What do these two QOS script statements mean

Question

I am studying the QOS utility of Linux -- TC -- recently, and today I encountered below two statments from a bandwidth-limit script on Linux:

     tc filter add dev eth0 parent 1: prio 1 protocol ip u32 \
     match ip tos 0x68 0xff match ip protocol 0x11 0xff flowid 1:1

     tc filter add dev eth0 parent 1: prio 1 protocol ip u32 \
     match ip tos 0xb8 0xff match ip protocol 0x11 0xff flowid 1:1

What does it mean by saying "match ip tos 0x68 0xff match ip protocol 0x11 0xff"?

Answer 1

With generic u32 selector, you define a pattern to be matched against IP packet header and a mask to match particular bits only at specific offset.

u32 match ip PATTERN MASK at OFFSET

Multiple matches are logically "anded" so a filter will be successful if all its matches are successful. Let's try to "decode" your filter and its selectors:

• ip tos 0x68 0xff

This can be read as check if TOS field of IP header has value of 0x68 (mask is 0xff so 0x68 AND 0xff = 0x68). I would say it has something to do with DSCP (more about value 0x68).

• ip protocol 0x11 0xff

This checks if protocol field has value of 0x11 which is UDP protocol. You can find protocol numbers in /etc/protocols (for UDP, 17 = 0x11)

ip tos and ip protocol are so-called specific selectors but you can rewrite them in generic form

• u8 0x68 0xff at 1
• u8 0x11 0xff at 9

Selector u8 defines lenght of pattern to be matched in bits (other ones are u32, u16). Then, use the same logic like above. Match value 0x68 at byte 1 in IP header which is TOS field and value 0x11 at byte 9 which is protocol field.

Finally, I would add one more example how to match IP packets with source IP address from particular network (192.168.123.0/24):

• u32 u32 0xc0a87b00 0xffffff00 at 12

This matches only first 3 bytes (0xc0 = 192, 0xa8 = 168, 0x7b = 123) of the field at byte 12 due to the mask value (0xc0 AND 0xff = 0xc0, ..., 0xXX AND 0x00 = 0x00). This can be written with specific selector as

• ip src 192.168.123/24

If you can use specific selectors as they are easier to understand. Finally. this picture can help with choosing right offsets if you have to use generic ones.