Tunnelling NTP Through a Firewall?

Franklin_DeMatto asks: "My ISP keeps my server behind a tight firewall, only allowing outgoing HTTP(S) and SMTP. I would like to sync the system's clock using NTP. Does anyone know of any public time servers that can do some type of NTP over HTTP, to get through the firewall? What about the software (preferably open source) to do it? (No, the ISP will not change the firewall rules.)"

Comments

Try the routers...

[h3]

I forget where I learned this tip, but it's useful and doesn't seem widely known: many routers provide NTP service. So you can do a traceroute from your server out to anywhere (say google.com) and get a list of upstream routers. Don't forget to try the "-I" option (or whatever the equiv is in your version of traceroute) to use ICMP instead of the default UDP datagrams if your firewall is blocking those.

If/once you have a list of routers, try time syncing against them. It's worth a shot.

-h3

TCP Over TCP Is A Bad Idea (Re:SSH?)

[alfaiomega]

If you have a shell account, they probably allow ssh through the firewall and so you can tunnel the NTP ports over SSH.

Read Why TCP Over TCP Is A Bad Idea by Olaf Titz:

A frequently occurring idea for IP tunneling applications is to run a protocol like PPP, which encapsulates IP packets in a format suited for a stream transport (like a modem line), over a TCP-based connection. This would be an easy solution for encrypting tunnels by running PPP over SSH, for which several recommendations already exist (one in the Linux HOWTO base, one on my own website, and surely several others). It would also be an easy way to compress arbitrary IP traffic, while datagram based compression has hard to overcome efficiency limits.

Unfortunately, it doesn't work well. Long delays and frequent connection aborts are to be expected. Here is why.

Very interesting read.