OpenBSD manual page server

NAME

ipsec.conf — IPsec configuration file

DESCRIPTION

The ipsec.conf file specifies rules and definitions for IPsec, which provides security services for IP datagrams. IPsec itself is a pair of protocols: Encapsulating Security Payload (ESP), which provides integrity and confidentiality; and Authentication Header (AH), which provides integrity. The IPsec protocol itself is described in ipsec(4).

In its most basic form, a flow is established between hosts and/or networks, and then Security Associations (SA) are established, which detail how the desired protection will be achieved. IPsec uses flows to determine whether to apply security services to an IP packet or not.

Generally speaking an automated keying daemon, such as isakmpd(8), is used to set up flows and establish SAs, by specifying an ‘ike’ line in ipsec.conf (see AUTOMATIC KEYING, below). An authentication method, such as public key authentication, will also have to be set up: see the PKI section of isakmpd(8) for information on the types of authentication available, and the procedures for setting them up.

The keying daemon, isakmpd(8), can be enabled to run at boot time via the isakmpd_flags variable in rc.conf.local(8). Note that it will probably need to be run with at least the -K option, to avoid keynote(4) policy checking. The ipsec.conf configuration itself is loaded at boot time if the variable ipsec is set to YES in rc.conf.local(8). A utility called ipsecctl(8) is also available to load ipsec.conf configurations, and can additionally be used to view and modify IPsec flows.

An alternative method of setting up SAs is also possible using manual keying. Manual keying is not recommended, but can be convenient for quick setups and testing. Those procedures are documented within this page.

IPSEC.CONF FILE FORMAT

The current line can be extended over multiple lines using a backslash (‘\’). Comments can be put anywhere in the file using a hash mark (‘#’), and extend to the end of the current line. Care should be taken when commenting out multi-line text: the comment is effective until the end of the entire block.

Argument names not beginning with a letter, digit, or underscore must be quoted.

Addresses can be specified in CIDR notation (matching netblocks), as symbolic host names, interface names, or interface group names.

Certain parameters can be expressed as lists, in which case ipsecctl(8) generates all the necessary combinations. For example:

ike esp from {192.168.1.1, 192.168.1.2} to \
	{10.0.0.17, 10.0.0.18} peer 192.168.10.1

Will expand to:

ike esp from 192.168.1.1 to 10.0.0.17 peer 192.168.10.1
ike esp from 192.168.1.1 to 10.0.0.18 peer 192.168.10.1
ike esp from 192.168.1.2 to 10.0.0.17 peer 192.168.10.1
ike esp from 192.168.1.2 to 10.0.0.18 peer 192.168.10.1

Macros can be defined that will later be expanded in context. Macro names must start with a letter, digit, or underscore, and may contain any of those characters. Macro names may not be reserved words (for example flow, from, esp). Macros are not expanded inside quotes.

For example:

remote_gw = "192.168.3.12"
flow esp from 192.168.7.0/24 to 192.168.8.0/24 peer $remote_gw

Additional configuration files can be included with the include keyword, for example:

include "/etc/macros.conf"

AUTOMATIC KEYING

In this scenario, ipsec.conf is used to set up flows and SAs automatically using isakmpd(8) with the ISAKMP/Oakley a.k.a. IKEv1 protocol. To configure automatic keying using the IKEv2 protocol, see iked.conf(5) instead. Some examples of setting up automatic keying:

# Set up a VPN:
# First between the gateway machines 192.168.3.1 and 192.168.3.2
# Second between the networks 10.1.1.0/24 and 10.1.2.0/24
ike esp from 192.168.3.1 to 192.168.3.2
ike esp from 10.1.1.0/24 to 10.1.2.0/24 peer 192.168.3.2

The commands are as follows:

ike [mode] [encap] [tmode]

mode specifies the IKEv1 mode to use: one of passive, active, or dynamic. When passive is specified, isakmpd(8) will not immediately start negotiation of this tunnel, but wait for an incoming request from the remote peer. When active or dynamic is specified, negotiation will be started at once. The dynamic mode will additionally enable Dead Peer Detection (DPD) and use the local hostname as the identity of the local peer, if not specified by the srcid parameter. dynamic mode should be used for hosts with dynamic IP addresses like road warriors or dialup hosts. If omitted, active mode will be used.

encap specifies the encapsulation protocol to be used. Possible protocols are esp and ah; the default is esp.

tmode describes the encapsulation mode to be used. Possible modes are tunnel and transport; the default is tunnel.

proto protocol

The optional proto parameter restricts the flow to a specific IP protocol. Common protocols are icmp(4), tcp(4), and udp(4). For a list of all the protocol name to number mappings used by ipsecctl(8), see the file /etc/protocols.

from src [port sport] [(srcnat)] to dst [port dport]

This rule applies for packets with source address src and destination address dst. The keyword any will match any address (i.e. 0.0.0.0/0). If the src argument specifies a fictional source ID, the srcnat parameter can be used to specify the actual source address. This can be used in outgoing NAT/BINAT scenarios as described below in OUTGOING NETWORK ADDRESS TRANSLATION. Host addresses are parsed as type “IPV4_ADDR”; adding the suffix /32 will change the type to “IPV4_ADDR_SUBNET”, which can improve interoperability with some IKEv1 implementations.

The optional port modifiers restrict the flows to the specified ports. They are only valid in conjunction with the tcp(4) and udp(4) protocols. Ports can be specified by number or by name. For a list of all port name to number mappings used by ipsecctl(8), see the file /etc/services.

local localip peer remote

The local parameter specifies the address or FQDN of the local endpoint. Unless we are multi-homed or have aliases, this option is generally not needed.

The peer parameter specifies the address or FQDN of the remote endpoint. For host-to-host connections where dst is identical to remote, this option is generally not needed as it will be set to dst automatically. If it is not specified or if the keyword any is given, the default peer is used.

mode auth algorithm enc algorithm group group lifetime time

These parameters define the mode and cryptographic transforms to be used for the phase 1 negotiation. During phase 1 the machines authenticate and set up an encrypted channel.

The mode can be either main, which specifies main mode, or aggressive, which specifies aggressive mode. Possible values for auth, enc, and group are described below in CRYPTO TRANSFORMS.

The lifetime parameter specifies the phase 1 lifetime in seconds. Two unit specifiers are recognized (ignoring case): ‘m’ and ‘h’ for minutes and hours, respectively.

If omitted, ipsecctl(8) will use the default values main, hmac-sha1, aes, modp1024, and 3600.

quick auth algorithm enc algorithm group group lifetime time

These parameters define the cryptographic transforms to be used for the phase 2 negotiation. During phase 2 the actual IPsec negotiations happen.

Possible values for auth, enc, and group are described below in CRYPTO TRANSFORMS. Perfect Forward Secrecy (PFS) is enabled unless group none is specified.

The lifetime parameter specifies the phase 2 lifetime in seconds. Two unit specifiers are recognized (ignoring case): ‘m’ and ‘h’ for minutes and hours, respectively.

If omitted, ipsecctl(8) will use the default values hmac-sha2-256 and aes; PFS will only be used if the remote side requests it. The default phase 2 lifetime value is 1200.

srcid string dstid string

srcid defines an ID of type “USER_FQDN” or “FQDN” that will be used by isakmpd(8) as the identity of the local peer. If the argument is an email address (bob@example.com), ipsecctl(8) will use USER_FQDN as the ID type. Anything else is considered to be an FQDN. If srcid is omitted, the default is to use the IP address of the connecting machine.

dstid is similar to srcid, but instead specifies the ID to be used by the remote peer.

psk string

Use a pre-shared key string for authentication. If this option is not specified, public key authentication is used (see isakmpd(8)).

tag string

Add a pf(4) tag to all packets of phase 2 SAs created for this connection. This will allow matching packets for this connection by defining rules in pf.conf(5) using the tagged keyword.

The following variables can be used in tags to include information from the remote peer on runtime:

$id

The remote phase 1 ID. It will be expanded to id-type/id-value, e.g. fqdn/foo.bar.org.

$domain

Extract the domain from IDs of type FQDN or UFQDN.

For example, if the ID is fqdn/foo.bar.org or ufqdn/user@bar.org, “ipsec-$domain” expands to “ipsec-bar.org”. The variable expansion for the tag directive occurs only at runtime, not during configuration file parse time.

PACKET FILTERING

IPsec traffic appears unencrypted on the enc(4) interface and can be filtered accordingly using the OpenBSD packet filter, pf(4). The grammar for the packet filter is described in pf.conf(5).

The following components are relevant to filtering IPsec traffic:

external interface

Interface for ISAKMP traffic and encapsulated IPsec traffic.

proto udp port 500

ISAKMP traffic on the external interface.

proto udp port 4500

ISAKMP NAT-Traversal traffic on the external interface.

proto ah | esp

Encapsulated IPsec traffic on the external interface.

enc0

Interface for outgoing traffic before it's been encapsulated, and incoming traffic after it's been decapsulated. State on this interface should be interface bound; see enc(4) for further information.

proto ipencap

[tunnel mode only] IP-in-IP traffic flowing between gateways on the enc0 interface.

tagged ipsec-example.org

Match traffic of phase 2 SAs using the tag keyword.

If the filtering rules specify to block everything by default, the following rule would ensure that IPsec traffic never hits the packet filtering engine, and is therefore passed:

set skip on enc0

In the following example, all traffic is blocked by default. IPsec-related traffic from gateways {192.168.3.1, 192.168.3.2} and networks {10.0.1.0/24, 10.0.2.0/24} is permitted.

block on sk0
block on enc0

pass  in on sk0 proto udp from 192.168.3.2 to 192.168.3.1 \
	port {500, 4500}
pass out on sk0 proto udp from 192.168.3.1 to 192.168.3.2 \
	port {500, 4500}

pass  in on sk0 proto esp from 192.168.3.2 to 192.168.3.1
pass out on sk0 proto esp from 192.168.3.1 to 192.168.3.2

pass  in on enc0 proto ipencap from 192.168.3.2 to 192.168.3.1 \
	keep state (if-bound)
pass out on enc0 proto ipencap from 192.168.3.1 to 192.168.3.2 \
	keep state (if-bound)
pass  in on enc0 from 10.0.2.0/24 to 10.0.1.0/24 \
	keep state (if-bound)
pass out on enc0 from 10.0.1.0/24 to 10.0.2.0/24 \
	keep state (if-bound)

pf(4) has the ability to filter IPsec-related packets based on an arbitrary tag specified within a ruleset. The tag is used as an internal marker which can be used to identify the packets later on. This could be helpful, for example, in scenarios where users are connecting in from differing IP addresses, or to support queue-based bandwidth control, since the enc0 interface does not support it.

The following pf.conf(5) fragment uses queues for all IPsec traffic with special handling for developers and employees:

queue std on sk0 bandwidth 100M
queue   deflt parent std bandwidth 10M default
queue   developers parent std bandwidth 75M
queue   employees parent std bandwidth 5M
queue   ipsec parent std bandwidth 10M

pass out on sk0 proto esp set queue ipsec

pass out on sk0 tagged ipsec-developers.bar.org set queue developers
pass out on sk0 tagged ipsec-employees.bar.org set queue employees

The tags will be assigned by the following ipsec.conf example:

ike esp from 10.1.1.0/24 to 10.1.2.0/24 peer 192.168.3.2 \
	tag ipsec-$domain

OUTGOING NETWORK ADDRESS TRANSLATION

In some network topologies it is desirable to perform NAT on traffic leaving through the VPN tunnel. In order to achieve that, the src argument is used to negotiate the desired network ID with the peer and the srcnat parameter defines the true local subnet, so that a correct SA can be installed on the local side.

For example, if the local subnet is 192.168.1.0/24 and all the traffic for a specific VPN peer should appear as coming from 10.10.10.1, the following configuration is used:

ike esp from 10.10.10.1 (192.168.1.0/24) to 192.168.2.0/24 \
	peer 10.10.20.1

Naturally, a relevant NAT rule is required in pf.conf(5). For the example above, this would be:

match out on enc0 from 192.168.1.0/24 to 192.168.2.0/24 \
	nat-to 10.10.10.1

From the peer's point of view, the local end of the VPN tunnel is declared to be 10.10.10.1 and all the traffic arrives with that source address.

CRYPTO TRANSFORMS

It is very important that keys are not guessable. One practical way of generating keys is to use openssl(1). The following generates a 160-bit (20-byte) key:

$ openssl rand -hex 20

The following authentication types are permitted with the auth keyword:

The following cipher types are permitted with the enc keyword:

Use of DES as an encryption algorithm is considered to be insecure since brute force attacks are practical due its short key length.

DES requires 8 bytes to form a 56-bit key and 3DES requires 24 bytes to form its 168-bit key. This is because the most significant bit of each byte is used for parity.

The keysize of AES-CTR can be 128, 192, or 256 bits. However as well as the key, a 32-bit nonce has to be supplied. Thus 160, 224, or 288 bits of key material, respectively, have to be supplied. The same applies to AES-GCM and AES-GMAC.

Using AES-GMAC or NULL with ESP will only provide authentication. This is useful in setups where AH can not be used, e.g. when NAT is involved.

The following group types are permitted with the group keyword:

MANUAL FLOWS

In this scenario, ipsec.conf is used to set up flows manually. IPsec uses flows to determine whether to apply security services to an IP packet or not. Some examples of setting up flows:

# Set up two flows:
# First between the machines 192.168.3.14 and 192.168.3.100
# Second between the networks 192.168.7.0/24 and 192.168.8.0/24
flow esp from 192.168.3.14 to 192.168.3.100
flow esp from 192.168.7.0/24 to 192.168.8.0/24 peer 192.168.3.12

The following types of flow are available:

flow esp

ESP can provide the following properties: authentication, integrity, replay protection, and confidentiality of the data. If no flow type is specified, this is the default.

flow ah

AH provides authentication, integrity, and replay protection, but not confidentiality.

flow ipip

IPIP does not provide authentication, integrity, replay protection, or confidentiality. However, it does allow tunnelling of IP traffic over IP, without setting up gif(4) interfaces.

The commands are as follows:

in or out

This rule applies to incoming or outgoing packets. If neither in nor out are specified, ipsecctl(8) will assume the direction out for this rule and will construct a proper in rule. Thus packets in both directions will be matched.

proto protocol

The optional proto parameter restricts the flow to a specific IP protocol. Common protocols are icmp(4), tcp(4), and udp(4). For a list of all the protocol name to number mappings used by ipsecctl(8), see the file /etc/protocols.

from src [port sport] to dst [port dport]

This rule applies for packets with source address src and destination address dst. The keyword any will match any address (i.e. 0.0.0.0/0). The optional port modifiers restrict the flows to the specified ports. They are only valid in conjunction with the tcp(4) and udp(4) protocols. Ports can be specified by number or by name. For a list of all port name to number mappings used by ipsecctl(8), see the file /etc/services.

local localip

The local parameter specifies the address or FQDN of the local endpoint of this flow and can be usually left out.

peer remote

The peer parameter specifies the address or FQDN of the remote endpoint of this flow. For host-to-host connections where dst is identical to remote, the peer specification can be left out as it will be set to dst automatically. Only if the keyword any is given is a flow without peer created.

type modifier

This optional parameter sets up special flows using modifiers. By default, ipsecctl(8) will automatically set up normal flows with the corresponding type. modifier may be one of the following:

acquire

Use IPsec and establish SAs dynamically. Unencrypted traffic is permitted until it is protected by IPsec.

bypass

Matching packets are not processed by IPsec.

deny

Matching packets are dropped.

dontacq

Use IPsec. If no SAs are available, does not trigger isakmpd(8).

require

Use IPsec and establish SAs dynamically. Unencrypted traffic is not permitted until it is protected by IPsec.

use

Use IPsec. Unencrypted traffic is permitted. Does not trigger isakmpd(8).

MANUAL SECURITY ASSOCIATIONS (SAs)

In this scenario, ipsec.conf is used to set up SAs manually. The security parameters for a flow are stored in the Security Association Database (SADB). An example of setting up an SA:

# Set up an IPsec SA for flows between 192.168.3.14 and 192.168.3.12
esp from 192.168.3.14 to 192.168.3.12 spi 0xdeadbeef:0xbeefdead \
	authkey file "auth14:auth12" enckey file "enc14:enc12"

Parameters specify the peers, Security Parameter Index (SPI), cryptographic transforms, and key material to be used. The following rules enter SAs in the SADB:

The commands are as follows:

mode

For ESP and AH the encapsulation mode can be specified. Possible modes are tunnel and transport. When left out, tunnel is chosen. For details on modes see ipsec(4).

from src to dst

This SA is for a flow between the peers src and dst.

spi number

The SPI identifies a specific SA. number is a 32-bit value and needs to be unique.

auth algorithm

For ESP and AH an authentication algorithm can be specified. Possible values are described above in CRYPTO TRANSFORMS.

If no algorithm is specified, ipsecctl(8) will choose hmac-sha2-256 by default.

enc algorithm

For ESP an encryption algorithm can be specified. Possible values are described above in CRYPTO TRANSFORMS.

If no algorithm is specified, ipsecctl(8) will choose aes by default.

authkey keyspec

keyspec defines the authentication key to be used. It is either a hexadecimal string or a path to a file containing the key. The filename may be given as either an absolute path to the file or a relative pathname, and is specified as follows:

authkey file "filename"

enckey keyspec

The encryption key is defined similarly to authkey.

tcpmd5 from src to dst spi number authkey keyspec

TCP MD5 signatures are generally used between BGP daemons, such as bgpd(8). Since bgpd(8) itself already provides this functionality, this option is generally not needed. More information on TCP MD5 signatures can be found in tcp(4), bgpd.conf(5), and RFC 2385.

This rule applies for packets with source address src and destination address dst. The parameter spi is a 32-bit value defining the Security Parameter Index (SPI) for this SA. The encryption key is defined similarly to authkey.

Since an SA is directional, a second SA is normally configured in the reverse direction. This is done by adding a second, colon-separated, value to spi, authkey, and enckey.

SEE ALSO

openssl(1), enc(4), ipcomp(4), ipsec(4), tcp(4), pf.conf(5), ipsecctl(8), isakmpd(8)

HISTORY

The ipsec.conf file format first appeared in OpenBSD 3.8.