I have attempted to create iptables rules to prevent my server connections limit to be filled up with in-completed SYN packets for which no ACK packet is returned by the client (SYN flood attack).
I have read the [RFC 4987 TCP SYN Flooding Attacks and Common Mitigations][1] and [Mitigate TCP SYN Flood Attacks with Red Hat Enterprise Linux 7 Beta][2] and finally tried to follow the instructions [here][3]: https://javapipe.com/blog/iptables-ddos-protection/
However, my site is still easily taken down with a DOS attack using the following command:
hping -S --flood -V -p 443 www.mydomain.com
At the end of the tutorial from Javapipe.com linked above, the command is mentioned to tell whether SYNPROXY is active or not.
To verify that SYNPROXY is working, you can do watch -n1 cat /proc/net/stat/synproxy. If the values change when you establish a new TCP connection to the port you use SYNPROXY on, it works.
The command is:
watch -n1 cat /proc/net/stat/synproxy
When I use that command on my server and try to load a page, the data does not change and is all zeros.
Every 1.0s: cat /proc/net/stat/synproxy Thu Jul 25 19:57:00 2019
entries syn_received cookie_invalid cookie_valid cookie_retrans conn_reopened
00000000 00000000 00000000 00000000 00000000 00000000
Here is the output of iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere
INPUT_direct all -- anywhere anywhere
INPUT_ZONES_SOURCE all -- anywhere anywhere
INPUT_ZONES all -- anywhere anywhere
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
REJECT tcp -- anywhere anywhere #conn src/32 > 80 reject-with tcp-reset
ACCEPT tcp -- anywhere anywhere ctstate NEW limit: avg 20/sec burst 20
DROP tcp -- anywhere anywhere ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp flags:RST/RST limit: avg 2/sec burst 2
DROP tcp -- anywhere anywhere tcp flags:RST/RST
DROP all -- anywhere anywhere ctstate INVALID
SYNPROXY tcp -- anywhere anywhere tcp ctstate INVALID,UNTRACKED SYNPROXY sack-perm timestamp wscale 7 mss 1460
DROP all -- anywhere anywhere ctstate INVALID
DROP all -- anywhere anywhere state INVALID
Chain FORWARD (policy ACCEPT)
target prot opt source destination
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere
FORWARD_direct all -- anywhere anywhere
FORWARD_IN_ZONES_SOURCE all -- anywhere anywhere
FORWARD_IN_ZONES all -- anywhere anywhere
FORWARD_OUT_ZONES_SOURCE all -- anywhere anywhere
FORWARD_OUT_ZONES all -- anywhere anywhere
DROP all -- anywhere anywhere ctstate INVALID
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
OUTPUT_direct all -- anywhere anywhere
Chain FORWARD_IN_ZONES (1 references)
target prot opt source destination
FWDI_public all -- anywhere anywhere [goto]
Chain FORWARD_IN_ZONES_SOURCE (1 references)
target prot opt source destination
Chain FORWARD_OUT_ZONES (1 references)
target prot opt source destination
FWDO_public all -- anywhere anywhere [goto]
Chain FORWARD_OUT_ZONES_SOURCE (1 references)
target prot opt source destination
Chain FORWARD_direct (1 references)
target prot opt source destination
Chain FWDI_public (1 references)
target prot opt source destination
FWDI_public_log all -- anywhere anywhere
FWDI_public_deny all -- anywhere anywhere
FWDI_public_allow all -- anywhere anywhere
REJECT icmp -- anywhere anywhere reject-with icmp-host-prohibited
Chain FWDI_public_allow (1 references)
target prot opt source destination
Chain FWDI_public_deny (1 references)
target prot opt source destination
Chain FWDI_public_log (1 references)
target prot opt source destination
Chain FWDO_public (1 references)
target prot opt source destination
FWDO_public_log all -- anywhere anywhere
FWDO_public_deny all -- anywhere anywhere
FWDO_public_allow all -- anywhere anywhere
Chain FWDO_public_allow (1 references)
target prot opt source destination
Chain FWDO_public_deny (1 references)
target prot opt source destination
Chain FWDO_public_log (1 references)
target prot opt source destination
Chain INPUT_ZONES (1 references)
target prot opt source destination
IN_public all -- anywhere anywhere [goto]
Chain INPUT_ZONES_SOURCE (1 references)
target prot opt source destination
Chain INPUT_direct (1 references)
target prot opt source destination
Chain IN_public (1 references)
target prot opt source destination
IN_public_log all -- anywhere anywhere
IN_public_deny all -- anywhere anywhere
IN_public_allow all -- anywhere anywhere
REJECT icmp -- anywhere anywhere reject-with icmp-host-prohibited
Chain IN_public_allow (1 references)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp dpt:http ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp dpt:https ctstate NEW
ACCEPT tcp -- anywhere anywhere tcp dpt:smtp ctstate NEW
Chain IN_public_deny (1 references)
target prot opt source destination
Chain IN_public_log (1 references)
target prot opt source destination
Chain OUTPUT_direct (1 references)
target prot opt source destination
The server is a VPS with Digital Ocean running Centos 7.6, Kernel: 3.10.0-862.2.3.el7.x86_64
Can anyone help explain why SYNPROXY may not be running, or else, why / how to protect my server from SYN flooding?
