Take the 2-minute tour ×
Server Fault is a question and answer site for professional system and network administrators. It's 100% free, no registration required.

I am using iptable's rule to filter & manipulate packets in my ubuntu server. but i cannot understand the mangle table.

from here:

This table should as we've already noted mainly be used for mangling packets. In other words, you may freely use the mangle matches etc that could be used to change TOS (Type Of Service) fields and so on.

You are strongly advised not to use this table for any filtering; nor will any DNAT, SNAT or Masquerading work in this table.

Can anybody describe me the iptable's mangle table, and provide some examples to understand when i should use mangle table?

share|improve this question
4  
The iptables(8) man page has all of the information you want, including several nice examples of mangle table usage. –  Michael Hampton Jan 14 '13 at 5:03
add comment

2 Answers 2

up vote 4 down vote accepted

Further to the other good answers, I recently had to use the mangle table to adjust for MTU (maximum transmission unit) discrepancies caused by traffic being brought through PPPoE, PPP, and ATM, each of which adds overhead that reduces the payload available for IP from the usual 1500 bytes of an Ethernet frame.

Systems on each end of the pipe, as is normal, would have their MTU at the regular default of 1500 and so they would try to send IP frames that large. Since the actual payload size available was smaller, this would have caused packet fragmentation, except that often the sender will request that packets not be fragmented, and as such they end up getting dropped entirely.

In an ideal world, path MTU discovery would have allowed the endpoints to adjust their MTU down as needed, but this discovery depends upon ICMP, and networks outside of my control were often configured to drop ICMP for security reasons.

The only choice was to use packet mangling in my router in order to modify TCP SYN packets to lower the maximum segment size at the transport layer:

iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1452

This sort of thing is messy and ideally should be avoided, but I had no other options and this did solve the problem.

Hope these examples help, as well as the man page.

share|improve this answer
add comment

I recently found a good explanation here. It is basically used to set specific headers for IP packets to affect the routing decision made further on. If any, the TTL option is probably the most interesting:

The TTL target is used to change the TTL (Time To Live) field of the packet. We could tell packets to only have a specific TTL and so on. One good reason for this could be that we don't want to give ourself away to nosy Internet Service Providers. Some Internet Service Providers do not like users running multiple computers on one single connection, and there are some Internet Service Providers known to look for a single host generating different TTL values, and take this as one of many signs of multiple computers connected to a single connection.

The other targets are TOS, MARK, SECMARK, CONNSECMARK.

share|improve this answer
    
Mangle is also used in QOS to handle marking of packet or in data load distribution to distribute packets to different routes. –  TomTom Jan 15 '13 at 17:55
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.