ENC(4) | Device Drivers Manual | ENC(4)

# 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 enc` N `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.

# SEE ALSO

ipsec(4), pf(4), ipsec.conf(5), pf.conf(5), tcpdump(8)

February 15, 2020 | Debian