conf/185229: ntpd with default /etc/ntp.conf can be used for NTP Reflection Attacks

[Florian Ermisch]

>Number:         185229
>Category:       conf
>Synopsis:       ntpd with default /etc/ntp.conf can be used for NTP Reflection Attacks
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          change-request
>Submitter-Id:   current-users
>Arrival-Date:   Fri Dec 27 13:40:00 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator:     Florian Ermisch
>Release:        9.2-RELEASE
>Organization:
>Environment:
FreeBSD $HOSTNAME 9.2-RELEASE FreeBSD 9.2-RELEASE #0 r255898: Thu Sep 26 22:50:31 UTC 2013     root at bake.isc.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64

>Description:
I've received an abuse report about a VM of mine participating in a NTP Reflection Attack (and being disabled by the provider). 
The VM was running FreeBSD 9.2 amd64 (fresh installation) and I enabled ntpd to have the system time synchronized for security/pam_google_authenticator a week ago. I didn't change the default /etc/ntp.conf, though.

>From the abuse report my provider forwarded to me:

Public NTP server used for an attack: 5.45.xxx.xxx
 You are running a public NTP server that participated a very large-scale attack against a
 customer of ours today, generating UDP responses to spoofed requests with bogus timestamps
 that claimed to be from the attack target. Your server was particularly active in the
 attack, sending a significant portion of the attack traffic we saw.

 Please consider reconfiguring your NTP server in one or more of these ways:

 - Set your NTP installation to act as a client only. With ntpd, that can be done with
 \"restrict default ignore\" in /etc/ntp.conf; other servers should have a similar
 configuration option. A firewall rule to block UDP to the public IP address on port 123
 would also work for this. More information can be found here:
https://www.team-cymru.org/ReadingRoom/Templates/secure-ntp-template.html
 - Adjust your firewall or NTP server configuration so that it only serves your customers
 and does not respond to outside IP addresses 
 - Rate-limit responses to individual source IP addresses, silently discarding those that
 exceed a low number, such as one request per IP address per second
 - Limit queries to TCP-only
 - Ignore particularly unlikely queries, such as those representing dates far in the future
 or past
 - Limit the size of allowed responses; today\'s were 440 bytes, which were very large
[...]

>How-To-Repeat:
 - Install FreeBSD 9.2 on a system with public IP (and no firewall blocking 123/udp between the system and the public internet)
 - Enable ntpd without changing the default /etc/ntp.conf
>Fix:
Add "restrict" statements like the following to the default /etc/ntp.conf on FreeBSD so the system cannot be used in a NTP Reflection Attack:

# by default act only as a basic NTP client
restrict -4 default nomodify nopeer noquery notrap
restrict -6 default nomodify nopeer noquery notrap
# allow NTP messages from the loopback address, useful for debugging
restrict 127.0.0.1
restrict ::1

(from https://www.team-cymru.org/ReadingRoom/Templates/secure-ntp-template.html)

Most systems are only NTP-clients and if the operator wants to run a NTP-server the ntp.conf will probably be tweaked anyway.

>Release-Note:
>Audit-Trail:
>Unformatted: