16 Oct

Network security consists of the provisions and policies adopted by a network administrator to prevent and monitor unauthorized access, misuse, modification, or denial of a computer network and network-accessible resources.

Control and configure Linux kernel and networking settings

sysctl.conf is a simple file containing sysctl values to be read in and set by sysctl.
$ sudo vim /etc/sysctl.conf

# Avoid a smurf attack
net.ipv4.icmp_echo_ignore_broadcasts = 1
# Turn on protection for bad icmp error messages
net.ipv4.icmp_ignore_bogus_error_responses = 1
# Turn on syncookies for SYN flood attack protection
net.ipv4.tcp_syncookies = 1
# Turn on and log spoofed, source routed, and redirect packets
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.log_martians = 1
# No source routed packets here
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
# Turn on reverse path filtering
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
# Make sure no one can alter the routing tables
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
# Don't act as a router
net.ipv4.ip_forward = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
# Turn on execshild
kernel.exec-shield = 1
kernel.randomize_va_space = 1
# Tuen IPv6
net.ipv6.conf.default.router_solicitations = 0
net.ipv6.conf.default.accept_ra_rtr_pref = 0
net.ipv6.conf.default.accept_ra_pinfo = 0
net.ipv6.conf.default.accept_ra_defrtr = 0
net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.default.dad_transmits = 0
net.ipv6.conf.default.max_addresses = 1
# Optimization for port usefor LBs
# Increase system file descriptor limit
fs.file-max = 65535
# Allow for more PIDs (to reduce rollover problems); may break some programs 32768
kernel.pid_max = 65536
# Increase system IP port limits
net.ipv4.ip_local_port_range = 2000 65000
# Increase TCP max buffer size setable using setsockopt()
net.ipv4.tcp_rmem = 4096 87380 8388608
net.ipv4.tcp_wmem = 4096 87380 8388608
# Increase Linux auto tuning TCP buffer limits
# min, default, and max number of bytes to use
# set max to at least 4MB, or higher if you use very high BDP paths
# Tcp Windows etc
net.core.rmem_max = 8388608
net.core.wmem_max = 8388608
net.core.netdev_max_backlog = 5000
net.ipv4.tcp_window_scaling = 1

Reload Changes:
# sysctl -p

Turn off nginx version number displayed

To turn off nginx version number displayed on all auto generated error pages, add the following in /etc/nginx/conf.d/default.conf:
server_tokens off

Limit Available Methods

If a web server does not require the implementation of all available methods, they should be disabled.
The following will filter and only allow GET, HEAD and POST methods:
## Only allow these request methods ##
if ($request_method !~ ^(GET|HEAD|POST)$ ) {
return 444;
## Do not accept DELETE, SEARCH and other methods ##

Controlling Buffer Overflow Attacks

Edit nginx.conf:

$ sudo vim /etc/nginx/nginx.conf

Set the buffer size limitations for all clients:
## Start: Size Limits & Buffer Overflows ##
client_body_buffer_size 1K;
client_header_buffer_size 1k;
client_max_body_size 1k;
large_client_header_buffers 2 1k;
## END: Size Limits & Buffer Overflows ##

  • client_body_buffer_size 1k – (default is 8k or 16k) The directive specifies the client request body buffer size.
  • client_header_buffer_size 1k – Directive sets the headerbuffer size for the request header from client. For the overwhelming majority of requests a buffer size of 1K is sufficient. Increase this if you have a custom header or a large cookie sent from the client (e.g., wap client).
  • client_max_body_size 1k- Directive assigns the maximum accepted body size of client request, indicated by the line Content-Length in the header of request. If size is greater the given one, then the client gets the error “Request Entity Too Large” (413). Increase this when you are getting file uploads via the POST method.
  • large_client_header_buffers 2 1k – Directive assigns the maximum number and size of buffers for large headers to read from client request. By default the size of one buffer is equal to the size of page, depending on platform this either 4K or 8K, if at the end of working request connection converts to state keep-alive, then these buffers are freed. 2x1k will accept 2kB data URI. This will also help combat bad bots and DoS attacks.

Control timeouts to improve server performance:
## Start: Timeouts ##
client_body_timeout 10;
client_header_timeout 10;
keepalive_timeout 5 5;
send_timeout 10;
## End: Timeouts ##

  • client_body_timeout 10; – Directive sets the read timeout for the request body from client. The timeout is set only if a body is not get in one readstep. If after this time the client send nothing, nginx returns error “Request time out” (408). The default is 60.
  • client_header_timeout 10; – Directive assigns timeout with reading of the title of the request of client. The timeout is set only if a header is not get in one readstep. If after this time the client send nothing, nginx returns error “Request time out” (408).
  • keepalive_timeout 5 5; – The first parameter assigns the timeout for keep-alive connections with the client. The server will close connections after this time. The optional second parameter assigns the time value in the header Keep-Alive: timeout=time of the response. This header can convince some browsers to close the connection, so that the server does not have to. Without this parameter, nginx does not send a Keep-Alive header (though this is not what makes a connection “keep-alive”).
  • send_timeout 10; – Directive assigns response timeout to client. Timeout is established not on entire transfer of answer, but only between two operations of reading, if after this time client will take nothing, then nginx is shutting down the connection.

 Nginx And PHP Security Tips

Edit /etc/php.ini as follows:
# Disallow dangerous functions
disable_functions = phpinfo, system, mail, exec
## Try to limit resources ##
# Maximum execution time of each script, in seconds
max_execution_time = 30
# Maximum amount of time each script may spend parsing request data
max_input_time = 60
# Maximum amount of memory a script may consume (8MB)
memory_limit = 8M
# Maximum size of POST data that PHP will accept.
post_max_size = 8M
# Whether to allow HTTP file uploads.
file_uploads = Off
# Maximum allowed size for uploaded files.
upload_max_filesize = 2M
# Do not expose PHP error messages to external users
display_errors = Off
# Turn on safe mode
safe_mode = On
# Only allow access to executables in isolated directory
safe_mode_exec_dir = php-required-executables-path
# Limit external access to PHP environment
safe_mode_allowed_env_vars = PHP_
# Restrict PHP information leakage
expose_php = Off
# Log all errors
log_errors = On
# Do not register globals for input data
register_globals = Off
# Minimize allowable PHP post size
post_max_size = 1K
# Ensure PHP redirects appropriately
cgi.force_redirect = 0
# Disallow uploading unless necessary
file_uploads = Off
# Enable SQL safe mode
sql.safe_mode = On
# Avoid Opening remote files
allow_url_fopen = Off

Restrictive Iptables Based Firewall

The following firewall script blocks everything and only allows:
Incoming HTTP (TCP port 80) requests
Incoming ICMP ping requests
Outgoing ntp (port 123) requests
Outgoing smtp (TCP port 25) requests
iptables -F
iptables -A INPUT -i lo -p all -j ACCEPT
iptables -A OUTPUT -o lo -p all -j ACCEPT
# allow outgoing smtp
iptables -A OUTPUT -o eth0 -p tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A INPUT -i eth0 -p tcp --sport 25 -m state --state ESTABLISHED -j ACCEPT
# allow incoming http port 80
iptables -A INPUT -i eth0 -p tcp -s 0/0 --sport 1024:65535 --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 80 -d 0/0 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
# allow incoming ssh port 22
iptables -A INPUT -i eth0 -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT

iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

Limiting access

By type of files

This refuses all requests for files beginning with the characters .ht from nginx:

#Contents of nginx.conf
location ~ /\.ht {
deny all;

By Ip Address

This directive limit access to the docs directory
#Contents of nginx.conf
location /docs/ {
## block one workstation
## allow anyone in
## drop rest of the world
deny all;

Password Protect The Directory

Creating the password file and adding a username:
# mkdir /usr/local/nginx/conf/.htpasswd/
# htpasswd -c /usr/local/nginx/conf/.htpasswd/passwd UserName

This directive protects the required directories:
### Password Protect /personal-files/ and /omega/ directories ###
location ~ /(personal-files/.*|omega/.*) {
auth_basic "Restricted";
auth_basic_user_file /usr/local/nginx/conf/.htpasswd/passwd;

Adding users:
# htpasswd -s /usr/local/nginx/conf/.htpasswd/passwd UserName

Load Balancing

Load balancing is a computer networking methodology to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid overload.

upstream myServer {
server {
location / {
proxy_pass http://myServer;

The ip_hash causes nginx to attempt to ensure that requests originating from a single IP address remain attached to a specific element of the cluster.

Remember that the ip_hash directive cannot be combined with server components that specify these additional arguments:
upstream appcluster {
server weight=1;
server weight=2 max_fails=2;
server weight=2 max_fails=2 fail_timeout=20;
server weight=4;
server weight=4 fail_timeout=4;
server weight=2 fail_timeout=20;

  • The argument weight=[number] specifies by default, unless explicitly specified, components in an upstream cluster have a weight of 1. If you would like certain servers to receive a greater proportion of the traffic, set weights for some or all of the components in the upstream.
  • The max_fails=[number] specifies the number of unsuccessful attempts at communication with an upstream component before it is considered inoperative. To prevent components from ever being marked as inoperative, even if they are unreachable, set this value to 0. The default value for max_fails is 1.
  • The fail_timeout=[time-in=seconds] argument determines the span of time within which the max_fails number of unsuccessful attempts must occur in order to mark a component of the server inoperative. Note, servers that return a 404 response are considered operative, and this value does not affect timeouts for established proxy connections.

NB: this article was originally posted on http://techwatch.keeward.com/.