--- name: "ipsec" section: "4" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "openbsd", "macos"] url: "https://new.carta.tech/ipsec/4" --- # ipsec(4) Internet Protocol Security protocol ## NAME **ipsec** – Internet Protocol Security protocol ## SYNOPSIS **options IPSEC** **options IPSEC\_SUPPORT** **device crypto** **#include \** **#include \** **#include \** **#include \** ## DESCRIPTION **ipsec** is a security protocol implemented within the Internet Protocol layer of the networking stack. **ipsec** is defined for both IPv4 and IPv6 ((inet) 4 and [inet6(4)](/inet6/4.md)). **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)](/crypto/4.md) subsystem. The packets can be passed to a virtual [enc(4)](/enc/4.md) 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 | | | | | --- | --- | --- | | **Name** | Default | Enable | | net.inet.ipsec.filtertunnel | 0 | 1 | | net.inet6.ipsec6.filtertunnel | 0 | 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 know 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)](/setsockopt/2.md) operations, and [sysctl(3)](/sysctl/3.md) 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)](/setsockopt/2.md) interface is used to define per-socket behavior, and [sysctl(3)](/sysctl/3.md) 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.** ### Policy management IPsec policies can be managed in one of two ways, either by configuring per-socket policies using the [setsockopt(2)](/setsockopt/2.md) system calls, or by configuring kernel level packet filter-based policies using the PF\_KEY interface, via the [setkey(8)](/setkey/8.md) you can define IPsec policies against packets using rules similar to packet filtering rules. Refer to [setkey(8)](/setkey/8.md) 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)](/ipsec_set_policy/3.md) function and used as socket option value for the [setsockopt(2)](/setsockopt/2.md) call. When setting policies using the [setkey(8)](/setkey/8.md) 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)](/ipsec_set_policy/3.md). | | | | | --- | --- | --- | | **Name** | Type | Changeable | | net.inet.ipsec.esp\_trans\_deflev | integer | yes | | net.inet.ipsec.esp\_net\_deflev | integer | yes | | net.inet.ipsec.ah\_trans\_deflev | integer | yes | | net.inet.ipsec.ah\_net\_deflev | integer | yes | | net.inet6.ipsec6.esp\_trans\_deflev | integer | yes | | net.inet6.ipsec6.esp\_net\_deflev | integer | yes | | net.inet6.ipsec6.ah\_trans\_deflev | integer | yes | | net.inet6.ipsec6.ah\_net\_deflev | integer | 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)](/sysctl/8.md) variables. 0 means “discard” which asks the kernel to drop the packet. 1 means “none”. | | | | | --- | --- | --- | | **Name** | Type | Changeable | | net.inet.ipsec.def\_policy | integer | yes | | net.inet6.ipsec6.def\_policy | integer | 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)](/sysctl/8.md). | | | | --- | --- | | **Name** | Default | | net.inet.esp.esp\_enable | On | | net.inet.ah.ah\_enable | On | | net.inet.ipcomp.ipcomp\_enable | On | In addition the following variables are accessible via [sysctl(8)](/sysctl/8.md), for tweaking the kernel's IPsec behavior: | | | | | | --- | --- | --- | --- | | **Name** | Type | Changeable | | net.inet.ipsec.ah\_cleartos | integer | yes | | net.inet.ipsec.ah\_offsetmask | integer | yes | | net.inet.ipsec.dfbit | integer | yes | | net.inet.ipsec.ecn | integer | yes | | net.inet.ipsec.debug | integer | yes | | net.inet.ipsec.natt\_cksum\_policy | integer | yes | | net.inet.ipsec.check\_policy\_history | integer | yes | | net.inet.ipsec.random\_id | | integer | yes | | net.inet6.ipsec6.ecn | integer | yes | | net.inet6.ipsec6.debug | integer | 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)](/gif/4.md) talks more about the behavior. - ipsec.debug If set to non-zero, debug messages will be generated via [syslog(3)](/syslog/3.md). - 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)](/inet/4.md) and [inet6(4)](/inet6/4.md) protocols and therefore supports most of the protocols defined upon those IP-layer protocols. The [icmp(4)](/icmp/4.md) and [icmp6(4)](/icmp6/4.md) protocols may behave differently with **ipsec** because **ipsec** can prevent [icmp(4)](/icmp/4.md) or [icmp6(4)](/icmp6/4.md) routines from looking into the IP payload. ## SEE ALSO [ioctl(2)](/ioctl/2.md), [socket(2)](/socket/2.md), [ipsec\_set\_policy(3)](/ipsec_set_policy/3.md), [crypto(4)](/crypto/4.md), [dtrace\_mib(4)](/dtrace_mib/4.md), [enc(4)](/enc/4.md), [icmp6(4)](/icmp6/4.md), [if\_ipsec(4)](/if_ipsec/4.md), [intro(4)](/intro/4.md), [ip6(4)](/ip6/4.md), [setkey(8)](/setkey/8.md), [sysctl(8)](/sysctl/8.md) S. Kent and R. Atkinson, *IP Authentication Header*, RFC 2404. S. Kent and R. Atkinson, *IP Encapsulating Security Payload (ESP)*, RFC 2406. ## 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 The original **ipsec** implementation appeared in the WIDE/KAME IPv6/IPsec stack. For FreeBSD 5.0 a fully locked IPsec implementation called fast\_ipsec was brought in. The protocols drew heavily on the OpenBSD implementation of the IPsec protocols. The policy management code was derived from the KAME implementation found in their IPsec protocols. The fast\_ipsec implementation lacked [ip6(4)](/ip6/4.md) support but made use of the [crypto(4)](/crypto/4.md) subsystem. For FreeBSD 7.0 [ip6(4)](/ip6/4.md) support was added to fast\_ipsec. After this the old KAME IPsec implementation was dropped and fast\_ipsec became what now is the only **ipsec** implementation in FreeBSD. ## BUGS There is no single standard for the policy engine API, so the policy engine API described herein is just for this implementation. AH and tunnel mode encapsulation may not work as you might expect. 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. This is because the policy check is enforced on the inner packet on reception, and AH authenticates encapsulating (outer) packet, not the encapsulated (inner) packet (so for the receiving kernel there is no sign of authenticity). The issue will be solved when we revamp our policy engine to keep all the packet decapsulation history. 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. The IPcomp protocol may occasionally error because of [zlib(3)](/zlib/3.md) problems. This documentation needs more review.