Richard> I use m0n0wall 1.3b18 on CDrom and USB key configuration as a wifi 
Richard> captive portal.
Richard> Some time, it does not accept any more connection.
Richard> I got messages  :
Richard> Logs/System :  kernel: pid 141 (dhcpd), uid 0 inumber 3205 on /: 
Richard> filesystem full
Richard> Logs/DHCP : dhcpd: commit_leases: unable to commit: No space left on device
Richard> What can I do ?

What type of hardware are you using?  You probably need more RAM on
the system so that you have more room for logs and the DHCP leases
file. 

Clean out your logs, or turn down the logging level as well. 

I assume you just reboot to clear the issue?  If so, before you do
that next time, try to look in the web interface to see how the memory
usage is, and what the logs look like.

John