ntpdcan be vulnerable to Sybil attacks. If a system is set up to use a trustedkey and if one is not using the feature introduced in ntp-4.2.8p6 allowing an optional 4th field in the
ntp.keysfile to specify which IPs can serve time, a malicious authenticated peer -- i.e. one where the attacker knows the private symmetric key -- can create arbitrarily-many ephemeral associations in order to win the clock selection of
ntpdand modify a victim's clock. Three additional protections are offered in ntp-4.2.8p11. One is the new
noepeerdirective, which disables symmetric passive ephemeral peering. Another is the new
ippeerlimitdirective, which limits the number of peers that can be created from an IP. The third extends the functionality of the 4th field in the
ntp.keysfile to include specifying a subnet range.
noepeerdirective to prohibit symmetric passive ephemeral associations.
ippeerlimitdirective to limit the number of peers that can be created from an IP.
ntpdstops running, auto-restart it without