IP-based access control (allow/deny) in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is IP-based access control in nginx?

It is a way to allow or deny access to a website or resource based on the visitor's IP address.

beginner

How do you allow access only from a specific IP address in nginx?

Use the directive allow [IP_address]; followed by deny all; to block others.

beginner

What does the directive deny all; do in nginx?

It blocks access to everyone except those explicitly allowed before it.

intermediate

Write a simple nginx config snippet to allow only 192.168.1.10 and deny all others.

location / { allow 192.168.1.10; deny all; }

intermediate

Can you allow multiple IP addresses in nginx? How?

Yes, by using multiple allow directives, one for each IP address or range.

Which nginx directive blocks all IPs except those allowed?

A reject all;

B allow all;

C block all;

D deny all;

The directive deny all; blocks all IPs except those explicitly allowed.

How do you allow access from IP 10.0.0.5 only?

A deny 10.0.0.5; allow all;

B allow 10.0.0.5; deny all;

C allow all; deny 10.0.0.5;

D deny all; allow 10.0.0.5;

You allow the specific IP first, then deny all others.

What happens if you only use deny all; without any allow?

A Blocks no one.

B Allows everyone to access.

C Blocks everyone from accessing.

D Allows only localhost.

Without any allow, deny all; blocks all access.

Can you allow an entire IP range in nginx?

A Yes, by specifying the range in CIDR notation in the allow directive.

B No, only single IPs are allowed.

C Yes, but only in the deny directive.

D No, nginx does not support IP ranges.

Nginx supports CIDR notation to allow or deny IP ranges.

Where in the nginx config do you place allow/deny directives?

A Inside a location, server, or http block.

B Only in the http block.

C Only in the server block.

D Only in the events block.

Allow and deny directives can be used inside location, server, or http blocks.

Explain how to restrict access to a website using IP addresses in nginx.

Think about allowing specific IPs first, then denying all others.

Describe how nginx handles multiple allow directives and what happens if no allow matches before deny all.

Consider how nginx processes access rules in order.

Practice

1. What does the allow directive do in an nginx configuration?

easy

A. It permits access to specified IP addresses or ranges.

B. It blocks access to specified IP addresses or ranges.

C. It redirects users to another URL.

D. It logs the IP addresses of visitors.

Solution

1. Step 1: Understand the purpose of allow

   The allow directive in nginx is used to specify which IP addresses or ranges are permitted to access the server or location.

2. Step 2: Compare with other directives

   Unlike deny which blocks IPs, allow explicitly permits them.

3. Final Answer:

   It permits access to specified IP addresses or ranges. -> Option A

4. Quick Check:

   allow means permit access [OK]

Hint: Allow means permit access, deny means block access [OK]

Common Mistakes:

• Confusing allow with deny
• Thinking allow redirects users
• Assuming allow logs IPs

2. Which of the following is the correct syntax to deny access to IP 192.168.1.10 in nginx?

easy

A. allow 192.168.1.10;

B. deny 192.168.1.10;

C. block 192.168.1.10;

D. reject 192.168.1.10;

Solution

1. Step 1: Identify the correct directive for blocking IPs

   In nginx, the directive to block or deny access is deny.

2. Step 2: Check syntax correctness

   The correct syntax is deny IP_address; with a semicolon at the end.

3. Final Answer:

   deny 192.168.1.10; -> Option B

4. Quick Check:

   deny blocks IP = deny 192.168.1.10; [OK]

Hint: Use 'deny IP;' to block access in nginx [OK]

Common Mistakes:

• Using 'allow' instead of 'deny'
• Using invalid directives like 'block' or 'reject'
• Missing semicolon at the end

3. Given this nginx config snippet inside a location /admin block:

allow 10.0.0.0/24;
deny all;

What happens when a user with IP 10.0.0.15 tries to access /admin?

medium

A. Access is denied because the IP is not explicitly allowed.

B. Access is denied because deny all overrides allow.

C. Access is allowed only if the user has a valid cookie.

D. Access is allowed because the IP is in the allowed range.

Solution

1. Step 1: Understand the order of allow and deny

   nginx checks allow and deny rules in order. If an IP matches an allow rule, access is granted even if deny all follows.

2. Step 2: Check if IP matches allowed range

   IP 10.0.0.15 is inside 10.0.0.0/24, so it matches the allow rule.

3. Final Answer:

   Access is allowed because the IP is in the allowed range. -> Option D

4. Quick Check:

   Allow matched before deny all = access allowed [OK]

Hint: Allow rules take priority before deny all [OK]

Common Mistakes:

• Assuming deny all always blocks
• Ignoring IP range matching
• Thinking cookies affect IP access control

4. You wrote this nginx config inside a server block:

deny 192.168.0.0/16
allow all;

But all IPs are denied access. What is the problem?

medium

A. You need to reload nginx after changes.

B. The order of deny and allow is wrong; allow should come first.

C. Missing semicolon after deny directive causes config error.

D. The IP range 192.168.0.0/16 is invalid.

Solution

1. Step 1: Check syntax for missing semicolons

   nginx requires semicolons at the end of each directive. The deny line is missing a semicolon, causing the config to fail.

2. Step 2: Understand effect of syntax error

   Without proper syntax, nginx may ignore or misinterpret rules, leading to unexpected blocking of all IPs.

3. Final Answer:

   Missing semicolon after deny directive causes config error. -> Option C

4. Quick Check:

   Every directive needs semicolon [OK]

Hint: Always end nginx directives with semicolon [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking order alone causes issue
• Assuming IP range is invalid

5. You want to allow access only from IP 203.0.113.5 and block everyone else for /secure location. Which nginx config snippet achieves this correctly?

hard

A. location /secure {
allow 203.0.113.5;
deny all;
}

B. location /secure {
deny all;
allow 203.0.113.5;
}

C. location /secure {
allow all;
deny 203.0.113.5;
}

D. location /secure {
deny 203.0.113.5;
allow all;
}

Solution

1. Step 1: Understand correct order of allow and deny

   nginx processes allow and deny in order. To allow one IP and block others, allow must come first, then deny all.

2. Step 2: Verify each option

   location /secure {
     allow 203.0.113.5;
     deny all;
   }

   correctly allows 203.0.113.5 then denies all others.

   location /secure {
     deny all;
     allow 203.0.113.5;
   }

   denies all first, so no one is allowed. Options A and D misuse allow and deny directives.

3. Final Answer:

   location /secure { allow 203.0.113.5; deny all; } -> Option A

4. Quick Check:

   Allow specific IP then deny all others [OK]

Hint: Allow first, then deny all to restrict access [OK]

Common Mistakes:

• Reversing allow and deny order
• Allowing all then denying one IP
• Misusing deny and allow directives