IPSEC(4) BSD Kernel Interfaces Manual IPSEC(4)
ipsec -- IP Security Protocol
IPsec is a pair of protocols, Encapsulating Security Payload (ESP) and
Authentication Header (AH), which provide security services for IP data-
Both protocols may be enabled or disabled using the following sysctl(3)
variables in /etc/sysctl.conf. By default, both protocols are enabled:
net.inet.esp.enable Enable the ESP IPsec protocol
net.inet.ah.enable Enable the AH IPsec protocol
There are four main security properties provided by IPsec:
Confidentiality - Ensure it is hard for anyone but the receiver to
understand what data has been communicated. For example, ensuring
the secrecy of passwords when logging into a remote machine over
Integrity - Guarantee that the data does not get changed in tran-
sit. If you are on a line carrying invoicing data you probably
want to know that the amounts and account numbers are correct and
have not been modified by a third party.
Authenticity - Sign your data so that others can see that it is
really you that sent it. It is clearly nice to know that documents
are not forged.
Replay protection - We need ways to ensure a datagram is processed
only once, regardless of how many times it is received. That is,
it should not be possible for an attacker to record a transaction
(such as a bank account withdrawal), and then by replaying it ver-
batim cause the peer to think a new message (withdrawal request)
had been received. WARNING: as per the standard's specification,
replay protection is not performed when using manual-keyed IPsec
(e.g. when using ipsecctl(8)).
IPsec provides these services using two new protocols: Authentication
Header (AH), and Encapsulating Security Payload (ESP).
ESP can provide the properties authentication, integrity, replay protec-
tion, and confidentiality of the data (it secures everything in the
packet that follows the IP header). Replay protection requires authenti-
cation and integrity (these two always go together). Confidentiality
(encryption) can be used with or without authentication/integrity. Simi-
larly, one could use authentication/integrity with or without confiden-
AH provides authentication, integrity, and replay protection (but not
confidentiality). The main difference between the authentication fea-
tures of AH and ESP is that AH also authenticates portions of the IP
header of the packet (such as the source/destination addresses). ESP
authenticates only the packet payload.
Authentication Header (AH)
AH works by computing a value that depends on all of the payload data,
some of the IP header data, and a certain secret value (the authentica-
tion key). This value is then sent with the rest of each packet. The
receiver performs the same computation, and if the value matches, he
knows no one tampered with the data (integrity), the address information
(authenticity) or a sequence number (replay protection). He knows this
because the secret authentication key makes sure no active attacker (man-
in-the-middle) can recompute the correct value after altering the packet.
The algorithms used to compute these values are called hash algorithms
and are parameters in the SA, just like the authentication key.
Encapsulating Security Payload (ESP)
ESP optionally does almost everything that AH does except that it does
not protect the outer IP header but furthermore it encrypts the payload
data with an encryption algorithm using a secret encryption key. Only
the ones knowing this key can decrypt the data, thus providing confiden-
tiality. Both the algorithm and the encryption key are parameters of the
Security Associations (SAs)
These protocols require certain parameters for each connection, describ-
ing exactly how the desired protection will be achieved. These parame-
ters are collected in an entity called a security association, or SA for
short. Typical SA parameters include encryption algorithm, hash algo-
rithm, encryption key, and authentication key, to name a few. When two
peers have established matching SAs (one at each end), packets protected
with one end's SA may be verified and/or decrypted using the information
in the other end's SA. The only issue remaining is to ensure that both
ends have matching SAs. This may be done manually, or automatically
using a key management daemon.
Further information on manual SA establishment is described in
ipsec.conf(5). Information on automated key management for IKEv1 can be
found in isakmpd(8) and for IKEv2 in iked.conf(5).
Security Parameter Indexes (SPIs)
In order to identify an SA we need to have a unique name for it. This
name is a triplet, consisting of the destination address, security param-
eter index (aka SPI) and the security protocol (ESP or AH). Since the
destination address is part of the name, an SA is necessarily a unidirec-
tional construct. For a bidirectional communication channel, two SAs are
required, one outgoing and one incoming, where the destination address is
our local IP address. The SPI is just a number that helps us make the
name unique; it can be arbitrarily chosen in the range 0x100 -
0xffffffff. The security protocol number should be 50 for ESP and 51 for
AH, as these are the protocol numbers assigned by IANA.
Modes of Operation
IPsec can operate in two modes, either tunnel or transport mode. In
transport mode the ordinary IP header is used to deliver the packets to
their endpoint; in tunnel mode the ordinary IP header just tells us the
address of a security gateway which knows how to verify/decrypt the pay-
load and forward the packet to a destination given by another IP header
contained in the protected payload. Tunnel mode can be used for estab-
lishing virtual private networks (VPNs), where parts of the networks can
be spread out over an unsafe public network, but security gateways at
each subnet are responsible for encrypting and decrypting the data pass-
ing over the public net. An SA will contain information specifying
whether it is a tunnel or transport mode SA, and for tunnels it will con-
tain values to fill in into the outer IP header.
The SA also holds a couple of other parameters, especially useful for
automatic keying, called lifetimes, which puts a limit on how much we can
use an SA for protecting our data. These limits can be in wall-clock
time or in volume of our data.
To better illustrate how IPsec works, consider a typical TCP packet:
[IP header] [TCP header] [data...]
If we apply ESP in transport mode to the above packet, we will get:
[IP header] [ESP header] [TCP header] [data...]
Everything after the ESP header is protected by whatever services of ESP
we are using (authentication/integrity, replay protection, confidential-
ity). This means the IP header itself is not protected.
If we apply ESP in tunnel mode to the original packet, we would get:
[IP header] [ESP header] [IP header] [TCP header] [data...]
Again, everything after the ESP header is cryptographically protected.
Notice the insertion of an IP header between the ESP and TCP header.
This mode of operation allows us to hide who the true source and destina-
tion addresses of a packet are (since the protected and the unprotected
IP headers don't have to be exactly the same). A typical application of
this is in Virtual Private Networks (or VPNs), where two firewalls use
IPsec to secure the traffic of all the hosts behind them. For example:
Net A <----> Firewall 1 <--- Internet ---> Firewall 2 <----> Net B
Firewall 1 and Firewall 2 can protect all communications between Net A
and Net B by using IPsec in tunnel mode, as illustrated above.
This implementation makes use of a virtual interface, enc0, which can be
used in packet filters to specify those packets that have been or will be
processed by IPsec.
NAT can also be applied to enc# interfaces, but special care should be
taken because of the interactions between NAT and the IPsec flow match-
ing, especially on the packet output path. Inside the TCP/IP stack,
packets go through the following stages:
UL/R -> [X] -> PF/NAT(enc0) -> IPsec -> PF/NAT(IF) -> IF
UL/R <-------- PF/NAT(enc0) <- IPsec <- PF/NAT(IF) <- IF
With IF being the real interface and UL/R the Upper Layer or Routing
code. The [X] stage on the output path represents the point where the
packet is matched against the IPsec flow database (SPD) to determine if
and how the packet has to be IPsec-processed. If, at this point, it is
determined that the packet should be IPsec-processed, it is processed by
the PF/NAT code. Unless PF drops the packet, it will then be IPsec-pro-
cessed, even if the packet has been modified by NAT.
Security Associations can be set up manually with ipsecctl(8) or automat-
ically with the isakmpd(8) or iked(8) key management daemons.
A number of sysctl(8) variables are relevant to ipsec. These are gener-
ally net.inet.ah.*, net.inet.esp.*, net.inet.ip.forwarding,
net.inet6.ip6.forwarding, and net.inet.ip.ipsec-*. Full explanations can
be found in sysctl(3), and variables can be set using the sysctl(8)
A number of kernel options are also relevant to ipsec. See options(4)
for further information.
The following IP-level setsockopt(2) and getsockopt(2) options are spe-
cific to ipsec. A socket can specify security levels for three different
IP_AUTH_LEVEL Specifies the use of authentication for packets
sent or received by the socket.
IP_ESP_TRANS_LEVEL Specifies the use of encryption in transport mode
for packets sent or received by the socket.
IP_ESP_NETWORK_LEVEL Specifies the use of encryption in tunnel mode.
For each of the categories there are five possible levels which specify
the security policy to use in that category:
IPSEC_LEVEL_BYPASS Bypass the default system security policy. This
option can only be used by privileged processes.
This level is necessary for the key management
IPSEC_LEVEL_AVAIL If a Security Association is available it will be
used for sending packets by that socket.
IPSEC_LEVEL_USE Use IP Security for sending packets but still
accept packets which are not secured.
IPSEC_LEVEL_REQUIRE Use IP Security for sending packets and also
require IP Security for received data.
IPSEC_LEVEL_UNIQUE The outbound Security Association will only be
used by this socket.
When a new socket is created, it is assigned the default system security
level in each category. These levels can be queried with getsockopt(2).
Only a privileged process can lower the security level with a
For example, a server process might want to accept only authenticated
connections to prevent session hijacking. It would issue the following
int level = IPSEC_LEVEL_REQUIRE;
error = setsockopt(s, IPPROTO_IP, IP_AUTH_LEVEL, &level, sizeof(int));
The system does guarantee that it will succeed at establishing the
required security associations. In any case a properly configured key
management daemon is required which listens to messages from the kernel.
A list of all security associations in the kernel tables can be obtained
using the ipsecctl(8) command.
A socket operation may fail with one of the following errors returned:
[EACCES] An attempt was made to lower the security level below the sys-
tem default by a non-privileged process.
[EINVAL] The length of option field did not match or an unknown security
level was given.
netstat(1) can be used to obtain some statistics about AH and ESP usage,
using the -p flag. Using the -r flag, netstat(1) displays information
about IPsec flows.
vmstat(8) displays information about memory use by IPsec with the -m flag
(look for ``tdb'' and ``xform'' allocations).
enc(4), options(4), iked(8), ipsecctl(8), isakmpd(8), sysctl(8)
IPsec was originally designed to provide security services for Internet
Protocol IPv6. It has since been engineered to provide those services
for the original Internet Protocol, IPv4.
The IPsec protocol design process was started in 1992 by John Ioannidis,
Phil Karn, and William Allen Simpson. In 1995, the former wrote an
implementation for BSD/OS. Angelos D. Keromytis ported it to OpenBSD and
NetBSD. The latest transforms and new features were implemented by Ange-
los D. Keromytis and Niels Provos.
The authors of the IPsec code proper are John Ioannidis, Angelos D.
Keromytis, and Niels Provos.
Niklas Hallqvist and Niels Provos are the authors of isakmpd(8).
Eric Young's libdeslite was used in this implementation for the DES algo-
Steve Reid's SHA-1 code was also used.
The setsockopt(2)/getsockopt(2) interface follows somewhat loosely the
draft-mcdonald-simple-ipsec-api (since expired, but still available from
There's a lot more to be said on this subject. This is just a beginning.
At the moment the socket options are not fully implemented.
BSD April 23, 2017 BSD