Routing for clusters

Here are notes I've found useful for managing routing on clusters of GNU/Linux boxes.

See more sections on Gnu/Linux .

Read some good free chapters on networking at

Diagnosing routing problems

Make sure your interface card is bound to an IP address (check with ifconfig -a). You should be able to ping all nodes from each other. Make sure tracepath or traceroute do not show more hops than necessary.

Make sure you have direct routes to all other nodes. Any of the following print route tables, with IP addresses or hostnames:
 $ route -n
 $ route
 $ netstat -rn 
 $ netstat -r

To diagnose problems with routing on a cluster, examine the following information on the mayor node and on a worker node.

Make sure that your default installation does not run a firewall packet filter by default. If you have the ipchains, iptables packages installed, you can disable firewall support by either uninstalling those packages, or running lokkit and selecting "no firewall".

See if you have identified hosts explicitly.
 $ cat /etc/hosts

If so, make sure "hosts" are resolved by "files" first:
 $ cat /etc/nsswitch.conf

We use the lookup order "hosts: files nis dns".

If you are not using DNS, then /etc/resolv.conf must be empty. If you are using DNS, then specify the DNS nameserver by IP address, and specify default domains for completing host names. DHCP may recreate this file for you every time you reboot.

Check DNS lookups from other machines with host hostname nslookup hostname or dig +short hostname

Check data transfer rates and lags between cluster nodes by running ttcp on two nodes. Here's the result of a good 100Mb/s connection. Start the receiver with -r first.
vacn2> ttcp -r -s -n 16384
ttcp-r: buflen=8192, nbuf=16384, align=16384/0, port=5001  tcp
ttcp-r: socket
ttcp-r: accept from
ttcp-r: 134217728 bytes in 11.41 real seconds = 11488.08 KB/sec +++
ttcp-r: 92640 I/O calls, msec/call = 0.13, calls/sec = 8119.62
ttcp-r: 0.1user 2.4sys 0:11real 22% 0i+0d 0maxrss 0+2pf 0+0csw
vacn7> ttcp -t -s vacn2 -n 16384
ttcp-t: buflen=8192, nbuf=16384, align=16384/0, port=5001  tcp  -> vacn2
ttcp-t: socket
ttcp-t: connect
ttcp-t: 134217728 bytes in 11.40 real seconds = 11495.07 KB/sec +++
ttcp-t: 16384 I/O calls, msec/call = 0.71, calls/sec = 1436.88
ttcp-t: 0.0user 0.8sys 0:11real 7% 0i+0d 0maxrss 0+2pf 0+0csw

Internal and external addresses

A special routing problem may occur if a Linux cluster mayor node has two ethernet interfaces --- one for an external address and one for an internal cluster IP address. If the mayor's hostname corresponds to the external address, then the machine will mis-identify itself to other cluster nodes. Those nodes will route through the external interface, if they are able to route at all. A conservative fix would use the internal address of the mayor node as the first-hop gateway for the external address of the mayor node.
  $ route add -net netmask gw

where is the internal IP address of the mayor node and is the external address of the mayor node.

Even better, set the route on all cluster nodes to use the internal address of the mayor as the first-hop gateway for any unknown external address:
  $ route add -net netmask gw

This fix makes the previous fix unnecessary.

Outside machines might not have a route to the cluster nodes either. To add a route to a PC that needs to see a cluster node, set the route to use the external address of the mayor node as the first-hop gateway to all cluster node addresses:
  $ route add mask

where with a mask of specifies the address range of the cluster nodes, and is the external address of the cluster mayor node.

Bill Harlan, 2002-2005

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