| /* |
| * This is a module which is used for setting the MSS option in TCP packets. |
| * |
| * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca> |
| * |
| * This program is free software; you can redistribute it and/or modify |
| * it under the terms of the GNU General Public License version 2 as |
| * published by the Free Software Foundation. |
| */ |
| |
| #include <linux/module.h> |
| #include <linux/skbuff.h> |
| |
| #include <linux/ip.h> |
| #include <net/tcp.h> |
| |
| #include <linux/netfilter_ipv4/ip_tables.h> |
| #include <linux/netfilter_ipv4/ipt_TCPMSS.h> |
| |
| MODULE_LICENSE("GPL"); |
| MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>"); |
| MODULE_DESCRIPTION("iptables TCP MSS modification module"); |
| |
| static inline unsigned int |
| optlen(const u_int8_t *opt, unsigned int offset) |
| { |
| /* Beware zero-length options: make finite progress */ |
| if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) |
| return 1; |
| else |
| return opt[offset+1]; |
| } |
| |
| static unsigned int |
| ipt_tcpmss_target(struct sk_buff **pskb, |
| const struct net_device *in, |
| const struct net_device *out, |
| unsigned int hooknum, |
| const struct xt_target *target, |
| const void *targinfo) |
| { |
| const struct ipt_tcpmss_info *tcpmssinfo = targinfo; |
| struct tcphdr *tcph; |
| struct iphdr *iph; |
| u_int16_t tcplen, newmss; |
| __be16 newtotlen, oldval; |
| unsigned int i; |
| u_int8_t *opt; |
| |
| if (!skb_make_writable(pskb, (*pskb)->len)) |
| return NF_DROP; |
| |
| iph = (*pskb)->nh.iph; |
| tcplen = (*pskb)->len - iph->ihl*4; |
| tcph = (void *)iph + iph->ihl*4; |
| |
| /* Since it passed flags test in tcp match, we know it is is |
| not a fragment, and has data >= tcp header length. SYN |
| packets should not contain data: if they did, then we risk |
| running over MTU, sending Frag Needed and breaking things |
| badly. --RR */ |
| if (tcplen != tcph->doff*4) { |
| if (net_ratelimit()) |
| printk(KERN_ERR |
| "ipt_tcpmss_target: bad length (%d bytes)\n", |
| (*pskb)->len); |
| return NF_DROP; |
| } |
| |
| if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) { |
| if (dst_mtu((*pskb)->dst) <= sizeof(struct iphdr) + |
| sizeof(struct tcphdr)) { |
| if (net_ratelimit()) |
| printk(KERN_ERR "ipt_tcpmss_target: " |
| "unknown or invalid path-MTU (%d)\n", |
| dst_mtu((*pskb)->dst)); |
| return NF_DROP; /* or IPT_CONTINUE ?? */ |
| } |
| |
| newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) - |
| sizeof(struct tcphdr); |
| } else |
| newmss = tcpmssinfo->mss; |
| |
| opt = (u_int8_t *)tcph; |
| for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) { |
| if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS && |
| opt[i+1] == TCPOLEN_MSS) { |
| u_int16_t oldmss; |
| |
| oldmss = (opt[i+2] << 8) | opt[i+3]; |
| |
| if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU && |
| oldmss <= newmss) |
| return IPT_CONTINUE; |
| |
| opt[i+2] = (newmss & 0xff00) >> 8; |
| opt[i+3] = (newmss & 0x00ff); |
| |
| tcph->check = nf_proto_csum_update(*pskb, |
| htons(oldmss)^htons(0xFFFF), |
| htons(newmss), |
| tcph->check, 0); |
| return IPT_CONTINUE; |
| } |
| } |
| |
| /* |
| * MSS Option not found ?! add it.. |
| */ |
| if (skb_tailroom((*pskb)) < TCPOLEN_MSS) { |
| struct sk_buff *newskb; |
| |
| newskb = skb_copy_expand(*pskb, skb_headroom(*pskb), |
| TCPOLEN_MSS, GFP_ATOMIC); |
| if (!newskb) |
| return NF_DROP; |
| kfree_skb(*pskb); |
| *pskb = newskb; |
| iph = (*pskb)->nh.iph; |
| tcph = (void *)iph + iph->ihl*4; |
| } |
| |
| skb_put((*pskb), TCPOLEN_MSS); |
| |
| opt = (u_int8_t *)tcph + sizeof(struct tcphdr); |
| memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr)); |
| |
| tcph->check = nf_proto_csum_update(*pskb, |
| htons(tcplen) ^ htons(0xFFFF), |
| htons(tcplen + TCPOLEN_MSS), |
| tcph->check, 1); |
| opt[0] = TCPOPT_MSS; |
| opt[1] = TCPOLEN_MSS; |
| opt[2] = (newmss & 0xff00) >> 8; |
| opt[3] = (newmss & 0x00ff); |
| |
| tcph->check = nf_proto_csum_update(*pskb, htonl(~0), *((__be32 *)opt), |
| tcph->check, 0); |
| |
| oldval = ((__be16 *)tcph)[6]; |
| tcph->doff += TCPOLEN_MSS/4; |
| tcph->check = nf_proto_csum_update(*pskb, |
| oldval ^ htons(0xFFFF), |
| ((__be16 *)tcph)[6], |
| tcph->check, 0); |
| |
| newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS); |
| iph->check = nf_csum_update(iph->tot_len ^ htons(0xFFFF), |
| newtotlen, iph->check); |
| iph->tot_len = newtotlen; |
| return IPT_CONTINUE; |
| } |
| |
| #define TH_SYN 0x02 |
| |
| static inline int find_syn_match(const struct ipt_entry_match *m) |
| { |
| const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data; |
| |
| if (strcmp(m->u.kernel.match->name, "tcp") == 0 && |
| tcpinfo->flg_cmp & TH_SYN && |
| !(tcpinfo->invflags & IPT_TCP_INV_FLAGS)) |
| return 1; |
| |
| return 0; |
| } |
| |
| /* Must specify -p tcp --syn/--tcp-flags SYN */ |
| static int |
| ipt_tcpmss_checkentry(const char *tablename, |
| const void *e_void, |
| const struct xt_target *target, |
| void *targinfo, |
| unsigned int hook_mask) |
| { |
| const struct ipt_tcpmss_info *tcpmssinfo = targinfo; |
| const struct ipt_entry *e = e_void; |
| |
| if (tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU && |
| (hook_mask & ~((1 << NF_IP_FORWARD) | |
| (1 << NF_IP_LOCAL_OUT) | |
| (1 << NF_IP_POST_ROUTING))) != 0) { |
| printk("TCPMSS: path-MTU clamping only supported in " |
| "FORWARD, OUTPUT and POSTROUTING hooks\n"); |
| return 0; |
| } |
| |
| if (IPT_MATCH_ITERATE(e, find_syn_match)) |
| return 1; |
| printk("TCPMSS: Only works on TCP SYN packets\n"); |
| return 0; |
| } |
| |
| static struct ipt_target ipt_tcpmss_reg = { |
| .name = "TCPMSS", |
| .target = ipt_tcpmss_target, |
| .targetsize = sizeof(struct ipt_tcpmss_info), |
| .proto = IPPROTO_TCP, |
| .checkentry = ipt_tcpmss_checkentry, |
| .me = THIS_MODULE, |
| }; |
| |
| static int __init ipt_tcpmss_init(void) |
| { |
| return ipt_register_target(&ipt_tcpmss_reg); |
| } |
| |
| static void __exit ipt_tcpmss_fini(void) |
| { |
| ipt_unregister_target(&ipt_tcpmss_reg); |
| } |
| |
| module_init(ipt_tcpmss_init); |
| module_exit(ipt_tcpmss_fini); |