SRX cannot send logs to syslog servers via interface in custom routing-instance.

In this case, the SRX has only fxp0 in the default routing instance inet.0. The SRX is unable to communicate with syslog servers via interfaces in routing-instance INSTANCE using the following:

set routing-options interface-routes rib-group inet syslog
set routing-options rib-groups syslog import-rib INSTANCE.inet.0
set routing-options rib-groups syslog import-rib inet.0
set routing-options static route <syslog server ip> next-hop <next-hop of the egress instance>

As the SRX performs route lookup for syslog only in the default routing instance, we need to share the interface route from the instance INSTANCE to default.

Here are the commands of the solution:

delete routing-options interface-routes rib-group inet syslog
set routing-instances INSTANCE routing-options interface-routes rib-group inet syslog
set routing-options rib-groups syslog import-rib INSTANCE.inet.0
set routing-options rib-groups syslog import-rib inet.0
set routing-options static route <syslog server ip> next-hop <next-hop of the egress instance>