/* * This is a module which is used for setting the MSS option in TCP packets. * * Copyright (C) 2000 Marc Boucher * * 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 #include #include #include #include #include MODULE_LICENSE("GPL"); MODULE_AUTHOR("Marc Boucher "); 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 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, void *targinfo, unsigned int targinfosize, unsigned int hook_mask) { const struct ipt_tcpmss_info *tcpmssinfo = targinfo; const struct ipt_entry *e = e_void; if (targinfosize != IPT_ALIGN(sizeof(struct ipt_tcpmss_info))) { DEBUGP("ipt_tcpmss_checkentry: targinfosize %u != %u\n", targinfosize, IPT_ALIGN(sizeof(struct ipt_tcpmss_info))); return 0; } 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 (e->ip.proto == IPPROTO_TCP && !(e->ip.invflags & IPT_INV_PROTO) && 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, .checkentry = ipt_tcpmss_checkentry, .me = THIS_MODULE, }; static int __init init(void) { return ipt_register_target(&ipt_tcpmss_reg); } static void __exit fini(void) { ipt_unregister_target(&ipt_tcpmss_reg); } module_init(init); module_exit(fini);