Dear and community, I have a conundrum. We have IPsec (transport mode) set-up between all our servers; we are also starting to use Rancher. Rancher sets up its own IPsec-protected network (tunnel mode).

So we end up with IPsec in IPsec, which sucks. Specifically, can't seem to get MTU right, there is *always* a window of packet sizes that simply will *not* get through.

Anybody knows of a way to tell the back-end IPsec "so, if it's a Rancher's IPsec packet, don't touch it"?

Fun fact, the window of packet sizes that will never go through is exactly 93 bytes. So, packets smaller than X go through, packets larger than X+92 go through, but packets with sizes of X to X-92 (inclusive) will not.

X depends on the MTU set on Rancher's virtual network interface. Specifically, it's exactly MTU - 121.

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@wxcafe you jest, but in my frustration and desperation, at 5AM, after 18 staight hours of debugging this bugger, I did set the MTU on the Rancher interface to 400.

Packets with sizes between 279 and 372 bytes (inclusive) never got through.

X = MTU - 121
279 = 400 - 121

:blobugh: 🤦

@rysiek solved? maybe you can use SPD entries to exclude traffic trom IPsec?

@saper solved-ish:

It's a weird one. I explored the idea of excluding IPsec on SPD level but did not find a way. I did not invest a lot of time into this. If you have an idea how to do this, do let tell, it does sound like the right way to deal with it.

@saper they're generated by:

To e specific:

So, something like:
spdadd "{LOCAL_IPV4}/32 ${REMOTE_IPV4}/32 any -P out ipsec
esp/transport//require ah/transport//require;

(and reverse, and same for IPv6)

@rysiek just to check, since I do not see it enabled in racoon.conf - you are not using UDP encapsulation for ESP?

@saper no; nat_traversal is not set and the default is, I believe, "yes" (which means "use when NAT is discovered during phase 1 negotiatnion"); there is no NAT between the peers though, and in tcpdump I only see ESP/AH.

@rysiek no ideas. what really wonders me how non-IP traffic (your already encrypted ESP) gets encapsulated by IPsec again. Maybe you can find out why.

@saper I am *GUESSING* it's IPsec transport mode on the outside, IPsec *tunnel* mode on the inside. Quite possible that the rancher IPsec (the one in tunnel mode, on the inside) uses NATT now that I think of it, but not sure.

@rysiek can you sniff stuff going out of rancher somehow?

@saper not really, no. I guess I would have to use Wireshark's IPsec decryptor.

@rysiek my tcpdump(1) has -E:

-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that
are addressed to addr and contain Security Parameter Index value
spi. This combination may be repeated with comma or newline sep‐

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