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IPSEC(4) Kernel Interfaces Manual IPSEC(4) NAME ipsec -- Internet Protocol Security protocol 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. ipsec is defined for both IPv4 and IPv6 (inet(4) and inet6(4)). ipsec is a set of protocols, ESP (for Encapsu- lating 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 par- ties 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 en- capsulates IP packets within other IP packets and is designed for secu- rity 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 in- bound. To properly filter on the inner packets of an ipsec tunnel with fire- walls, 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 asso- ciating keys with security associations, also know as SAs. Policy man- agement 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 imple- ments an extended version of the PF_KEY interface and allows the pro- grammer to define IPsec policies which are similar to the per-packet filters. The setsockopt(2) interface is used to define per-socket be- havior, and sysctl(3) interface is used to define host-wide default be- havior. 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) system calls, or by config- uring kernel level packet filter-based policies using the PF_KEY inter- face, 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). 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) 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 in- cluded in the system. To selectively enable/disable protocols, use sysctl(8). 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), 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 integeryes 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 confor- mance. ipsec.ah_offsetmask During AH authentication data computation, the kernel will in- clude 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 im- plement RFC1826 AH. It should be set to zero (clear the frag- ment offset field during computation) for RFC2402 conformance. ipsec.dfbit This variable configures the kernel behavior on IPv4 IPsec tun- nel 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/decapsula- tion behavior will be friendly to ECN (explicit congestion no- tification), 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 decapsu- lated 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 de- fault, 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 de- fault, 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) proto- cols and therefore supports most of the protocols defined upon those IP-layer protocols. The icmp(4) and icmp6(4) protocols may behave dif- ferently with ipsec because ipsec can prevent icmp(4) or icmp6(4) rou- tines from looking into the IP payload. SEE ALSO ioctl(2), socket(2), ipsec_set_policy(3), crypto(4), enc(4), icmp6(4), if_ipsec(4), intro(4), ip6(4), setkey(8), sysctl(8) 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 implementa- tion 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) support but made use of the crypto(4) subsystem. For FreeBSD 7.0 ip6(4) 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 recep- tion, and AH authenticates encapsulating (outer) packet, not the encap- sulated (inner) packet (so for the receiving kernel there is no sign of authenticity). The issue will be solved when we revamp our policy en- gine 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 sock- ets 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) problems. This documentation needs more review. FreeBSD 15.0 March 4, 2025 IPSEC(4)
NAME | SYNOPSIS | DESCRIPTION | PROTOCOLS | SEE ALSO | STANDARDS | HISTORY | BUGS
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