IPv6 related kernel settings are documented here:
Enable IPv4 forwarding between all interfaces
Enable IPv6 forwarding between all interfaces
/proc/sys/net/ipv4/conf/all/rp_filter
/proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
If set to true, then the kernel will ignore ICMP echo requests sent to broadcast/multicast addresses.
Accept packets with SRR option.
Accept ICMP Redirects.
/proc/sys/net/ipv4/icmp_ignore_bogus_error_responses
Some routers violate RFC 1122 by sending bogus responses to broadcast frames. Such violations are normally logged via a kernel warning. If this is set to TRUE, the kernel will not give such warnings, which will avoid log file clutter.
/proc/sys/net/ipv4/ip_dynaddr
If set, enables support for dynamic addresses.
Log and drop "Martian" packets. A "Martian" packet is one for which the host does not have a route back to the source IP address (it apparently dropped in from Mars).
/proc/sys/net/ipv4/tcp_fin_timeout
Time to hold socket in state FIN-WAIT-2, if it was closed by our side. Peer can be broken and never close its side, or even died unexpectedly. Default value is 60sec. Usual value used in 2.2 was 180 seconds, you may restore it, but remember that if your machine is even underloaded WEB server, you risk to overflow memory with kilotons of dead sockets, FIN-WAIT-2 sockets are less dangerous than FIN-WAIT-1, because they eat maximum 1.5K of memory, but they tend to live longer.
/proc/sys/net/ipv4/tcp_keepalive_intvl
How often TCP sends out keepalive messages when keepalive is enabled. Default: 2hours.
/proc/sys/net/ipv4/tcp_window_scaling
Enable window scaling as defined in RFC1323.
/proc/sys/net/ipv4/tcp_sack
Enable select acknowledgments (SACKS).
/proc/sys/net/ipv4/tcp_fack
Enable FACK congestion avoidance and fast retransmission. The value is not used, if tcp_sack is not enabled.
/proc/sys/net/ipv4/tcp_ecn
Enable Explicit Congestion Notification in TCP.
/proc/sys/net/ipv4/tcp_syncookies
Only valid when the kernel was compiled with CONFIG_SYNCOOKIES Send out syncookies when the syn backlog queue of a socket overflows. This is to prevent against the common 'syn flood attack' Default: FALSE
Note, that syncookies is fallback facility. It MUST NOT be used to help highly loaded servers to stand against legal connection rate. If you see synflood warnings in your logs, but investigation shows that they occur because of overload with legal connections, you should tune another parameters until this warning disappear. See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
syncookies seriously violate TCP protocol, do not allow to use TCP extensions, can result in serious degradation of some services (f.e. SMTP relaying), visible not by you, but your clients and relays, contacting you. While you see synflood warnings in logs not being really flooded, your server is seriously misconfigured.
/proc/sys/net/ipv4/tcp_timestamps
Enable timestamps as defined in RFC1323.
In this tab you can set path to the system command line tools used by generated iptables script. Use these if tools you expect to use are located in non-standard directories (such as "/usr/local/bin", "/use/local/sbin" etc.) Leave these blank if tools you expect to use are in standard system directories.
In this tab you can adjust parameters of the conntrack module to tune its performance. This is useful for firewalls passing a lot of traffic but probably is not necessary in most cases. See http://www.wallfire.org/misc/netfilter_conntrack_perf.txt for the detailed explanation.
You can adjust the following parameters: