IPSEC (4) | Special files and drivers | Unix Manual Pages

IPSEC (4) | Special files and drivers | Unix Manual Pages | :man▋

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

Policy engine can be controlled by extended part of PF_KEY API, setsockopt(2) operations, and sysctl(3) interface. The kernel implements extended version of PF_KEY interface, and allows you to define IPsec policy like per-packet filters. 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 dynamic encryption key exchange protocol like IKE (Internet Key Exchange). That should be implemented as userland programs (usually as daemons), by using the above described APIs.

Policy management

The kernel implements experimental policy management code. You can manage the IPsec policy in two ways. One is to configure per-socket policy using setsockopt(2). The other is to configure kernel packet filter-based policy using PF_KEY interface, via setkey(8). In both cases, IPsec policy must be specified with syntax described in ipsec_set_policy(3).

With setsockopt(2), you can define IPsec policy in per-socket basis. You can enforce particular IPsec policy onto packets that go through particular socket.

With setkey(8) you can define IPsec policy against packets, using sort of packet filtering rule. Refer to setkey(8) on how to use it.

In the latter case, "default" policy is allowed for use with setkey(8). By configuring policy to default, you can refer system-wide sysctl(8) variable for default settings. The following variables are available. 1 means "use", and 2 means "require" in the syntax.

"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_deflevinteger yes" "net.inet6.ipsec6.ah_net_deflev integer yes"

If kernel finds no matching policy system wide default value is applied. System wide default 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

The following variables are accessible via sysctl(8), for tweaking kernel 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 type-of-service field in the IPv4 header during AH authentication data computation. The variable is for tweaking AH behavior to interoperate 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 16bit fragment offset field (including flag bits) in IPv4 header, after computing logical AND with the variable. The variable is for tweaking AH behavior to interoperate with devices that implement RFC1826 AH. It should be set to zero (clear the fragment offset field during computation) for RFC2402 conformance.
ipsec.dfbit
	The variable configures the kernel behavior on IPv4 IPsec tunnel encapsulation. If set to 0, DF bit on the outer IPv4 header will be cleared. 1 means that the outer DF bit is set regardless from the inner DF bit. 2 means that the DF bit is copied from the inner header to the outer. 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).