As long as you are doing "vpn debug" style commands and not kernel debugs (fw ctl debug) it is pretty safe as "vpn debug" is just switching on debugs in the vpnd daemon. Even if there is a runaway debug it will not impact the bulk of traffic operations happening in the kernel including encrypt/decrypt operations for existing VPN tunnels. If somehow vpnd crashes or becomes impaired new IKE negotiations cannot occur, and certain types of Remote Access VPN traffic (such as Visitor mode & NAT-T) will be impacted. However vpnd is a child process of fwd who will instantly restart vpnd if it dies.
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