Книга: Linux Network Administrator Guide, Second Edition

Setting the TOS Bits Using ipfwadm or ipchains

Setting the TOS Bits Using ipfwadm or ipchains

The ipfwadm and ipchains commands deal with the TOS bits in much the same manner. In both cases you specify a rule that matches the datagrams with particular TOS bits set, and use the -t argument to specify the change you wish to make.

The changes are specified using two-bit masks. The first of these bit masks is logically ANDed with the IP options field of the datagram and the second is logically eXclusive-ORd with it. If this sounds complicated, we'll give you the recipes required to enable each of the types of service in a moment.

The bit masks are specified using eight-bit hexadecimal values. Both ipfwadm and ipchains use the same argument syntax:

- t andmask xormask

Fortunately the same mask arguments can be used each time you wish to set a particular type of service, to save you having to work them out. They are presented with some suggested uses in Table 9.3.

Table 9.3: Suggested Uses for TOS Bitmasks

TOS ANDmask XORmask Suggested Use
Minimum Delay 0x01 0x10 ftp, telnet, ssh
Maximum Throughput 0x01 0x08 ftp-data, www
Maximum Reliability 0x01 0x04 snmp, dns
Minimum Cost 0x01 0x02 nntp, smtp

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