2 - Raspberry PI Community Projects

➨ http://www.faqs.org/rfcs/rfc777.html
➨ http://www.faqs.org/rfcs/rfc792.html
For reference, a receive buffer is a small memory zone storing data between
the time it arrives from the network and the time the kernel handles it. If this
zone is full, new data cannot be received, and ICMP signals the problem, so
that the emier can slow down its transfer rate (which should ideally reach
an equilibrium aer some time).
Note that although an IPv4 network can work without ICMP, ICMPv6 is
strictly required for an IPv6 network, since it combines several functions that
were, in the IPv4 world, spread across ICMPv4, IGMP (Internet Group Membership
Protocol) and ARP (Address Resolution Protocol). ICMPv6 is defined in
RFC4443.
➨ http://www.faqs.org/rfcs/rfc4443.html
• ACCEPT: allow the packet to go on its way;
• REJECT: reject the packet with an ICMP error packet (the --reject-with type option to
iptables allows selecting the type of error);
• DROP: delete (ignore) the packet;
• LOG: log (via syslogd) a message with a description of the packet; note that this action
does not interrupt processing, and the execution of the chain continues at the next rule,
which is why logging refused packets requires both a LOG and a REJECT/DROP rule;
• ULOG: log a message via ulogd, which can be better adapted and more efficient than
syslogd for handling large numbers of messages; note that this action, like LOG, also
returns processing to the next rule in the calling chain;
• chain_name: jump to the given chain and evaluate its rules;
• RETURN: interrupt processing of the current chain, and return to the calling chain; in
case the current chain is a standard one, there's no calling chain, so the default action
(defined with the -P option to iptables) is executed instead;
• SNAT (only in the nat table, therefore only in IPv4): apply Source NAT (extra options describe
the exact changes to apply);
• DNAT (only in the nat table, therefore only in IPv4): apply Destination NAT (extra options
describe the exact changes to apply);
• MASQUERADE (only in the nat table, therefore only in IPv4): apply masquerading (a
special case of Source NAT);
• REDIRECT (only in the nat table, therefore only in IPv4): redirect a packet to a given
port of the firewall itself; this can be used to set up a transparent web proxy that works
with no configuration on the client side, since the client thinks it connects to the recipient
whereas the communications actually go through the proxy.
378 The Debian Administrator's Handbook