check_dhcp permissions issue even when run as root

I'm having a strange issue with the check_dhcp plugin v1.4.16 on SL6. 
Permissions on the file are 4750, owner is root, and group is nagios; 
the nagios user is a member of the nagios group. The partition was 
mounted with the suid option enabled. SELinux is disabled.

$ ls -l /usr/local/nagios/libexec/check_dhcp
-rwsr-x--- 1 root nagios 123851 Jun  3 12:08 
/usr/local/nagios/libexec/check_dhcp

When I run check_dhcp as the nagios user, I get the following error:

$ /usr/local/nagios/libexec/check_dhcp
Error: Could not bind socket to interface eth0.  Check your privileges...

Even when I run the plugin as the root user, I get the same error, so I 
suspect this is not really a file permissions issue.

I found a similar post on this list from 2008 
(http://sourceforge.net/mailarchive/message.php?msg_id=20407079), but 
when that poster changed the permissions to 0775 he was able to run the 
plugin successfully as root. But when I chmod 0775 and run as root I 
still get the same error. The last post in the thread simply says "was a 
permission issue on the file" with no additional explanation, which is 
oh so helpful. Does anyone have any idea what could cause this?

--Max Carey

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