Currently, sk_lookup allows an ebpf program to run on the ingress socket lookup path, and accept traffic not only on a range of addresses, but also on a range of ports. At Cloudflare we use sk_lookup for two main cases: 1. Sharing a single port between multiple services - i.e. two services (or more) use disjoint IP ranges but share the same port; 2. Receiving traffic on all ports - i.e. a service which accepts traffic on specific IP ranges but any port [1].
However, one main challenge we face while using sk_lookup for these use cases is how to source return UDP traffic: - On point 1. above, sometimes this range of addresses are not local (i.e. there's no local routes for these in the server), which means we need IP_TRANSPARENT set to be able to egress traffic from addresses we've received traffic on (or simply IP_FREEBIND in the case of IPv6); - And on point 2. above, allowing traffic to a range of ports means a service could get traffic on multiple ports, but currently there's no way to set the source UDP port egress traffic should be sourced from - it's possible to receive the original destination port using the IP_ORIGDSTADDR ancilliary message in recvmsg, but not set it in sendmsg.
Both of these limitations can be worked around, but in a sub-optimal way. Using IP_TRANSPARENT, for instance, requires special privileges. And while one could use UDP connected sockets to send return traffic, creating a connected socket for each different address a UDP traffic is received on does have performance implications.
Given sk_lookup allows services to accept traffic on a range of addresses or ports, it seems sensible to also allow return traffic to proceed through as well, without needing extra configurations / set ups.
This patch set allows to do exactly this by performing a reverse socket lookup on the egress path - where it looks to see if the egress socket matches a socket in the attached sk_lookup ebpf program for the traffic that's being sent. If it does, traffic is allowed to proceed.
The downsides to this is that this runs on the egress hot path, although this work tries to minimise its impact by only performing the reverse socket lookup when necessary. Further performance measurements are to be taken, but we're reaching out early for feedback to see what the technical concerns are and if we can address them.
[1] https://blog.cloudflare.com/how-we-built-spectrum/
Suggested-by: Jakub Sitnicki jakub@cloudflare.com Signed-off-by: Tiago Lam tiagolam@cloudflare.com --- Tiago Lam (3): ipv4: Run a reverse sk_lookup on sendmsg. ipv6: Run a reverse sk_lookup on sendmsg. bpf: Add sk_lookup test to use ORIGDSTADDR cmsg.
include/net/ip.h | 1 + net/ipv4/ip_sockglue.c | 11 ++++ net/ipv4/udp.c | 33 +++++++++- net/ipv6/datagram.c | 76 ++++++++++++++++++++++ net/ipv6/udp.c | 8 ++- tools/testing/selftests/bpf/prog_tests/sk_lookup.c | 70 +++++++++++++------- 6 files changed, 174 insertions(+), 25 deletions(-) --- base-commit: da3ea35007d0af457a0afc87e84fddaebc4e0b63 change-id: 20240909-reverse-sk-lookup-f7bf36292bc4
Best regards,