Solved ip link set help

[CaffeineAddict]

ip link set help

The following options among others are printed by the above help command:

• multicast
• allmulticast
• trailers
• carrier

To toggle these, the command is ip link set <dev> trailers on or ip link set <dev> allmulticast off etc.

Once ip link set ... command is run to toggle specific option, then running ip link alone will show some flags, for instance:

if you run trailers on and allmulticast on output will be (pay attention to bold flags):

7: wlan0: <BROADCAST,ALLMULTI,NOTRAILERS,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode D
EFAULT group default qlen 1000

If you toggle them off, then these 2 flags won't be present.

Question 1:
What each of these 4 options mean exactly?
btw. I know carrier off (NO-CARRIER) means the NIC is not connected to gateway.
And I know multicast means that NIC will be able to send/receive multicast packets.
Correct me if wrong...

But I don't know about trailers (NOTRAILERS) and allmulticast (ALLMULTI)

possible hint: allmulticast may be used or required in promiscuous or monitor mode e.g. ip link set <dev> promisc on

Question 2:
When I attempt to modify carrier off or on I get this response:

sudo ip link set wlan0 carrier on

RTNETLINK answers: Operation not supported

The same output happens regardless if I'm connected to WI-FI or not, and regardless if carrier option is initially on or off.
Why?

 

[dos2unix]

Copilot

Question 1: Meaning of Options​

1. multicast: This option allows the network interface to send and receive multicast packets. You are correct about this.
2. allmulticast (ALLMULTI): When this option is enabled, the network interface will receive all multicast packets, not just those it is subscribed to. This can be useful in promiscuous or monitor mode
3. trailers (NOTRAILERS): This option is related to the use of trailer encapsulations. When trailers is off (NOTRAILERS), the network interface avoids using trailer encapsulations, which are a method of encapsulating network packets
4. carrier (NO-CARRIER): As you mentioned, this indicates whether the network interface is connected to a network. carrier off (NO-CARRIER) means the NIC is not connected to a gateway or network
   

Question 2: RTNETLINK answers: Operation not supported​

The error message RTNETLINK answers: Operation not supported typically indicates that the operation you are trying to perform is not supported by the network interface or the driver. Here are a few possible reasons:

1. Driver Limitations: The network driver for your wireless interface might not support toggling the carrier state. This is common with many wireless drivers
2. Kernel Configuration: Certain kernel configurations might not support this operation. Ensuring that your kernel and drivers are up-to-date might help
3. Hardware Limitations: Some network interfaces, especially wireless ones, might not support certain operations that are typically available on wired interfaces
   

 

[CaffeineAddict]

Thank you very much, I knew it will be you to respond.

I understand everything except trailer encapsulation, but found this blog post talking about it:

Conproly - Why does layer 2 have trailers?

A student asked me a question after seeing diagrams about ethernet headers and encapsulation: why does layer 2 have trailers? In this post, I will explain why.

www.conproly.com

It's layer 2 trailer, as opposite to a header and is used for checksum calculation in ethernet frame.