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RPC and SE Linux

One ongoing problem with TCP networking is the combination of RPC services and port based services on the same host. If you have an RPC service that uses a port less than 1024 then typically it will start at 1023 and try lower ports until it finds one that works. A problem that I have had in the past is that an RPC service used port 631 and I then couldn’t start CUPS (which uses that port). A similar problem can arise in a more insidious manner if you have strange networking devices such as a BMC [1] which uses the same IP address as the host and just snarfs connections for itself (as documented by pantz.org [2]), this means that according to the OS the port in question is not in use, but connections to that port will go to the hardware BMC and the OS won’t see them.

Another solution is to give a SE Linux security context to the port which prevents the RPC service from binding to it. RPC applications seem to be happy to make as many bind attempts as necessary to get an available port (thousands of attempts if necessary) so reserving a few ports is not going to cause any problems. As far as I recall my problems with CUPS and RPC services was a motivating factor in some of my early work on writing SE Linux policy to restrict port access.

Of course the best thing to do is to assign IP addresses for IPMI that are different from the OS IP addresses. This is easy to do and merely requires an extra IP address for each port. As a typical server will have two Ethernet ports on the baseboard (one for the front-end network and one for the private network) that means an extra two IP addresses (you want to use both interfaces for redundancy in case the problem which cripples a server is related to one of the Ethernet ports). But for people who don’t have spare IP addresses, SE Linux port labeling could really help.

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