read() builtin doesn't work with /proc files containing only an integer value

[Micah Stetson]

Binary package hint: munin

Ubuntu 10.04 LTS, munin-node 1.4.4-1ubuntu1. On my newly installed Lucid server, Munin's fw_conntrack plugin reports the "critical" number of network connects as 5 or 6. This means that I get constant alerts about having too many network connections. I think this is actually caused by a bug in dash's built-in read command. Fw_conntrack gets its max connection number by reading /proc/sys/net/ipv4/ip_conntrack_max or /proc/sys/net/ipv4/netfilter/ip_conntrack_max around line 110 of /usr/share/munin/plugins/fw_conntrack. Like this:

    read MAX < /proc/sys/net/ipv4/netfilter/ip_conntrack_max

If I run this in bash:

    bash# read MAX < /proc/sys/net/ipv4/netfilter/ip_conntrack_max && echo good: $MAX || echo bad: $MAX
    good: 7852

but in dash:

    dash# read MAX < /proc/sys/net/ipv4/netfilter/ip_conntrack_max && echo good: $MAX || echo bad: $MAX
    bad: 7

So dash's read is failing AND setting $MAX. I imagine this is a bug in dash, but fw_conntrack is also ignoring the fact that read is returning failure.

Changing fw_conntrack's #! line to #!/bin/bash works around the problem, but I wouldn't call that a fix.