Andryan’s Random Notes

If you wonder why your VPS or specifically OpenVZ VE’s ntpd is stuck at stratum 16 or stratum 0 while ntpq -p shows no abnormality (use ntpq -c rv to check state, it should say state=3 when abnormal), then try executing ntpdate manually. If it says “ntpdate: step-systime: Operation not permitted”, then you need to reconfigure your OpenVZ VE or ask your host to do it for you: vzctl set 123 –capability sys_time:on –save

If everything is fine but this message “ntpdate: no server suitable for synchronization found” keeps coming up, then it could be your ISP blocking UDP port 123. Use ntpdate -u to verify, this option should circumvent the firewall.

References:
http://blog.haite.ch/2009/05/22/1242984900000.html

PREROUTING in nat table: DNAT, REDIRECT
POSTROUTING in nat table: SNAT/MASQUERADE

PREROUTING in mangle table: alter routing (e.g. source-based routing)
FORWARD in mangle table: traffic shaping for tc (flowid)

In MikroTik RouterOS, /ip firewall nat:
srcnat chain = PREROUTING
dstnat chain = POSTROUTING

/ip firewall mangle:
prerouting chain = global-in
forward chain = global-out (support NAT traffic shaping, higher load on router)

Update: I used to think that global-in literally means global input, while global-out literally means global output.  Apparently I was mistaken.  The parent queues MUST be matched to the right parent, either global-in, global-out, global-total, or one of the network interfaces depending on the mangle rules.  Top-most queue parent (global-in, global-out, global-total) doesn’t decide whether it’s up/down.  The mangle rules do the direction decision trick, whether a packet is incoming/download or outgoing/upload.  This makes perfect sense now! (doh)  I kept wondering why half of my queues (download queues’ top-most parent was global-in) weren’t working, when all my mangle rules were in forward chain.  Obviously they wouldn’t, because global-in marks both direction in prerouting chain.

Update 2: mangle: download rules first then upload rules. global-in, connection mark: prerouting; packet mark: prerouting. global-out, connection mark: forward, packet mark: postrouting.

This didn’t make sense until I read the URL listed below under References.  Silly me!  Assumption is the root of most, if not all, problems indeed.

References:
http://wiki.ispadmin.eu/index.php/Documentation/Mikrotik_guide#.22Global-in.22_vs._.22global-out.22_setup