows.net
2025-06-02 13:49:20 [worker 0] dns_send_request()-1430
2025-06-02 13:49:20 [worker 0] dns_send_resol_request()-1266: orig id: 0x005c local id: 0x005c domain=login.windows.net
2025-06-02 13:49:20 [worker 0] dns_find_best_server()-654: found server: 10.5.20.107 (vfid=0 vrf=0) 

After 5 seconds:

2025-06-02 13:49:25 [worker 0] dns_retransmit_func()-1703: jiffies=212908 created=212408 wait_cat=0 wait_res=1 profile=last_tx=212408 ftg_last_tx=0 domain=login.windows.net (orig id: 0x005c local id:0x005c active)
2025-06-02 13:49:25 [worker 0] dns_send_request()-1430
2025-06-02 13:49:25 [worker 0] dns_send_resol_request()-1266: orig id: 0x005c local id: 0x005c domain=login.windows.net
2025-06-02 13:49:25 [worker 0] dns_send_resol_request()-1315: retransmission (domain=login.windows.net)
2025-06-02 13:49:25 [worker 0] dns_server_downgrade()-393: ip=10.5.20.107 encrypt=none rating=0 failure=0 last_failed=0

After 10 seconds from first query:

2025-06-02 13:49:30 [worker 0] dns_send_resol_request()-1266: orig id: 0x005c local id: 0x005c domain=login.windows.net
2025-06-02 13:49:30 [worker 0] dns_send_resol_request()-1315: retransmission (domain=login.windows.net)
2025-06-02 13:49:30 [worker 0] dns_server_downgrade()-393: ip=10.5.20.107 encrypt=none rating=0 failure=2 last_failed=406
2025-06-02 13:49:30 [worker 0] dns_find_best_server()-654: found server: 96.45.46.46 (vfid=0 vrf=0)
2025-06-02 13:49:30 [worker 0] dns_tcp_forward_request()-1111: vdom=root req_type=1 domain=login.windows.net
2025-06-02 13:49:30 [worker 0] dns_tcps_schedule_query_write()-374: orig id: 0x005c local id: 0x005c domain=login.windows.net mode=0
2025-06-02 13:49:30 [worker 0] dns_tcps_schedule_query_write()-395: schedule query (domain=login.windows.net) to connection 96.45.46.46:53 mode=0 

Firewall considers failover time=retry*timeout.

After the timeout is changed to 1, and the retry is 2 failover takes 2 seconds:

Initial query to api.akamai.com:

2025-06-04 17:41:47 [worker 0] dns_local_lookup()-2529: vfid=0, real_vfid=0, qname=api.akamai.com, qtype=1, qclass=1, offset=32, map#=3 max_sz=131072
2025-06-04 17:41:47 [worker 0] dns_lookup_aa_zone()-608: vfid=0, fqdn=api.akamai.com
2025-06-04 17:41:47 [worker 0] dns_send_request()-1430
2025-06-04 17:41:47 [worker 0] dns_send_resol_request()-1266: orig id: 0x0044 local id: 0x0044 domain=api.akamai.com
2025-06-04 17:41:47 [worker 0] dns_find_best_server()-654: found server: 10.5.20.107 (vfid=0 vrf=0)
2025-06-04 17:41:47 [worker 0] dns_udp_forward_request()-1092: vdom=root req_type=1 domain=api.akamai.com oif=0
2025-06-04 17:41:47 [worker 0] dns_udp_forward_request()-1212: Send 32B to [10.5.20.107]:53 via fd=19 request:1

After 1 second:

2025-06-04 17:41:48 [worker 0] dns_retransmit_func()-1703: jiffies=18887136 created=18887036 wait_cat=0 wait_res=1 profile=
last_tx=18887036 ftg_last_tx=0 domain=api.akamai.com (orig id: 0x0044 local id:0x0044 active)
2025-06-04 17:41:48 [worker 0] dns_send_request()-1430
2025-06-04 17:41:48 [worker 0] dns_send_resol_request()-1266: orig id: 0x0044 local id: 0x0044 domain=api.akamai.com
2025-06-04 17:41:48 [worker 0] dns_send_resol_request()-1315: retransmission (domain=api.akamai.com)
2025-06-04 17:41:48 [worker 0] dns_server_downgrade()-393: ip=10.5.20.107 encrypt=none rating=0 failure=0 last_failed=0

After 2 seconds from the initial query:

2025-06-04 17:41:49 [worker 0] dns_retransmit_func()-1703: jiffies=18887243 created=18887141 wait_cat=0 wait_res=1 profile=
last_tx=18887143 ftg_last_tx=0 domain=api.akamai.com (orig id: 0x0044 local id:0x0044 active)
2025-06-04 17:41:49 [worker 0] dns_send_request()-1430
2025-06-04 17:41:49 [worker 0] dns_send_resol_request()-1266: orig id: 0x0044 local id: 0x0044 domain=api.akamai.com
2025-06-04 17:41:49 [worker 0] dns_send_resol_request()-1315: retransmission (domain=api.akamai.com)
2025-06-04 17:41:49 [worker 0] dns_server_downgrade()-393: ip=10.5.20.107 encrypt=none rating=0 failure=1 last_failed=107
2025-06-04 17:41:49 [worker 0] dns_find_best_server()-654: found server: 96.45.46.46 (vfid=0 vrf=0)
2025-06-04 17:41:49 [worker 0] dns_tcp_forward_request()-1111: vdom=root req_type=1 domain=api.akamai.com
2025-06-04 17:41:49 [worker 0] dns_tcps_schedule_query_write()-374: orig id: 0x0044 local id: 0x0044 domain=api.akamai.com mode=0
2025-06-04 17:41:49 [worker 0] dns_tcps_schedule_query_write()-395: schedule query (domain=api.akamai.com) to connection 96.45.46.46:53 mode=0 
2025-06-04 17:41:49 [worker 0] _dns_tcps_conn_connect()-736: connecting to 96.45.46.46:53 via vfid=0 status=0 

2025-06-04 17:41:49 [worker 0] tcp_handle_response()-164: domain=api.akamai.com (id=0x0044)
2025-06-04 17:41:49 [worker 0] dns_query_handle_response()-2650: vfid=0 real_vfid=0 vrf=0 id=0x0044 domain=api.akamai.com pktlen=530
2025-06-04 17:41:49 [worker 0] dns_query_save_response()-2631: domain=api.akamai.com pktlen=530 
2025-06-04 17:41:49 [worker 0] dns_set_min_ttl()-190: QR: api.akamai.com

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