RFC 8229 - TCP support for IKEv2 and ESP

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As of version 4.0, libreswan supports RFC 8229 that supports sending IKE packets and encapsulating ESP packets over the TCP protocol. The IKEv1 protocol does not support TCP support. TCP support is only available when IKEv2 is used.

The IPsec TCP kernel support was merged in Linux kernel 5.6. See LWN: RFC 8229 (TCP Encapsulation for IPsec) support merged. Note that some important bugfixes have since been merged in and the Libreswan Team has found and reported some remaining issues.

You can watch the https://netdevconf.info/0x13/index.html NetDev 0x13] presentation IPsec encapsulation over TCP by Red Hat kernel developer Sabrina Dubroca if you are interested in the kernel implementation details.

IPsec connections are negotiated using IKE. This protocol is based on UDP and uses UDP port 500 and 4500. Once the IKE negotiation has completed, IP packets are encrypted and transported using the ESP protocol (protocol 50). Many routers and NAT gateways only support sending UDP and TCP packets and would drop ESP packets. A method was standardized in 2005 by RFC 3947 and RFC 3948 that defines how negotiate and encapsulate ESP packets to be transported within UDP. This is often written as ESPinUDP. These UDP packets are send over UDP port 4500.

Unfortunately, a number of networks block all non-DNS UDP packets and some networks specifically block IPsec VPNs by blocking UDP port 500 and 4500. With the introduction of RFC 8229 IKE and ESP can now be encapsulated in TCP on any (preconfigured) port. Furthermore, the TCP stream could further be encrypted as HTTPS traffic, so it becomes indistinguishable from generic web traffic. Note that libreswan does not currently support using HTTPS, only TCP directly. But this resolves the vast majority of network blocking issues seen in the wild. We will use IKEoverTCP and ESPinTCP to refer to the implementation of RFC 8223.

When to use TCP for IKE and EPS

As Section 12 of RFC 8229 clearly states, TCP should be avoided when possible. The performance is significantly reduced, especially when there is packet loss on the TCP link. Furthermore, any on-path attacker can just send spoofed garbage or an RST packet onto the TCP stream to break it. Do not switch all your UDP based connections to TCP just because sometimes UDP packets are dropped.

The RFC recommends to always try UDP first, and only later fallback to TCP. And once on TCP, to keep trying to see if UDP starts working and if it does to migrate back to UDP. Currently, libreswan only implements using TCP as fallback or using only TCP. It does not implement checking of it can switch back to UDP. There are also some interactions with MOBIKE that might prevent connections that were established on TCP to move back to UDP when a network change happens (eg switching from wifi to mobile data or back).

New configuration options

In the config setup section of the /etc/ipsec.conf there is a new option listen-tcp=yes|no (default: no) Note that the listen= option was renamed to listen-udp=yes|no, so it is possible to only offer TCP and not UDP.

In the connection section, there are two new options:

  • enable-tcp=yes|no|fallback (default: no)
  • tcp-remoteport= (default: 4500)

Note that the local TCP port is obtained like any other regular application - it will ask the operating system for an ephemeral port to use.

Note that leftikeport= and rightikeport= only affect UDP ports.

Note that currently, there is a bug which means libreswan can only listen as a responder on TCP port 4500


For some example configurations, see our TCP testcases: