MTU for LINUX

Show MTU running ---------------- # ip a | grep mtu Temporarily changing the MTU Size – Using ifconfig command We can use the ifconfig command to change the MTU size of a system’s network interface. However, remember that this change does survive a reboot and returns to the default value i.e. 1500. Setup MTU ----------------- ifconfig mtu up Oracle Linux: How to Change MTU Size (Doc ID 2520148.1) To BottomTo Bottom Solution Check the current MTU settings You can use both ifconfig and ip command to check it: # ifconfig eth0 eth0 Link encap:Ethernet HWaddr 02:00:17:00:XX:XX inet addr: Bcast:XXX.XXX.XXX.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:2786436 errors:0 dropped:0 overruns:0 frame:0 TX packets:3744195 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:789337509 (752.7 MiB) TX bytes:654466831 (624.1 MiB) [root@j-ol6-8324 opc]# ip link list 1: lo: mtu 65536 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: eth0: mtu 9000 qdisc mq state UP qlen 1000 link/ether 02:00:17:00:XX:XX brd ff:ff:ff:ff:ff:ff Change the MTU size by ifconfig or ip commands # ifconfig $DEV mtu 1400 or # ip link set $DEV mtu 1400 For instance: # ifconfig eth0 mtu 1500 Or: # ip link set dev eth0 mtu 1500 # ip link list 1: lo: mtu 65536 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: eth0: mtu 1500 qdisc mq state UP qlen 1000 link/ether 02:00:17:00:XX:XX brd ff:ff:ff:ff:ff:ff Make the setting permanent by appending the line 'MTU=' in /etc/sysconfig/network-scripts/ifcfg-*: # grep MTU /etc/sysconfig/network-scripts/ifcfg-* /etc/sysconfig/network-scripts/ifcfg-eth0:MTU=9000 Restart the network service(for Oralce Linux 6): # service network restart Or (OL7) # systemctl restart network Test it from a remote machine by ping to probe the packet path: $ for x in 1462 1463 1472 1473 1500 9000; do echo Lenth $x; ping -c 3 -M do -s $x xxx.xxx.xxx.xxx; done Lenth 1462 PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx) 1462(1490) bytes of data. 1470 bytes from xxx.xxx.xxx.xxx: icmp_seq=1 ttl=39 time=231 ms 1470 bytes from xxx.xxx.xxx.xxx: icmp_seq=2 ttl=39 time=231 ms 1470 bytes from xxx.xxx.xxx.xxx: icmp_seq=3 ttl=39 time=243 ms --- xxx.xxx.xxx.xxx ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2244ms rtt min/avg/max/mdev = 231.508/235.597/243.448/5.553 ms Lenth 1463 PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx) 1463(1491) bytes of data. 1471 bytes from xxx.xxx.xxx.xxx: icmp_seq=1 ttl=39 time=243 ms 1471 bytes from xxx.xxx.xxx.xxx: icmp_seq=2 ttl=39 time=231 ms 1471 bytes from xxx.xxx.xxx.xxx: icmp_seq=3 ttl=39 time=243 ms --- xxx.xxx.xxx.xxx ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2246ms rtt min/avg/max/mdev = 231.495/239.439/243.632/5.634 ms Lenth 1472 >>>>>>>>>>>> Remote instance MTU Size=Data Size+IP header(20 bytes) +ICMP header(8 bytes)=1472+20+8=1500 PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx) 1472(1500) bytes of data. 1480 bytes from xxx.xxx.xxx.xxx: icmp_seq=1 ttl=39 time=243 ms 1480 bytes from xxx.xxx.xxx.xxx: icmp_seq=2 ttl=39 time=231 ms 1480 bytes from xxx.xxx.xxx.xxx: icmp_seq=3 ttl=39 time=231 ms --- xxx.xxx.xxx.xxx ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2234ms rtt min/avg/max/mdev = 231.344/235.478/243.590/5.749 ms Lenth 1473 PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx) 1473(1501) bytes of data. --- xxx.xxx.xxx.xxx ping statistics --- 3 packets transmitted, 0 received, 100% packet loss, time 11999ms Lenth 1500 PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx) 1500(1528) bytes of data. --- xxx.xxx.xxx.xxx ping statistics --- 3 packets transmitted, 0 received, 100% packet loss, time 11999ms Lenth 9000 PING xxx.xxx.xxx.xxx (xxx.xxx.xxx.xxx) 9000(9028) bytes of data. ping: local error: Message too long, mtu=9000 ping: local error: Message too long, mtu=9000 ping: local error: Message too long, mtu=9000 --- xxx.xxx.xxx.xxx ping statistics --- 3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2999ms