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cls_redirect is a TC clsact based replacement for the glb-redirect iptables module available at [1]. It enables what GitHub calls "second chance" flows [2], similarly proposed by the Beamer paper [3]. In contrast to glb-redirect, it also supports migrating UDP flows as long as connected sockets are used. cls_redirect is in production at Cloudflare, as part of our own L4 load balancer. We have modified the encapsulation format slightly from glb-redirect: glbgue_chained_routing.private_data_type has been repurposed to form a version field and several flags. Both have been arranged in a way that a private_data_type value of zero matches the current glb-redirect behaviour. This means that cls_redirect will understand packets in glb-redirect format, but not vice versa. The test suite only covers basic features. For example, cls_redirect will correctly forward path MTU discovery packets, but this is not exercised. It is also possible to switch the encapsulation format to GRE on the last hop, which is also not tested. There are two major distinctions from glb-redirect: first, cls_redirect relies on receiving encapsulated packets directly from a router. This is because we don't have access to the neighbour tables from BPF, yet. See forward_to_next_hop for details. Second, cls_redirect performs decapsulation instead of using separate ipip and sit tunnel devices. This avoids issues with the sit tunnel [4] and makes deploying the classifier easier: decapsulated packets appear on the same interface, so existing firewall rules continue to work as expected. The code base started it's life on v4.19, so there are most likely still hold overs from old workarounds. In no particular order: - The function buf_off is required to defeat a clang optimization that leads to the verifier rejecting the program due to pointer arithmetic in the wrong order. - The function pkt_parse_ipv6 is force inlined, because it would otherwise be rejected due to returning a pointer to stack memory. - The functions fill_tuple and classify_tcp contain kludges, because we've run out of function arguments. - The logic in general is rather nested, due to verifier restrictions. I think this is either because the verifier loses track of constants on the stack, or because it can't track enum like variables. 1: https://github.com/github/glb-director/tree/master/src/glb-redirect 2: https://github.com/github/glb-director/blob/master/docs/development/second-chance-design.md 3: https://www.usenix.org/conference/nsdi18/presentation/olteanu 4: https://github.com/github/glb-director/issues/64 Signed-off-by: Lorenz Bauer <lmb@cloudflare.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20200424185556.7358-2-lmb@cloudflare.com |
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