Go to main content

man pages section 7: Device and Network Interfaces

Exit Print View

Updated: July 2017



ipsecesp, ESP - IPsec Encapsulating Security Payload




The ipsecesp module provides confidentiality, integrity, authentication, and partial sequence integrity (replay protection) to IP datagrams. The encapsulating security payload (ESP) encapsulates its data, enabling it to protect data that follows in the datagram. For TCP packets, ESP encapsulates the TCP header and its data only. If the packet is an IP in IP datagram, ESP protects the inner IP datagram. Per-socket policy allows "self-encapsulation" so ESP can encapsulate IP options when necessary. See ipsec(7P).

Unlike the authentication header (AH), ESP allows multiple varieties of datagram protection. (Using a single datagram protection form can expose vulnerabilities.) For example, only ESP can be used to provide confidentiality. But protecting confidentiality alone exposes vulnerabilities in both replay attacks and cut-and-paste attacks. Similarly, if ESP protects only integrity and does not fully protect against eavesdropping, it may provide weaker protection than AH. See ipsecah(7P).

ESP Device

ESP is implemented as a module that is auto-pushed on top of IP. Use the /dev/ipsecesp entry to tune ESP with ndd(1M).


ESPuses encryption and authentication algorithms. Authentication algorithms include HMAC-MD5 and HMAC-SHA-1. Encryption algorithms include DES, Triple-DES, Blowfish and AES. Each authentication and encryption algorithm contain key size and key format properties. You can obtain a list of authentication and encryption algorithms and their properties by using the ipsecalgs(1M) command. You can also use the functions described in the getipsecalgbyname(3NSL) man page to retrieve the properties of algorithms. Because of export laws in the United States, not all encryption algorithms are available outside of the United States.

Security Considerations

ESP without authentication exposes vulnerabilities to cut-and-paste cryptographic attacks as well as eavesdropping attacks. Like AH, ESP is vulnerable to eavesdropping when used without confidentiality.


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also

ipsecalgs(1M), ipsecconf(1M), ndd(1M), attributes(5), getipsecalgbyname(3NSL), ip(7P), ipsec(7P), ipsecah(7P)

Kent, S. and Atkinson, R.RFC 2406, IP Encapsulating Security Payload (ESP), The Internet Society, 1998.