Get the IP address of the machine

Question

This Question is almost the same as the previously asked Get the IP Address of local computer-Question  However I need to find the IP address es  of a Linux Machine   So  How do I - programmatically in C   - detect the IP addresses of the linux server my application is running on  The servers will have at least two IP addresses and I need a specific one  the one in a given network  the public one     I m sure there is a simple function to do that - but where     To make things a bit clearer    The server will obviously have the  localhost   127 0 0 1 The server will have an internal  management  IP address  172 16 x x The server will have an external  public  IP address  80 190 x x   I need to find the external IP address to bind my application to it  Obviously I can also bind to INADDR ANY  and actually that s what I do at the moment   I would prefer to detect the public address  though

User · Answer

I found the ioctl solution problematic on os x (which is POSIX compliant so should be similiar to linux). However getifaddress() will let you do the same thing easily, it works fine for me on os x 10.5 and should be the same below.

I've done a quick example below which will print all of the machine's IPv4 address, (you should also check the getifaddrs was successful ie returns 0).

I've updated it show IPv6 addresses too.

#include <stdio.h>      
#include <sys/types.h>
#include <ifaddrs.h>
#include <netinet/in.h> 
#include <string.h> 
#include <arpa/inet.h>

int main (int argc, const char * argv[]) {
    struct ifaddrs * ifAddrStruct=NULL;
    struct ifaddrs * ifa=NULL;
    void * tmpAddrPtr=NULL;

    getifaddrs(&ifAddrStruct);

    for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) {
        if (!ifa->ifa_addr) {
            continue;
        }
        if (ifa->ifa_addr->sa_family == AF_INET) { // check it is IP4
            // is a valid IP4 Address
            tmpAddrPtr=&((struct sockaddr_in *)ifa->ifa_addr)->sin_addr;
            char addressBuffer[INET_ADDRSTRLEN];
            inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } else if (ifa->ifa_addr->sa_family == AF_INET6) { // check it is IP6
            // is a valid IP6 Address
            tmpAddrPtr=&((struct sockaddr_in6 *)ifa->ifa_addr)->sin6_addr;
            char addressBuffer[INET6_ADDRSTRLEN];
            inet_ntop(AF_INET6, tmpAddrPtr, addressBuffer, INET6_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } 
    }
    if (ifAddrStruct!=NULL) freeifaddrs(ifAddrStruct);
    return 0;
}

User · Answer

An elegant scripted solution for Linux can be found at  http   www damnsmalllinux org f topic-3-23-17031-0 html

User · Answer

You can do some integration with curl as something as easy as  curl www whatismyip org from the shell will get you your global ip  You re kind of reliant on some external server  but you will always be if you re behind a NAT

User · Answer

Further to what Steve Baker has said  you can find a description of the SIOCGIFCONF ioctl in the netdevice 7  man page   Once you have the list of all the IP addresses on the host  you will have to use application specific logic to filter out the addresses you do not want and hope you have one IP address left

User · Answer

As you have found out there is no such thing as a single  local IP address   Here s how to find out the local address that can be sent out to a specific host    Create a UDP socket Connect the socket to an outside address  the host that will eventually receive the local address  Use getsockname to get the local address

User · Answer

You can do some integration with curl as something as easy as  curl www whatismyip org from the shell will get you your global ip  You re kind of reliant on some external server  but you will always be if you re behind a NAT

User · Answer

I found the ioctl solution problematic on os x (which is POSIX compliant so should be similiar to linux). However getifaddress() will let you do the same thing easily, it works fine for me on os x 10.5 and should be the same below.

I've done a quick example below which will print all of the machine's IPv4 address, (you should also check the getifaddrs was successful ie returns 0).

I've updated it show IPv6 addresses too.

#include <stdio.h>      
#include <sys/types.h>
#include <ifaddrs.h>
#include <netinet/in.h> 
#include <string.h> 
#include <arpa/inet.h>

int main (int argc, const char * argv[]) {
    struct ifaddrs * ifAddrStruct=NULL;
    struct ifaddrs * ifa=NULL;
    void * tmpAddrPtr=NULL;

    getifaddrs(&ifAddrStruct);

    for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) {
        if (!ifa->ifa_addr) {
            continue;
        }
        if (ifa->ifa_addr->sa_family == AF_INET) { // check it is IP4
            // is a valid IP4 Address
            tmpAddrPtr=&((struct sockaddr_in *)ifa->ifa_addr)->sin_addr;
            char addressBuffer[INET_ADDRSTRLEN];
            inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } else if (ifa->ifa_addr->sa_family == AF_INET6) { // check it is IP6
            // is a valid IP6 Address
            tmpAddrPtr=&((struct sockaddr_in6 *)ifa->ifa_addr)->sin6_addr;
            char addressBuffer[INET6_ADDRSTRLEN];
            inet_ntop(AF_INET6, tmpAddrPtr, addressBuffer, INET6_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } 
    }
    if (ifAddrStruct!=NULL) freeifaddrs(ifAddrStruct);
    return 0;
}

User · Answer

Don t hard code it  this is the sort of thing that can change  Many programs figure out what to bind to by reading in a config file  and doing whatever that says  This way  should your program sometime in the future need to bind to something that s not a public IP  it can do so

User · Answer

I found the ioctl solution problematic on os x (which is POSIX compliant so should be similiar to linux). However getifaddress() will let you do the same thing easily, it works fine for me on os x 10.5 and should be the same below.

I've done a quick example below which will print all of the machine's IPv4 address, (you should also check the getifaddrs was successful ie returns 0).

I've updated it show IPv6 addresses too.

#include <stdio.h>      
#include <sys/types.h>
#include <ifaddrs.h>
#include <netinet/in.h> 
#include <string.h> 
#include <arpa/inet.h>

int main (int argc, const char * argv[]) {
    struct ifaddrs * ifAddrStruct=NULL;
    struct ifaddrs * ifa=NULL;
    void * tmpAddrPtr=NULL;

    getifaddrs(&ifAddrStruct);

    for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) {
        if (!ifa->ifa_addr) {
            continue;
        }
        if (ifa->ifa_addr->sa_family == AF_INET) { // check it is IP4
            // is a valid IP4 Address
            tmpAddrPtr=&((struct sockaddr_in *)ifa->ifa_addr)->sin_addr;
            char addressBuffer[INET_ADDRSTRLEN];
            inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } else if (ifa->ifa_addr->sa_family == AF_INET6) { // check it is IP6
            // is a valid IP6 Address
            tmpAddrPtr=&((struct sockaddr_in6 *)ifa->ifa_addr)->sin6_addr;
            char addressBuffer[INET6_ADDRSTRLEN];
            inet_ntop(AF_INET6, tmpAddrPtr, addressBuffer, INET6_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } 
    }
    if (ifAddrStruct!=NULL) freeifaddrs(ifAddrStruct);
    return 0;
}

User · Answer

Create a socket  Perform ioctl  lt socketfd gt   SIOCGIFCONF   struct ifconf  amp buffer     Read  usr include linux if h for information on the ifconf and ifreq structures   This should give you the IP address of each interface on the system   Also read  usr include linux sockios h for additional ioctls

User · Answer

Further to what Steve Baker has said  you can find a description of the SIOCGIFCONF ioctl in the netdevice 7  man page   Once you have the list of all the IP addresses on the host  you will have to use application specific logic to filter out the addresses you do not want and hope you have one IP address left

User · Answer

Create a socket  Perform ioctl  lt socketfd gt   SIOCGIFCONF   struct ifconf  amp buffer     Read  usr include linux if h for information on the ifconf and ifreq structures   This should give you the IP address of each interface on the system   Also read  usr include linux sockios h for additional ioctls

User · Answer

Don t hard code it  this is the sort of thing that can change  Many programs figure out what to bind to by reading in a config file  and doing whatever that says  This way  should your program sometime in the future need to bind to something that s not a public IP  it can do so

User · Answer

This has the advantage of working on many flavors of unix    and you can modify it trivially to work on any o s    All of the solutions above give me compiler errors depending on the phase of the moon   The moment there s a good POSIX way to do it    don t use this  at the time this was written  that wasn t the case       ifconfig   perl -ne  print   1 n  if  inet addr    d         include  lt stdlib h gt   int main             setenv  LANG   C  1           FILE   fp   popen  ifconfig    r            if  fp                    char  p NULL   e  size t n                  while   getline  amp p   amp n  fp   gt  0   amp  amp  p                       if  p   strstr p   inet                               p  5                          if  p   strchr p                                        p                              if  e   strchr p                                            e   0                                    printf   s n   p                                                                                                                  pclose fp           return 0

User · Answer

You can do some integration with curl as something as easy as  curl www whatismyip org from the shell will get you your global ip  You re kind of reliant on some external server  but you will always be if you re behind a NAT

User · Answer

An elegant scripted solution for Linux can be found at  http   www damnsmalllinux org f topic-3-23-17031-0 html

User · Answer

Use a HTTP request to a well known server that echo s back the public IP address void GetPublicIP CString  amp  csIP           Initialize COM     bool bInit   false      if  SUCCEEDED CoInitialize NULL                     COM was initialized         bInit   true          Create a HTTP request object     MSXML2  IXMLHTTPRequestPtr HTTPRequest      HRESULT hr   HTTPRequest CreateInstance  MSXML2 XMLHTTP        if  SUCCEEDED hr                    Build a request to a web site that returns the public IP address         VARIANT Async          Async vt   VT BOOL          Async boolVal   VARIANT FALSE          CComBSTR ccbRequest   L http   whatismyipaddress com                Open the request         if  SUCCEEDED HTTPRequest- gt raw open L GET  ccbRequest Async                             Send the request             if  SUCCEEDED HTTPRequest- gt raw send                                      Get the response                 CString csRequest   HTTPRequest- gt GetresponseText                        Parse the IP address                 CString csMarker     lt  -- contact us before using a script to get your IP address -- gt                    int iPos   csRequest Find csMarker                   if  iPos    -1                      return                  iPos    csMarker GetLength                    int iPos2   csRequest Find csMarker iPos                   if  iPos2    -1                      return                      Build the IP address                 int nCount   iPos2 - iPos                  csIP   csRequest Mid iPos nCount                                       Unitialize COM if  bInit      CoUninitialize

User · Answer

Further to what Steve Baker has said  you can find a description of the SIOCGIFCONF ioctl in the netdevice 7  man page   Once you have the list of all the IP addresses on the host  you will have to use application specific logic to filter out the addresses you do not want and hope you have one IP address left

User · Answer

I do not think there is a definitive right answer to your question   Instead there is a big bundle of ways how to get close to what you wish   Hence I will provide some hints how to get it done   If a machine has more than 2 interfaces  lo counts as one  you will have problems to autodetect the right interface easily   Here are some recipes on how to do it   The problem  for example  is if hosts are in a DMZ behind a NAT firewall which changes the public IP into some private IP and forwards the requests   Your machine may have 10 interfaces  but only one corresponds to the public one   Even autodetection does not work in case you are on double-NAT  where your firewall even translates the source-IP into something completely different   So you cannot even be sure  that the default route leads to your interface with a public interface   Detect it via the default route     This is my recommended way to autodetect things   Something like ip r get 1 1 1 1 usually tells you the interface which has the default route   If you want to recreate this in your favourite scripting programming language  use strace ip r get 1 1 1 1 and follow the yellow brick road   Set it with  etc hosts     This is my recommendation if you want to stay in control   You can create an entry in  etc hosts like  80 190 1 3 publicinterfaceip   Then you can use this alias publicinterfaceip to refer to your public interface      Sadly haproxy does not grok this trick with IPv6   Use the environment     This is a good workaround for  etc hosts in case you are not root   Same as  etc hosts  but use the environment for this   You can try  etc profile or    profile for this   Hence if your program needs a variable MYPUBLICIP then you can include code like  this is C  feel free to create C   from it     define MYPUBLICIPENVVAR  MYPUBLICIP   const char  mypublicip   getenv MYPUBLICIPENVVAR    if   mypublicip    fprintf stderr   please set environment variable  s n   MYPUBLICIPENVVAR   exit 3       So you can call your script program  path to your script like this  MYPUBLICIP 80 190 1 3  path to your script  this even works in crontab   Enumerate all interfaces and eliminate those you do not want     The desperate way if you cannot use ip   If you do know what you do not want  you can enumerate all interfaces and ignore all the false ones   Here already seems to be an answer https   stackoverflow com a 265978 490291 for this approach   Do it like DLNA     The way of the drunken man who tries to drown himself in alcohol   You can try to enumerate all the UPnP gateways on your network and this way find out a proper route for some  external  thing   This even might be on a route where your default route does not point to   For more on this perhaps see https   en wikipedia org wiki Internet Gateway Device Protocol  This gives you a good impression which one is your real public interface  even if your default route points elsewhere   There are even more     Where the mountain meets the prophet   IPv6 routers advertise themselves to give you the right IPv6 prefix   Looking at the prefix gives you a hint about if it has some internal IP or a global one   You can listen for IGMP or IBGP frames to find out some suitable gateway   There are less than 2 32 IP addresses   Hence it does not take long on a LAN to just ping them all   This gives you a statistical hint on where the majority of the Internet is located from your point of view   However you should be a bit more sensible than the famous https   de wikipedia org wiki SQL Slammer  ICMP and even ARP are good sources for network sideband information   It might help you out as well   You can use Ethernet Broadcast address to contact to all your network infrastructure devices which often will help out  like DHCP  even DHCPv6  and so on   This additional list is probably endless and always incomplete  because every manufacturer of network devices is busily inventing new security holes on how to auto-detect their own devices   Which often helps a lot on how to detect some public interface where there shouln t be one    Nuff said   Out

User · Answer

Create a socket  Perform ioctl  lt socketfd gt   SIOCGIFCONF   struct ifconf  amp buffer     Read  usr include linux if h for information on the ifconf and ifreq structures   This should give you the IP address of each interface on the system   Also read  usr include linux sockios h for additional ioctls

User · Answer

Further to what Steve Baker has said  you can find a description of the SIOCGIFCONF ioctl in the netdevice 7  man page   Once you have the list of all the IP addresses on the host  you will have to use application specific logic to filter out the addresses you do not want and hope you have one IP address left

User · Answer

As you have found out there is no such thing as a single  local IP address   Here s how to find out the local address that can be sent out to a specific host    Create a UDP socket Connect the socket to an outside address  the host that will eventually receive the local address  Use getsockname to get the local address

User · Answer

Use a HTTP request to a well known server that echo s back the public IP address void GetPublicIP CString  amp  csIP           Initialize COM     bool bInit   false      if  SUCCEEDED CoInitialize NULL                     COM was initialized         bInit   true          Create a HTTP request object     MSXML2  IXMLHTTPRequestPtr HTTPRequest      HRESULT hr   HTTPRequest CreateInstance  MSXML2 XMLHTTP        if  SUCCEEDED hr                    Build a request to a web site that returns the public IP address         VARIANT Async          Async vt   VT BOOL          Async boolVal   VARIANT FALSE          CComBSTR ccbRequest   L http   whatismyipaddress com                Open the request         if  SUCCEEDED HTTPRequest- gt raw open L GET  ccbRequest Async                             Send the request             if  SUCCEEDED HTTPRequest- gt raw send                                      Get the response                 CString csRequest   HTTPRequest- gt GetresponseText                        Parse the IP address                 CString csMarker     lt  -- contact us before using a script to get your IP address -- gt                    int iPos   csRequest Find csMarker                   if  iPos    -1                      return                  iPos    csMarker GetLength                    int iPos2   csRequest Find csMarker iPos                   if  iPos2    -1                      return                      Build the IP address                 int nCount   iPos2 - iPos                  csIP   csRequest Mid iPos nCount                                       Unitialize COM if  bInit      CoUninitialize

User · Answer

You can do some integration with curl as something as easy as  curl www whatismyip org from the shell will get you your global ip  You re kind of reliant on some external server  but you will always be if you re behind a NAT

User · Answer

As you have found out there is no such thing as a single  local IP address   Here s how to find out the local address that can be sent out to a specific host    Create a UDP socket Connect the socket to an outside address  the host that will eventually receive the local address  Use getsockname to get the local address

User · Answer

As the question specifies Linux  my favourite technique for discovering the IP-addresses of a machine is to use netlink  By creating a netlink socket of the protocol NETLINK ROUTE  and sending an RTM GETADDR  your application will received a message s  containing all available IP addresses  An example is provided here    In order to simply parts of the message handling  libmnl is convenient  If you are curios in figuring out more about the different options of NETLINK ROUTE  and how they are parsed   the best source is the source code of iproute2  especially the monitor application  as well as the receive functions in the kernel  The man page of rtnetlink also contains useful information

User · Answer

I like jjvainio s answer  As Zan Lnyx says  it uses the local routing table to find the IP address of the ethernet interface that would be used for a connection to a specific external host  By using a connected UDP socket  you can get the information without actually sending any packets  The approach requires that you choose a specific external host  Most of the time  any well-known public IP should do the trick  I like Google s public DNS server address 8 8 8 8 for this purpose  but there may be times you d want to choose a different external host IP  Here is some code that illustrates the full approach   void GetPrimaryIp char  buffer  size t buflen         assert buflen  gt   16        int sock   socket AF INET  SOCK DGRAM  0       assert sock    -1        const char  kGoogleDnsIp    8 8 8 8       uint16 t kDnsPort   53      struct sockaddr in serv      memset  amp serv  0  sizeof serv        serv sin family   AF INET      serv sin addr s addr   inet addr kGoogleDnsIp       serv sin port   htons kDnsPort        int err   connect sock   const sockaddr    amp serv  sizeof serv        assert err    -1        sockaddr in name      socklen t namelen   sizeof name       err   getsockname sock   sockaddr    amp name   amp namelen       assert err    -1        const char  p   inet ntop AF INET   amp name sin addr  buffer  buflen       assert p        close sock

User · Answer

I found the ioctl solution problematic on os x (which is POSIX compliant so should be similiar to linux). However getifaddress() will let you do the same thing easily, it works fine for me on os x 10.5 and should be the same below.

I've done a quick example below which will print all of the machine's IPv4 address, (you should also check the getifaddrs was successful ie returns 0).

I've updated it show IPv6 addresses too.

#include <stdio.h>      
#include <sys/types.h>
#include <ifaddrs.h>
#include <netinet/in.h> 
#include <string.h> 
#include <arpa/inet.h>

int main (int argc, const char * argv[]) {
    struct ifaddrs * ifAddrStruct=NULL;
    struct ifaddrs * ifa=NULL;
    void * tmpAddrPtr=NULL;

    getifaddrs(&ifAddrStruct);

    for (ifa = ifAddrStruct; ifa != NULL; ifa = ifa->ifa_next) {
        if (!ifa->ifa_addr) {
            continue;
        }
        if (ifa->ifa_addr->sa_family == AF_INET) { // check it is IP4
            // is a valid IP4 Address
            tmpAddrPtr=&((struct sockaddr_in *)ifa->ifa_addr)->sin_addr;
            char addressBuffer[INET_ADDRSTRLEN];
            inet_ntop(AF_INET, tmpAddrPtr, addressBuffer, INET_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } else if (ifa->ifa_addr->sa_family == AF_INET6) { // check it is IP6
            // is a valid IP6 Address
            tmpAddrPtr=&((struct sockaddr_in6 *)ifa->ifa_addr)->sin6_addr;
            char addressBuffer[INET6_ADDRSTRLEN];
            inet_ntop(AF_INET6, tmpAddrPtr, addressBuffer, INET6_ADDRSTRLEN);
            printf("%s IP Address %s\n", ifa->ifa_name, addressBuffer); 
        } 
    }
    if (ifAddrStruct!=NULL) freeifaddrs(ifAddrStruct);
    return 0;
}

User · Answer

Create a socket  Perform ioctl  lt socketfd gt   SIOCGIFCONF   struct ifconf  amp buffer     Read  usr include linux if h for information on the ifconf and ifreq structures   This should give you the IP address of each interface on the system   Also read  usr include linux sockios h for additional ioctls

User · Answer

I like jjvainio s answer  As Zan Lnyx says  it uses the local routing table to find the IP address of the ethernet interface that would be used for a connection to a specific external host  By using a connected UDP socket  you can get the information without actually sending any packets  The approach requires that you choose a specific external host  Most of the time  any well-known public IP should do the trick  I like Google s public DNS server address 8 8 8 8 for this purpose  but there may be times you d want to choose a different external host IP  Here is some code that illustrates the full approach   void GetPrimaryIp char  buffer  size t buflen         assert buflen  gt   16        int sock   socket AF INET  SOCK DGRAM  0       assert sock    -1        const char  kGoogleDnsIp    8 8 8 8       uint16 t kDnsPort   53      struct sockaddr in serv      memset  amp serv  0  sizeof serv        serv sin family   AF INET      serv sin addr s addr   inet addr kGoogleDnsIp       serv sin port   htons kDnsPort        int err   connect sock   const sockaddr    amp serv  sizeof serv        assert err    -1        sockaddr in name      socklen t namelen   sizeof name       err   getsockname sock   sockaddr    amp name   amp namelen       assert err    -1        const char  p   inet ntop AF INET   amp name sin addr  buffer  buflen       assert p        close sock

User · Answer

As you have found out there is no such thing as a single  local IP address   Here s how to find out the local address that can be sent out to a specific host    Create a UDP socket Connect the socket to an outside address  the host that will eventually receive the local address  Use getsockname to get the local address

User · Answer

This has the advantage of working on many flavors of unix    and you can modify it trivially to work on any o s    All of the solutions above give me compiler errors depending on the phase of the moon   The moment there s a good POSIX way to do it    don t use this  at the time this was written  that wasn t the case       ifconfig   perl -ne  print   1 n  if  inet addr    d         include  lt stdlib h gt   int main             setenv  LANG   C  1           FILE   fp   popen  ifconfig    r            if  fp                    char  p NULL   e  size t n                  while   getline  amp p   amp n  fp   gt  0   amp  amp  p                       if  p   strstr p   inet                               p  5                          if  p   strchr p                                        p                              if  e   strchr p                                            e   0                                    printf   s n   p                                                                                                                  pclose fp           return 0

User · Answer

As the question specifies Linux  my favourite technique for discovering the IP-addresses of a machine is to use netlink  By creating a netlink socket of the protocol NETLINK ROUTE  and sending an RTM GETADDR  your application will received a message s  containing all available IP addresses  An example is provided here    In order to simply parts of the message handling  libmnl is convenient  If you are curios in figuring out more about the different options of NETLINK ROUTE  and how they are parsed   the best source is the source code of iproute2  especially the monitor application  as well as the receive functions in the kernel  The man page of rtnetlink also contains useful information

User · Answer

I do not think there is a definitive right answer to your question   Instead there is a big bundle of ways how to get close to what you wish   Hence I will provide some hints how to get it done   If a machine has more than 2 interfaces  lo counts as one  you will have problems to autodetect the right interface easily   Here are some recipes on how to do it   The problem  for example  is if hosts are in a DMZ behind a NAT firewall which changes the public IP into some private IP and forwards the requests   Your machine may have 10 interfaces  but only one corresponds to the public one   Even autodetection does not work in case you are on double-NAT  where your firewall even translates the source-IP into something completely different   So you cannot even be sure  that the default route leads to your interface with a public interface   Detect it via the default route     This is my recommended way to autodetect things   Something like ip r get 1 1 1 1 usually tells you the interface which has the default route   If you want to recreate this in your favourite scripting programming language  use strace ip r get 1 1 1 1 and follow the yellow brick road   Set it with  etc hosts     This is my recommendation if you want to stay in control   You can create an entry in  etc hosts like  80 190 1 3 publicinterfaceip   Then you can use this alias publicinterfaceip to refer to your public interface      Sadly haproxy does not grok this trick with IPv6   Use the environment     This is a good workaround for  etc hosts in case you are not root   Same as  etc hosts  but use the environment for this   You can try  etc profile or    profile for this   Hence if your program needs a variable MYPUBLICIP then you can include code like  this is C  feel free to create C   from it     define MYPUBLICIPENVVAR  MYPUBLICIP   const char  mypublicip   getenv MYPUBLICIPENVVAR    if   mypublicip    fprintf stderr   please set environment variable  s n   MYPUBLICIPENVVAR   exit 3       So you can call your script program  path to your script like this  MYPUBLICIP 80 190 1 3  path to your script  this even works in crontab   Enumerate all interfaces and eliminate those you do not want     The desperate way if you cannot use ip   If you do know what you do not want  you can enumerate all interfaces and ignore all the false ones   Here already seems to be an answer https   stackoverflow com a 265978 490291 for this approach   Do it like DLNA     The way of the drunken man who tries to drown himself in alcohol   You can try to enumerate all the UPnP gateways on your network and this way find out a proper route for some  external  thing   This even might be on a route where your default route does not point to   For more on this perhaps see https   en wikipedia org wiki Internet Gateway Device Protocol  This gives you a good impression which one is your real public interface  even if your default route points elsewhere   There are even more     Where the mountain meets the prophet   IPv6 routers advertise themselves to give you the right IPv6 prefix   Looking at the prefix gives you a hint about if it has some internal IP or a global one   You can listen for IGMP or IBGP frames to find out some suitable gateway   There are less than 2 32 IP addresses   Hence it does not take long on a LAN to just ping them all   This gives you a statistical hint on where the majority of the Internet is located from your point of view   However you should be a bit more sensible than the famous https   de wikipedia org wiki SQL Slammer  ICMP and even ARP are good sources for network sideband information   It might help you out as well   You can use Ethernet Broadcast address to contact to all your network infrastructure devices which often will help out  like DHCP  even DHCPv6  and so on   This additional list is probably endless and always incomplete  because every manufacturer of network devices is busily inventing new security holes on how to auto-detect their own devices   Which often helps a lot on how to detect some public interface where there shouln t be one    Nuff said   Out

[c++] Get the IP address of the machine

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