BPF sk_lookup program

BPF sk_lookup program type (BPF_PROG_TYPE_SK_LOOKUP) introduces programmability into the socket lookup performed by the transport layer when a packet is to be delivered locally.

When invoked BPF sk_lookup program can select a socket that will receive the incoming packet by calling the bpf_sk_assign() BPF helper function.

Hooks for a common attach point (BPF_SK_LOOKUP) exist for both TCP and UDP.


BPF sk_lookup program type was introduced to address setup scenarios where binding sockets to an address with bind() socket call is impractical, such as:

  1. receiving connections on a range of IP addresses, e.g., when binding to a wildcard address INADRR_ANY is not possible due to a port conflict,

  2. receiving connections on all or a wide range of ports, i.e. an L7 proxy use case.

Such setups would require creating and bind()’ing one socket to each of the IP address/port in the range, leading to resource consumption and potential latency spikes during socket lookup.


BPF sk_lookup program can be attached to a network namespace with bpf(BPF_LINK_CREATE, ...) syscall using the BPF_SK_LOOKUP attach type and a netns FD as attachment target_fd.

Multiple programs can be attached to one network namespace. Programs will be invoked in the same order as they were attached.


The attached BPF sk_lookup programs run whenever the transport layer needs to find a listening (TCP) or an unconnected (UDP) socket for an incoming packet.

Incoming traffic to established (TCP) and connected (UDP) sockets is delivered as usual without triggering the BPF sk_lookup hook.

The attached BPF programs must return with either SK_PASS or SK_DROP verdict code. As for other BPF program types that are network filters, SK_PASS signifies that the socket lookup should continue on to regular hashtable-based lookup, while SK_DROP causes the transport layer to drop the packet.

A BPF sk_lookup program can also select a socket to receive the packet by calling bpf_sk_assign() BPF helper. Typically, the program looks up a socket in a map holding sockets, such as SOCKMAP or SOCKHASH, and passes a struct bpf_sock * to bpf_sk_assign() helper to record the selection. Selecting a socket only takes effect if the program has terminated with SK_PASS code.

When multiple programs are attached, the end result is determined from return codes of all the programs according to the following rules:

  1. If any program returned SK_PASS and selected a valid socket, the socket is used as the result of the socket lookup.

  2. If more than one program returned SK_PASS and selected a socket, the last selection takes effect.

  3. If any program returned SK_DROP, and no program returned SK_PASS and selected a socket, socket lookup fails.

  4. If all programs returned SK_PASS and none of them selected a socket, socket lookup continues on.


In its context, an instance of struct bpf_sk_lookup, BPF sk_lookup program receives information about the packet that triggered the socket lookup. Namely:

  • IP version (AF_INET or AF_INET6),

  • L4 protocol identifier (IPPROTO_TCP or IPPROTO_UDP),

  • source and destination IP address,

  • source and destination L4 port,

  • the socket that has been selected with bpf_sk_assign().

Refer to struct bpf_sk_lookup declaration in linux/bpf.h user API header, and bpf-helpers(7) man-page section for bpf_sk_assign() for details.


See tools/testing/selftests/bpf/prog_tests/sk_lookup.c for the reference implementation.