netfilter: xt_TCPMSS: SYN packets are allowed to contain data (10a19939) · Commits · 戴 / test

The TCPMSS target is dropping SYN packets where: 1) There is data, or 2) The data offset makes the TCP header larger than the packet. Both of these result in an error level printk. This printk has been removed. This change avoids dropping SYN packets containing data. If there is also no MSS option (as well as data), one will not be added because of possible complications due to the increased packet size. Signed-off-by:

Simon Arlott <simon@fire.lp0.eu> Signed-off-by:

Patrick McHardy <kaber@trash.net>

net/netfilter/xt_TCPMSS.c

+8 −10

Original line number	Diff line number	Diff line
		@@ -60,17 +60,9 @@ tcpmss_mangle_packet(struct sk_buff *skb,
		tcplen = skb->len - tcphoff;
		tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);

		/* 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 "xt_TCPMSS: bad length (%u bytes)\n",
		skb->len);
		/* Header cannot be larger than the packet */
		if (tcplen < tcph->doff*4)
		return -1;
		}

		if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
		if (dst_mtu(skb_dst(skb)) <= minlen) {
		@@ -115,6 +107,12 @@ tcpmss_mangle_packet(struct sk_buff *skb,
		}
		}

		/* There is data after the header so the option can't be added
		without moving it, and doing so may make the SYN packet
		itself too large. Accept the packet unmodified instead. */
		if (tcplen > tcph->doff*4)
		return 0;

		/*
		* MSS Option not found ?! add it..
		*/

Admin message