18

How can I check the MTU value for my current connection?

In my network connection settings, the MTU is set to automatic and I would like to know wich value was automatic selected for it.

27
ifconfig

to view the current MTU. Example (just searching for MTU):

ifconfig| grep -i MTU
      UP BROADCAST MULTICAST  MTU:1500  Metric:1
      UP LOOPBACK RUNNING  MTU:65536  Metric:1
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
  • On my system it's mtu not MTU. Suggest to grep in a case-insensitive way with grep -i . – krlmlr Apr 23 at 7:30
  • 1
    @krlmlr hmm odd :D added! Good catch. Have some upvotes as a reward ;) – Rinzwind Apr 23 at 7:37
17

Alternative method:

ip addr | grep mtu

For example:

 ip addr | grep mtu
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
2: enp0s31f6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1300 qdisc fq_codel state UP group default qlen 1000
3: wlp4s0: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN group default qlen 1000
4: veth0@if5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1400 qdisc noqueue state UP group default qlen 1000
  • Trying to avoid a pure "+1!" response, but this alternate answer deserves some love in modern environments where containerization means the running OS of an application may be stripped of common networking tools, like ipconfig. – Marc Bollinger May 26 '17 at 22:50
  • 1
    @firo can you please write ip addr instead of ip ad. While abbreviation is possible, I think it's better to use full name in answers. – pevik Oct 26 '18 at 7:35
  • @marcbollinger ipconfig may be "common", but it's also deprecated - the real reason for it being stripped out IMO :) It can omit important information at times, such as secondary IPs; IIRC the method it uses to communicate with the kernel (ioctls) limits the amount of data that can be sent, while the tools in iproute2 use a different method (netlink) with far higher limits. – mark999 Nov 26 '18 at 17:35
4

You can read the value from the /sys kernel file system:

cat /sys/class/net/<interface>/mtu

Replace <interface> with the name of the interface to query.

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