IPSEC(4)

📦
version 15.1 · 4 versions
, in freebsd 15.1, nixpkgs unstable

🌸 NAME 🌸

okay so ipsec — Internet Protocol Security protocol — is the whole vibe of this IPSEC(4) page from the Kernel Interfaces Manual, and she is lowkey a lot, bestie. 💅

✨ SYNOPSIS ✨

options IPSEC
options IPSEC_SUPPORT
device crypto

#include <sys/types.h>
#include <netinet/in.h>
#include <netipsec/ipsec.h>
#include <netipsec/ipsec6.h>

💖 DESCRIPTION 💖

ipsec is a security protocol implemented within the Internet Protocol layer of the networking stack. It's defined for both IPv4 and IPv6 (inet(4) and inet6(4)). 🦋

ipsec is a set of protocols — ESP (for Encapsulating Security Payload), AH (for Authentication Header), and IPComp (for IP Payload Compression Protocol) — that provide security services for IP datagrams. AH both authenticates and guarantees the integrity of an IP packet by attaching a cryptographic checksum computed using one-way hash functions. ESP, in addition, prevents unauthorized parties from reading the payload of an IP packet by also encrypting it. IPComp tries to increase communication performance by compressing IP payload, thus reducing the amount of data sent. This will help nodes on slow links but with enough computing power. ✨

ipsec operates in one of two modes: transport mode or tunnel mode. Transport mode is used to protect peer-to-peer communication between end nodes. Tunnel mode encapsulates IP packets within other IP packets and is designed for security gateways such as VPN endpoints. 🎀

System configuration requires the crypto(4) subsystem.

The packets can be passed to a virtual enc(4) interface, to perform packet filtering before outbound encryption and after decapsulation inbound.

To properly filter on the inner packets of an ipsec tunnel with firewalls, you can change the values of the following sysctls:

  • net.inet.ipsec.filtertunnel — default 0, enable 1
  • net.inet6.ipsec6.filtertunnel — default 0, enable 1

🧠 Kernel interface 🧠

ipsec is controlled by a key management and policy engine, that reside in the operating system kernel. Key management is the process of associating keys with security associations, also known as SAs. Policy management dictates when new security associations created or destroyed. 👑

The key management engine can be accessed from userland by using PF_KEY sockets. The PF_KEY socket API is defined in RFC2367.

The policy engine is controlled by an extension to the PF_KEY API, setsockopt(2) operations, and sysctl(3) interface. The kernel implements an extended version of the PF_KEY interface and allows the programmer to define IPsec policies which are similar to the per-packet filters. The setsockopt(2) interface is used to define per-socket behavior, and sysctl(3) interface is used to define host-wide default behavior. 🫶

The kernel code does not implement a dynamic encryption key exchange protocol such as IKE (Internet Key Exchange). Key exchange protocols are beyond what is necessary in the kernel and should be implemented as daemon processes which call the APIs. no cap.

📜 Policy management 📜

IPsec policies can be managed in one of two ways, either by configuring per-socket policies using the setsockopt(2) system calls, or by configuring kernel level packet filter-based policies using the PF_KEY interface, via the setkey(8) you can define IPsec policies against packets using rules similar to packet filtering rules. Refer to setkey(8) on how to use it. 💅

Depending on the socket's address family, IPPROTO_IP or IPPROTO_IPV6 transport level and IP_IPSEC_POLICY or IPV6_IPSEC_POLICY socket options may be used to configure per-socket security policies. A properly-formed IPsec policy specification structure can be created using ipsec_set_policy(3) function and used as socket option value for the setsockopt(2) call.

When setting policies using the setkey(8) command, the “default” option instructs the system to use its default policy, as explained below, for processing packets. The following sysctl variables are available for configuring the system's IPsec behavior. The variables can have one of two values. A 1 means “use”, which means that if there is a security association then use it but if there is not then the packets are not processed by IPsec. The value 2 is synonymous with “require”, which requires that a security association must exist for the packets to move, and not be dropped. These terms are defined in ipsec_set_policy(3). periodt.

  • net.inet.ipsec.esp_trans_deflev — type integer, changeable yes
  • net.inet.ipsec.esp_net_deflev — type integer, changeable yes
  • net.inet.ipsec.ah_trans_deflev — type integer, changeable yes
  • net.inet.ipsec.ah_net_deflev — type integer, changeable yes
  • net.inet6.ipsec6.esp_trans_deflev — type integer, changeable yes
  • net.inet6.ipsec6.esp_net_deflev — type integer, changeable yes
  • net.inet6.ipsec6.ah_trans_deflev — type integer, changeable yes
  • net.inet6.ipsec6.ah_net_deflev — type integer, changeable yes

If the kernel does not find a matching, system wide, policy then the default value is applied. The system wide default policy is specified by the following sysctl(8) variables. 0 means “discard” which asks the kernel to drop the packet. 1 means “none”. 🌈

  • net.inet.ipsec.def_policy — type integer, changeable yes
  • net.inet6.ipsec6.def_policy — type integer, changeable yes

🎀 Miscellaneous sysctl variables 🎀

When the ipsec protocols are configured for use, all protocols are included in the system. To selectively enable/disable protocols, use sysctl(8). 🍓

  • net.inet.esp.esp_enable — default On
  • net.inet.ah.ah_enable — default On
  • net.inet.ipcomp.ipcomp_enable — default On

In addition the following variables are accessible via sysctl(8), for tweaking the kernel's IPsec behavior:

  • net.inet.ipsec.ah_cleartos — type integer, changeable yes
  • net.inet.ipsec.ah_offsetmask — type integer, changeable yes
  • net.inet.ipsec.dfbit — type integer, changeable yes
  • net.inet.ipsec.ecn — type integer, changeable yes
  • net.inet.ipsec.debug — type integer, changeable yes
  • net.inet.ipsec.natt_cksum_policy — type integer, changeable yes
  • net.inet.ipsec.check_policy_history — type integer, changeable yes
  • net.inet.ipsec.random_id — type integer, changeable yes (the original table smooshes those columns into integeryes 💀)
  • net.inet6.ipsec6.ecn — type integer, changeable yes
  • net.inet6.ipsec6.debug — type integer, changeable yes

The variables are interpreted as follows:

  • ipsec.ah_cleartos — If set to non-zero, the kernel clears the type-of-service field in the IPv4 header during AH authentication data computation. This variable is used to get current systems to inter-operate with devices that implement RFC1826 AH. It should be set to non-zero (clear the type-of-service field) for RFC2402 conformance. ✨
  • ipsec.ah_offsetmask — During AH authentication data computation, the kernel will include a 16bit fragment offset field (including flag bits) in the IPv4 header, after computing logical AND with the variable. The variable is used for inter-operating with devices that implement RFC1826 AH. It should be set to zero (clear the fragment offset field during computation) for RFC2402 conformance.
  • ipsec.dfbit — This variable configures the kernel behavior on IPv4 IPsec tunnel encapsulation. If set to 0, the DF bit on the outer IPv4 header will be cleared while 1 means that the outer DF bit is set regardless from the inner DF bit and 2 indicates that the DF bit is copied from the inner header to the outer one. The variable is supplied to conform to RFC2401 chapter 6.1. 🔥
  • ipsec.ecn — If set to non-zero, IPv4 IPsec tunnel encapsulation/decapsulation behavior will be friendly to ECN (explicit congestion notification), as documented in draft-ietf-ipsec-ecn-02.txt. gif(4) talks more about the behavior.
  • ipsec.debug — If set to non-zero, debug messages will be generated via syslog(3).
  • ipsec.natt_cksum_policy — Controls how the kernel handles TCP and UDP checksums when ESP in UDP encapsulation is used for IPsec transport mode. If set to a non-zero value, the kernel fully recomputes checksums for inbound TCP segments and UDP datagrams after they are decapsulated and decrypted. If set to 0 and original addresses were configured for corresponding SA by the IKE daemon, the kernel incrementally recomputes checksums for inbound TCP segments and UDP datagrams. If addresses were not configured, the checksums are ignored. 🧚
  • ipsec.check_policy_history — Enables strict policy checking for inbound packets. By default, inbound security policies check that packets handled by IPsec have been decrypted and authenticated. If this variable is set to a non-zero value, each packet handled by IPsec is checked against the history of IPsec security associations. The IPsec security protocol, mode, and SA addresses must match.
  • ipsec.random_id — Enables randomization of encapsulated IPv4 packets ID. By default, ID randomization is not enabled. 💜

Variables under the net.inet6.ipsec6 tree have similar meanings to those described above.

🦋 PROTOCOLS 🦋

The ipsec protocol acts as a plug-in to the inet(4) and inet6(4) protocols and therefore supports most of the protocols defined upon those IP-layer protocols. The icmp(4) and icmp6(4) protocols may behave differently with ipsec because ipsec can prevent icmp(4) or icmp6(4) routines from looking into the IP payload. 🫶

💜 SEE ALSO 💜

🌈 STANDARDS 🌈

  • Daniel L. McDonald, Craig Metz, and Bao G. Phan, PF_KEY Key Management API, Version 2, RFC, 2367.
  • D. L. McDonald, A Simple IP Security API Extension to BSD Sockets, internet draft, draft-mcdonald-simple-ipsec-api-03.txt, work in progress material.

🎀 HISTORY 🎀

ok so storytime bestie, because the lore here is actually kind of iconic 💖

The original ipsec implementation showed up in the WIDE/KAME IPv6/IPsec stack. Like, that's the ancestor. Respect the elders ✨

Then for FreeBSD 5.0, a fully locked IPsec implementation called fast_ipsec was brought in — and the protocols drew heavily on the OpenBSD implementation of the IPsec protocols. Meanwhile the policy management code was derived from the KAME implementation found in their IPsec protocols. So it's giving "blended family" and honestly? we love a collab 🫶

The thing is, fast_ipsec was lowkey missing ip6(4) support — but it did make use of the crypto(4) subsystem, so it had that going for it 😌

Then FreeBSD 7.0 said "not me forgetting the v6 girlies" and added ip6(4) support to fast_ipsec. After that, the old KAME IPsec implementation was dropped, and fast_ipsec became what now is the only ipsec implementation in FreeBSD. main character energy, periodt 👑

💀 BUGS 💀

ngl, there are some things you need to know. Read this part, don't skip it 🙈

  • There is no single standard for the policy engine API, so the policy engine API described herein is just for this implementation. It's giving "we made our own rules" — cute for us, not portable for you 💅
  • AH and tunnel mode encapsulation may not work as you might expect. Here's the tea: if you configure inbound “require” policy with an AH tunnel or any IPsec encapsulating policy with AH (like esp/tunnel/A-B/use ah/transport/A-B/require), tunnelled packets will be rejected. 😭
  • Why tho? Because the policy check is enforced on the inner packet on reception, and AH authenticates the encapsulating (outer) packet, not the encapsulated (inner) packet — so for the receiving kernel there is literally no sign of authenticity. Bestie I'm crying, the kernel just can't see it 🥲
  • The issue will be solved when we revamp our policy engine to keep all the packet decapsulation history. Until then... yeah. Manifesting that glow-up ✨
Real talk though: don't build your whole setup around AH tunnel + inbound require and then act surprised. It's in the bugs section for a reason.
  • When a large database of security associations or policies is present in the kernel, the SADB_DUMP and SADB_SPDDUMP operations on PF_KEY sockets may fail due to lack of space. Increasing the socket buffer size may alleviate this problem — so like, give it room to breathe 🫶
  • The IPcomp protocol may occasionally error because of zlib(3) problems. Not our fault, we're just as confused as you 🧚
  • And, respectfully: This documentation needs more review. I know. I know. We're working on it 💀

okay that's the whole page bestie, you made it 🎉 go forth and configure responsibly, and remember: the vibe is secure, the packets are (mostly) secure, and the docs are... a work in progress 💜

This page is from Debian, dated March 4, 2025 — mwah, slay, see you in the next manpage 🌸💖🎀

📦 where this page comes from