.\" $OpenBSD: ipsec.4,v 1.53 2003/03/20 07:26:33 jmc Exp $
.\"
.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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.\"      This product includes software developed by Niels Provos.
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.\" Manual page, using -mandoc macros
.\"
.Dd September 5, 1997
.Dt IPSEC 4
.Os
.Sh NAME
.Nm IPsec
.Nd IP Security Protocol
.Sh NOTE
.Tn IPsec
may be enabled or disabled using the following
.Xr sysctl 3
variables in
.Pa /etc/sysctl.conf .
By default, both protocols are enabled:
.Bl -tag -width xxxxxxxxxxxxxxxxxxxxx
.It net.inet.esp.enable
Enable the ESP IPsec protocol
.It net.inet.ah.enable
Enable the AH IPsec protocol
.El
.Sh DESCRIPTION
.Tn IPsec
is a pair of protocols,
.Tn ESP
(for Encapsulating Security
Payload) and
.Tn AH
(for Authentication Header), which provide
security services for
.Tn IP
datagrams.
.Pp
The original Internet Protocol -
.Tn IPv4 -
does not inherently provide any
protection to transferred data.
Furthermore, it does not even guarantee that the sender is who he
claims to be.
.Tn IPsec
tries to remedy this by providing the required security services for
.Tn IP
datagrams.
There are four main security properties provided by
.Tn IPsec :
.Bl -inset -offset indent
.It	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 the Internet.
.It	Integrity
- Guarantee that the data does not get changed
in transit.
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.
.It	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.
.It	Replay protection
- We need ways to ensure a datagram is processed only once, regardless
of how many times it is received.
I.e. it should not be possible for an attacker
to record a transaction (such as a bank account withdrawal), and then
by replaying it verbatim cause the peer to think a new message
(withdrawal request) had been received.
WARNING: as per the standards specification, replay protection is not
performed when using manual-keyed IPsec (e.g., when using
.Xr ipsecadm 8 ) .
.El
.Pp
.Ss IPsec Protocols
.Tn IPsec
provides these services using two new protocols:
.Tn AH ,
Authentication Header, and
.Tn ESP ,
Encapsulating Security Payload.
.Pp
.Tn ESP
can provide the properties authentication, integrity, replay protection, and
confidentiality of the data (it secures everything in the packet that
follows the
.Tn IP
header).
Replay protection requires authentication and
integrity (these two always go together).
Confidentiality (encryption) can be used with or without
authentication/integrity.
Similarly, one could use authentication/integrity with or without
confidentiality.
.Pp
.Tn AH
provides authentication, integrity, and replay protection (but not
confidentiality).
The main difference between the authentication features of
.Tn AH
and
.Tn ESP
is that
.Tn AH
also authenticates portions of the
.Tn IP
header of the packet (such as the source/destination
addresses).
.Tn ESP
authenticates only the packet payload.
.Pp
.Ss Security Associations (SAs)
These protocols require certain parameters for each connection, describing
exactly how the desired protection will be achieved.
These parameters are collected in an entity called a security association,
or
.Tn SA
for short.
Typical
.Tn SA
parameters include encryption algorithm, hash algorithm,
encryption key, and authentication key, to name a few.
When two peers have established matching 
.Tn SAs
(one at each end),
packets protected with one end's
.Tn SA
may be verified and/or decrypted
using the information in the other end's
.Tn SA.
The only issue remaining is to ensure that both ends have matching
.Tn SAs .
This may be done manually, or automatically using a key management daemon.
.Pp
Further information on manual
.Tn SA
establishment is described in
.Xr ipsecadm 8 .
Information on automated key management may be found in
.Xr isakmpd 8 .
.Pp
.Ss Authentication Header (AH)
.Tn AH
works by computing a value that depends on all of the payload
data, some of the
.Tn IP
header data, and a certain secret value (the
authentication 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.
.Pp
.Ss Encapsulating Security Payload (ESP)
.Tn ESP
optionally does almost everything that
.Tn AH
does except that it does not
protect the outer
.Tn 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
confidentiality.
Both the algorithm and the encryption key are parameters of the SA.
.Pp
.Ss 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
parameter index (aka SPI) and the security protocol (ESP or AH).
Since the destination address is part of the name, an SA is necessarily a
unidirectional 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
.Tn ESP
and 51 for
.Tn AH ,
as these are the protocol numbers assigned by IANA.
.Pp
.Ss Modes of Operation
.Tn IPsec
can operate in two modes, either tunnel or transport mode.
In transport mode the ordinary
.Tn IP
header is used to deliver the packets to their endpoint,
in tunnel mode the ordinary
.Tn IP
header just tells us the address of a
security gateway, knowing how to verify/decrypt the payload and forward the
packet to a destination given by another
.Tn IP
header contained in the protected payload.
Tunnel mode can be used for establishing VPNs, virtual
private networks, 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 passing over the public net.
An SA will hold information telling if it is a tunnel or transport mode SA,
and for tunnels, it will contain values to fill in into the outer
.Tn IP
header.
.Pp
.Ss Lifetimes
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.
.Pp
.Ss IPsec Examples
To better illustrate how
.Tn IPsec
works, consider a typical
.Tn TCP
packet:
.Bd -literal -offset indent
[IP header] [TCP header] [data...]
.Ed
.Pp
If we apply
.Tn ESP
in transport mode to the above packet, we will get:
.Bd -literal -offset indent
[IP header] [ESP header] [TCP header] [data...]
.Ed
.Pp
where everything after the
.Tn ESP
header is protected by whatever services of
.Tn ESP
we are using (authentication/integrity, replay protection,
confidentiality).
This means the
.Tn IP
header itself is not protected.
.Pp
If we apply
.Tn ESP
in tunnel mode to the original packet, we would get:
.Bd -literal -offset indent
[IP header] [ESP header] [IP header] [TCP header] [data...]
.Ed
.Pp
where, again, everything after the
.Tn ESP
header is cryptographically
protected.
Notice the insertion of an
.Tn IP
header between the
.Tn ESP
and
.Tn TCP
header.
This mode of operation allows us to hide who the true
source and destination addresses of a packet are (since the protected
and the unprotected
.Tn 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
.Tn IPsec
to secure the traffic of all the hosts behind them.
For example:
.Bd -literal -offset indent
Net A <----> Firewall 1 <--- Internet ---> Firewall 2 <----> Net B
.Ed
.Pp
Firewall 1 and Firewall 2 can protect all communications between Net A
and Net B by using
.Tn IPsec
in tunnel mode, as illustrated above.
.Pp
This implementation makes use of a virtual interface
.Nm enc0 ,
which can be used in packet filters to specify those packets that have
been or will be processed by
.Tn IPsec.
.Pp
NAT can also be applied to
.Nm enc#
interfaces, but special care should be taken because of the interactions
between NAT and the IPsec flow matching, especially on the packet output path.
Inside the TCP/IP stack, packets go through the following stages:
.Bd -literal -offset indent
UL/R -> [X] -> PF/NAT(enc0) -> IPsec -> PF/NAT(IF) -> IF
UL/R <-------- PF/NAT(enc0) <- IPsec -> PF/NAT(IF) <- IF
.Ed
.Pp
With
.Tn IF
being the real interface and
.Tn UL/R
the Upper Layer or Routing code.
The
.Tn [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-processed, even if the
packet has been modified by NAT.
.Pp
Security Associations can be set up manually with the
.Xr ipsecadm 8
utility or automatically with the
.Xr isakmpd 8
key management daemon.
.Pp
.Ss API Details
The following
.Tn IP-level
.Xr setsockopt 2
and
.Xr getsockopt 2
options are specific to
.Xr ipsec 4 .
A socket can specify security levels for three different categories:
.Bl -tag -width IP_ESP_NETWORK_LEVEL
.It IP_AUTH_LEVEL
Specifies the use of authentication for packets sent or received by the
socket.
.It IP_ESP_TRANS_LEVEL
Specifies the use of encryption in transport mode for packets sent or
received by the socket.
.It IP_ESP_NETWORK_LEVEL
Specifies the use of encryption in tunnel mode.
.El
.Pp
For each of the categories there are five possible levels which
specify the security policy to use in that category:
.Bl -tag -width IPSEC_LEVEL_REQUIRE
.It 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 daemon,
.Xr isakmpd 8 .
.It IPSEC_LEVEL_AVAIL
If a Security Association is available it will be used for sending packets
by that socket.
.It IPSEC_LEVEL_USE
Use IP Security for sending packets but still accept packets which are not
secured.
.It IPSEC_LEVEL_REQUIRE
Use IP Security for sending packets and also require IP Security for
received data.
.It IPSEC_LEVEL_UNIQUE
The outbound Security Association will only be used by this socket.
.El
.Pp
When a new socket is created, it is assigned the default system security
level in each category.
These levels can be queried with
.Xr getsockopt 2 .
Only a privileged process can lower the security level with a
.Xr setsockopt 2
call.
.Pp
For example, a server process might want to accept only authenticated
connections to prevent session hijacking.
It would issue the following
.Xr setsockopt 2
call:
.Bd -literal -offset 4n
int level = IPSEC_LEVEL_REQUIRE;
error = setsockopt(s, IPPROTO_IP, IP_AUTH_LEVEL, &level, sizeof(int));
.Ed
.Pp
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.
.Pp
A list of all security associations in the kernel tables can be
obtained via the kernfs file
.Aq Pa ipsec
(typically in
.Aq Pa /kern/ipsec ) .
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EINVAL]
.It Bq Er EACCES
when an attempt is made to lower the security level below the system default
by a non-privileged process.
.It Bq Er EINVAL
The length of option field did not match or an unknown security level
was given.
.El
.Pp
.Xr netstat 1
can be used to obtain some statistics about
.Tn AH
and
.Tn ESP
usage, using the
.Fl p
flag.
Using the
.Fl r
flag,
.Xr netstat 1
displays information about
.Tn IPsec
flows.
.Pp
.Xr vmstat 8
displays information about memory use by IPsec with the
.Fl m
flag (look for ``tdb'' and ``xform'' allocations).
.Sh BUGS
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.
.Sh SEE ALSO
.Xr enc 4 ,
.Xr icmp 4 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr netintro 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr ipsecadm 8 ,
.Xr isakmpd 8
.Xr vpn 8
.Sh ACKNOWLEDGMENTS
The authors of the
.Tn IPsec
code proper are John Ioannidis, Angelos D. Keromytis, and Niels Provos.
.Pp
Niklas Hallqvist and Niels Provos are the authors of
.Xr isakmpd 8 .
.Pp
Eric Young's libdeslite was used in this implementation for the
DES algorithm.
.Pp
Steve Reid's SHA-1 code was also used.
.Pp
The
.Xr setsockopt 2 / Ns Xr getsockopt 2
interface follows somewhat loosely the 
draft-mcdonald-simple-ipsec-api (since expired, but
still available from
.Pa ftp://ftp.kame.net/pub/internet-drafts/ )
.Sh HISTORY
The
.Tn 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
.Bsx .
Angelos D. Keromytis ported it to
.Ox
and
.Nx .
The latest transforms and new features were
implemented by Angelos D. Keromytis and Niels Provos.