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Linux

Linux is technically a Kernel, but “knowing linux” involves far more.

Some books about the internals of Linux:

  • Linux Kernel Development by Robert Love
  • The Linux Programming Interface by Michael Kerrisk

Files Link to heading

Linux Networking Link to heading

See Also: page on Networking in general

DDoS Protection using iptables Link to heading

  • You can’t block everything using iptables. If you’re under real attack you need to be behind real routers and protection like Cloudflare and etc.
  • However you can do a lot to limit the damage and avoid getting your server overwhelmed before needing to do that.
Example Rules 
### 1: Drop invalid packets ### 
/sbin/iptables -t mangle -A PREROUTING -m conntrack --ctstate INVALID -j DROP  

### 2: Drop TCP packets that are new and are not SYN ### 
/sbin/iptables -t mangle -A PREROUTING -p tcp ! --syn -m conntrack --ctstate NEW -j DROP 
 
### 3: Drop SYN packets with suspicious MSS value ### 
/sbin/iptables -t mangle -A PREROUTING -p tcp -m conntrack --ctstate NEW -m tcpmss ! --mss 536:65535 -j DROP  

### 4: Block packets with bogus TCP flags ### 
/sbin/iptables -t mangle -A PREROUTING -p tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
/sbin/iptables -t mangle -A PREROUTING -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
/sbin/iptables -t mangle -A PREROUTING -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
/sbin/iptables -t mangle -A PREROUTING -p tcp --tcp-flags FIN,ACK FIN -j DROP
/sbin/iptables -t mangle -A PREROUTING -p tcp --tcp-flags ACK,URG URG -j DROP
/sbin/iptables -t mangle -A PREROUTING -p tcp --tcp-flags ACK,PSH PSH -j DROP
/sbin/iptables -t mangle -A PREROUTING -p tcp --tcp-flags ALL NONE -j DROP

### 5: Block spoofed packets ### 
/sbin/iptables -t mangle -A PREROUTING -s 224.0.0.0/3 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 169.254.0.0/16 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 172.16.0.0/12 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 192.0.2.0/24 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 192.168.0.0/16 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 10.0.0.0/8 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 0.0.0.0/8 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 240.0.0.0/5 -j DROP 
/sbin/iptables -t mangle -A PREROUTING -s 127.0.0.0/8 ! -i lo -j DROP  

### 6: Drop ICMP (you usually don't need this protocol) ### 
/sbin/iptables -t mangle -A PREROUTING -p icmp -j DROP  

### 7: Drop fragments in all chains ### 
/sbin/iptables -t mangle -A PREROUTING -f -j DROP  

### 8: Limit connections per source IP ### 
/sbin/iptables -A INPUT -p tcp -m connlimit --connlimit-above 111 -j REJECT --reject-with tcp-reset  

### 9: Limit RST packets ### 
/sbin/iptables -A INPUT -p tcp --tcp-flags RST RST -m limit --limit 2/s --limit-burst 2 -j ACCEPT 
/sbin/iptables -A INPUT -p tcp --tcp-flags RST RST -j DROP  

### 10: Limit new TCP connections per second per source IP ### 
/sbin/iptables -A INPUT -p tcp -m conntrack --ctstate NEW -m limit --limit 60/s --limit-burst 20 -j ACCEPT 
/sbin/iptables -A INPUT -p tcp -m conntrack --ctstate NEW -j DROP  

### 11: Use SYNPROXY on all ports (disables connection limiting rule) ### 
# Hidden - unlock content above in "Mitigating SYN Floods With SYNPROXY" section

Observability and Performance Link to heading

  • Always measure before making changes

Tools Link to heading

  • htop - Easy to use alternative to top for monitoring cpu and memory load
  • iostat - Easily view I/O load by process. Good for monitoring restores or seeing if an application is I/O bound.
    • In ubuntu part of the sysstat package

Books Link to heading

  • Systems Performance: Enterprise and the Cloud by Brendan Gregg
    • “2nd Ed will be released Dec 9, 2020”
  • BPF Performance Tools by Brendan Gregg
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