Add ip6tables TCPMSS extension (Arnaud Ebalard <arno@natisbad.org>)

Kernel part will go in 2.6.21.

diff --git a/extensions/libip6t_TCPMSS.c b/extensions/libip6t_TCPMSS.c
new file mode 100644 (file)
index 0000000..7fcccd5
--- /dev/null
+++ b/extensions/libip6t_TCPMSS.c
@@ -0,0 +1,134 @@
+/* Shared library add-on to iptables to add TCPMSS target support.
+ *
+ * Copyright (c) 2000 Marc Boucher
+*/
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <getopt.h>
+
+#include <ip6tables.h>
+#include <linux/netfilter_ipv6/ip6_tables.h>
+#include <linux/netfilter_ipv6/ip6t_TCPMSS.h>
+
+struct mssinfo {
+       struct ip6t_entry_target t;
+       struct ip6t_tcpmss_info mss;
+};
+
+/* Function which prints out usage message. */
+static void
+help(void)
+{
+       printf(
+"TCPMSS target v%s mutually-exclusive options:\n"
+"  --set-mss value               explicitly set MSS option to specified value\n"
+"  --clamp-mss-to-pmtu           automatically clamp MSS value to (path_MTU - 60)\n",
+IPTABLES_VERSION);
+}
+
+static struct option opts[] = {
+       { "set-mss", 1, 0, '1' },
+       { "clamp-mss-to-pmtu", 0, 0, '2' },
+       { 0 }
+};
+
+/* Initialize the target. */
+static void
+init(struct ip6t_entry_target *t, unsigned int *nfcache)
+{
+}
+
+/* Function which parses command options; returns true if it
+   ate an option */
+static int
+parse(int c, char **argv, int invert, unsigned int *flags,
+      const struct ip6t_entry *entry,
+      struct ip6t_entry_target **target)
+{
+       struct ip6t_tcpmss_info *mssinfo
+               = (struct ip6t_tcpmss_info *)(*target)->data;
+
+       switch (c) {
+               unsigned int mssval;
+
+       case '1':
+               if (*flags)
+                       exit_error(PARAMETER_PROBLEM,
+                                  "TCPMSS target: Only one option may be specified");
+               if (string_to_number(optarg, 0, 65535 - 60, &mssval) == -1)
+                       exit_error(PARAMETER_PROBLEM, "Bad TCPMSS value `%s'", optarg);
+
+               mssinfo->mss = mssval;
+               *flags = 1;
+               break;
+
+       case '2':
+               if (*flags)
+                       exit_error(PARAMETER_PROBLEM,
+                                  "TCPMSS target: Only one option may be specified");
+               mssinfo->mss = IP6T_TCPMSS_CLAMP_PMTU;
+               *flags = 1;
+               break;
+
+       default:
+               return 0;
+       }
+
+       return 1;
+}
+
+static void
+final_check(unsigned int flags)
+{
+       if (!flags)
+               exit_error(PARAMETER_PROBLEM,
+                          "TCPMSS target: At least one parameter is required");
+}
+
+/* Prints out the targinfo. */
+static void
+print(const struct ip6t_ip6 *ip6,
+      const struct ip6t_entry_target *target,
+      int numeric)
+{
+       const struct ip6t_tcpmss_info *mssinfo =
+               (const struct ip6t_tcpmss_info *)target->data;
+       if(mssinfo->mss == IP6T_TCPMSS_CLAMP_PMTU)
+               printf("TCPMSS clamp to PMTU ");
+       else
+               printf("TCPMSS set %u ", mssinfo->mss);
+}
+
+/* Saves the union ip6t_targinfo in parsable form to stdout. */
+static void
+save(const struct ip6t_ip6 *ip, const struct ip6t_entry_target *target)
+{
+       const struct ip6t_tcpmss_info *mssinfo =
+               (const struct ip6t_tcpmss_info *)target->data;
+
+       if(mssinfo->mss == IP6T_TCPMSS_CLAMP_PMTU)
+               printf("--clamp-mss-to-pmtu ");
+       else
+               printf("--set-mss %u ", mssinfo->mss);
+}
+
+static struct ip6tables_target mss = {
+       .next           = NULL,
+       .name           = "TCPMSS",
+       .version        = IPTABLES_VERSION,
+       .size           = IP6T_ALIGN(sizeof(struct ip6t_tcpmss_info)),
+       .userspacesize  = IP6T_ALIGN(sizeof(struct ip6t_tcpmss_info)),
+       .help           = &help,
+       .init           = &init,
+       .parse          = &parse,
+       .final_check    = &final_check,
+       .print          = &print,
+       .save           = &save,
+       .extra_opts     = opts
+};
+
+void _init(void)
+{
+       register_target6(&mss);
+}

diff --git a/extensions/libip6t_TCPMSS.man b/extensions/libip6t_TCPMSS.man
new file mode 100644 (file)
index 0000000..b4c357e
--- /dev/null
+++ b/extensions/libip6t_TCPMSS.man
@@ -0,0 +1,42 @@
+This target allows to alter the MSS value of TCP SYN packets, to control
+the maximum size for that connection (usually limiting it to your
+outgoing interface's MTU minus 60).  Of course, it can only be used
+in conjunction with
+.BR "-p tcp" .
+It is only valid in the
+.BR mangle
+table.
+.br
+This target is used to overcome criminally braindead ISPs or servers
+which block ICMPv6 Packet Too Big packets or are unable to send them.
+The symptoms of this problem are that everything works fine from your 
+Linux firewall/router, but machines behind it can never exchange large
+packets:
+.PD 0
+.RS 0.1i
+.TP 0.3i
+1)
+Web browsers connect, then hang with no data received.
+.TP
+2)
+Small mail works fine, but large emails hang.
+.TP
+3)
+ssh works fine, but scp hangs after initial handshaking.
+.RE
+.PD
+Workaround: activate this option and add a rule to your firewall
+configuration like:
+.nf
+ ip6tables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\
+             -j TCPMSS --clamp-mss-to-pmtu
+.fi
+.TP
+.BI "--set-mss " "value"
+Explicitly set MSS option to specified value.
+.TP
+.B "--clamp-mss-to-pmtu"
+Automatically clamp MSS value to (path_MTU - 60).
+.TP
+These options are mutually exclusive.
+

diff --git a/include/linux/netfilter_ipv6/ip6t_TCPMSS.h b/include/linux/netfilter_ipv6/ip6t_TCPMSS.h
new file mode 100644 (file)
index 0000000..412d1cb
--- /dev/null
+++ b/include/linux/netfilter_ipv6/ip6t_TCPMSS.h
@@ -0,0 +1,10 @@
+#ifndef _IP6T_TCPMSS_H
+#define _IP6T_TCPMSS_H
+
+struct ip6t_tcpmss_info {
+       u_int16_t mss;
+};
+
+#define IP6T_TCPMSS_CLAMP_PMTU 0xffff
+
+#endif /*_IP6T_TCPMSS_H*/