[strongSwan] DPD and rekeying useless for road^w home warriors

Kajetan Staszkiewicz vegeta at tuxpowered.net
Tue Aug 10 16:06:34 CEST 2021

Hello group!

I'm running 2 Strongswan-based VPN IKEv2 servers runnig on Linux (Debian
Buster with kernel 5.10 and Strongswan 5.8). Together they grant access
to my corporate network to around 200 users each day since the pandemics
has started.

One of the servers is available only over IPv4 and the other over IPv4
and IPv6 and we see more than half of clients connecting over IPv6.

I have came across a problem of server talking to client for reasons of
DPD (see [1]) or rekeying (see [2]):

1. Client is using IPv6.
2. Client sends some data (ikev2_init) over UDP 500.
3. Client's home router creates a firewall state for UDP 500.
4. Client sends some data (ikev2_auth) over UDP 4500.
5. Client's home router creates a firewall state for UDP 4500.
6. IPsec is established.
7. Client tunnels traffic in ESP.
8. Client's home router creates a firewall state for ESP.
9. Some time passes.
10. States for UDP 500 and UDP 4500 are removed from client's
    home router.
11. State for ESP is kept because there is active traffic
    flowing through it.
11. More time passes.
10. Server decides it's time for DPD or rekeying.
11. Server sends packets (parent_sa inf2 in case of DPD) to client
    over UDP 4500.
12. This data never reaches the client due to his home router
    not forwarding them.
13. No responses come to the server.
14. Served decides the client is dead and removes his policies.
15. Client comes to us complaining that his "vpn broke". This also
    sometimes leaves Windows VPN client unable to reconnect.

How is it supposed to work in real life? For clients connecting over
IPv4 I *think* this works fine because of active traffic being
encapsulated in UDP 4500. Support for IPv6 UDP IPsec has been added only
recently to Linux kernel and I'm not even sure if Windows or Mac OS
clients can do this. Also - why go with UDP at all? Pure ESP has lower
overhead, doesn't it?

When I started developing the VPN solution, I came across the problem
that clients (mostly Windows and Mac) sometimes loose connection
(problems at ISP, at home WiFi, having the laptop sleep, etc), reconnect
and demand the same IP address. This address is granted by Strongswan
but it is impossible to install policies in Kernel as the old ones still
exist. At least that's what I've documented over a year ago. In order to
ensure that a client will be able to reconnect, I've pushed DPD timers
quite low:

dpd_delay = 40s # There's around 20 seconds from retransmit
                # algorithm, makes total ~60s
dpd_action = clear

So far so good, clients now can reconnect. And for most clients doing
real work, this is fine - they will generate some traffic once every 40

But when I'm connecting from a test system (tested on Windows and Linux
Ubuntu), which does nothing, it is easy to have no data sent over 40

I have given up on rekeying, timers are set to some absurd values
ensuring that clients can work fine the whole day.

| pozdrawiam / greetings | Powered by macOS, Debian and FreeBSD |
|  Kajetan Staszkiewicz  |  www: http://vegeta.tuxpowered.net   |

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