Re: ipfw nat and smaller wan mtu
- In reply to: John Hay : "ipfw nat and smaller wan mtu"
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Date: Thu, 08 Dec 2022 06:57:08 UTC
Hi,
Adding this patch does make it work for me. There might be better ways to
do it. I have tested with ping and ssh. In ping's case, ping reported:
frag needed and DF set (MTU 1392)
In ssh's case I could see with tcpdump that the "need to frag (mtu 1392)"
was sent back and the next packet's length was adjusted.
#####
06:29:59.869677 IP (tos 0x48, ttl 64, id 0, offset 0, flags [DF], proto TCP
(6), length 1500)
10.10.1.3.64344 > 10.10.7.7.22: Flags [.], cksum 0xb64d (correct), seq
39:1487, ack 39, win 1027, options [nop,nop,TS val 260430893 ecr
926374970], length 1448
06:29:59.869954 IP (tos 0x0, ttl 63, id 62454, offset 0, flags [none],
proto ICMP (1), length 596)
10.10.2.2 > 10.10.1.3: ICMP 10.10.7.7 unreachable - need to frag (mtu
1392), length 576
IP (tos 0x48, ttl 63, id 0, offset 0, flags [DF], proto TCP (6), length
1500, bad cksum e081 (->19b7)!)
10.10.1.3.64344 > 10.10.7.7.22: Flags [.], seq 39:1487, ack 39, win
1027, options [nop,nop,TS val 260430893 ecr 926374970], length 1448
06:29:59.871301 IP (tos 0x48, ttl 64, id 0, offset 0, flags [DF], proto TCP
(6), length 1392)
10.10.1.3.64344 > 10.10.7.7.22: Flags [.], cksum 0x6841 (correct), seq
39:1379, ack 39, win 1027, options [nop,nop,TS val 260430893 ecr
926374970], length 1340
#####
--- sys/netinet/libalias/alias.c.orig 2022-05-12 04:54:03.000000000 +0000
+++ sys/netinet/libalias/alias.c 2022-12-08 05:42:25.127980000 +0000
@@ -365,6 +365,19 @@
lnk = NULL;
if (lnk != NULL) {
+ /*
+ If the packet was locally generated, it will have a
+ loopback address as source, which will not be handled
+ correctly. For now use the destination address as source
+ address. The correct source address might be the the
+ interface address that the packet will be going out on.
+ */
+ if (IN_LOOPBACK(ntohl(pip->ip_src.s_addr)) &&
+ !IN_LOOPBACK(ntohl(pip->ip_dst.s_addr))) {
+ DifferentialChecksum(&pip->ip_sum,
+ &pip->ip_dst, &pip->ip_src, 2);
+ pip->ip_src = pip->ip_dst;
+ }
if (ip->ip_p == IPPROTO_UDP || ip->ip_p == IPPROTO_TCP) {
int accumulate, accumulate2;
struct in_addr original_address;
On Wed, 7 Dec 2022 at 16:33, John Hay <john@sanren.ac.za> wrote:
> Hi,
>
> What would the proper ipfw rules be to make nat work and properly get the
> icmp too big packets back to a local host if the wan interface needs a
> smaller mtu?
>
> I'm using a FreeBSD machine as router/firewall, but its wan interface
> needs a smaller mtu (1392) than the default ethernet mtu. I have replicated
> this in a VM so I can test it. My simplified ipfw rules make it work for
> packets that are smaller than the wan mtu:
>
> #####
> net.inet.ip.fw.one_pass=0
> net.inet.ip.fw.verbose=1
> #####
> fwcmd="/sbin/ipfw -q"
> wan="vtnet0"
> lan="vtnet1"
> ${fwcmd} nat 123 config if ${wan} log
> ${fwcmd} add 1000 count log all from any to any
> ${fwcmd} add 5000 nat 123 ip4 from any to any via ${wan}
> ${fwcmd} add 6000 allow log all from any to any
> #####
> The wan ip of the firewall is 10.10.2.2 and the ip address of the host (on
> the lan side) I'm testing from is 10.10.1.3. And I did a ping to 10.10.5.5,
> which is on the other side of the wan interface.
>
> This works for packets smaller than the wan mtu. But if the packet is
> larger than the wan mtu, the icmp too big is generated, but with 127.0.0.1
> as the source and the wan ip as the destination and then sent via lo0 and
> it looks like this in the ipfw log:
>
> Dec 7 13:24:59 rtr kernel: ipfw: 1000 Count ICMP:3.4 127.0.0.1 10.10.2.2
> out via lo0
>
> So I added a nat ipfw rule to catch that:
>
> ${fwcmd} add 5050 nat 123 ip4 from any to not 127.0.0.1 via lo0
>
> That helped partly because it was then able to recover the address of the
> host I was testing from and tried to send the packet out on the correct
> interface (vtnet1). Unfortunately it still had the source address of
> 127.0.0.1, which means it did not actually make it to the wire:
>
> ######
> Dec 7 14:17:31 rtr kernel: ipfw: 1000 Count ICMP:8.0 10.10.1.3 10.10.5.5
> in via vtnet1
> Dec 7 14:17:31 rtr kernel: ipfw: 6000 Accept ICMP:8.0 10.10.1.3 10.10.5.5
> in via vtnet1
> Dec 7 14:17:31 rtr kernel: ipfw: 1000 Count ICMP:8.0 10.10.1.3 10.10.5.5
> out via vtnet0
> Dec 7 14:17:31 rtr kernel: ipfw: 6000 Accept ICMP:8.0 10.10.2.2 10.10.5.5
> out via vtnet0
> Dec 7 14:17:31 rtr kernel: ipfw: 1000 Count ICMP:3.4 127.0.0.1 10.10.2.2
> out via lo0
> Dec 7 14:17:31 rtr kernel: ipfw: 6000 Accept ICMP:3.4 127.0.0.1 10.10.2.2
> out via lo0
> Dec 7 14:17:31 rtr kernel: ipfw: 1000 Count ICMP:3.4 127.0.0.1 10.10.2.2
> in via lo0
> Dec 7 14:17:31 rtr kernel: ipfw: 6000 Accept ICMP:3.4 127.0.0.1 10.10.1.3
> in via lo0
> Dec 7 14:17:31 rtr kernel: ipfw: 1000 Count ICMP:3.4 127.0.0.1 10.10.1.3
> out via vtnet1
> Dec 7 14:17:31 rtr kernel: ipfw: 6000 Accept ICMP:3.4 127.0.0.1 10.10.1.3
> out via vtnet1
> ######
>
> Once I have this sorted, there seems to be a similar problem with nptv6.
>
> Regards
>
> John
>
>