This target will process TCP three-way-handshake parallel in netfilter
context to protect either local or backend system. This target requires
connection tracking because sequence numbers need to be translated.
The kernels ability to absorb SYNFLOOD was greatly improved starting with
Linux 4.4, so this target should not be needed anymore to protect Linux servers.
.TP
\fB\-\-mss\fP \fImaximum segment size\fP
Maximum segment size announced to clients. This must match the backend.
.TP
\fB\-\-wscale\fP \fIwindow scale\fP
Window scale announced to clients. This must match the backend.
.TP
\fB\-\-sack\-perm\fP
Pass client selective acknowledgement option to backend (will be disabled
if not present).
.TP
\fB\-\-timestamps\fP
Pass client timestamp option to backend (will be disabled if not present,
also needed for selective acknowledgement and window scaling).
.PP
Example:
.PP
Determine tcp options used by backend, from an external system
.IP
tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-ack)'
.br
    port 80 &
.br
telnet 192.0.2.42 80
.br
18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
.br
    Flags [S.], seq 360414582, ack 788841994, win 14480,
.br
    options [mss 1460,sackOK,
.br
    TS val 1409056151 ecr 9690221,
.br
    nop,wscale 9],
.br
    length 0
.PP
Switch tcp_loose mode off, so conntrack will mark out\-of\-flow
packets as state INVALID.
.IP
echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose
.PP
Make SYN packets untracked
.IP
iptables \-t raw \-A PREROUTING \-i eth0 \-p tcp \-\-dport 80
    \-\-syn \-j CT \-\-notrack
.PP
Catch UNTRACKED (SYN packets) and INVALID (3WHS ACK packets) states
and send them to SYNPROXY. This rule will respond to SYN packets with
SYN+ACK syncookies, create ESTABLISHED for valid client response (3WHS ACK
packets) and drop incorrect cookies. Flags combinations not expected
during 3WHS will not match and continue (e.g. SYN+FIN, SYN+ACK).
.IP
iptables \-A INPUT \-i eth0 \-p tcp \-\-dport 80
    \-m state \-\-state UNTRACKED,INVALID \-j SYNPROXY
    \-\-sack\-perm \-\-timestamp \-\-mss 1460 \-\-wscale 9
.PP
Drop invalid packets, this will be out\-of\-flow packets that were not
matched by SYNPROXY.
.IP
iptables \-A INPUT \-i eth0 \-p tcp \-\-dport 80 \-m state \-\-state INVALID \-j DROP