In this slide, we discussed the architecture of iptables and also showed how to implement your own IPTABLES module. Upon the understanding of iptables, we implemented the DNS layer 7 parse in iptables module. After that, we studied how Kubernetes service works and also explained why Kubernetes can't do layer7 load-balancer in TCP connection but UDP.

1. IPTABLES & Kubernetes
HungWei Chiu

2. HungWei Chiu
•MTS @ ONF
•SDNDS-TW/CNTUG
•Linux/Network/Container/
Kubernetes
•Kuberentes Courses @Hiskio

3. IPTABLES Series
Introduction to IPTABLES
Learn IPTABLES by Docker environment.
Implementation of IPTABLES
User Space/Kernel Space
Implement our own iptables modules
Kubernetes Service discussion
Layer4 load-balancing, why ?
Modify the kernel module to make it support Layer7, really ?

4. IPTABLES/EBTABLES
Example
iptables -t nat -A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
ebtables -t filter -I INPUT --log --log-prefix 'ctc/ebtable/filter-input' --log-level debug
Components
Chain. -> Insert/Append (-I/-A)
Table
Match (Module) -> build-in/module
Target (Module). -> build-in/module

5. Last Time

6. Demo Environment
ContainerA
Linux Bridge
Eth0
Eth0
Veth0
https://github.com/hwchiu/
network-study/tree/master/
iptables/k8s
Customize IPTABLES module
Drop DNS packet if its request
domain is what we expect
Layer 7 processing.

7. Architecture
UserSpace
Parameter, usage
Kernel space
Implementation
Communicate by
getsockopt
setsockopt

8. User Space
Repo: git.netfilter.org/iptables
Version: 1.6.1 (In my demo)
Includes
iptables commands, modules,
Parses parameters and then send out to kernel
Written by C language

9. User Space
Directory
extension
old/new style
iptables
Extension
Parse the argument and then store to a memory.
iptables send data to kernel space later.

10. Implement
Implement a basic module
Support one argument
domain (string)
Name-> hwchiu
Output
Iptables -L
Iptables-save

11. Kernel Space
Repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Version: 4.15 (In my demo)
Whole Linux kernel
Implement the function (match/target)
Build as build-in function or kernel module.
Written by C language

12. Kernel Space
net/netfilter/
Implement the function then register to kernel.
Be called when someone(iptables) calls it by
setSockOpt
getSockOpt

13. DNS Format (RFC 1035)

14. Summary
You can inspect the packet in iptables modules.
Do anything you want
Coding in kernel space

15. Kubernetes Service
Why People called it a Layer 4 Load-Balancer ?
We could do layer7 parser in iptables.

16. Kubernetes
Service
ClusterIP
NodePort
LoadBalancer
Headless

17. Demo Environment
Kubernetes
Pod
Service
ClusterIP
Deployment
Three Pods
Pod Pod
ClusterIP

18. Workflow
PRE_ROUTING
Packets
KUBE-SERVICES KUBE-SVC-XXX KUBE-SEP-XXX
DNAT
Jump
Jump if match protocol/clusterIP Jump if random module return true
Match protocol
Module: TCP/UDP Module: Random
Choose ENDPOINTS
OUTPUT

19. Again

20. Random
If P < 0.25
If P < 0.33
If P < 0.5
Endpoint3 Endpoint4
Endpoint2
Endpoint1
Endpoint1
Endpoint2
Endpoint3
Endpoint4
10.244.1.3
10.244.2.32
10.244.3.63
10.244.3.23
Request
P=1/4 = 0.25
P= 3/4 * 1/3 = 0.25
P= 3/4 * 2/3 * 1/2 = 0.25 1-0.75 = 0.25

21. Workflow
PRE_ROUTING
Packets
KUBE-SERVICES KUBE-SVC-XXX KUBE-SEP-XXX
DNAT
Jump
Jump if match protocol/clusterIP Jump if random module return true
Match protocol
Module: UDP Module: Random
Choose ENDPOINTS
dest=10.96.121.30
dest=10.96.121.30 dest=10.96.121.30
dest=10.244.0.3

22. Kubernetes Service
Two types of load-balancer
Client <----> LB <----> Server
Two connections
Client <----LB-----> Server
One connection.

23. Kubernetes
Conntrack help to cache the NAT result.
Packets will skip NAT table if there is a conntrack entry for it.
For TCP packets
Kubernetes did DNAT in three-way handshake.
Without application data
For UDP packets
No three-way handshake

24. Iptables
Layer 7 Firewall (IPS/IDS)
Nfqueue,NDPI
Layer 7 load-balancer
Connectionless protocol (limited)

25. (Demo)Kubernetes
Modify the statistic modules to support UDP load-balancing.
Just for fun, to demonstrate how can we do from iptables.
Parameters (/proc)
ClusterIP (focus on specific service)
Content, PodIP (forward to PodIP if it's data include content)
PodIP list (to know where we are)
Match workflow (limited by Random and iptables structure)
Rollback if it's TCP packet.
Rollback if it's destination IP != ClusterIP
Return true if (1) data contains content, (2) PodIP equals iptables rule's target.
Else, return false

26. One Question
If we send packets to ClusterIP from Kubernetes node, what
happen?
UDP (Statistic + UDP)
How about ARP?

27. Again

28. Socket Programming (TCP)
https://wrytin.com/ramathakur/tcpip-reference-model-jvb79y39

29. (fs/read_write.c) SYSCALL_DEFINE3(....)
(fs/read_write.c) vsf_write(....)
(fs/read_write.c) do_sync_write(....)
(net/socket.c) sock_aio_write(....)
(net/socket.c) __sock_sendmsg(....)
(security/security.c) security_socket_sendmsg(...)
(net/socket.c) __sock_sendmsg_nosec(...)
(net/ipv4/af_inet.c) inet_sendmsg(....)
(net/ipv4/tcp.c) tcp_sendmsg(...)
(net/ipv4/tcp.c) __tcp_push_pending_frames(...)
(net/ipv4/tcp.c) tcp_push_one(...)
(net/ipv4/tcp.c) tcp_write_xmit(...)
(net/ipv4/tcp.c) tcp_transmit_skb(...)
sk_buff

30. (net/ipv4/ip_output.c) ip_queue_xmit(...)
(include/net/route.h) ip_route_output_ports(...)
(net/ipv4/route.c) ip_route_output_flow(...)
(net/xfrm/xfrm_policy.c) xfrm_lookup(...)
(net/ipv4/ip_output.c) ip_local_out(...)
(include/net/dst.h) dst_output(...)
(net/ipv4/ip_output.c) ip_output(....)
(net/ipv4/ip_output.c) ip_finish_output(...)
(net/ipv4/ip_output.c) ip_fragment(...)
(net/ipv4/ip_output.c) ip_finish_output2(...)
(include/net/neighbour.h) neigh_output(...)
(net/core/dev.c) dev_queue_xmit(...)
(net/core/dev.c) dev_hard_start_xmit(...)
(net/core/neighbour.c) neigh_resolve_output(...)
(net/ipv4/ip_output.c) __ip_local_out(....)
nf_hook(NFPROTO_IPV4, NF_INET_LOCAL_OUT...)
Call IPTABLES
(net/ipv4/ip_output.c) __ipv4_neigh_lookup_noref(....)
(include/net/neighbour.h) __neigh_create(...)
Lookup ARP Table
Create ARP Record
sk_buff

31. One more thing
COSCUP CFP
https://j.mp/2WmOemq
Cloud Native Hub
Telegram: https://t.me/cntug
Github: https://github.com/cloud-native-taiwan/meetups
MyBlog: https://www.hwchiu.com

32. Q&A