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How to get HTCondor and a NAT Firewall to Cooperate

The setup here is that you have a HTCondor machine on which you want to do useful work either as a submit node or an execute node. You also would like to share your new computer resources with others outside your department or division, but you do not want crooks using your systems to wreak havoc. So, you install a firewall between your HTCondor resource and the Internet. In the setup below, we will assume that the firewall is a separate host from the HTCondor resource. Indeed, will assume explicitly that it is a bastion host running Linux, and the firewall is iptables, with Network Address Translation (NAT); this is to be explicit about which commands to run. Either you, or your firewall admin, should be able to translate these instructions to your firewall installation. We are also assuming that you are not going to be using CCB.

Let us first assume that you have HTCondor installed and running. For this, you should follow the instructions in section 3.2 of the HTCondor manual.

Let us assume you have the following setup. The HTCondor schedd is installed on a machine (named S) with IP address 192.168.0.1; this is your submit machine. An HTCondor startd is installed on a machine (named E) at 192.168.0.2; this is your execute node. The firewall has an external---that is, facing the Internet---IP address of 10.0.0.1, and an internal---that is, facing toward your local network---IP address of 192.168.0.250; we will call this machine F. We know 10.0.0.1 is actually not a routable address, but pretend that it is for the duration of this document. S and E are in the domain mydomain.net.

Then we will make the following changes to condor_config.local on S (the schedd). To find your HTCondor configuration files, the command condor_config_val -dump will be a big help, as the files are listed in the header of the output 

USE_SHARED_PORT = True
SHARED_PORT_ARGS = -p 9617
PRIVATE_NETWORK_NAME = mydomain.net
PRIVATE_NETWORK_INTERFACE = eth0
TCP_FORWARDING_HOST = 10.0.0.1
In the configuration settings above, the port 9617 was chosen out of a hat; there is no reason it cannot be any port on the system. 9618 is often chosen; it is the well-known port of the HTCondor collector. In our setup, we are not assuming that there is a collector in the 192.168.0.0/24 network that will be contacted from outside 192.168.0.0/24, so 9618 is also a valid port number; but you may well want to avoid 9618 if you have an internal collector. Note that the TCP_FORWARDING_HOST must match the external address of the collector.

On the execute node E, we have similar configuration changes, except for the shared port: 

USE_SHARED_PORT = True
SHARED_PORT_ARGS = -p 9616
PRIVATE_NETWORK_NAME = mydomain.net
PRIVATE_NETWORK_INTERFACE = eth0
TCP_FORWARDING_HOST = 10.0.0.1
The use of PRIVATE_NETWORK_NAME on S and E allow them to communicate directly without going through the firewall F.

Now, on the firewall F, we run the following commands to redirect connections from the Internet to ports 9617 and 9616 on F to the corresponding ports on S and E: 

iptables -t nat -A PREROUTING -p tcp -d 10.0.0.1 --dport 9617 -j DNAT --to-destination 192.168.0.1
iptables -t nat -A PREROUTING -p tcp -d 10.0.0.1 --dport 9616 -j DNAT --to-destination 192.168.0.2
iptables -A POSTROUTING -o eth1 -j SNAT --to-source 10.0.0.1
The first rule above says that inbound connections to 10.0.0.1 on port 9617 will be rewritten to be connections to 192.168.0.1, port 9617. The second line states a similar rule for 192.168.0.2, port 9616. The third rule is probably superfluous, as it is likely already in the firewall rules: all outbound connections from 10.0.0.1 will look like they emanate from 10.0.0.1.

Finally, run condor_reconfig on S and E to incorporate the configuration changes.

One may well ask how this works for HTCondor. Simply put, HTCondor wraps all the information in a "sinful string". condor_status -schedd -l will return a Schedd ClassAd, which will contain a line that looks something like: 

MyAddress = "<10.0.0.1:9617?PrivAddr=%3c192.168.0.1:9617%3fsock%3d936_480b_8%3e&PrivNet=mydomain.net&noUDP&sock=936_480b_8>"
This string contains the needed information for another HTCondor client or daemon to contact the schedd and begin using high throughput computing.