NAME
enc —
encapsulating interface
SYNOPSIS
pseudo-device enc
DESCRIPTION
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: 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.