Technical Tip: Working of Link Monitor ICMP probing

[ekrishnan]

Description	This article describes the working of Link Monitor ICMP probing when it is being active and inactive. To show that FortiGate still probes the health check server via the FortiGate interface.
Scope	FortiGate.
Solution	When the link monitor is inactive, can remove the failed link from the routing table and this article shows what is the background when there is no default route on routing table and how does probing still occurs, the following examples are provided below, Config done for PORT3 and set server to 8.8.8.8. FGT # show sys link-monitor config system link-monitor     edit "1"         set srcintf "port3"         set server "8.8.8.8"         set gateway-ip 10.47.15.254         set update-policy-route disable     next end Routing table showing the defaul route via port3. FGT # get router info routing-table all Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default Routing table for VRF=0 S* 0.0.0.0/0 [10/0] via 10.47.31.254, port3, [3/0] <----- DEFAULT ROUTE. C 10.47.0.0/20 is directly connected, port3 C 10.47.16.0/20 is directly connected, port1 C 10.232.0.0/22 is directly connected, port2 FGT # diagnose sniffer packet port3 'host 8.8.8.8' 4 0 l <----- TESTING PROBING FOR THE INTERFACE, IT IS POSSIBLE TO SEE THE REQUEST AND REPLY PACKET AS THE LINK MONITOR IS ACTIVE. Using Original Sniffing Mode interfaces=[port3] filters=[host 8.8.8.8] 2022-09-12 15:11:05.111151 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request 2022-09-12 15:11:05.113606 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply 2022-09-12 15:11:05.614420 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request 2022-09-12 15:11:05.617194 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply 2022-09-12 15:11:06.117835 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request 2022-09-12 15:11:06.119121 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply 2022-09-12 15:11:06.612113 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request 2022-09-12 15:11:06.614613 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply 2022-09-12 15:11:07.115256 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request 2022-09-12 15:11:07.117122 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply 2022-09-12 15:11:07.617935 port3 -- 10.47.2.106 -> 8.8.8.8: icmp: echo request 2022-09-12 15:11:07.620604 port3 -- 8.8.8.8 -> 10.47.2.106: icmp: echo reply ^C 14 packets received by filter 0 packets dropped by kernel Changing the health-check to a non_working server. FGT# config sys link-monitor FGT (link-monitor) # edit 1 FGT (1) # set server 4.4.4.4 <----- Non-working server. FGT (1) # end FGT # get router info routing-table all Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default Routing table for VRF=0<----- ROUTE ASSOCIATED WITH PORT3 IS REMOVED. C 10.47.0.0/20 is directly connected, port3 C 10.47.16.0/20 is directly connected, port1 C 10.232.0.0/22 is directly connected, port2 The below justifies that even when the static route is associated with the port, the FortiGate interface will still send probe to the server. FGT # diagnose sniffer packet port3 'icmp' 4 0 l <-----TESTING PROBING FOR THE PORT3 INTERFACE, IT IS ONLY POSSIBLE TO SEE REQUEST PACKETS. Using Original Sniffing Mode interfaces=[port3] filters=[icmp] 2022-09-12 15:12:12.846581 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:13.347184 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:13.847876 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:14.348557 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:14.839245 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:15.339899 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:15.840569 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:16.341178 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:16.841901 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request 2022-09-12 15:12:17.342536 port3 -- 10.47.2.106 -> 4.4.4.4: icmp: echo request ^C 10 packets received by filter 0 packets dropped by kernel CORRECTING THE SERVER TO 8.8.8.8. FGT # config sys link-monitor FGT (link-monitor) # edit 1 FGT (1) # set server 8.8.8.8 FGT (1) # end Default route associated with the Port3 is added back to routing table. FGT # get router info routing-table all Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default Routing table for VRF=0 S* 0.0.0.0/0 [10/0] via 10.47.31.254, port3, [3/0]<----- ROUTE IS ADDED BACK. C 10.47.0.0/20 is directly connected, port3 C 10.47.16.0/20 is directly connected, port1 C 10.232.0.0/22 is directly connected, port2