[CentOS] IP forwarding

Recently, someone or something has been turning off IP forwarding
on my CentOS server:
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[tim at william NumberTheory]$ sudo sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 0
[tim at william NumberTheory]$ sudo sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1
[tim at william NumberTheory]$ sudo sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 1
--------------------------
Who or what can this be?

-- 
Timothy Murphy  
gayleard /at/ eircom.net
School of Mathematics, Trinity College, Dublin