ENC(4) OpenBSD Programmer's Manual ENC(4)NAMEenc - encapsulating interface
SYNOPSIS
pseudo-device encDESCRIPTION
The enc interface is a virtual interface for ipsec(4) traffic. It allows
packet filtering using pf(4); prior to encapsulation and after
decapsulation, packets may be monitored using tcpdump(8).
An enc interface can be created at runtime using the ifconfig encN create
command or by setting up a hostname.if(5) configuration file for
netstart(8). The enc0 interface will always exist and cannot be
destroyed using ifconfig(8).
Packet filtering is documented in greater detail in pf.conf(5), however
some details relevant to filtering on the enc interface are documented
below.
Firstly, pf(4) is a stateful packet filter, which means it can track the
state of a connection. It does this automatically. States are normally
floating, which means they can match packets on any interface. However
this is a potential problem for filtering IPsec traffic: states need to
be interface bound, to avoid permitting unencrypted traffic should the
SAs expire and not be replaced. Therefore all rules on the enc interface
should explicitly set ``keep state (if-bound)''. For example:
pass in on enc0 proto ipencap from 172.25.0.45 to 1.2.3.4 \
keep state (if-bound)
Secondly, the enc interface does not directly support bandwidth control
via pf(4) queueing. Instead, IPsec packets must be tagged and the tagged
packets are assigned to queues. ipsec.conf(5) provides an example of
tag-based queueing and further information on packet tagging.
Finally, the use of translation rules to map and redirect network traffic
requires some care. Packets destined to be IPsec processed are seen by
the filter/translation engine twice, both before and after being IPsec
processed. If a packet's translated address on the way back fails to
match an existing IPsec flow, from the translated address to the original
source address, it will be discarded by the filter. It is best to avoid
this situation where possible, though a flow may be explicitly created to
work around it.
As noted above, tcpdump(8) may be invoked on the enc interface to see
packets prior to encapsulation and after decapsulation. For example:
# tcpdump -envps 1500 -i enc0 -l | grep 10.0.0.33
tcpdump: WARNING: enc0: no IPv4 address assigned
tcpdump: listening on enc0, link-type ENC
15:05:08.934708 (authentic,confidential): SPI 0x6bcac587: \
172.25.0.45 > 1.2.3.4: 10.9.9.28.7001 > 10.0.0.33.7000: \
[udp sum ok] udp 52 (ttl 64, id 5672, len 80) \
(ttl 64, id 30009, len 100, bad cksum 0!)
15:05:09.063517 (authentic,confidential): SPI 0x4b70c05a: \
1.2.3.4 > 172.25.0.45: 10.0.0.33.7000 > 10.9.9.28.7001: \
[udp sum ok] udp 156 (ttl 63, id 14880, len 184) \
(ttl 51, id 19689, len 204)
The packets above show (for each direction): date, ESP (not AH), SPI,
direction, and encapsulated part. The first packet is headed from
172.25.0.45 to 1.2.3.4 and the encapsulated part from 10.9.9.28 to
10.0.0.33.
Negotiations can be watched on the physical interface too:
# tcpdump -envps 1500 -i wi0 port 500 or port 4500
tcpdump: listening on wi0, link-type EN10MB
15:15:58.188747 0:2:6f:3a:3f:3e 0:10:f3:3:bd:8a 0800 226: \
172.25.0.45.500 > 1.2.3.4.500: [udp sum ok] \
[...]
attribute ENCRYPTION_ALGORITHM = AES_CBC
attribute HASH_ALGORITHM = SHA
attribute AUTHENTICATION_METHOD = RSA_SIG
attribute GROUP_DESCRIPTION = MODP_1024
attribute LIFE_TYPE = SECONDS
attribute LIFE_DURATION = 3600
attribute KEY_LENGTH = 128
[...]
15:15:59.080058 0:10:f3:3:bd:8a 0:2:6f:3a:3f:3e 0800 226: \
1.2.3.4.500 > 172.25.0.45.500: [udp sum ok] \
[...]
attribute ENCRYPTION_ALGORITHM = AES_CBC
attribute HASH_ALGORITHM = SHA
attribute AUTHENTICATION_METHOD = RSA_SIG
attribute GROUP_DESCRIPTION = MODP_1024
attribute LIFE_TYPE = SECONDS
attribute LIFE_DURATION = 3600
attribute KEY_LENGTH = 128
[...]
The attribute lines for the negotiation must match.
SEE ALSOipsec(4), pf(4), ipsec.conf(5), pf.conf(5), tcpdump(8)OpenBSD 4.9 June 29, 2010 OpenBSD 4.9