There are many debug commands that you can run to troubleshoot problems on Juniper firewalls. The most common debugs used are:
Debug flow basic shows the flow of traffic through the firewall, allowing for troubleshooting route selection, policy selection, any address translation and whether the packet is recieved or dropped by the firewall.
Debug ike detail show all of the vpn messages between two devices when setting up the vpn. Useful to see problems between Juniper and 3rd party firewalls or configuration porblems.
Other debugs are ones allowing troubleshooting of routing protocols, av, deep inspection, higher availability, NAT and loads of others. You can find out what others there are by typing 'debug ?' on the cli.
Debug captures are sent to a buffer by default but can be sent the console.
Buffer commands: get dbuf info - Displays debug buffer size in bytes
get dbuf stream - Displays the contents of the debug buffer
set dbuf size - Allocates system memory for the debug buffer
clear dbuf - Clears the contents of the debug buffer
Here are best practices when debug flow basic and how to read the output.
Debug Flow basic
1) get ffilter - see if an filters have been set already, if they have you use 'unset ffilter' to remove
2) set ffilter - allows you to limit the traffic that you capture using src-ip, src-port, dst-ip, dst-port, ip-proto Recommeded as debug flow basic can be intensive on the firewall especially if it is under heavy load.
filters that are written on one line are and 'AND' statement. eg set ffilter scr-ip 10.1.1.1 dst-ip 220.127.116.11 will match 10.1.1.1 AND 18.104.22.168
filters that are on seperate lines are 'OR' statements. eg set ffilter scr-ip 10.1.1.1, set ffilter dst-ip 22.214.171.124 will match 10.1.1.1 OR 126.96.36.199
3) debug flow basic - turns on flow debuging with a level of basic logging
4) clear db - make sure there is nothing in the debug buffer from previous debugs
5) Begin the test, do a ping or try to access the resource that you are having problems with.
6) undebug all or press Esc key - turns off debug
7) get db str - reads the debug buffer and outputs it to the screen for reading.