network utils

ifconfig

Gathering network related data.

# example output of ifconfig
eno1      Link encap:Ethernet  HWaddr 0c:c4:7a:e3:5b:fe
          inet addr:222.29.2.44  Bcast:222.29.2.255  Mask:255.255.255.0
          inet6 addr: 2001:da8:201:1302:ec4:7aff:fee3:5bfe/64 Scope:Global
          inet6 addr: fe80::ec4:7aff:fee3:5bfe/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1000529332 errors:0 dropped:310819822 overruns:3907 frame:0
          TX packets:1016086867 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:291468156454 (291.4 GB)  TX bytes:1366810427542 (1.3 TB)
          Memory:fb120000-fb13ffff

eno2      Link encap:Ethernet  HWaddr 0c:c4:7a:e3:5b:ff
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
          Memory:fb100000-fb11ffff

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:157678206 errors:0 dropped:0 overruns:0 frame:0
          TX packets:157678206 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:2013281906725 (2.0 TB)  TX bytes:2013281906725 (2.0 TB)

Explain:

Link encap: link encapsulation. e.g., Ethernet, Local Loopback.

almost all encapsulations are Ethernet.

I don’t know what the networking protocol of the future is, but it will be called ‘Ethernet.’

HWaddr: hardware address (MAC address)

unique to each ethernet card.

inet addr: IP address

Bast: broadcast address

Mask: network mask

UP: kernel modules related to ethernet interface.
- BROADCAST: support broadcast
- NOTRAILERS: trailer encapsulation is disabled. Linux usually ignore this.
- RUNNING: ready to accept data.
- MULTICAST: support multicast.

MTU: Maximum Transmission Unit. (size of each packet received by the ethernet card)

Metric: priority of this device.

RX Packets, TX Packets: total number of received, transmitted packets.

collisions: collisions for congestion, should ideally be 0.

RX Bytes, TX Bytes: total amount of data received, transmitted.

arp

command to show the ARP (Address Resolution Protocol) cache.

ARP cache is a collection of ARP entries (IP to MAC) for efficiently communicate with the IP address.

# show all entries
arp -a 
# verbose
arp -v

route, ip, netstat

show routing table

# show routing table
route
netstat -r
# replace domain name for numerical IP address
route -n
netstat -nr
# explain the table
ip route show

### output example of `route -n`
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         222.29.2.1      0.0.0.0         UG    0      0        0 enp129s0f0 # default gateway
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 enp129s0f0
172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 docker0
222.29.2.0      0.0.0.0         255.255.255.0   U     0      0        0 enp129s0f0

# 0.0.0.0 or * means None (not set)
# genmask: netmask
# flags: U = up, G = gateway

# BTW, equivalent in win10 powershell is `route PRINT`
# 10.1.0.1: default router IP reserved for wireless routers.

### output example of `ip route show`
default via 222.29.2.1 dev enp129s0f0 onlink
169.254.0.0/16 dev enp129s0f0  scope link  metric 1000
172.17.0.0/16 dev docker0  proto kernel  scope link  src 172.17.0.1 linkdown
222.29.2.0/24 dev enp129s0f0  proto kernel  scope link  src 222.29.2.65

modifying route:

# add a new route
ip route add <target/mask> via <next hop> dev <interface>

# delete route
ip route del <target/mask> via <next hop> dev <interface>

# add reject route
route add -host <target> reject # `ping <target>` will show unreachable

subnet mask

Each host is configured with a unique IP address and subnet mask.

The IP address is divided by the subnet mask to 2 parts: Network ID + Host ID

# example
inet addr:222.29.2.44  Mask:255.255.255.0

# CIDR format is 222.29.2.44/24 
# 24 ones = 11111111.11111111.11111111.00000000 = 255.255.255.0

# network id = 222.29.2.0
# host id    = 0.0.0.44

what router does ?

When presented with a packet bound for an IP address

the router needs to determine which of its network interfaces will best get that packet closer to its destination.

default gateway

The node that serves as the default router when no other route specification matches the destination IP address.

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         222.29.2.1      0.0.0.0         UG    0      0        0 enp129s0f0 # default gateway
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 enp129s0f0
172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 docker0
222.29.2.0      0.0.0.0         255.255.255.0   U     0      0        0 enp129s0f0

# 0.0.0.0, or * means ANY IP.
# 169.254.0.0 or link-local is for communication within the network segment.

traceroute

install by sudo apt install traceroute.

The name is not traceroute6 !

# trace route to an IP
traceroute bilibili.com
traceroute 8.8.8.8 # google DNS

# BTW, equivalent in win10 powershell is `tracert`

iptables

firewall.

# list ip tables 
iptables -L