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IPSEC(4) BSD Kernel Interfaces Manual IPSEC(4) NAME ipsec -- IP security protocol SYNOPSIS #include <sys/types.h> #include <netinet/in.h> #include <netinet6/ipsec.h> DESCRIPTION ipsec is a security protocol implemented within the Internet Protocol layer of the TCP/IP stack. ipsec is defined for both IPv4 and IPv6 (inet(4) and inet6(4)). ipsec contains two protocols, ESP, the encapsu- lated security payload protocol and AH, the authentication header proto- col. ESP prevents unauthorized parties from reading the payload of an IP packet by encrypting it using secret key cryptography algorithms. AH both authenticates guarantees the integrity of an IP packet by attaching a cryptographic checksum computed using one-way hash functions. ipsec has operates in one of two modes: transport mode or tunnel mode. Trans- port 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. 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 associat- ing 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) 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 pro- tocol such as IKE (Internet Key Exchange). Key exchange protocols are beyond what is necessary in the kernel and should be implemented as dae- mon 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 configur- ing kernel level packet filter-based policies using the PF_KEY interface, via the setkey(8) command. In either case, IPsec policies must be speci- fied using the syntax described in ipsec_set_policy(3). Please refer to the setkey(8) man page for instructions on its use. When setting policies using the setkey(8) command the "default" option you can have the system use its default policy, explained below, for pro- cessing packets. The following sysctl variables are available for con- figuring 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 associ- ation 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(8). 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 de- fault value is applied. The system wide default policy is specified by the following sysctl(8) variables. 0 means "discard" which asks the ker- nel 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 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.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 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 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 re- gardless 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 notifica- tion), 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). 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 differ- ently with ipsec because ipsec can prevent icmp(4) or icmp6(4) routines from looking into the IP payload. SEE ALSO ioctl(2), socket(2), ipsec_set_policy(3), icmp6(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 implementation described herein appeared in WIDE/KAME IPv6/IPsec stack. BUGS The IPsec support is subject to change as the IPsec protocols develop. There is no single standard for the policy engine API, so the policy en- gine API described herein is just for KAME 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 en- capsulating 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's no sign of authentic- ity). 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 al- leviate this problem. BSD February 14, 2006 BSD
NAME | SYNOPSIS | DESCRIPTION | PROTOCOLS | SEE ALSO | STANDARDS | HISTORY | BUGS
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