failover-dhcpclient.script

# WAN interface names and default distances
:local ISPConfig {
  {interface="internet"; distance=20};
  {interface="huawei"; distance=21};
}
# google public dns
:local PingTarget 8.8.8.8
:local FailThreshold 3
# Define the distance increase of a route when it fails
:local DistanceIncrease 2

# Script notes:
# Requires "pure IP" connectivity, as it relies on /ip dhcp-client; pppoe will likely not work.

# Setup notes:
# Create this script as script named "failover", permissions=write.
# Create schedule: /system schedule add interval=5s name=failover on-event="/system script run failover" policy=read,write,test

# ----------------------------------------------------------------------------
:global FailoverState
# first time init
:if ([:typeof $FailoverState] = "nothing") do={
  :set FailoverState {{ failcount=0 }; { failcount=0 }}
}

:foreach isp,ispConfig in=$ISPConfig do={
    # This variable will be used to keep results of individual ping attempts
    :local Table ("isp" . [:tostr ($isp + 1)])
    :local Interface ($ispConfig->"interface")
    :local Distance ($ispConfig->"distance")

    :local Gateway
    :foreach i in [/ip route find dst-address="0.0.0.0/0" && vrf-interface=$Interface && static] do={
        :set Gateway [/ip route get $i gateway]
    }
    :put "found $Interface Gateway: $Gateway"
    :if ([:typeof $Gateway] != "nothing") do={
        # Create check table if it does not exist yet.
        :if ([:len [/ip route find dst-address="0.0.0.0/0" && routing-mark=$Table && static]] = 0) do={
            /ip route add dst-address="0.0.0.0/0" routing-mark=$Table gateway=$Gateway
        }
        # If gateway for $Interface has changed, update our check table route.
        :foreach i in [/ip route find dst-address="0.0.0.0/0" && routing-mark=$Table && static] do={
            :if ([/ip route get $i gateway] != $Gateway) do={
                :put "update $Interface check route in table $Table"
                /ip route set $i gateway=$Gateway
            }
        }

        # Run Ping check
        :local PingResult [ping $PingTarget count=1 routing-table=$Table interface=$Interface]
        :put "$Table ping result: $PingResult"
        :if ($PingResult = 0) do={
            :if (($FailoverState->$isp->"failcount") < ($FailThreshold)) do={
                :set ($FailoverState->$isp->"failcount") (($FailoverState->$isp->"failcount") + 1)
                :if (($FailoverState->$isp->"failcount") = $FailThreshold) do={
                    :put "$Interface went bad"
                    :log warning "Failover: $Interface can NOT reach $PingTarget, increasing distance."
                    :foreach i in=[/ip dhcp-client find interface=$Interface] do={
                        /ip dhcp-client set $i default-route-distance=($Distance + $DistanceIncrease)
                    }
                }
            }
        }
        :if ($PingResult = 1) do={
            :if (($FailoverState->$isp->"failcount") > 0) do={
                :set ($FailoverState->$isp->"failcount") (($FailoverState->$isp->"failcount") - 1)
                :if (($FailoverState->$isp->"failcount") = 0) do={
                    :put "$Interface went good"
                    :log warning "Failover: $Interface can reach $PingTarget again, restoring distance."
                    :foreach i in=[/ip dhcp-client find interface=$Interface] do={
                        /ip dhcp-client set $i default-route-distance=$Distance
                    }
                }
            }
        }
        :put ("$Interface failcount: ".[:tostr ($FailoverState->$isp->"failcount")])
    } else={
        :log warning "Failover: No gateway found for interface $Interface"
    }
}