- Network Configuration
- IPv4/IPv6 networking
- Domain Name System(DNS)
Validating Network Configuration
- The /sbin/ip command is used to show device and address information:
$ ip addr show em1 2: em1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether 68:f7:28:1b:3d:c2 brd ff:ff:ff:ff:ff:ff inet 192.168.1.107/24 scope global dynamic em1 valid_lft 6249sec preferred_lft 6249sec inet6 fe80::6af7:28ff:fe1b:3dc2/64 scope link valid_lft forever preferred_lft forever
- Display the current IP address and netmask for all interfaces:
$ ip addr 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: em1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000 link/ether 68:f7:28:1b:3d:c2 brd ff:ff:ff:ff:ff:ff inet 192.168.1.107/24 scope global dynamic em1 valid_lft 6931sec preferred_lft 6931sec inet6 fe80::6af7:28ff:fe1b:3dc2/64 scope link valid_lft forever preferred_lft forever
- The ip command may also be used to show statistics about newwork performance, The received(RA) and transmitted(TX) packets, errors, and dropped count etc:
$ ip -s link show em1 2: em1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000 link/ether 68:f7:28:1b:3d:c2 brd ff:ff:ff:ff:ff:ff RX: bytes packets errors dropped overrun mcast 183544 833 0 0 0 89 TX: bytes packets errors dropped carrier collsns 162430 1195 0 0 0 0
- Show routing information:
$ ip route default via 192.168.1.1 dev em1 proto static metric 1024 172.16.209.0/24 dev vmnet8 proto kernel scope link src 172.16.209.1 172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.42.1 192.168.1.0/24 dev em1 proto kernel scope link src 192.168.1.107 192.168.1.0/24 dev wlp3s0 proto kernel scope link src 192.168.1.108 192.168.173.0/24 dev vmnet1 proto kernel scope link src 192.168.173.1
- The ping command is used to test connectivity:
$ ping -c2 192.168.1.1 PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data. 64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.597 ms 64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=0.372 ms --- 192.168.1.1 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 1000ms rtt min/avg/max/mdev = 0.372/0.484/0.597/0.114 ms
- To trace the path to a remote host, use either traceroute or tracepath
$ traceroute 192.168.1.1 traceroute to 192.168.1.1 (192.168.1.1), 30 hops max, 60 byte packets 1 192.168.1.1 (192.168.1.1) 0.448 ms 0.399 ms 0.596 ms
$ tracepath 192.168.1.1 1: 192.168.1.107 0.177ms pmtu 1500 1: 192.168.1.1 1.155ms reached 1: 192.168.1.1 1.084ms reached Resume: pmtu 1500 hops 1 back 64
- Troubleshooting ports and services via ss command:
$ ss -ta
Note that ss command is meant to replace to the older netstat:
$ netstat -ta
Options for ss and netstat:
- -n - Show numbers instead of names for interfaces and ports.
- -t - Show TCP sockets
- -u - Show UDP sockets
- -l - Show only listening sockets
- -a - Show all(listening and established) sockets
- -p - Show the process using the socket
Configuring Networking with nmcli
NetworkManager is a daemon that monitors and manages network settings. The network configuration files in the
Viewing network information with nmcli
# nmcli connection show NAME UUID TYPE DEVICE enp0s25 131d803f-e480-4512-8f4a-299d275e4a37 802-3-ethernet enp0s25 virbr0 4a9927cf-fd88-42eb-b60f-2863e2baecd1 bridge virbr0
# nmcli connection show --active NAME UUID TYPE DEVICE enp0s25 131d803f-e480-4512-8f4a-299d275e4a37 802-3-ethernet enp0s25 virbr0 4a9927cf-fd88-42eb-b60f-2863e2baecd1 bridge virbr0
# nmcli connection show enp0s25 connection.id: enp0s25 connection.uuid: 131d803f-e480-4512-8f4a-299d275e4a37 connection.stable-id: -- connection.interface-name: enp0s25 connection.type: 802-3-ethernet connection.autoconnect: yes ...
A device is a network interface. A connection is a configuration used for a device which is made up of a collection of settings. Multiple connections may exist for a device, but only one may be active at a time.
# nmcli device status DEVICE TYPE STATE CONNECTION virbr0 bridge connected virbr0 enp0s25 ethernet connected enp0s25 wlp3s0 wifi unavailable -- lo loopback unmanaged --
# nmcli device show enp0s25 GENERAL.DEVICE: enp0s25 GENERAL.TYPE: ethernet GENERAL.HWADDR: 00:21:CC:71:3B:09 GENERAL.MTU: 1500 GENERAL.STATE: 100 (connected) ...
Creating network connections with nmcli
- Define a new connection named “default” which will autoconnect as an Ethernet connecton on eth0 device using DHCP:
# nmcli connection add con-name "default" type ethernet ifname eth0
- Change back to the DHCP connection.
# nmcli connection up default
- Delete a connection
# nmcli connection delete default
- Create a static connection with the same IPv4 address, network prefix, and default gateway. Name the new connection static-eth0.
# nmcli connection add con-name "static-eth0" ifname enp0s25 type ethernet ipv4.addresses 10.66.192.121/24 ipv4.gateway 10.66.193.254
- Modify the new connection to add the DNS setting.
# nmcli connection modify "static-eth0" ipv4.dns 10.72.17.5
- Display and activate the new connection.
# nmcli connection show # nmcli connection show --active # nmcli connection up "static-eth0" # nmcli connection show --active # ip addr show enp0s25 # ip route # ping 10.72.17.5 # nmcli connection modify "static-eth0" connection.autoconnect no # reboot # nmcli connection show --active
Editing Network Configuration Files
Interface configuration files control the software interfaces for individual network devices. These files are usually named /etc/sysconfig/network-scripts/ifcfg-
Example of static-eth0:
# cat /etc/sysconfig/network-scripts/ifcfg-static-eth0 TYPE=Ethernet BOOTPROTO=dhcp DEFROUTE=yes IPV4_FAILURE_FATAL=no IPV6INIT=yes IPV6_AUTOCONF=yes IPV6_DEFROUTE=yes IPV6_FAILURE_FATAL=no IPV6_ADDR_GEN_MODE=stable-privacy NAME=static-eth0 UUID=57e917f1-9f9a-4c35-8fba-366876eb608e DEVICE=enp0s25 ONBOOT=no DNS1=10.72.17.5 IPADDR=10.66.192.121 PREFIX=24 GATEWAY=10.66.193.254 PEERDNS=yes PEERROUTES=yes IPV6_PEERDNS=yes IPV6_PEERROUTES=yes
After modify this configuration file, run nmcli reload and restart the interface, make sure the changes to take effect:
# nmcli connection reload # nmcli connection down "System eth0" # nmcli connection up "System eth0"
Configuring Host Names and Name Resolution
Changing the system host name
# hostname ksoong.org
A static host name may be specified in the /etc/hostname file. The
hostnamectl command is used to modify this file and may be used to view the status of the system’s fully qualified host name.
# cat /etc/hostname ksoong.org
# hostnamectl set-hostname ksoong.com # hostnamectl status
Configuring name resolution
The stub resolver is used to convert host names to IP addresses or the reverse. The contents of the file /etc/hosts are checked first.
# cat /etc/hosts 127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4 ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
# getent hosts ksoong.com # host ksoong.com
IPv4 Networking Configuration
In Linux, the configuration of network interfaces is managed by a system daemon called NetworkManager. For NetworkManager:
deviceis a network interface.
connectionis a connection of settings that can be configured for a device.
- Only one connection is
activefor any one device at a time. Multiple connections may exist, for use by different devices or to allow a ocnfiguration to be altered for the same device.
- Each connection has a name or ID that identifies it.
- The persistent configuration for a connection is stored in /etc/sysconfig/network-scripts/ifcfg-name, which
nameis the name of the connection(although spaces are normally replaced with underscored in the file name). This file can be edited by hand if desired.
nmcliutility can be used to create and edit connection files from the shell prompt.
Domain Name System(DNS)
- Domain - A domain is a collection of resource records that ends in a common name and represents an entire subtree of the DNS name space, eg,
- Subdomain - A subdomain is a domain that is a subtree of another domain, eg,
lab.example.comis a subdomain of
- Zone - A zone is the portion of a domain for which a particular nameserver is directly responsible, or authoritative. This may be an entire domain, or just part of a domain with some or all of its subdomains delegated to other nameserver(s)
When a system needs to perform name resolution using a DNS server, it begins by sending queries to the servers listed in
/etc/resolv.conf in order, until it gets a response or runs out of servers. The
dig commands can be used to manually look up DNS names.
- Local authoritative data
- Local cached non-authoritative data
- Remote non-authoritative data via recursion
DNS resource records(RRs) are entries in a DNS zone that specify information about a particular name or object in the zone.
- A (IPv4 address) records
- AAAA (IPv6 address) records
- CNAME(canonical name) records
- PTR (pointer) records
- NS (name server) records
- SOA (start of authority) records
- TXT(text) records
- SRV (service) records
Hosts and resource records
A typical host, whether a client or a server, will have the following records:
- One or more
AAAArecords mapping its host name to its IP addresses
PTRrecord for each of its IP addresses, reverse mapping them to its host name
- Optionally, one or more
CNAMErecords mapping alternate names to its canonical host name
A DNS zone will typically have, in addition to the records for the hosts in the zone:
NSrecord for each of its authoritative name servers
- One or more
MXrecords mapping the domain name to the mail exchange which receives email for addresses endng in the domain name
- Optionally, one or more
TXTrecords for functions such as SPF or Google Site Verification
- Optionally, one or more
SRVrecords to locate services in the domain
Caching nameservers and DNSSEC
Caching nameservers store DNS query results in a local cache and removes resource records from the cache when their TTLs expire. It is common to set up caching nameservers to perform queries on behalf of clients on the local network. This greatly improves the efficiency of DNS name resolutions by reducing DNS traffic across the Internet. As the cache grows, DNS performance improves as the caching nameserver answers more and more client queries from its local cache.
DNSSEC validation given the stateless nature of UDP, DNS transactions are prone to spoofing and tampering. Caching nameservers have historically been favored targets of attackers looking to redirect or hijack network traffic. This is often achieved by exploiting vulnerabilities in DNS server software to fool a DNS server into accepting and populating malicious data into its cache, a technique commonly referred to as cache poisoning. Once the attacker succeeds in poisoning a DNS server’s cache, they effectively compromise the DNS data received by the numerous clients utilizing the caching name service on the DNS server and can consequently redirect or hijack the clients’ network traffic. While a caching nameserver can greatly improve DNS performance on the local network, they can also provide improved security by
Domain Name System Security Extensions (DNSSEC) validation. DESSEC validation enabled at the caching nameserver allows the authenticity and integrity of resource records to be validated prior to being places in the cache for use by clients, and therefore protects clients against the consequences of cache poisoning.
Configuring and administering unbound as a caching nameserver
Name resolution methods
On Linux systems, name resolution is attempted first with the hosts file
/etc/hosts by default, per order specified in
/etc/nsswitch.conf. Therefore, when beginning name resolution troubleshooting, do not leap to the assumption that the issue resides with DNS. Begin first by identifying the name resolution mechanism which is in play, rather than simply starting with DNS.
gethostip commands from the syslinux package, can both be used to perform name resolution, mirroring the process used by most applications in following the order of host name resolution as dictated by
# getent hosts github.com 188.8.131.52 github.com 184.108.40.206 github.com
Client-server network connectivity
dig to troubleshoot a DNS issue, if a response is not received from a DNS server, it is a clear indication that the cause lies with the client-server network connectivity to the DNS server.
# dig A baidu.com ; <<>> DiG 9.9.4-RedHat-9.9.4-8.fc20 <<>> A baidu.com ;; global options: +cmd ;; connection timed out; no servers could be reached