| /* |
| * 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"); |
| |
| #if 0 |
| #define DEBUGP printk |
| #else |
| #define DEBUGP(format, args...) |
| #endif |
| |
| static u_int16_t |
| cheat_check(u_int32_t oldvalinv, u_int32_t newval, u_int16_t oldcheck) |
| { |
| u_int32_t diffs[] = { oldvalinv, newval }; |
| return csum_fold(csum_partial((char *)diffs, sizeof(diffs), |
| oldcheck^0xFFFF)); |
| } |
| |
| 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, |
| void *userinfo) |
| { |
| const struct ipt_tcpmss_info *tcpmssinfo = targinfo; |
| struct tcphdr *tcph; |
| struct iphdr *iph; |
| u_int16_t tcplen, newtotlen, oldval, newmss; |
| unsigned int i; |
| u_int8_t *opt; |
| |
| if (!skb_make_writable(pskb, (*pskb)->len)) |
| return NF_DROP; |
| |
| if ((*pskb)->ip_summed == CHECKSUM_HW && |
| skb_checksum_help(*pskb, out == NULL)) |
| 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(!(*pskb)->dst) { |
| if (net_ratelimit()) |
| printk(KERN_ERR |
| "ipt_tcpmss_target: no dst?! can't determine path-MTU\n"); |
| return NF_DROP; /* or IPT_CONTINUE ?? */ |
| } |
| |
| 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 = cheat_check(htons(oldmss)^0xFFFF, |
| htons(newmss), |
| tcph->check); |
| |
| DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu" |
| "->%u.%u.%u.%u:%hu changed TCP MSS option" |
| " (from %u to %u)\n", |
| NIPQUAD((*pskb)->nh.iph->saddr), |
| ntohs(tcph->source), |
| NIPQUAD((*pskb)->nh.iph->daddr), |
| ntohs(tcph->dest), |
| oldmss, newmss); |
| goto retmodified; |
| } |
| } |
| |
| /* |
| * 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) { |
| if (net_ratelimit()) |
| printk(KERN_ERR "ipt_tcpmss_target:" |
| " unable to allocate larger skb\n"); |
| 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 = cheat_check(htons(tcplen) ^ 0xFFFF, |
| htons(tcplen + TCPOLEN_MSS), tcph->check); |
| tcplen += TCPOLEN_MSS; |
| |
| opt[0] = TCPOPT_MSS; |
| opt[1] = TCPOLEN_MSS; |
| opt[2] = (newmss & 0xff00) >> 8; |
| opt[3] = (newmss & 0x00ff); |
| |
| tcph->check = cheat_check(~0, *((u_int32_t *)opt), tcph->check); |
| |
| oldval = ((u_int16_t *)tcph)[6]; |
| tcph->doff += TCPOLEN_MSS/4; |
| tcph->check = cheat_check(oldval ^ 0xFFFF, |
| ((u_int16_t *)tcph)[6], tcph->check); |
| |
| newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS); |
| iph->check = cheat_check(iph->tot_len ^ 0xFFFF, |
| newtotlen, iph->check); |
| iph->tot_len = newtotlen; |
| |
| DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu" |
| "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n", |
| NIPQUAD((*pskb)->nh.iph->saddr), |
| ntohs(tcph->source), |
| NIPQUAD((*pskb)->nh.iph->daddr), |
| ntohs(tcph->dest), |
| newmss); |
| |
| retmodified: |
| 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 targinfosize, |
| 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); |