TCP keepalives are do-nothing packets the TCP layer can send to see ifa connection is still alive or if the remote end has gone unreachable(due to a machine crash, a network problem, or whatever). Keepalivesare not default TCP behavior (at least not in any TCP stack thatconforms to the RFCs), so you have to specifically turn them on.(There are various reasons why this is sensible.)

In Python you can do this with the .setsockopt() socket method,using

Chris Lewis of Nortel recently posted a breakdown of CBL listings byISP in news.admin.net-abuse.email. Here's the top ten of his listing:

375649 chinanet.cn.net130245 cnc-noc.net102931 telekom.gov.tr80936 kornet.net67721 tpnet.pl51671 dtag.de47246 rain.fr3

I recently discovered that not all of the iptables extension modules are documented in the iptables manualpage, at least the versions installed on Debian Sarge or Fedora Core4; they're only documented in the netfilter.org extensions HOWTO here ,or sometimes not even there.

I care about this because this means there's interesting things I coulddo with iptables that I can't find out by reading the manpage. So for myfuture reference and anyone else's use, here's a quick summary of