Network permissions (--allow-net) in Deno - Performance & Optimization

Performance: Network permissions (--allow-net)

MEDIUM IMPACT

Controls network access for scripts, impacting security and load times by limiting external requests.

Allowing network access only when necessary

Deno

deno run --allow-net=example.com script.ts

Allows network access only to trusted domains, reducing unnecessary network overhead.

📈 Performance GainLimits network calls, reducing load time and improving security.

Allowing network access only when necessary

Deno

deno run script.ts

No network permission granted, script fails or blocks network calls causing delays.

📉 Performance CostBlocks network requests, causing script errors or delays in loading remote resources.

Performance Comparison

Pattern	Network Requests	Security Risk	Load Delay	Verdict
No --allow-net flag	0 (blocked)	Low (no network)	High (script errors/delays)	[X] Bad
--allow-net full access	Many (all domains)	High	Medium to High	[!] Bad
--allow-net limited domains	Few (trusted only)	Low	Low	[OK] Good

Rendering Pipeline

Network permissions control whether scripts can make external requests, affecting resource loading and script execution timing.

→Resource Loading

→Script Execution

⚠️ BottleneckNetwork requests can delay script execution if permissions are too broad or missing.

Optimization Tips

1Only grant --allow-net permissions to domains your script truly needs.

2Avoid full network access to reduce unnecessary external requests and improve load speed.

3Use DevTools Network panel to monitor which requests your script makes.

Performance Quiz - 3 Questions

What is the main performance benefit of restricting --allow-net to specific domains?

A Increases the number of network requests

B Reduces unnecessary network requests and speeds up script execution

C Blocks all network requests causing errors

D Automatically caches all network responses

DevTools: Network panel

How to check: Run the script with different --allow-net flags, open DevTools Network panel, observe which requests are made or blocked.

What to look for: Look for blocked requests or excessive external calls indicating permission issues or over-permission.

Practice

1. What does the --allow-net flag do when running a Deno program?

easy

A. It allows the program to access the network.

B. It disables all network access for the program.

C. It grants permission to read local files.

D. It enables debugging mode for the program.

Solution

1. Step 1: Understand the purpose of --allow-net

   This flag is used to grant network access to a Deno program, which is otherwise restricted for security.

2. Step 2: Compare options with the flag's function

   Only "It allows the program to access the network." correctly states that --allow-net allows network access. Other options describe unrelated permissions or features.

3. Final Answer:

   It allows the program to access the network. -> Option A

4. Quick Check:

   Network permission = Allow network access [OK]

Hint: Remember: --allow-net means allow network access [OK]

Common Mistakes:

• Confusing --allow-net with file permissions
• Thinking it disables network access
• Mixing it up with debugging flags

2. Which of the following is the correct way to allow network access only to example.com when running a Deno script named app.ts?

easy

A. deno run app.ts --allow-net=example.com

B. deno run --allow-net app.ts example.com

C. deno run --allow-net=example.com app.ts

D. deno run --allow-net example.com app.ts

Solution

1. Step 1: Identify correct flag syntax for domain restriction

   The correct syntax is --allow-net=domain placed before the script name.

2. Step 2: Check command order and placement

   deno run --allow-net=example.com app.ts correctly places --allow-net=example.com before app.ts. Other options have incorrect order or syntax.

3. Final Answer:

   deno run --allow-net=example.com app.ts -> Option C

4. Quick Check:

   Correct flag syntax = deno run --allow-net=example.com app.ts [OK]

Hint: Put --allow-net=domain before script name [OK]

Common Mistakes:

• Placing domain after script name
• Using space instead of '=' between flag and domain
• Putting script name before flags

3. Consider this Deno command and script:

deno run --allow-net=deno.land app.ts

And app.ts contains:

const response = await fetch('https://deno.land/std/http/file_server.ts');
console.log(response.status);

What will be the output?

medium

A. SyntaxError

B. PermissionDenied: network access to "deno.land" is not allowed

C. 404

D. 200

Solution

1. Step 1: Check network permission for the domain

   The command allows network access only to "deno.land", which matches the fetch URL domain.

2. Step 2: Understand fetch behavior with permission

   Since permission is granted, fetch succeeds and returns HTTP status 200 for a valid resource.

3. Final Answer:

   200 -> Option D

4. Quick Check:

   Allowed domain fetch = 200 status [OK]

Hint: Allowed domain fetch returns 200 status [OK]

Common Mistakes:

• Assuming permission error despite allowed domain
• Confusing 404 with permission issues
• Expecting syntax errors from fetch

4. You run this command:

deno run --allow-net=example.com app.ts

Inside app.ts you fetch https://google.com. What error will you get?

medium

A. PermissionDenied: network access to "google.com" is not allowed

B. No error, fetch succeeds

C. TypeError: Invalid URL

D. ReferenceError: fetch is not defined

Solution

1. Step 1: Analyze allowed domains

   The command only allows network access to "example.com" but the script tries to fetch "google.com".

2. Step 2: Understand Deno's permission enforcement

   Deno blocks network requests to domains not allowed, causing a PermissionDenied error.

3. Final Answer:

   PermissionDenied: network access to "google.com" is not allowed -> Option A

4. Quick Check:

   Disallowed domain fetch = PermissionDenied error [OK]

Hint: Fetch outside allowed domain causes PermissionDenied [OK]

Common Mistakes:

• Thinking fetch will succeed without permission
• Confusing permission error with URL or reference errors
• Ignoring domain restrictions in --allow-net

5. You want to run a Deno script that fetches data from both api.example.com and data.service.org. Which command correctly grants network access to both domains using a single --allow-net flag?

hard

A. deno run --allow-net=api.example.com --allow-net=data.service.org app.ts

B. deno run --allow-net=api.example.com,data.service.org app.ts

C. deno run --allow-net app.ts api.example.com data.service.org

D. deno run --allow-net=api.example.com;data.service.org app.ts

Solution

1. Step 1: Understand multiple domain syntax for --allow-net

   Deno accepts multiple domains separated by commas in a single --allow-net flag.

2. Step 2: Evaluate each option's correctness

   deno run --allow-net=api.example.com,data.service.org app.ts uses comma-separated domains correctly. deno run --allow-net=api.example.com --allow-net=data.service.org app.ts repeats the flag (not a single flag). deno run --allow-net app.ts api.example.com data.service.org misplaces domains as arguments. deno run --allow-net=api.example.com;data.service.org app.ts uses semicolon which is incorrect.

3. Final Answer:

   deno run --allow-net=api.example.com,data.service.org app.ts -> Option B

4. Quick Check:

   Multiple domains = comma-separated in one flag [OK]

Hint: Use commas to list multiple domains in --allow-net [OK]

Common Mistakes:

• Using multiple --allow-net flags
• Separating domains with semicolons
• Placing domains after script name