Test grep result
grep "somestring" somefile if [ "$?" -eq 0 ]; then echo "processing somefile" fi
Test environment variable
if [ -n "${ENV_VAR+x}" ]
then
... do something ...
else
... do something else ...
fi
Test if a program is installed
if which program >/dev/null; then
echo program found
else
echo program not found
exit 1
fi
Array initialization
int a[4] = {[2] = 6, [3] = 7};
int grid[100][100] = [0][0] = 8, [50][25] = 7};
Structure initialization
struct address {
int street_no;
char *street_name;
char *city;
char *prov;
char *postal_code;
};
struct address temp_address = { .city = "Hamilton", .prov = "Ontario" };
struct a {
struct b {
int c;
int d;
} e;
float f;
} g = {.e.c = 3 };
Union initialization
union {
char birthday[9];
int age;
float weight;
} people = { .age = 14 };
printf format
%[flags][min field width][precision][length]conversion specifier
where:
flags:
# Alternate
0 zero pad
- left align
+ explicit + - sign
space for + sign
' locale thousands grouping
I Use locale's alt digits
min field width: #,*
precision: .#, .*
length:
hh char
h short
l long
ll long long
j [u]intmax_t
z size_t
t ptrdiff_t
L long double
conversion specifier:
c unsigned char
d signed int
u unsigned int
x unsigned hex int
X unsigned HEX int
e [-]d.ddde±dd double,
E [-]d.dddE±dd double
Examples:
printf("%08X", var); 00001234
printf("%20s","string"); string (right aligned)
printf("%*s", 20, "string"); string (right aligned, the alignment is specified as an argument)
printf("%-20s", "string"); string (left aligned)
printf("%-20.20s", "string"); string (the string is truncated if it is too long)
Equal Cost Multipath (ECMP) is a network load-balancing method that enables the coexistence of multiple network paths form one source node to a destination node. The two or more paths between the nodes have the same routing cost, thus the traffic will be split evenly across, avoiding congestion and increasing the bandwidth.
ECMP is also a network redundancy method. In case one ECMP link fails, the traffic will move on the remaining links with minimal interruption in service.
For ECMP to work, a router will need special support in the forwarding plane and in the routing protocols deployed in the network. For experimentation I will use two Linux virtual machines on my host computer. The Linux kernel has an excellent ECMP implementation, as for the routing protocol I will use OSPF.
The virtual machines are set using Easy LXC. Each machine owns a slice of the Linux kernel running on the host computer, with full network and process separation. In each virtual machine I run one instance of RCP100. RCP100 is a router control plane for Linux platforms, supporting among other things OSPF and ECMP. RCP100 also features a CISCO-like command line interface (CLI) which simplifies router operation for people already skilled in configuring commercial routers.
The network diagram is as follows:
ECMP test network
The two virtual machines are plugged into two Linux bridges, br0 and br1. On router connections I use network addresses in the range 10.20.x.0/24 range. Router IDs are defined in 192.168.10.x range and are implemented as loopback interfaces on the router. The loopback interfaces are redistributed into OSPF and should be advertised by OSPF throughout the network. The relevant configuration for the two routers looks like this:
Router r0:
r0#show running-config | no-more hostname r0 [snip] ! router ospf router-id 192.168.10.0 network 10.20.0.0/16 area 0 redistribute connected loopback ! interface ethernet eth0 ip address 10.20.0.2/24 ip mtu 1500 no shutdown ! interface ethernet eth1 ip address 10.20.1.2/24 ip mtu 1500 no shutdown ! interface loopback 0 ip address 192.168.10.0/32 ip mtu 16436 ! [snip] r0#
Router r1:
r1#show running-config | no-more hostname r1 [snip] router ospf router-id 192.168.10.1 network 10.20.0.0/16 area 0 redistribute connected loopback ! interface ethernet eth0 ip address 10.20.0.3/24 ip mtu 1500 no shutdown ! interface ethernet eth1 ip address 10.20.1.3/24 ip mtu 1500 no shutdown ! interface loopback 0 ip address 192.168.10.1/32 ip mtu 16436 ! r1#
We login into the routers by telnet 0 in the virtual machine control terminals. The routing tables will show the equal cost multipath routes installed for the neighbor’s router ID. As we configured them earlier as loopbacks, router IDs are real routes, we can ping them for example.
Router r0:
r0#show ip route
Codes: C - connected, S - static, R - RIP, B - blackhole, O - OSPF
IA - OSPF inter area, E1 - OSPF external type 1, E2 - OSPF external type 2
C 10.20.0.0/24 is directly connected, eth0
C 10.20.1.0/24 is directly connected, eth1
O E2 192.168.10.1/32[110/20] via 10.20.0.3, eth0
via 10.20.1.3, eth1
r0#
Router r1:
r1#show ip route
Codes: C - connected, S - static, R - RIP, B - blackhole, O - OSPF
IA - OSPF inter area, E1 - OSPF external type 1, E2 - OSPF external type 2
C 10.20.0.0/24 is directly connected, eth0
C 10.20.1.0/24 is directly connected, eth1
O E2 192.168.10.0/32[110/20] via 10.20.0.2, eth0
via 10.20.1.2, eth1
r1#
With the ECMP routes installed, we can try now to see how the traffic is distributed across the two links. We login into router 0 and from there we telnet several times into r1 at 192.168.10.1.
r0#telnet 192.168.10.1 Trying 192.168.10.1... Connected to 192.168.10.1. Escape character is '^]'. User: rcp Password: r1>exit Connection closed by foreign host. r0#
Each telnet connection goes trough one path or the other alternatively. Once the telnet session is established, the traffic stays on the specific path until the telnet connection is closed. This is accomplished in the Linux kernel by hashing source/destination address and port numbers, and associating specific hash values with specific connections.
The following picture shows two Wireshark windows side by side, while pinging r1 from r0. The ICMP requests are spread over the two links, r1 will respond on the link it received the request.
ECMP traffic in two WIreshark windows
As the Internet goes slower and slower and your Internet Service Provider refuses to go faster and faster, these are three easy things you can do to speed up Mozilla Firefox:
1. Disable IPv6
For a regular web page such as slashdot.org, Firefox needs to resolve more than 40 domain names. Each domain name is resolved twice, once for an IPv4 address and once for an IPv6 address. This results in lots of DNS requests, slowing down your web access. If you are like 99.999% of the population without IPv6 access, translating domain names in IPv6 addresses is useless.
To disable this functionality, type about:config into the address bar. Type ipv6 into the search bar and toggle network.dns.disableIPv6 to true.
2. Install Adblock Plus extension
The extension will cut down most, if not all, advertisements and annoying banners.
3. Install Ghostery extension
The extension removes “invisible” trackers, web bugs, pixels, and beacons placed on web pages by Facebook, Google Analytics, and over 1,000 other ad networks.
